Body Stuff Adaptação neural

1: Clin Physiol Funct Imaging. 2007 Mar;27(2):91-100.

Comparative effects of resistance training on peak isometric torque, muscle hypertrophy, voluntary activation and surface EMG between young and elderly women.

Cannon J, Kay D, Tarpenning KM, Marino FE.

Exercise and Sports Science Laboratories, School of Human Movement Studies, Charles Sturt University, Bathurst, NSW, Australia. jcannon@csu.edu.au

We compared the effect of a 10-week resistance training program on peak isometric torque, muscle hypertrophy, voluntary activation and electromyogram signal amplitude (EMG) of the knee extensors between young and elderly women. Nine young women (YW; range 20-30 years) and eight elderly women (EW; 64-78 years) performed three sets of ten repetitions at 75% 1 repetition maximum for the bilateral leg extension and bilateral leg curl 3 days per week for 10 weeks. Peak isometric torque, EMG and voluntary activation were assessed before, during, and after the training period, while knee extensor lean muscle cross-sectional area (LCSA) and lean muscle volume (LMV) were assessed before and after the training period only. Similar increases in peak isometric torque (16% and 18%), LCSA (13% and 12%), LMV (10% and 9%) and EMG (19% and 21%) were observed between YW and EW, respectively, at the completion of training (P<0.05), while the increase in voluntary activation in YW (1.9%) and EW (2.1%) was not significant (P>0.05). These findings provide evidence to indicate that participation in regular resistance exercise can have significant neuromuscular benefits in women independent of age. The lack of change in voluntary activation following resistance training in both age groups despite the increase in EMG may be related to differences between measurements in their ability to detect resistance training-induced changes in motor unit activity. However, it is possible that neural adaptation did not occur and that the increase in EMG was due to peripheral adaptations.

Publication Types:

PMID: 17309529 [PubMed - indexed for MEDLINE]

2: Eur J Appl Physiol. 2006 Jul;97(4):486-93. Epub 2006 Jun 9.

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Excitability of the soleus reflex arc during intensive stretch-shortening cycle exercise in two power-trained athlete groups.

Avela J, Finni J, Komi PV.

Neuromuscular Research Center, Department of Biology of Physical Activity, University of Jyväskylä, 40100 Jyvaskyla, Finland. janne.avela@sport.jyu.fi

In several explosive types of sport events the leg extensor muscles are subjected to very high impact loads. Thus, extreme requirements exist for the neuromuscular system to develop sufficient muscle stiffness in the lower extremities in order to tolerate these high impact loads. Therefore, it would be challenging to measure reflex modulation during high impact activities, and with different athlete populations. In the present experiment, H-reflex and short latency reflex (M1) sensitivity was measured during drop jump exercises among high jumpers and sprinters. The changes in both reflex peak-to-peak amplitudes showed a significant (P < 0.05) reduction towards the end of the exercise for the sprinters. In addition, the same subject group showed a remarkable increase in serum creatine kinase (CK) activity 2 h after the jumps. Similar changes could not be observed for the high jumpers. These results clearly indicate different neural adaptation strategies for the two athlete groups. Reduction in H-reflex sensitivity and an increase in CK-activity in sprinters were taken as evidence for presynaptic inhibition, probably induced by substances related to muscle damage. Since high jump training includes more high impact loading, it was assumed that it could lead to some structural adaptation and, thus, prevents exercise induced reflex modification to a certain extent.

Publication Types:

Comparative Study

PMID: 16763835 [PubMed - indexed for MEDLINE]

3: Eur J Appl Physiol. 2006 Apr;96(6):722-8. Epub 2006 Feb 28.

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Chronic neural adaptation induced by long-term resistance training in humans.

del Olmo MF, Reimunde P, Viana O, Acero RM, Cudeiro J.

Department of Medicine-INEF-Galicia, Laboratory of Neuroscience and Motor Control (NEUROcom), Universidad de A Coruña, 15006 A Coruña, Spain.

While it is known that resistance training causes changes in the central nervous system (CNS) in the initial stages of training, there have been few studies of cumulative or sustained neural adaptation to resistance training beyond the initial periods. To further investigate this we compared the electromyographic (EMG) response to transcranial magnetic stimulation (TMS) during voluntary contractions of ten subjects who have been training for more than 2 years, resistance-training (RT) group, and ten subjects that have never participated in resistance training (NT). The active motor threshold for biceps brachii was obtained during voluntary elbow flexion at 10% of maximal voluntary contraction (MVC). TMS was also delivered at 100% of the maximal stimulator output while the participants exerted forces ranging from 10 to 90% of MVC. Evoked force, motor-evoked potential (MEP) amplitude and latency from biceps brachii was recorded for each condition to explore changes in corticospinal excitability. The evoked force was significantly lower in the RT group in comparison with the NT group between 30 and 70% of MVC intensity (P<0.05). At 90% of MVC, nine subjects from the RT group showed an absence in the evoked force while this occurred in only five subjects from the NT group. The MEP amplitude and latency changed significantly (P<0.001) with increasing levels of contraction, without significant difference between groups. These results indicate that changes in the CNS are sustained in the log-term practices of resistance training and permit a higher voluntary activation at several intensities of the MVC.

Publication Types:

Research Support, Non-U.S. Gov't

PMID: 16506058 [PubMed - indexed for MEDLINE]

4: Eur J Appl Physiol. 2005 Aug;94(5-6):593-601. Epub 2005 May 26.

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Changes in agonist EMG activation level during MVC cannot explain early strength improvement.

Holtermann A, Roeleveld K, Vereijken B, Ettema G.

Human Movement Sciences Program, Faculty of Social Sciences and Technology Management, Norwegian University of Science and Technology (NTNU), Dragvoll Idrettssenter, 7491 Trondheim, Norway.

A substantial gain in strength is often observed in the early phase of resistance training. The aim of this study was to address whether improved strength in the early phase of resistance training, can be attributed to increased activation, or to intra-muscular changes of the agonist muscle during maximal isometric torque production. Fourteen male subjects trained maximal isometric dorsiflexion during 5 days. Each subject performed 9 sessions with 25 maximal voluntary contractions in a device that registered the dorsiflexion torque. Surface electromyography (SEMG) of the tibialis anterior (TA) was recorded with a 130-channel grid electrode. SEMG of the extensor digitorum longus, gastrocnemius and soleus muscles were recorded with bipolar electrodes. The main finding was that all subjects gained in strength while the SEMG activation level of the primer agonist, TA, decreased with no apparent intra-muscular spatial changes following 5 days of resistance training. The other muscles that influence dorsiflexion torque did not modify their activation level with training. These findings reject an increase in agonist activation level as the main source for early strength gain, and illustrate the need for further research to reveal the specific sites of neural adaptation and other physiological mechanisms that might contribute to increased strength during the early phase of resistance training.

Publication Types:

Clinical Trial

PMID: 15918059 [PubMed - indexed for MEDLINE]

5: J Gerontol A Biol Sci Med Sci. 2003 Oct;58(10):B889-94.

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Differential effectiveness of low-intensity exercise in young and old rats.

Brown M, Taylor J, Gabriel R.

Program in Physical Therapy, Washington University School of Medicine, St Louis, Missouri 65211, USA. brownmb@health.missouri.edu

Low-intensity exercise increases strength and function in old adults, but it is unclear if change occurs secondary to "neural adaptation" or to intrinsic muscle adaptation. Whether function and strength change concomitantly is also unclear. We examined effects of a modest intensity, 10-session exercise program on muscle mass, contractile force, and function (gait) in 6-month-old and 30-month-old rats. Animals underwent 45 minutes of activity (e.g., ramp walking, balancing) 5 days/week. In old animals, a significant increase in muscle mass and peak contractile force occurred with exercise in soleus, plantaris, extensor digitorum longus, and peroneus longus compared with controls, but did not restore values to those for young controls. The increase in muscle force in old rats was accompanied by a significant lengthening of stride (90 +/- 9 to 103 +/- 15 mm), which was still 23% less than stride values for young rats. Changes in muscle function and gait with exercise were not apparent in young rats. Results suggest that (a). rapid and significant changes in muscle mass and strength in an aged organism can occur with a modest activity program, (b). the threshold for muscle adaptation may differ in young versus old rats, and (c). changes in strength and function in old rats may occur concomitantly.

Publication Types:

Research Support, U.S. Gov't, P.H.S.

PMID: 14570854 [PubMed - indexed for MEDLINE]

: A single set of low intensity resistance exercise immediately following high intensity resistance exercise stimulates growth hormone secretion in men.

Goto K, Sato K, Takamatsu K.

Doctoral Program in Health and Sport Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan.

AIM: The purpose of the present study was to examine the effects of an additional set immediately following high intensity resistance exercise on growth hormone (GH) response. METHODS: Subjects (n=8) performed 4 resistance exercise protocols (bilateral knee extension exercise) on separate days. The protocols were categorized into 2 types of protocol, namely "Strength-up type (S-type)" and "Combination type (Combi-type)". The S-type was resistance exercise which consisted of 5 sets at 90% of 1 repetition maximum (RM) with 3-min rest periods between sets, whereas the Combi-type is a training protocol which adds an additional set (either 50% of 1 RM [C50-type], 70% of 1 RM [C70-type] or 90% of 1 RM [C90-type]) to the S-type. Serum GH concentration and blood lactate concentration were determined pre-exercise and at 0-60 min postexercise. Relative changes in thigh girth and maximal unilateral isometric strength were determined pre-exercise and immediately postexercise. RESULTS: The increasing values of GH concentration (DGH) in the S-type was the lowest of all protocols. On the other hand, DGH in the C50-type showed a significantly (p<0.05) higher increase than in the S-type and C90-type, and a relatively higher increase than in the C70-type. CONCLUSION: These results suggests that a high intensity, low volume training protocol to induce neural adaptation resulted in little GH response, but GH secretion was increased by performing a single set of low intensity resistance exercise at the end of a series of high intensity resistance sets.

PMID: 12853908 [PubMed - indexed for MEDLINE]

7: Scand J Med Sci Sports. 2003 Apr;13(2):88-97.

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Recent advances in the understanding of the repeated bout effect: the protective effect against muscle damage from a single bout of eccentric exercise.

McHugh MP.

The Nicholas Institute of Sports Medicine and Athletic Trauma, Lenox Hill Hospital, New York, NY, USA. mchugh@nismat.org

The repeated bout effect refers to the adaptation whereby a single bout of eccentric exercise protects against muscle damage from subsequent eccentric bouts. While the mechanism for this adaptation is poorly understood there have been significant recent advances in the understanding of this phenomenon. The purpose of this review is to provide an update on previously proposed theories and address new theories that have been advanced. The potential adaptations have been categorized as neural, mechanical and cellular. There is some evidence to suggest that the repeated bout effect is associated with a shift toward greater recruitment of slow twitch motor units. However, the repeated bout effect has been demonstrated with electrically stimulated contractions, indicating that a peripheral, non-neural adaptation predominates. With respect to mechanical adaptations there is evidence that both dynamic and passive muscle stiffness increase with eccentric training but there are no studies on passive or dynamic stiffness adaptations to a single eccentric bout. The role of the cytoskeleton in regulating dynamic stiffness is a possible area for future research. With respect to cellular adaptations there is evidence of longitudinal addition of sarcomeres and adaptations in the inflammatory response following an initial bout of eccentric exercise. Addition of sarcomeres is thought to reduce sarcomere strain during eccentric contractions thereby avoiding sarcomere disruption. Inflammatory adaptations are thought to limit the proliferation of damage that typically occurs in the days following eccentric exercise. In conclusion, there have been significant advances in the understanding of the repeated bout effect, however, a unified theory explaining the mechanism or mechanisms for this protective adaptation remains elusive.

Publication Types:

Review

PMID: 12641640 [PubMed - indexed for MEDLINE]

8: Sports Med. 2000 Dec;30(6):385-94.

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A proposed model for examining the interference phenomenon between concurrent aerobic and strength training.

Docherty D, Sporer B.

School of Physical Education, University of Victoria, British Columbia, Canada. docherty@uvic.ca

A review of the current research on the interference phenomenon between concurrent aerobic and strength training indicates modest support for the model proposed in this article. However, it is clear that without a systematic approach to the investigation of the phenomenon there is lack of control and manipulation of the independent variables, which makes it difficult to test the validity of the model. To enhance the understanding of the interference phenomenon, it is important that researchers are precise and deliberate in their choice of training protocols. Clear definition of the specific training objectives for strength (muscle hypertrophy or neural adaptation) and aerobic power (maximal aerobic power or anaerobic threshold) are required. In addition, researchers should equate training volumes as much as possible for all groups. Care needs to be exercised to avoid overtraining individuals. There should be adequate recovery and regeneration between the concurrent training sessions as well as during the training cycle. The model should be initially tested by maintaining the same protocols throughout the duration of the study. However, it is becoming common practice to use a periodised approach in a training mesocycle in which there is a shift from high volume and moderate intensity training to tower volume and higher intensity. The model should be evaluated in the context of a periodised mesocycle provided the investigators are sensitive to the potential impact of the loading parameters on the interference phenomenon. It may be that the periodised approach is one way of maintaining the training stimulus and minimising the amount of interference. The effects of gender, training status, duration and frequency of training, and the mode of training need to be regarded as potential factors effecting the training response when investigating the interference phenomenon. Other experimental design factors such as unilateral limb training or training the upper body for one attribute and the lower body for another attribute, may help establish the validity of the model.

Publication Types:

Review

PMID: 11132121 [PubMed - indexed for MEDLINE]

9: Eur J Appl Physiol. 2000 Sep;83(1):51-62.

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Neuromuscular adaptation during prolonged strength training, detraining and re-strength-training in middle-aged and elderly people.

Häkkinen K, Alen M, Kallinen M, Newton RU, Kraemer WJ.

Neuromuscular Research Centre and Department of Biology of Physical Activity, University of Jyväskylä, Finland. Hakkinen@Maila.jyu.fi

Effects of a 24-week strength training performed twice weekly (24 ST) (combined with explosive exercises) followed by either a 3-week detraining (3 DT) and a 21-week re-strength-training (21 RST) (experiment A) or by a 24-week detraining (24 DT) (experiment B) on neural activation of the agonist and antagonist leg extensors, muscle cross-sectional area (CSA) of the quadriceps femoris, maximal isometric and one repetition maximum (1-RM) strength and jumping (J) and walking (W) performances were examined. A group of middle-aged (M, 37-44 years, n = 12) and elderly (E, 62-77, n = 10) and another group of M (35-45, n = 7) and E (63-78, n = 7) served as subjects. In experiment A, the 1-RM increased substantially during 24 ST in M (27%, P<0.001) and E (29%, P<0.001) and in experiment B in M (29%, P<0.001) and E (23%, P<0.01). During 21 RST the 1-RM was increased by 5% at week 48 (P<0.01) in M and 3% at week 41 in E (n.s., but P<0.05 at week 34). In experiment A the integrated electromyogram (IEMG) of the vastus muscles in the 1-RM increased during 24 ST in both M (P<0.05) and E (P<0.001) and during 21 RST in M for the right (P<0.05) and in E for both legs (P<0.05). The biceps femoris co-activation during the 1-RM leg extension decreased during the first 8-week training in M (from 29+/-5% to 25+/-3%, n.s.) and especially in E (from 41+/-11% to 32+/-9%, P<0.05). The CSA increased by 7% in M (P<0.05) and by 7% in E (P<0.001), and by 7% (n.s.) in M and by 3% in E (n.s.) during 24 ST periods. Increases of 18% (P<0.001) and 12% (P<0.05) in M and 22% (P<0.001) and 26% (P<0.05) in E occurred in J. W speed increased (P<0.05) in both age groups. The only decrease during 3 DT was in maximal isometric force in M by 6% (P<0.05) and by 4% (n.s.) in E. During 24 DT the CSA decreased in both age groups (P<0.01), the 1-RM decreased by 6% (P<0.05) in M and by 4% (P<0.05) in E and isometric force by 12% (P<0.001) in M and by 9% (P<0.05) in E, respectively, while J and W remained unaltered. The strength gains were accompanied by increased maximal voluntary neural activation of the agonists in both age groups with reduced antagonist co-activation in the elderly during the initial training phases. Neural adaptation seemed to play a greater role than muscle hypertrophy. Short-term detraining led to only minor changes, while prolonged detraining resulted in muscle atrophy and decreased voluntary strength, but explosive jumping and walking actions in both age groups appeared to remain elevated for quite a long time by compensatory types of physical activities when performed on a regular basis.

Publication Types:

Research Support, Non-U.S. Gov't

PMID: 11072774 [PubMed - indexed for MEDLINE]

: Plasma catecholamine responses and neural adaptation during short-term resistance training.

Pullinen T, Huttunen P, Komi PV.

Neuromuscular Research Center, Department of Biology of Physical Activity, University of Jyväkylä, Finland. tpulline@pallo.jyu.fi

Low exercise-induced plasma adrenaline (A) responses have been reported in resistance-trained individuals. In the study reported here, we investigated the interaction between strength gain and neural adaptation of the muscles, and the plasma A response in eight healthy men during a short-term resistance-training period. The subjects performed 5 resistance exercises (E1-E5), consisting of 6 sets of 12 bilateral leg extensions performed at a 50% load, and with 2 days rest in between. Average electromyographic (EMG) signal amplitude was recorded before and after the exercises, from the knee extensor muscles in isometric maximal voluntary contraction (MVC) as well as during the exercises (aEMGmax and aEMGexerc, respectively). Total oxygen consumed during the exercises (VO2tot) was also measured. All of the exercises were exhaustive and caused significant decreases in MVC (34-36%, P < 0.001). As expected, the concentric one-repetition maximum (1-RM), MVC and aEMGmax were all higher before the last exercise (E5) than before the first exercise (E1; 7, 9 and 19%, respectively, P < 0.05). In addition, in E5 the aEMGexerc:load and VO2tot:load ratios were lower than in E1 (-5 and -14%, P < 0.05), indicating enhanced efficiency of the muscle contractions, However, the post-exercise plasma noradrenaline (NA) and A were not different in these two exercises [mean (SD) 10.2 (3.8) nmol x l(-1) vs 11.3 (6.0) nmol x l(-1), ns, and 1.2 (1.0) nmol x l(-1) vs 1.9 (1.1) nmol x l(-1), ns, respectively]. However, although NA increased similarly in every exercise (P < 0.01), the increase in A reached the level of statistical significance only in E1 (P < 0.05). The post-exercise A was also already lower in E2 [0.7 (0.7) nmol x l(-1), P < 0.05) than in E1, despite the higher post-exercise blood lactate concentration than in the other exercises [9.4 (1.1) mmol x l(-1), P < 0.05]. Thus, the results suggest that the observed attenuation in the A response can not be explained by reduced exercise-induced strain due to the strength gain and neural adaptation of the muscles. Correlation analysis actually revealed that those individuals who had the highest strength gain during the training period even tended to have an increased post-exercise A concentration in the last exercise as compared to first one (r = 0.76, P < 0.05).

PMID: 10879445 [PubMed - indexed for MEDLINE]

11: Sports Med. 1999 Mar;27(3):157-70.

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Exercise-induced muscle damage and potential mechanisms for the repeated bout effect.

McHugh MP, Connolly DA, Eston RG, Gleim GW.

School of Sport, Health and Physical Education Sciences, University of Wales, Bangor, Gwynedd, Wales. mchugh@nismat.org

Unfamiliar, predominantly eccentric exercise, frequently results in muscle damage. A repeated bout of similar eccentric exercise results in less damage and is referred to as the 'repeated bout effect'. Despite numerous studies that have clearly demonstrated the repeated bout effect, there is little consensus as to the actual mechanism. In general, the adaptation has been attributed to neural, connective tissue or cellular adaptations. Other possible mechanisms include, adaptation in excitation-contraction coupling or adaptation in the inflammatory response. The 'neural theory' predicts that the initial damage is a result of high stress on a relatively small number of active fast-twitch fibres. For the repeated bout, an increase in motor unit activation and/or a shift to slow-twitch fibre activation distributes the contractile stress over a larger number of active fibres. Although eccentric training results in marked increases in motor unit activation, specific adaptations to a single bout of eccentric exercise have not been examined. The 'connective tissue theory' predicts that muscle damage occurs when the noncontractile connective tissue elements are disrupted and myofibrillar integrity is lost. Indirect evidence suggests that remodelling of the intermediate filaments and/or increased intramuscular connective tissue are responsible for the repeated bout effect. The 'cellular theory' predicts that muscle damage is the result of irreversible sarcomere strain during eccentric contractions. Sarcomere lengths are thought to be highly non-uniform during eccentric contractions, with some sarcomeres stretched beyond myofilament overlap. Loss of contractile integrity results in sarcomere strain and is seen as the initial stage of damage. Some data suggest that an increase in the number of sarcomeres connected in series, following an initial bout, reduces sarcomere strain during a repeated bout and limits the subsequent damage. It is unlikely that one theory can explain all of the various observations of the repeated bout effect found in the literature. That the phenomenon occurs in electrically stimulated contractions in an animal model precludes an exclusive neural adaptation. Connective tissue and cellular adaptations are unlikely explanations when the repeated bout effect is demonstrated prior to full recovery, and when the fact that the initial bout does not have to cause appreciable damage in order to provide a protective effect is considered. It is possible that the repeated bout effect occurs through the interaction of various neural, connective tissue and cellular factors that are dependent on the particulars of the eccentric exercise bout and the specific muscle groups involved.

Publication Types:

Review

PMID: 10222539 [PubMed - indexed for MEDLINE]

12: Eur J Appl Physiol Occup Physiol. 1998;77(1-2):170-5.

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A comparison of strength and muscle mass increases during resistance training in young women.

Chilibeck PD, Calder AW, Sale DG, Webber CE.

Department of Kinesiology, McMaster University, Hamilton, Ontario, Canada.

Strength gains with resistance training are due to muscle hypertrophy and nervous system adaptations. The contribution of either factor may be related to the complexity of the exercise task used during training. The purpose of this investigation was to measure the degree to which muscle hypertrophy contributes to gains in strength during exercises of varying complexity. Nineteen young women resistance trained twice a week for 20 weeks, performing exercises designed to provide whole-body training. The lean mass of the trunk, legs and arms was measured by dual energy x-ray absorptiometry and compared to strength gains (measured as the 1-repetition maximum) in bench press, leg press and arm curl exercises, pre-, mid- (10 weeks) and post-training. No changes were found in a control group of ten women. For the exercise group, increases in bench press, leg press and arm curl strength were significant from pre- to mid-, and from mid- to post-training (P < 0.05). In contrast, increases in the lean mass of the body segments used in these exercises followed a different pattern. Increases in the lean mass of the arms were significant from pre- to mid-training, while increases in the lean mass of the trunk and legs were delayed and significant from mid- to post-training only (P < 0.05). It is concluded that a more prolonged neural adaptation related to the more complex bench and leg press movements may have delayed hypertrophy in the trunk and legs. With the simpler arm curl exercise, early gains in strength were accompanied by muscle hypertrophy and, presumably, a faster neural adaptation.

Publication Types:

PMID: 9459538 [PubMed - indexed for MEDLINE]

13: J Orthop Sports Phys Ther. 1998 Jan;27(1):3-8.

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Weight training of the thigh muscles using closed vs. open kinetic chain exercises: a comparison of performance enhancement.

Augustsson J, Esko A, Thomeé R, Svantesson U.

Department of Rehabilitation Medicine, Göteborg University, Sweden.

Resistance training is commonly used in sports for prevention of injuries and in rehabilitation. The purpose of this study was to compare closed vs. open kinetic chain weight training of the thigh muscles and to determine which mode resulted in the greatest performance enhancement. Twenty-four healthy subjects were randomized into a barbell squat or a knee extension and hip adduction variable resistance weight machine group and performed maximal, progressive weight training twice a week for 6 weeks. All subjects were tested prior to training and at the completion of the training period. A barbell squat 3-repetition maximum, an isokinetic knee extension 1-repetition maximum, and a vertical jump test were used to monitor effects of training. Significant improvements were seen in both groups in the barbell squat 3-repetition maximum test. The closed kinetic chain group improved 23 kg (31%), which was significantly more than the 12 kg (13%) seen in the open kinetic chain group. In the vertical jump test, the closed kinetic chain group improved significantly, 5 cm (10%), while no significant changes were seen in the open kinetic chain group. A large increase of training load was observed in both subject groups; however, improvements in isotonic strength did not transfer to the isokinetic knee extension test. The results may be explained by neural adaptation, weight training mode, and specificity of tests.

Publication Types:

PMID: 9440034 [PubMed - indexed for MEDLINE]

14: Clin Physiol. 1997 May;17(3):311-24.

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Dynamic muscle strength alterations to detraining and retraining in elderly men.

Taaffe DR, Marcus R.

Musculoskeletal Research Laboratory, Veterans Affairs Medical Center, Palo Alto, California, USA.

To investigate the effects of cessation and subsequent resumption of training on muscle strength in elderly men, 11 men (aged 65-77 years), just completing a 24-week randomized controlled trial of recombinant human growth hormone (rhGH) and resistance exercise (rhGH, n = 6; placebo, n = 5), detrained for 12 weeks and subsequently retrained for 8 weeks. During the detraining and retraining phase, subjects did not receive rhGH. The resistance programme included three sets of eight repetitions at 75% of one-repetition maximum (1-RM), three times per week, for 10 upper and lower body exercises. Dynamic muscle strength was assessed by the 1-RM method every 2 weeks for 44 weeks. Needle biopsies of vastus lateralis muscle were obtained from seven men. Muscle strength increased during initial training by 40.4 +/- 5.5% (mean +/- SEM), ranging from 26.0 +/- 5.0 to 83.9 +/- 15.6%, depending on muscle group. Increased strength was accompanied by hypertrophy (P < 0.05) of type I (17.4 +/- 4.1%) and II (25.8 +/- 12.4%) muscle fibres. Of initial strength gains, only 29.9 +/- 5.2% was lost with detraining. However, type I and II fibre cross-sectional area reverted to pretraining values. After 8 weeks of retraining, muscle strength returned to trained values, but without a significant change in fibre morphology. The results indicate that elderly men lose some muscle strength following short-term detraining, but that only a brief period of retraining suffices to regain maximal strength. Reversal of fibre cross-sectional area with detraining, and only modest improvement with retraining, suggests that much of the retention in strength with detraining and reacquisition of lost strength with retraining reflects neural adaptation.

Publication Types:

PMID: 9171971 [PubMed - indexed for MEDLINE]

15: J Appl Physiol. 1996 Mar;80(3):765-72.

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Adaptive responses to muscle lengthening and shortening in humans.

Hortobágyi T, Hill JP, Houmard JA, Fraser DD, Lambert NJ, Israel RG.

Biomechanics Laboratory and Department of Medicine and Physical Therapy, East Carolina University, Greenville, North Carolina 27858, USA.

We tested the hypothesis that exercise training with maximal eccentric (lengthening) muscle actions results in greater gains in muscle strength and size than training with concentric (shortening) actions. Changes in muscle strength, muscle fiber size, and surface electromyographic (EMG) activity of the quadriceps muscle were compared after 36 sessions of isokinetic concentric (n = 8) or eccentric (n = 7) exercise training over 12 wk with use of a one-leg model. Eccentric training increased eccentric strength 3.5 times more (pre/post 46%, P < 0.05) than concentric training increased concentric strength (pre/post 13%). Eccentric training increased concentric strength and concentric training increased eccentric strength by about the same magnitude (5 and 10%, respectively, P > 0.05). Eccentric training increased EMG activity seven times more during eccentric testing (pre/post 86%, P < 0.05) than concentric training increased EMG activity during concentric testing (pre/post 12%). Eccentric training increased the EMG activity measured during concentric tests and concentric training increased the EMG activity measured during eccentric tests by about the same magnitude (8 and 11%, respectively, P > 0.05). Type I muscle fiber percentages did not change significantly, but type IIa fibers increased and type IIb fibers decreased significantly (P < 0.05) in both training groups. Type I fiber areas did not change significantly (P > 0.05), but type II fiber area increased approximately 10 times more (P < 0.05) in the eccentric than in the concentric group. It is concluded that adaptations to training with maximal eccentric contractions are specific to eccentric muscle actions that are associated with greater neural adaptation and muscle hypertrophy than concentric exercise.

Publication Types:

PMID: 8964735 [PubMed - indexed for MEDLINE]

16: Aviat Space Environ Med. 1995 Mar;66(3):251-5.

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Training effects of electrically induced dynamic contractions in human quadriceps muscle.

Kim CK, Takala TE, Seger J, Karpakka J.

Department of Physiology and Pharmacology, Karolinska Institute, Stockholm, Sweden.

The effects of electrically induced dynamic muscle contractions on muscle endurance, strength, morphology and enzymatic adaptation were studied in seven male physical education students. The training program consisted of electrically induced one-leg extensions on a modified Krogh cycle with a 30-Watt (W) load for 60 min, 3 times a week for 4 weeks. Muscle fiber type composition was unchanged, but diffusional capacity was increased after electromyostimulation training. The endurance capacity in the trained leg increased by 82% (p < 0.01), but there were no significant changes in citrate synthase, phosphofructokinase activities, and carbonic anhydrase III and myoglobin contents, suggesting that neural adaptation and learning were more important factors for the increased endurance capacity than enzymatic adaptation. Prolyl 4-hydroxylase activity, a marker of collagen biosynthesis, increased 3-fold (p < 0.01) as a result of the training. This could be due to muscle damage caused by electrically induced muscle contractions. In conclusion, electrically induced dynamic muscle contractions can increase muscle endurance without clear concominant changes in muscle morphologic and enzymatic adaptation. Increased prolyl 4-hydroxylase activity could suggest muscle damage caused by electrically induced muscle contractions.

Publication Types:

PMID: 7661836 [PubMed - indexed for MEDLINE]

17: Acta Physiol Scand. 1990 Sep;140(1):31-9.

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Effects of eccentric and concentric muscle actions in resistance training.

Colliander EB, Tesch PA.

Department of Environmental Medicine, Karolinska Institute, Stockholm, Sweden.

The adaptive responses to two different resistance training regimens were compared. Healthy males performed five sets of either 12 maximum bilateral concentric (Grp CON; n = 11) or six pairs of maximum bilateral eccentric and concentric (Grp ECCON; n = 11) quadriceps muscle actions three times per week for 12 weeks. Uni- and bilateral eccentric and concentric peak torque at various angular velocities, vertical jump height and three-repetition maximum half-squat were measured before and after training. Muscle biopsies were obtained from m. vastus lateralis and analysed for fibre type composition and area using histochemical techniques. In contrast to a control group (n = 7), performing no training, Grps CON and ECCON demonstrated marked increases (P less than 0.05) in overall eccentric (19 and 37% respectively) and concentric (15 and 26% respectively) peak torques. Grp ECCON, however, showed greater (P less than 0.05) increases in peak torque, vertical jump height and three repetition maximum than Grp CON. The 7% increases in slow-twitch fibre area in Grps CON and ECCON and in fast-twitch fibre area in Grp CON were non-significant. This study suggests that increases in peak torque and strength-related performance parameters were greater following a programme consisting of maximum concentric and eccentric muscle actions than resistance training using concentric muscle actions only. Because increases in muscle fibre areas were small it is also suggested that the increased muscle strength shown subsequent to short-term accommodated resistance training is mainly due to neural adaptation.

Publication Types:

Research Support, Non-U.S. Gov't

PMID: 2275403 [PubMed - indexed for MEDLINE]

18: Med Sci Sports Exerc. 1988 Oct;20(5 Suppl):S135-45.

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Neural adaptation to resistance training.

Sale DG.

Department of Physical Education, McMaster University, Hamilton, Ontario, Canada.

Strength performance depends not only on the quantity and quality of the involved muscles, but also upon the ability of the nervous system to appropriately activate the muscles. Strength training may cause adaptive changes within the nervous system that allow a trainee to more fully activate prime movers in specific movements and to better coordinate the activation of all relevant muscles, thereby effecting a greater net force in the intended direction of movement. The evidence indicating neural adaptation is reviewed. Electromyographic studies have provided the most direct evidence. They have shown that increases in peak force and rate of force development are associated with increased activation of prime mover muscles. Possible reflex adaptations related to high stretch loads in jumping and rapid reciprocal movements have also been revealed. Other studies, including those that demonstrate the "cross-training" effect and specificity of training, provide further evidence of neural adaptation. The possible mechanisms of neural adaptation are discussed in relation to motor unit recruitment and firing patterns. The relative roles of neural and muscular adaptation in short- and long-term strength training are evaluated.

Publication Types:

Review

PMID: 3057313 [PubMed - indexed for MEDLINE]

1: J Physiol. 2002 Oct 15;544(Pt 2):641-52.

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The sites of neural adaptation induced by resistance training in humans.

Carroll TJ, Riek S, Carson RG.

Perception and Motor Systems Laboratory, The School of Human Movement Studies, The University of Queensland, Brisbane, Queensland 4072, Australia. tcarroll@ualberta.ca

Although it has long been supposed that resistance training causes adaptive changes in the CNS, the sites and nature of these adaptations have not previously been identified. In order to determine whether the neural adaptations to resistance training occur to a greater extent at cortical or subcortical sites in the CNS, we compared the effects of resistance training on the electromyographic (EMG) responses to transcranial magnetic (TMS) and electrical (TES) stimulation. Motor evoked potentials (MEPs) were recorded from the first dorsal interosseous muscle of 16 individuals before and after 4 weeks of resistance training for the index finger abductors (n = 8), or training involving finger abduction-adduction without external resistance (n = 8). TMS was delivered at rest at intensities from 5 % below the passive threshold to the maximal output of the stimulator. TMS and TES were also delivered at the active threshold intensity while the participants exerted torques ranging from 5 to 60 % of their maximum voluntary contraction (MVC) torque. The average latency of MEPs elicited by TES was significantly shorter than that of TMS MEPs (TES latency = 21.5 +/- 1.4 ms; TMS latency = 23.4 +/- 1.4 ms; P < 0.05), which indicates that the site of activation differed between the two forms of stimulation. Training resulted in a significant increase in MVC torque for the resistance-training group, but not the control group. There were no statistically significant changes in the corticospinal properties measured at rest for either group. For the active trials involving both TMS and TES, however, the slope of the relationship between MEP size and the torque exerted was significantly lower after training for the resistance-training group (P < 0.05). Thus, for a specific level of muscle activity, the magnitude of the EMG responses to both forms of transcranial stimulation were smaller following resistance training. These results suggest that resistance training changes the functional properties of spinal cord circuitry in humans, but does not substantially affect the organisation of the motor cortex.

Publication Types:

Research Support, Non-U.S. Gov't

PMID: 12381833 [PubMed - indexed for MEDLINE]

PMCID: PMC2290590

: Mechanomyographic response to transcranial magnetic stimulation from biceps brachii and during transcutaneous electrical nerve stimulation on extensor carpi radialis.

Reza MF, Ikoma K, Chuma T, Mano Y.

Department of Rehabilitation and Physical Medicine, Hokkaido University Graduate School of Medicine, N15 W7, Sapporo 060-8638, Japan.

Transcranial magnetic stimulation (TMS) elicits short latency excitatory responses in the target muscles, termed motor evoked potential (MEP). When TMS is delivered during a voluntary contraction, the MEP is followed by a period of silence called silent period (SP). These MEP parameters are in general recordable by electromyography (EMG). Mechanomyography (MMG) on the other hand is the mechanical counterpart of EMG. Thus, this study has been conducted to observe whether the MEP parameters from MMG signals showed similar trait of EMG recordings. Five normal healthy male subjects were included in this study. The subjects were required to perform right biceps brachii muscles contraction at diverse graded of load level at 5, 10, 20, 30, 40, 60, and 100% maximum voluntary contraction (MVC). MEPs by single pulse TMS on left hemisphere were obtained from both EMG electrode and MMG accelerometer at rest and at different levels of predetermined load level. MEP amplitude and area obtained both from EMG and MMG record were increased with the increase of muscle contraction with a maximum of 60% MVC. With increasing the level of contraction there was a shortening of onset latency and decreasing in the length of silent period in both EMG and MMG signals. We also recorded the EMG- and MMG-MEP from the right extensor carpi radialis muscle during transcutaneous electric nerve stimulation in order to observe neural changes in sensory stimulation from both EMG and MMG responses. The EMG-MEP was not visible in electrical artifact whereas it was obvious in MMG responses. In accordance with other study, this study showed that the voluntary contraction of biceps brachii muscle influenced the MEP parameter which are moreover obtainable by MMG even in electrical noise may provide insight for future study.

PMID: 16026847 [PubMed - indexed for MEDLINE]

3: J Appl Physiol. 2005 Oct;99(4):1558-68. Epub 2005 May 12.

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Motor skill training and strength training are associated with different plastic changes in the central nervous system.

Jensen JL, Marstrand PC, Nielsen JB.

Department of Medical Physiology, Panum Institute, University of Copenhagen, Denmark. j.b.nielsen@mfi.ku.dk

Changes in corticospinal excitability induced by 4 wk of heavy strength training or visuomotor skill learning were investigated in 24 healthy human subjects. Measurements of the input-output relation for biceps brachii motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation were obtained at rest and during voluntary contraction in the course of the training. The training paradigms induced specific changes in the motor performance capacity of the subjects. The strength training group increased maximal dynamic and isometric muscle strength by 31% (P < 0.001) and 12.5% (P = 0.045), respectively. The skill learning group improved skill performance significantly (P < 0.001). With one training bout, the only significant change in transcranial magnetic stimulation parameters was an increase in skill learning group maximal MEP level (MEP(max)) at rest (P = 0.02) for subjects performing skill training. With repeated skill training three times per week for 4 wk, MEP(max) increased and the minimal stimulation intensity required to elicit MEPs decreased significantly at rest and during contraction (P < 0.05). In contrast, MEP(max) and the slope of the input-output relation both decreased significantly at rest but not during contraction in the strength-trained subjects (P < or = 0.01). No significant changes were observed in a control group. A significant correlation between changes in neurophysiological parameters and motor performance was observed for skill learning but not strength training. The data show that increased corticospinal excitability may develop over several weeks of skill training and indicate that these changes may be of importance for task acquisition. Because strength training was not accompanied by similar changes, the data suggest that different adaptive changes are involved in neural adaptation to strength training.

Publication Types:

Research Support, Non-U.S. Gov't

PMID: 15890749 [PubMed - indexed for MEDLINE]

4: Sports Med. 2006;36(2):133-49.

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Neural adaptations to resistive exercise: mechanisms and recommendations for training practices.

Gabriel DA, Kamen G, Frost G.

Department of Physical Education and Kinesiology, Brock University, St Catharines, Ontario, Canada. dgabriel@brocku.ca

It is generally accepted that neural factors play an important role in muscle strength gains. This article reviews the neural adaptations in strength, with the goal of laying the foundations for practical applications in sports medicine and rehabilitation.An increase in muscular strength without noticeable hypertrophy is the first line of evidence for neural involvement in acquisition of muscular strength. The use of surface electromyographic (SEMG) techniques reveal that strength gains in the early phase of a training regimen are associated with an increase in the amplitude of SEMG activity. This has been interpreted as an increase in neural drive, which denotes the magnitude of efferent neural output from the CNS to active muscle fibres. However, SEMG activity is a global measure of muscle activity. Underlying alterations in SEMG activity are changes in motor unit firing patterns as measured by indwelling (wire or needle) electrodes. Some studies have reported a transient increase in motor unit firing rate. Training-related increases in the rate of tension development have also been linked with an increased probability of doublet firing in individual motor units. A doublet is a very short interspike interval in a motor unit train, and usually occurs at the onset of a muscular contraction. Motor unit synchronisation is another possible mechanism for increases in muscle strength, but has yet to be definitely demonstrated.There are several lines of evidence for central control of training-related adaptation to resistive exercise. Mental practice using imagined contractions has been shown to increase the excitability of the cortical areas involved in movement and motion planning. However, training using imagined contractions is unlikely to be as effective as physical training, and it may be more applicable to rehabilitation.Retention of strength gains after dissipation of physiological effects demonstrates a strong practice effect. Bilateral contractions are associated with lower SEMG and strength compared with unilateral contractions of the same muscle group. SEMG magnitude is lower for eccentric contractions than for concentric contractions. However, resistive training can reverse these trends. The last line of evidence presented involves the notion that unilateral resistive exercise of a specific limb will also result in training effects in the unexercised contralateral limb (cross-transfer or cross-education). Peripheral involvement in training-related strength increases is much more uncertain. Changes in the sensory receptors (i.e. Golgi tendon organs) may lead to disinhibition and an increased expression of muscular force.Agonist muscle activity results in limb movement in the desired direction, while antagonist activity opposes that motion. Both decreases and increases in co-activation of the antagonist have been demonstrated. A reduction in antagonist co-activation would allow increased expression of agonist muscle force, while an increase in antagonist co-activation is important for maintaining the integrity of the joint. Thus far, it is not clear what the CNS will optimise: force production or joint integrity.The following recommendations are made by the authors based on the existing literature. Motor learning theory and imagined contractions should be incorporated into strength-training practice. Static contractions at greater muscle lengths will transfer across more joint angles. Submaximal eccentric contractions should be used when there are issues of muscle pain, detraining or limb immobilisation. The reversal of antagonists (antagonist-to-agonist) proprioceptive neuromuscular facilitation contraction pattern would be useful to increase the rate of tension development in older adults, thus serving as an important prophylactic in preventing falls. When evaluating the neural changes induced by strength training using EMG recording, antagonist EMG activity should always be measured and evaluated.

Publication Types:

PMID: 16464122 [PubMed - indexed for MEDLINE]

5: J Neurosci. 2003 Nov 12;23(32):10224-30.

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Responses of human motoneurons to corticospinal stimulation during maximal voluntary contractions and ischemia.

Butler JE, Taylor JL, Gandevia SC.

Prince of Wales Medical Research Institute and University of New South Wales, Randwick, New South Wales, 2031 Australia.

The discharge frequency of human motoneurons declines during a sustained isometric maximal voluntary contraction (MVC) of elbow flexor muscles, but the cause is unresolved. We aimed to determine whether motoneurons were inhibited during a sustained fatiguing contraction of the elbow flexor muscles and whether this inhibition was caused by the discharge of group III and IV muscle afferents. Subjects performed brief MVCs before and after a fatiguing 2 min MVC. During maximal efforts, electromyographic responses recorded from the elbow flexor muscles were evoked by stimulation of the corticospinal tracts at the cervicomedullary level [cervicomedullary motor evoked potentials (CMEPs)] and by supramaximal stimulation over the brachial plexus (Mmax). This revealed a novel decrease in the size of the muscle response to corticospinal tract stimulation during fatigue. During the sustained MVCs, the size of CMEPs decreased to 81 +/- 15 and 78 +/- 15% of the control value for brachioradialis and biceps brachii, respectively (mean +/- SEM; n = 8). This recovered within 15 sec after the fatiguing contraction. In a second set of studies, input from group III and IV muscle afferents was prolonged after the end of the fatiguing contraction by holding the muscle ischemic with a cuff inflated above arterial pressure. Despite the maintained discharge of group III and IV afferents, the CMEPs again recovered within 15 sec of the end of the sustained contraction. These results show a diminished output of spinal motoneurons to stimulation of corticospinal tracts during a fatiguing MVC; however, the mechanisms responsible for this decline are not attributable to activity in group III and IV muscle afferents.

Publication Types:

PMID: 14614080 [PubMed - indexed for MEDLINE]

6: J Neurosci. 2006 May 3;26(18):4796-802.

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Fatigue-sensitive afferents inhibit extensor but not flexor motoneurons in humans.

Martin PG, Smith JL, Butler JE, Gandevia SC, Taylor JL.

Prince of Wales Medical Research Institute, University of New South Wales, Randwick, New South Wales 2031, Australia.

The role of group III and IV muscle afferents in controlling the output from human muscles is poorly understood. We investigated the effects of these afferents from homonymous or antagonist muscles on motoneuron pools innervating extensor and flexor muscles of the elbow. In study 1, subjects (n = 8) performed brief maximal voluntary contractions (MVCs) of elbow extensors before and after a 2 min MVC of the extensors. During MVCs, electromyographic responses from triceps were evoked by stimulation of the corticospinal tracts [cervicomedullary motor evoked potentials (CMEPs)]. The same subjects repeated the protocol, but input from fatigue-sensitive afferents was prolonged after the fatiguing contraction by maintained muscle ischemia. In study 2, CMEPs were evoked in triceps during brief extensor MVCs before and after a 2 min sustained flexor MVC (n = 7) or in biceps during brief flexor MVCs before and after a sustained extensor MVC (n = 7). Again, ischemia was maintained after the sustained contractions. During sustained MVCs of the extensors, CMEPs in triceps decreased by approximately 35%. Without muscle ischemia, CMEPs recovered within 15 s, but with maintained ischemia, they remained depressed (by approximately 28%; p < 0.001). CMEPs in triceps were also depressed (by approximately 20%; p < 0.001) after fatiguing flexor contractions, whereas CMEPs in biceps were facilitated (by approximately 25%; p < 0.001) after fatiguing extensor contractions. During fatigue, inputs from group III and IV muscle afferents from homonymous or antagonist muscles depress extensor motoneurons but facilitate flexor motoneurons. The more pronounced inhibitory influence of these afferents on extensors suggests that these muscles may require greater cortical drive to generate force during fatigue.

Publication Types:

PMID: 16672652 [PubMed - indexed for MEDLINE]

: Maximal motor unit firing rates during isometric resistance training in men.

Pucci AR, Griffin L, Cafarelli E.

School of Kinesiology and Health Science, York University, Toronto, ON, Canada.

This study measured changes in maximal voluntary contraction (MVC) force, percentage maximal activation, maximal surface EMG, M-wave amplitude and average motor unit firing rates during the initial 3 weeks of isometric resistance training of the quadricep