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Neuromuscular Aspects of Sport Performance, Volume XVII

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IntroductionBasic Theories of Strength and Power TrainingMotor Units Characteristics and Muscle Activation LevelNeural AdaptationsMuscular AdaptationsConclusions References

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... Although increases in maximal strength are due to both muscle and neural adaptations, the purpose of our article is not to provide an in-depth review of the molecular mechanisms underlying muscle hypertrophy or to discuss the numerous neural adaptations induced by strength training. Such information can be found in the following reviews on muscle (13)(14)(15)(16)(17) and neural (18)(19)(20)(21)(22) adaptations. ...
... High mechanical loading of the muscle (≥70% of 1RM) traditionally is considered to be an essential stimulus for increases in maximal strength (2)(3)(4)(5)21,23). At the muscle level, the primary adaptation is an increase in muscle size (i.e., hypertrophy). ...
... In contrast, greater gains in maximal strength are observed for training with higher load (>60% of 1RM) compared with lower load (≤60% of 1RM). The relative greater increase in maximal strength than in muscle hypertrophy with high-load training is usually explained primarily by a greater contribution of neural changes (2,18,21) and muscle cytoskeletal remodeling associated with force transmission (29). ...
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Training with low-load exercise performed under blood-flow restriction can augment muscle hypertrophy and maximal strength to a similar extent as the classical high-load strength training method. However, the blood-flow restriction method elicits only minor neural adaptations. In an attempt to maximize training-related gains, we propose using other protocols that combine high voluntary activation, mechanical tension and metabolic stress.
... This may increase variability due to less developed neuromuscular capacity to perform more complex movement patterns resulting in more variation in performance. 90,91 Additionally, stopwatch measures reliability was limited to two sporting populations (American football and tennis). Therefore, while present data does not support the above assumption, we suggest additional research be conducted to establish a meaningful conclusion. ...
... Elite athletes are likely faster as a result of more developed neuromuscular systems, enabling greater force capacity to perform more complex movement skills with high synergistic muscle activation and high rates of force development. 90,91 Interestingly, findings from the similar spread in performance, indicate that sub-elite and novice athletes are characterised by similar athletic capabilities when it comes to 180°COD performance. 82 It is important to note however, the variation in number of studies found for the data presented in each sport, in terms of skill level and player positions. ...
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Change of direction (COD) is an important component of athlete performance and measuring and comparing athletes is an integral aspect of strength and conditioning practice. This article aimed to determine pro-agility shuttle utility, by quantifying variability and normative values for different sports, skill-levels and positions. Limitations of the pro-agility shuttle are identified, as are future research directions. A total of 67 studies were included for review. Pro-agility shuttle reliability was reported in 10 studies across 6 sports; however, comprehensive reliability statistics were absent in most papers. Additionally, only reliability of total-time from stopwatch and timing lights were reported. Data of 32,891 subjects in 12 sports (American football, basketball, cricket, general athletes, hockey, lacrosse, recreational athletes, resistance-trained athletes, rugby, soccer, swimming, and tennis) were extracted and aggregated, establishing sport, skill-level (elite, sub-elite, and novice) and positional normative values, where practical. Elite athletes showed the fastest performance times, whereas sub-elite and novice athletes showed similar spreads in performance, suggesting similar athletic capabilities. In conclusion, the pro-agility shuttle currently has limited diagnostic value and the variability of smaller performance sub-components within pro-agility shuttle should be examined. Furthermore, the value of other technologies such as smart phone, inertial sensor or radar should be investigated.
... Figure 11 : Comparaison de la force sur une contraction volontaire et tétanique (Duchateau et Baudry, 2011) ; données issues de (Duchateau et Hainaut, 1988) Des études montrent qu'un entrainement spécifique engendre des adaptations nerveuses notamment au niveau du recrutement des unités motrices 3 (UM) [ (Carroll et al., 2001) ; (Duchateau et al., 2006)]. Ces dernières sont recrutées selon un ordre précis qui suit le principe de taille d'Henneman (Henneman, 1957). ...
... Facteurs engendrant une évolution de la production de la force maximale(Duchateau et Baudry, 2011) Il a été fait le choix d'aborder uniquement l'influence du facteur nerveux et du facteur structural. Néanmoins, il est important de préciser que la transmission des forces lors du mouvement dû au système tendineux a un impact certain dans la production de force. ...
Thesis
Ce travail de thèse s’est intéressé à la production de force externe ainsi qu’à la relation force-vitesse-puissance des membres supérieurs chez les sportifs de haut niveau en escalade.La première partie de l'étude s'est focalisée sur deux aspects clefs dans la performance : la force maximale volontaire (FMV) des muscles fléchisseurs des doigts ainsi que le taux de développement de la force (RFD). Un protocole de mesure a été mis au point et sa fiabilité a été vérifiée. Cela a permis de montrer qu'un entrainement spécifique de seulement 4 semaines permettait déjà d'engendrer des évolutions spécifiques au niveau du RFD.Ce protocole a également vérifié l'hypothèse que la force maximale et le RFD étaient supérieurs chez les grimpeurs de haut niveau : la différence par rapport aux autres sportifs, y compris les grimpeurs confirmés, est significative. De même, ce protocole a confirmé que la force maximale des muscles fléchisseurs des doigts ainsi que le RFD diffèrent significativement selon la discipline : ils sont plus élevés chez les grimpeurs de bloc que chez les spécialistes de la difficulté. La seconde partie de ce travail a étudié un troisième déterminant clef de la performance en escalade : la production de force externe des muscles des bras à différentes vitesses. Un test de tractions lestées a été mené sur des groupes de grimpeurs de haut niveau représentatifs de chacune des 3 disciplines : bloc, difficulté et vitesse. Il a ainsi été montré qu'il existe d'importantes différences, du point de vue de la relation force-vitesse-puissance, selon la discipline. Les grimpeurs de bloc présentent notamment une production de force à vitesse rapide significativement supérieure à celle des deux autres disciplines. Par contre, la production de force à vitesse lente n'est pas discriminante entre les 3 groupes.En conclusion, ce travail a démontré que l'évaluation quantitative des facteurs déterminants biomécaniques pourrait compléter utilement les outils qualitatifs à la disposition de l'entraineur. Ainsi, selon la discipline et les prochaines échéances sportives, il pourra définir un entrainement spécifique et suivre de façon objective les résultats obtenus par le grimpeur.
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1 Finland HÄKKINEN K., KOMI P.V. & TESCH P.A. Effect of ccmbined concentric ard eccentric strength training and detraining on force-time, muscle fiber-and metabolic characteristics of leg extensor muscles. Scand. J ,Sports Sci. 3 (2): 50-58, 1981. Prog¡essive strength training of combined concentric and eccentric contractions were performed three times a week for 16 weeks by 14 males {20-30 yrs of age) accustomed to weight training. The training peeriod was iollowed bv 8 weeks of detraining. The training program consisted mainly of dynamic exeicises for the ieg-extensovs with loads of 80 to 120 of one maximum repetition The training caused significant improvements in-maximal force (p < 0.001) and various force-time (p (0.05-4.01) para¡àeters. Du¡ing thg I'ast trarning àionìh tbe inãrease in force was gireatly tri¡nited' and there was ¿ decrease in th,e force-time parameters. The marked improvements in mwcle strength were accompanied by ccnsiderable intemål qdaptatioos ,Ín-ttre tnaCned muscle, as Judged from l¡rcreases (p < 0-001) ,iqr. the fibet ãeas ôt tËe Ïast fi¡¡itch (FT) and slow twitch (ST) fibers. Durlng early conditioning improvement i! the qqgs! jump w,as related to tl.e relãtive hypertrop]ty of tr1l ii¡eis fo <0.01). No sier¡j-Êi,cå,r¡t ct¡anges ,in tJre er¡zyme aittv¡tiês oi mÍoki¡¡ase-a¡¡d creatine kirmse were found as a result of-tra¡rrir}g, but i,ndividt¡al charrges in my-o-kinase activity $/ere related to the relative. hypertrop'hy of FT fibers-(p ç 0.05) and Improvernent i+ the squat jump (p < O.Of)-during early conditiontuag. All the ada,p-iatlo:ns'-incilcating musõle hypertrophy occurred. prtm@lv during the last two training mo¡rths. Decreases (p (0.001) in maxirnal force during the detrairring were accompâ-nied bv a sisrificår¡t rediuction in the fi¡b,er areas of ttle fC tp < 0.01) and ST (p < 0.05) tvpes end by a change in bödy-antliropometry.-A periodiè-and partial usage. of àccentr-ic contráctions,-together with conèentric training' is suggested to be effectiùe in training for-maximal force and äso for force-time eharacteristics. In training of longer durations the specific effects of strength trainlng are-obviot¡s and explaiñable by adaptatlons in the trained muscle. Keg tenns: erìzJûne actlvities, muscle mechanics, muscle metabollsn, muscle streng:th.
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We examined motor unit (MU) discharge properties (mean interspike interval, ISI, discharge variability, short-term synchronization, common drive) and force tremor in the first dorsal interosseous (FDI) muscle of five musicians (skill-trained), five weight-lifters (strength-trained) and six untrained subjects during low-force isometric abduction of the index finger. Mean MU ISI was slightly shorter in skill-trained subjects than in untrained subjects. Discharge variability of FDI MUs did not differ significantly between groups. The mean strength of MU synchronization (expressed as the frequency of extra synchronous discharges above chance) was different in skill-trained (0.22+/-0.02 s(-1), 162 MU pairs), untrained (0.32+/-0.02 s(-1), 199 MU pairs) and strength-trained subjects (0.44+/-0.03 s(-1), 183 MU pairs). FDI MU synchrony was weak and of equivalent strength in both hands of skill-trained subjects and the dominant (skilled) hand of untrained subjects. The stronger FDI MU synchrony in the non-dominant hand of untrained subjects was equivalent to that found in both hands of strength-trained subjects. The extent of common modulation of firing rates (common drive) was assessed for a subset of MU pairs and was weaker in skill-trained subjects (0.30+/-0.04, n=14) than untrained (0.43+/-0.3, n=14) and strength-trained (0.48+/-0.03, n=21) subjects. Force tremor was quantified for each hand in the same subjects during isometric index finger abduction at target forces of 0.5 N and 3.5 N. Tremor rms amplitude and peak power in the force frequency spectrum were significantly lower in skill-trained subjects than strength-trained subjects with the 3.5-N target force. The peak tremor frequency was similar in the three groups. The relatively more independent discharge of pairs of FDI MUs in skill-trained subjects was not responsible for the reduced tremor amplitudes in these subjects. Correlations between the overall extent of MU synchrony and common drive in FDI muscles and tremor measures obtained during the same experimental session were all non-significant. Differences in the central descending command signals are the most likely explanation for the more independent discharge of FDI MUs in skill-trained hands, while neural or peripheral muscular factors may be responsible for the weaker tremor.
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Six women who had participated in a previous 20-wk strength training study for the lower limb detrained for 30-32 wk and subsequently retrained for 6 wk. Seven untrained women also participated in the 6-wk "retraining" phase. In addition, four women from each group volunteered to continue training an additional 7 wk. The initial 20-wk training program caused an increase in maximal dynamic strength, hypertrophy of all three major fiber types, and a decrease in the percentage of type IIb fibers. Detraining had relatively little effect on fiber cross-sectional area but resulted in an increased percentage of type IIb fibers with a concomitant decrease in IIa fibers. Maximal dynamic strength decreased but not to pretraining levels. Retraining for 6 wk resulted in significant increases in the cross-sectional areas of both fast fiber types (IIa and IIab + IIb) compared with detraining values and a decrease in the percentage of type IIb fibers. The 7-wk extension accentuated these trends such that cross-sectional areas continued to increase (nonsignificant) and no IIb fibers could be found. Similar results were found for the nonpreviously trained women. These data suggest that rapid muscular adaptations occur as a result of strength training in previously trained as well as non-previously trained women. Some adaptations (fiber area and maximal dynamic strength) may be retained for long periods during detraining and may contribute to a rapid return to "competitive" form.
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Ten active males performed reactive drop jumps from a height of 40 cm in six experimental conditions: jumps with additional loads of 100 N (BW+ 100 N) and 200 N (BW+ 200 N), an ordinary jump with body weight (BW) and three jumps in which the body weight was artificially reduced (BW-172 N, BW-337 N and BW-495 N). The vertical ground reaction forces, the angular displacement in the knee and ankle joints as well as the surface electromyogram (EMGs) of the triceps surae muscles and tibialis ant. muscle were recorded. When compared to the control condition (BW) in the jumps with extra load and in the jumps with reduced body weight, both the take-off velocity as well as the mean vertical ground reaction force were decreased during the push-off phase. The integrated EMG before ground contact as well as the duration of the preactivation phase was significantly reduced as a function of the load condition. Upon the touchdown, the coactivation of the muscles acting around the ankle joint was greatest in the control jump. Through all experimental conditions, the mean activation amplitude remained rather constant both for the impact as well as for the push-off phase of the contact. It is concluded that the centrally programmed activity prior to the contact can be seen as the decisive mechanism in the regulation of the stiffness behavior of the tendomuscular system. The extent of the preprogrammed activity determines mainly the physical output of the entire jump exercise.
Article
Despite full voluntary effort, neuromuscular activation of the quadriceps femoris muscle appears inhibited during slow concentric and eccentric contractions. Our aim was to compare neuromuscular activation during maximal voluntary concentric and eccentric quadriceps contractions, hypothesizing that inhibition of neuromuscular activation diminishes with resistance training. In 15 men, pretraining electromyographic activity of the quadriceps muscles [vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF)] was 17–36% lower during slow and fast (30 and 240°/s) eccentric and slow concentric contractions compared with fast concentric contractions. After 14 wk of heavy resistance training, neuromuscular inhibition was reduced for VL and VM and was completely removed for RF. Concurrently, electromyographic activity increased 21–52, 22–29, and 16–32% for VL, VM, and RF, respectively. In addition, median power frequency decreased for VL and RF. Eccentric quadriceps strength increased 15–17%, whereas slow and fast concentric strength increased 15 and 8%, respectively. Pre- and posttraining median power frequency did not differ between eccentric and concentric contractions. In conclusion, quadriceps motoneuron activation was lower during maximal voluntary eccentric and slow concentric contractions compared with during fast concentric contraction in untrained subjects, and, after heavy resistance training, this inhibition in neuromuscular activation was reduced.
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McCall, G. E., W. C. Byrnes, A. Dickinson, P. M. Pattany, and S. J. Fleck. Muscle fiber hypertrophy, hyperplasia, and capillary density in college men after resistance training. J. Appl. Physiol. 81(5): 2004–2012, 1996.—Twelve male subjects with recreational resistance training backgrounds completed 12 wk of intensified resistance training (3 sessions/wk; 8 exercises/session; 3 sets/exercise; 10 repetitions maximum/set). All major muscle groups were trained, with four exercises emphasizing the forearm flexors. After training, strength (1-repetition maximum preacher curl) increased by 25% ( P < 0.05). Magnetic resonance imaging scans revealed an increase in the biceps brachii muscle cross-sectional area (CSA) (from 11.8 ± 2.7 to 13.3 ± 2.6 cm ² ; n = 8; P < 0.05). Muscle biopsies of the biceps brachii revealed increases ( P < 0.05) in fiber areas for type I (from 4,196 ± 859 to 4,617 ± 1,116 μm ² ; n = 11) and II fibers (from 6,378 ± 1,552 to 7,474 ± 2,017 μm ² ; n = 11). Fiber number estimated from the above measurements did not change after training (293.2 ± 61.5 × 10 ³ pretraining; 297.5 ± 69.5 × 10 ³ posttraining; n = 8). However, the magnitude of muscle fiber hypertrophy may influence this response because those subjects with less relative muscle fiber hypertrophy, but similar increases in muscle CSA, showed evidence of an increase in fiber number. Capillaries per fiber increased significantly ( P < 0.05) for both type I (from 4.9 ± 0.6 to 5.5 ± 0.7; n = 10) and II fibers (from 5.1 ± 0.8 to 6.2 ± 0.7; n = 10). No changes occurred in capillaries per fiber area or muscle area. In conclusion, resistance training resulted in hypertrophy of the total muscle CSA and fiber areas with no change in estimated fiber number, whereas capillary changes were proportional to muscle fiber growth.
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List of Contributors. Preface. Units of Measurement and Terminology. Part 1: Definitions. 1 Basic Definitions for Exercise. H.G. KNUTTGEN AND P.V. KOMI. . Part 2: Biological Basis for Strength and Power. 2 Neuronal control of functional movement. VOLKER DIETZ. 3 Motor Unit and Motor Neuron Excitability during Explosive Movement. TOSHIO MORITANI. 4 Muscular Basis of Strength. R. BILLETER AND H. HOPPELER. 5 Hormonal Mechanisms Related to the Expression of Muscular Strength and Power. WILLIAM J. KRAEMER AND SCOTT A. MAZZETTI. 6 Exercise--related Adaptations in Connective Tissue. RONALD F. ZERNICKE AND BARBARA LOITZ--RAMAGE. 7 Contractile Performance of Skeletal Muscle Fibres. K.A. PAUL EDMAN. 8 Skeletal Muscle and Motor Unit Architecture: Effect on Performance. RONALD R. ROY, RYAN J. MONTI, ALEX LAI AND V. REGGIE EDGERTON. 9 Mechanical Muscle Models and Their Application to Force and Power Production. WALTER HERZOG. 10 Stretch--shortening Cyle. PAAVO V. KOMI. 11 Stretch--shortening Cycle Fatigue and its Influence on Force and Power Production. CAROLINE NICOL AND PAAVO. V. KOMI. . Part 3: Mechanism for Adaptation in Strength and Power Training. 12 Cellular and Molecular Aspects of Adaptation in Skeletal Muscle. GEOFFREY GOLDSPINK AND STEPHEN HARRIDGE. 13 Hypertrophy and Hyperplasia. J. DUNCAN MACDOUGALL. 14 Acute and Chronic Muscle Metabolic Adaptations to Strength Training. PER A. TESCH AND BJORN A. ALKNER. 15 Neural Adaptation to Strength Training. DIGBY G. SALE. 16 Mechanism of Muscle and Motor Unit Adaptation to Explosive Power Training. JACQUES DUCHATEAU AND KARL HAINAUT. 17 Proprocetive Training: Considerations for Strength and Power Production. ALBERT GOLLHOFER. 18 Connective Tissue and Bone Response to Strength Training. MICHAEL H. STONE AND CHRISTINA KARATZAFERI. 19 Endocrine Responses and Adaptations to Strength and Power Training. WILLIAM KRAEMER AND NICHOLASS A. RATAMESS. 20 Cardiovascular Responses to Training. STEVEN J. FLECK. . Part 4: Special Problems in Strength and Power Training. 21 Aging and Neuromuscular Adaptation to Strength Training. KEIJO HAKKINEN. 22 Use of Electrical Stimulation in Strength and Power Training. GARY A. DUDLEY AND SCOTT W. STEVENSON. Part 5: Strength and Power Training for Sports. 23 Biomechanics of Strength and Strength Training. VLADIMIR M. ZATSIORSKY. 24 Vibration Loads: Potential for Strength and Power Development. JOACHIM MESTER P.(?) SPITZENPFEIL AND ZENGYOUAN YUE. 25 Training for Weightlifting. JOHN GARHAMMER AND BOB TAKANO
Article
Effects of a 12-week power training of upper limbs on neuromuscular performance and mechanical efficiency (ME) was studied in 7 women. Isometric maximum, pure concentric (Cone), pure eccentric (Ecc), and stretch-shortening cycle (SSC) exercises with maximal and submaximal efforts were undertaken. The results showed that improvement in maximal performance due to training was seen as increased maximal isometric force and its rate of development (p < 0.05), and as improvement in maximal Cone and SSC force productions. ME, measured during submaximal exercise, did not change during the follow-up period. The improvements observed in the maximal isometric condition could not be explained by the increase in agonist EMG-activity. In the dynamic movements, however, the improved force production could have resulted from the simple improvement of coordination between agonist and antagonist muscles. The fact that the greatest improvements took place at higher shortening velocities of the concentric action (lightest load) and at the respective velocities of the SSC exercise emphasizes the specificity aspect of power training. (C) 1998 National Strength and Conditioning Association
Article
Effects of power training with stretch-shortening cycle (SSC) exercises on mechanical efficiency (ME) were investigated with 9 young women who trained 3 times a week for 4 months. The training included various types of jumping exercises. Before and after the training as well as after the detraining (2 months) the subjects performed 6 different submaximal exercises with a special sledge apparatus. Each exercise involved 60 muscle actions lasting for a total of 3 min per testing condition. The work intensities were determined individually according to the recordings of distance obtained during the single maximal concentric exercises. The training caused the greatest changes of ME in conditions of higher prestretch intensities. The ME values changed from 49.3 ± 12.9% to 55.4 ± 12.1% in pure eccentric exercises and from 39.5 ± 4.6% to 46.1 ± 5.0% in SSC exercises during the training. After the training, the subjects preactivated their leg extensor muscles earlier before the impact, and the eccentric working phase was more powerful, because of higher tendomuscular stiffness. Higher preactivation of the measured muscles, higher flexion of knee and increased dorsiflexion of ankle joints in the beginning of contact caused the increased stiffness, possibly through more powerful reflex activation. At the same time the metabolic demands of muscles decreased, causing the increases of ME.
Article
Kernell, D. The adaptation and the relation between discharge frequency and current strength of cat lumbosacral motoneurones stimulated by long-lasting injected currents. Acta physiol. scand. 1965. 65. 65–73. — Cat lumbosacral motoneurones were stimulated by steady currents injected through the tip of an intracellular micro-electrode. The results strongly indicate that the majority of undamaged motoneurones are capable of responding to steady currents by long-lasting and well maintained repetitive discharges. Such discharges from various motoneurones were studied with regard to the adaptation and the slope of the linear relation between discharge frequency and current strength (cf. Granit, Kernell and Shortess 1963 a, b). Measurements were also made of the lowest current strength (in units of rheobase) which was needed for eliciting a steady repetitive discharge. None of these characteristics relating to repetitive firing were found to be correlated with the duration of afterhyperpolarization, the latter being measured by single spike discharges (generally antidromic). The results have been discussed also with reference to earlier findings concerning tonic and phasic repetitive firing of motoneurones stimulated by muscle stretch.
Chapter
IntroductionMuscle Size and StrengthSpecific TensionEvidence for a Role of the Nervous System in Strength GainsNeural Activation of MuscleConclusion AcknowledgementsReferences
The effects of a 1 year training period on 13 elite weight-lifters were investigated by periodical tests of electromyographic, muscle fibre and force production characteristics. A statistically non-significant increase of 3.5% in maximal isometric strength of the leg extensors, from 48411104 to 50101012 N, occured over the year. Individual changes in the high force portions of the force-velocity curve correlated (p<0.05–0.01) with changes in weight-lifting performance. Training months 5–8 were characterized by the lowest average training intensity (77.1+2.0%), and this resulted in a significant (p<0.05) decrease in maximal neural activation (IEMG) of the muscles, while the last four month period, with only a slightly higher average training intensity (79.13.0%), led to a significant (p<0.01) increase in maximum IEMG. Individual increases in training intensity between these two training periods correlated with individual increases both in muscular strength (p<0.05) and in the weight lifted in the clean & jerk (p<0.05). A non-significant increase of 3.9% in total mean muscle fibre area occurred over the year. The present findings demonstrate the limited potential for strength development in elite strength athletes, and suggest that the magnitudes and time courses of neural and hypertrophic adaptations in the neuromuscular system during their training may differ from those reported for previously untrained subjects. The findings additionally indicate the importance of training intensity for modifying training responses in elite strength athletes.
Article
Effects of 6 mo of heavy-resistance training combined with explosive exercises on neural activation of the agonist and antagonist leg extensors, muscle cross-sectional area (CSA) of the quadriceps femoris, as well as maximal and explosive strength were examined in 10 middle-aged men (M40; 42 +/- 2 yr), 11 middle-aged women (W40; 39 +/- 3 yr), 11 elderly men (M70; 72 +/- 3 yr) and 10 elderly women (W70; 67 +/- 3 yr). Maximal and explosive strength remained unaltered during a 1-mo control period with no strength training. After the 6 mo of training, maximal isometric and dynamic leg-extension strength increased by 36 +/- 4 and 22 +/- 2% (P < 0. 001) in M40, by 36 +/- 3 and 21 +/- 3% (P < 0.001) in M70, by 66 +/- 9 and 34 +/- 4% (P < 0.001) in W40, and by 57 +/- 10 and 30 +/- 3% (P < 0.001) in W70, respectively. All groups showed large increases (P < 0.05-0.001) in the maximum integrated EMGs (iEMGs) of the agonist vastus lateralis and medialis. Significant (P < 0.05-0.001) increases occurred in the maximal rate of isometric force production and in a squat jump that were accompanied with increased (P < 0.05-0. 01) iEMGs of the leg extensors. The iEMG of the antagonist biceps femoris muscle during the maximal isometric leg extension decreased in both M70 (from 24 +/- 6 to 21 +/- 6%; P < 0.05) and in W70 (from 31 +/- 9 to 24 +/- 4%; P < 0.05) to the same level as recorded for M40 and W40. The CSA of the quadriceps femoris increased in M40 by 5% (P < 0.05), in W40 by 9% (P < 0.01), in W70 by 6% (P < 0.05), and in M70 by 2% (not significant). Great training-induced gains in maximal and explosive strength in both middle-aged and elderly subjects were accompanied by large increases in the voluntary activation of the agonists, with significant reductions in the antagonist coactivation in the elderly subjects. Because the enlargements in the muscle CSAs in both middle-aged and elderly subjects were much smaller in magnitude, neural adaptations seem to play a greater role in explaining strength and power gains during the present strength-training protocol.
Article
To investigate the effect of initial conditions on the modulation of motor unit discharge during fast voluntary contractions, we compared ballistic isometric contractions of the ankle dorsiflexor muscles that were produced from either a resting state or superimposed on a sustained contraction. The torque of the dorsiflexors and the surface and intramuscular EMGs from the tibialis anterior were recorded. The results showed that the performance of a ballistic contraction from a sustained contraction ( approximately 25% maximal voluntary contraction (MVC)) had a negative effect on the maximal rate of torque development. Although the electromechanical delay was shortened, the EMG activity during the ballistic contraction was less synchronized. These observations were associated with a significant decline in the average discharge rate of single motor units (89.8 +/- 3.8 versus 115 +/- 5.8 Hz) and in the percentage of units (6.2 versus 15.5% of the whole sample) that exhibited double discharges at brief intervals (= 5 ms). High-threshold units that were not recruited during the sustained contraction displayed the same activation pattern, which indicates that the mechanisms responsible for the decline in discharge rate were not restricted to previously activated units, but appear to influence the entire motor unit pool. When a premotor silent period (SP) was observed at the transition from the sustained muscular activity to the ballistic contraction (19% of the trials), these adjustments in motor unit activity were not present, and the ballistic contractions were similar to those performed from a resting state. Together, these results indicate that initial conditions can influence the capacity for motor unit discharge rate and hence the performance of a fast voluntary contraction.
Article
Although the performance capabilities of muscle differ during shortening and lengthening contractions, realization of these differences during functional tasks depends on the characteristics of the activation signal discharged from the spinal cord. Fundamentally, the control strategy must differ during the two anisometric contractions due to the lesser force that each motor unit exerts during a shortening contraction and the greater difficulty associated with decreasing force to match a prescribed trajectory during a lengthening contraction. The activation characteristics of motor units during submaximal contractions depend on the details of the task being performed. Indexes of the strategy encoded in the descending command, such as coactivation of antagonist muscles and motor unit synchronization, indicate differences in cortical output for the two types of anisometric contractions. Furthermore, the augmented feedback from peripheral sensory receptors during lengthening contractions appears to be suppressed by centrally and peripherally mediated presynaptic inhibition of Ia afferents, which may also explain the depression of voluntary activation that occurs during maximal lengthening contractions. Although modulation of the activation during shortening and lengthening contractions involves both supraspinal and spinal mechanisms, the association with differences in performance cannot be determined without more careful attention to the details of the task.
Article
Address reprint requests to Dr. J. R. Basford, Mayo Clinic, 200 First Street S.W., Rochester, MN 55905. The purpose of this study was to compare the effects of eccentric and concentric isokinetic training exercises on eccentric and concentric quadriceps performance in 63 healthy subjects during a six-week training period. The subjects were randomly divided into three groups: eccentric training, concentric training, and no training. Members of the training groups exercised their right quadriceps only. Orientation, training, and testing were performed on a computer-assisted isokinetic dynamometer (KIN-COM) at 100 degrees /sec. All subjects were tested concentrically and eccentrically on both quadriceps; and peak torque, total work, and position angle at peak torque were measured. Analysis of variance and post hoc analyses were performed on the mean changes (pretest and posttest differences) for all groups. The concentric training group demonstrated the most significant increases in concentric torque and work; the eccentric training group demonstrated the most significant increases in eccentric torque and work. No significant changes were seen in the untrained left limbs of any group. These results support the concept of mode specificity in isokinetic exercise training. J Orthop Sports Phys Ther 1991;14(1):31-36.
Some normal human subjects show definite synchronization of the motor units in hand muscles (i.e., the impulses from two or more motor units coincide in time more frequently than expected for independent random processes). Subjects who show synchronization tend to use their hands to exert large, brief forces, either in their work (e.g., manual laborers) or recreational activities (e.g., weightlifters). In this study all seven weightlifters examined showed a significant degree of synchronization. Furthermore, after 6 weeks of using the first dorsal interosseus muscle of the hand to exert maximal, voluntary contractions, the level of synchronization increased substantially in four control subjects, and the average level became significantly different from zero. Weightlifters also showed greater late reflex responses than control subjects, but no significant difference in earlier spinal reflexes. Two late reflex waves are described which probably involve fast pathways to and from motor cortes. We suggest that supraspinal connections from motor cortex directly to spinal motoneurons may be enhanced as a result of training to the point where they produce a significant synchronization of motor units during steady, voluntary contractions.
Article
The purpose of this study was to determine whether differences in isometric twitch contraction times of skeletal muscles are related more closely to myosin adenosine triphosphatase (ATPase) activity or to the ability of the sarcotubules to accumulate Ca2+. The isometric contraction time was observed to be shorter in the crureus muscle than in the soleus muscle of the rabbit. These muscles were found to have similar myosin ATPase activities, but sarcotubules isolated from crureus had a faster rate of Ca2+ uptake than soleus sarcotubules. Furthermore, the yield of sarcotubules was 41% higher from crureus. Likewise, the isometric contraction time of rat soleus decreased during the first 3 weeks of life, but no change was observed in myosin ATPase activity. However, sarcotubular uptake of Ca2+ increased as did the yield of sarcotubules (44% higher). It is suggested that the differences in isometric contraction time between skeletal muscles are related more closely to the sarcotubular Ca2+ uptake than to the activity of myosin ATPase.
Article
Needle biopsies were taken from triceps brachii of 6 healthy males before and after a 6 month intensive weight training programme. The tissue was sectioned, photographed under a Philips EM200 and subjected to stereological analysis. Cross sectional fibre areas were also calculated from cryostat sections stained for ATPase activity. Morphometric analysis indicated that training resulted in a significant 26% reduction in mitochondrial volume density and a 25% reduction in the mitochondrial volume to myofibrillar volume ratio. These changes were accompanied by significant increases in fibre area for both FT (33%) and ST (27%) fibres as determined from the light microscope. There was a significant correlation between the reduction in mitochondrial volume density and the increase in FT fibre area following training (r=0.845). It was concluded that heavy resistance training leads to a dilution of the mitochondrial volume density through an increase in myofibrillar size with hypertrophy.
Article
Progressive strength training was performed 3 times a week for 8 weeks by 14 male students (19-31 yrs.). The training program consisted mainly of dynamic exercises for the leg extensors with maximal or close to maximal loads. The training caused significant improvements in dynamic and isometric strength. One repetition maximum in squats increased with 67%, Sargent jump with 22%, and maximal voluntary isometric contraction (MVC) with 13%, respectively. Body weight and leg muscle circumferences remained unchanged after training, whereas total body potassium, lean body mass and calculated total muscle mass increased, suggesting a change in body composition with training. Muscle biopsies were obtained from vastus lateralis for fibre analyses and determination of enzyme activities. There were no changes in muscle fibre composition or fibre area with training. The activities of Mg2+ stimulated ATPase, creatine phosphokinase and phosphofructokinase remained unchanged, whereas myokinase activity was increased after training from (1.41 to 1.52 moles x 10(-4) x g-1 x min-1, p less than 0.05). After training significant correlations (p less than 0.01) were demonstrated between Mg2+ stimulated ATPase activity and % fast twitch fibres (% FT) (r = 0.67), as well as between myokinase activity and % FT (r = 0.86).
Article
The time course of strength gain with respect to the contributions of neural factors and hypertrophy was studied in seven young males and eight females during the course of an 8 week regimen of isotonic strength training. The results indicated that neural factors accounted for the larger proportion of the initial strength increment and thereafter both neural factors and hypertrophy took part in the further increase in strength, with hypertrophy becoming the dominant factor after the first 3 to 5 weeks. Our data regarding the untrained contralateral arm flexors provide further support for the concept of cross education. It was suggested that the nature of this cross education effect may entirely rest on the neural factors presumably acting at various levels of the nervous system which could result in increasing the maximal level of muscle activation.
Article
1. Single motor units were recorded from the masseter, soleus and first dorsal interosseous muscles of normal adult man. An analysis of discharge patterns was carried out either during slow ramp voluntary contractions, or during self-initiated isolated ballistic voluntary contractions. The isometric myogram was simultaneously recorded.2. Each motor unit was only recruited when the peak force of a brisk contraction exceeded a certain value and a ;ballistic force threshold' (in kg) was estimated for the unit from a large series of brisk contractions of different strengths. For each muscle, the ranking order for recruitment of different motor units recorded from one electrode position was virtually identical in slow ramp versus brisk ballistic contractions of different force (Kendall rank correlation coefficient = 0.91-1.0). There was no evidence for any consistent selective activation of fast twitch motor units in ballistic contractions.3. The ballistic force threshold is considerably reduced with respect to the slow ramp force threshold for the motor units of the soleus muscle. This drop is also marked for the units of the first interosseous and tibialis anterior muscles, whereas it is only small for the units of the masseter muscle. These data have been validated after consideration of the complicating factor related to the possible differential involvement of synergic muscles in ramp or ballistic contractions.4. In the masseter and first interosseous muscles, the time to peak is about 80 msec in small ballistic voluntary contractions and it increases to about 150 msec in strong contractions. This effect appears related to repetitive discharges of single motor units when their force threshold is exceeded. By contrast, in the soleus muscle, the time to peak remains at about 150 msec both in small and in strong ballistic contractions and most soleus motor units fire only one spike in the ballistic burst.5. Brisk ballistic contractions are graded in force by the recruitment of additional motor units according to their usual rank order. The importance of rate gradation through the repetitive firing of motor units varies in the different human muscles investigated, being quite significant in isometric brisk contractions of the masseter and first interosseous muscles but much less so in the soleus muscle in which little repetitive firing of single motor units was observed over the range of ballistic forces studied.
Article
1. Single motor units were recorded with highly selective electrodes from intact tibialis anterior muscle in the adult man. A detailed parametric analysis was made of the discharge patterns during voluntary isometric contractions of different peak forces carried out at various rates of force development. 2. During the smooth tracking of a ramp force, the different motor units recorded from a given muscle site were recruited in a consistant order, each unit becoming active when the muscle developed a certain level of force. The threshold of some of the units in such slow ramp contractions exceeded 8 kg. By contrast, in brisk ballistic contractions reaching a peak force of 12 kg in less than 0-15 sec, the same motor units discharged in a transient burst which largely preceded the muscle force production. 3. In slow tracking ramp contractions, the instantaneous frequency of single motor units was initially rather low (5-15/sec) and it increased as the ramp force augmented. By contrast, in (strong) ballistic contractions, the same units discharged at an unusually high instantaneous frequency (60-120/sec) early in the burst and the firing frequency decreased thereafter. Such hitherto unknown pattern appears characteristic of ballistic contractions and it was not found in even fast tracking ramp contractions achieving 12 kg in only 0-4 sec. 4. The potentials of the different motor units activated are rather crowded at intervals of a few msec in the early burst of a strong ballistic contraction and observations on the rank activation of the different motor units do not provide reliable data for the analysis of the recruitment order of units in ballistic contractions. 5. A new method is described for estimating ballistic force threshold of single motor units. When a large series of brisk ballistic contractions with peak forces ranging from 0-05 to 12 kg was carried out any given motor unit only became active when the ballistic peak force exceeded a certain reproducible value. A detailed analysis of the recruitment order based on these ballistic force thresholds showed it to be virtually identical to the recruitment order of the same units in slow tracking ramp contractions (correlation=0-95). 6. Ballistic contractions are graded in force both by the recruitment of additional motor units in stronger contractions, and by an increase in their rate of firing. These gradation mechanisms are discussed.
Article
The training effects of static and dynamic exercise programs on the firing patterns of 450 single motor units (SMU) in the human tibialis anterior muscle were investigated. In a six week program, the static group (N = 5) participated in daily high intensity, short duration exercises (isometric) while the dynamic group (N = 5) participated three times weekly in low intensity, moderate duration endurance exercises (isotonic). The control group (N = 4) did not participate in an exercise program. Pre- and posttest SMU firing patterns were recorded by means of 26 gauge tygon coated monopolar needle electrodes. In order to assess the degree of impersistence of periodicity of motor unit firing, an interspike interval (ISI) longer than 260 msec was defined as a lapse in motor unit firing. This value is >5 S.D. longer than the mean ISI at onset of firing for motor units in tibialis anterior. An individual ratio (ILU) of normal ISIs to lapses for each SMU three minute recording run and the mean ratio for each subject (MLR) were calculated. Significant differences were found between MLR pre- and posttest values for both groups (p<0.05). These results indicate that changes in motor unit firing can be produced by specific exercise programs; high intensity, short duration and low intensity, long duration exercises producing firing rates more and less variable than normal respectively.
Article
When a homonymous motoneuron pool is volitionally activated during a gradually changing isometric contraction, there exist systematic relationships between the sizes of motor units, their recruitment, and their firing behavior. These relationships can be used to develop an accurate functional model of the voluntary force-production process.
Article
The study refers mainly to recordings made from the first dorsal interosseus muscle (FDI). This muscle was thought to fit particularly well for this investigation since the nerve fibers innervating its extrafusal muscle fibers are known to be unimodally distributed. But in contrast, histochemically the muscle is composed of type I (57.4%) and type II (42.6%) fibers, so that both possibilities considered above appear equally possible. The results suggest that the features of the motor units studied in the course of these experiments can be explained on the basis of excitability differences between cells of different size.
Article
Twenty sedentary male university students were randomly assigned to an experimental or a control group. The experimental group trained the knee extensors of one leg by producing 30 isometric extension maximal voluntary contractions (MVC) per day, three times per week for 8 wk. After 8 wk of training, extensor MVC in the trained leg increased 32.8% (P less than 0.05), but there was no change in vastus lateralis maximal integrated electromyographic activity (IEMGmax). The most important finding was that the degree of hamstring coactivation during extension MVC decreased by approximately 20% (P less than 0.05) after the 1st wk of training. Less pronounced adaptations occurred in the untrained leg: extension MVC force increased 16.2% (P less than 0.05), hamstring coactivity decreased 13% (P less than 0.05) after 2 wk of training, and vastus lateralis IEMGmax was unchanged. The same measures in legs of the control group were not changed during the study. There were no changes in flexion MVC, biceps femoris IEMGmax, or the degree of quadriceps coactivity during flexion MVC in either leg of the control or experimental group. A reduction in hamstring coactivity in the trained and untrained legs indicates that these muscles provide less opposing force to the contracting quadriceps. We conclude that this small but significant decrease in hamstring coactivation that occurs during the early stages of training is a nonhypertrophic adaptation of the neuromuscular system in response to static resistance training of this type.
Article
The purpose of this investigation was to determine the effect of high velocity resistance (HVR) training on peak torque (PT), cross sectional area (CSA) and myofibrillar ATPase activity of the knee extensors. HVR training was performed in a circuit on hydraulic exercise equipment, 4 times a week for 5 weeks at an angular velocity of approximately 3.14 rad.s-1. Knee extension PT was determined on a Cybex II isokinetic dynamometer and CSA of the quadriceps femoris muscle was assessed using computer tomography (CT) scanning. Muscle biopsies were obtained from the lateral quadriceps muscle and were analyzed for myofibrillar ATPase activity. Knee extension peak torque was significantly increased at 1.57, 2.09, 3.14, 3.66 and 4.19 rad.s-1. Myofibrillar ATPase activity and CSA was also significantly increased after HVR training. These findings showed that short-term high velocity resistance training enhances the in vivo torque/velocity curve especially at fast angular velocities and these changes are partly attributed to an increase in muscle CSA and activity of myofibrillar ATPase.
Real-time ultrasound scanning was used to measure the angles of fibre pennation of vastus lateralis (VL) and vastus intermedius (VI) of the human quadriceps (n = 12) in vivo. The maximum isometric force and cross-sectional area of the quadriceps were also measured. With the knee at right-angles the mean fibre angles for VL and VI respectively were 0.133 (0.021) rad [7.6 degrees (1.2 degrees)] and 0.143 (0.028) rad [8.2 degrees (1.6 degrees)] [mean (SD)], which is within the range of angles measured on cadavers. The mean angle decreased in going from the contracted [VL, 0.244 rad (14 degrees); VI, 0.279 rad (16 degrees)] to the stretched [VL, 0.105 rad (6 degrees); VI, 0.122 rad (7 degrees)] position. There was a significant positive correlation between fibre angle and muscle cross-sectional area but no relationship between fibre angle and force per cross-sectional area. No increase in fibre angle was detected after 3 months strength training. We conclude that ultrasound can be used to measure pennation angles of superficial muscle groups but we could not demonstrate a relationship between pennation and force-generating capacity.
Article
The purpose of this experiment was to determine whether training-induced increases in maximal voluntary contraction (MVC) can be completely accounted for by increases in muscle cross-sectional area. Fifteen female university students were randomly divided into a control (N = 7) and an experimental (N = 8) group. The experimental group underwent 8 wk of isometric resistance training of the knee extensors of one leg; the other leg was the untrained control. Training consisted of 30 MVC.d-1 x 3 d.wk-1 x 8 wk. Extensor cross-sectional area (CSA), assessed by computerized tomographic (CT) scanning of a cross-sectional slice at mid-thigh, was used as a measure of muscle hypertrophy. After 8 wk of training, MVC increased by 28% (P < 0.05), CSA increased by 14.6% (P < 0.05), and the amplitude of the electromyogram at MVC (EMGmax) was unchanged in the trained leg of the experimental subjects. The same measures in the untrained legs of the experimental subjects and in both legs of the control subjects were not changed after training. Although there was an apparent discrepancy between the increase in MCV (28%) and CSA (14.6%), the ratio between the two, the specific tension (N.cm-2), was not significantly different after training. As a result of these findings, we conclude that in these subjects there is no evidence of nonhypertrophic adaptations to resistance training of this type and magnitude, and that the increase in force-generating capacity of the muscle is due to the synthesis of additional contractile proteins.
Article
1. The relationship between the motor unit discharge pattern (rate and variability) and synchronization of motor unit pairs was studied in the first dorsal interosseus muscle of human subjects. In separate trials of up to 4 min duration, subjects voluntarily controlled the mean discharge rate of an identified motor unit at one of several prescribed rates (range 7.5-17.5 Hz). 2. The effect of discharge rate on the synchronous peak in the cross-correlogram was examined in eighty motor unit pairs from six subjects. Five commonly used synchronization indices were used to quantify synchrony in the cross-correlograms constructed from different discharge-rate trials. For each synchronization index, the apparent magnitude of synchrony increased at lower motor unit discharge rates. The synchronization indices were not equally sensitive to discharge rate; increases in the different indices ranged from 72 to 494% between the highest and lowest discharge rates. 3. A model of the membrane potential trajectory underlying rhythmic motoneuron discharge was used to explain the observed increase in the magnitude of the synchronization indices at lower discharge rates. The essential feature of this model is that the probability of a common-input EPSP causing a synchronous discharge in two motoneurons is independent of discharge rate. This means that the number of synchronous action potentials in excess of chance in any trial depends on the properties of the common-input EPSPs and the duration of the trial, but is not related to motor unit discharge rates. The model also demonstrated that when the excess synchronous counts are normalized to motor unit discharge rate, or baseline counts in the histogram (as in the conventional synchronization indices), the magnitude of the index increases when the motor unit discharge rates are low. 4. The strength of common input to motoneurons could be misinterpreted if conventional synchronization indices are used because of discharge-rate effects. The model was used to derive an index of the strength of common input to motoneurons (CIS) that was independent of motor unit discharge rate. CIS is the frequency of synchronous action potentials in the motor unit pair in excess of those expected due to chance (calculated during periods of tonic discharge in both units). The mean CIS in first dorsal interosseus motor unit pairs ranged from 0.052 to 1.005 extra synchronous action potentials per second across subjects. 5. Discharge variability was correlated with each of the synchronization indices and the CIS.(ABSTRACT TRUNCATED AT 400 WORDS)
Article
The influence of gender on muscular adaptation of the elbow flexors to 24 wk of heavy resistance training was studied in five male bodybuilders (MB) and five female bodybuilders (FB) who were highly competitive. Muscle cross-sectional area (CSA), fiber area, and fiber number were determined from the biceps brachii, and voluntary elbow flexor torque was obtained at velocities of contraction between 0 and 300 degrees/s. Biceps and flexor CSA was 75.8 and 81% greater, respectively, in MB than in FB, but muscle CSA was not significantly altered by the training program in either group. Because estimated fiber number and the volume density of nonmuscle tissue were similar in MB and FB, most of the gender difference in muscle CSA appeared to be due to greater absolute mean fiber areas in MB (10.51 and 10.68 x 10(3) microns 2 pre- and posttraining, respectively) than in FB (5.33 and 5.96 x 10(3) microns 2 pre- and posttraining, respectively). In neither MB nor FB did fiber type achieve further hypertrophy during the 24-wk training program. These data suggest that the extent of any change in muscle mass or muscle fiber characteristics is minimal after a bodybuilder of either gender has attained a high degree of muscle mass and a highly competitive status.
Article
Three different training regimens were performed to study the influence of eccentric muscle actions on skeletal muscle adaptive responses to heavy resistance exercise. Middle-aged males performed the leg press and leg extension exercises two days each week. The resistance was selected to induce failure within six to twelve repetitions of each set. Group CON/ECC (n = 8) performed coupled concentric and eccentric actions while group CON (n = 8) used concentric actions only. They did four or five sets of each exercise. Group CON/CON (n = 10) performed twice as many sets with only concentric actions. Eight subjects did not train and served as controls. Tissue samples were obtained from m. vastus lateralis using the biopsy technique before and after 19 weeks of training, and after four weeks of detraining. Histochemical analyses were performed to assess fibre type composition, fibre area and capillarization. Training increased (P less than 0.05) Type IIA and decreased (P less than 0.05) Type IIB fibre percentage. Only group CON/ECC increased Type I area (14%, P less than 0.05). Type II area increased (P less than 0.05) 32 and 27%, respectively, in groups CON/ECC and CON/CON, but not in group CON. Mean fibre area increased (P less than 0.05) 25 and 20% in groups CON/ECC and CON/CON, respectively. Capillaries per fibre increased (P less than 0.05) equally for Type I and Type II fibres. Capillaries per fibre area for both fibre types, however, increased (P less than 0.05) only in groups CON and CON/CON. The changes in fibre type composition and capillary frequency were manifest after detraining.(ABSTRACT TRUNCATED AT 250 WORDS)
Article
The purpose of this study was to examine the effects of electrical stimulation on torque output during knee extension. Nine well-trained males (19-43 years) performed maximal voluntary, electrically evoked and superimposed eccentric and concentric knee extensions at velocities of 60, 180 and 360 degrees s-1, plus an isometric test (torque was always recorded at a 60 degree knee angle). Fifty-hertz stimulation was applied percutaneously at the maximum tolerated voltage (140-200 V). By superimposing electrical stimulation, eccentric torque could be increased by an average of 21-24% above the voluntary level (P less than 0.05). No corresponding differences were observed between superimposed and voluntary torques under isometric or concentric conditions. Electrically evoked torque also exceeded voluntary torque under eccentric conditions (11-12%, P less than 0.05), but was less under isometric and concentric conditions (-10 to -52%, P less than 0.05). Within the limitations of the study, it was concluded that eccentric knee extension torque under maximal voluntary conditions does not represent the maximal torque-producing capacity. The action of a neural inhibitory mechanism was proposed as an explanation for this finding. If active, this mechanism may protect against the extreme muscle tension that could otherwise develop under truly maximal eccentric conditions.
Article
The effect of an eccentric strength training programme on the muscular series elastic component (SEC) was studied on the flexors of the human elbow. The characteristics of the SEC were determined using an in situ technique derived from methods commonly used on isolated muscles. The results were expressed in terms of compliance-force and tension-extension relationships. These relationships indicate a sharp increase in compliance when tension decreases. Furthermore, for a given value of tension, the SEC compliance of the trained muscles is found to be lower than that of the untrained muscles. These results are discussed in relation to the active and passive parts of the SEC.
Article
1. The contractile properties, recruitment and firing rates of motor units from the human adductor pollicis and the first dorsal interosseous were studied during voluntary isometric contractions after 6-8 weeks' immobilization of the corresponding limbs. 2. In both muscles, motor units of different force thresholds showed a proportionally identical twitch tension decrease and slowing of their time course after immobilization. 3. When expressed as a percentage of the maximal voluntary contraction, more high-threshold motor units were recorded in disused muscles than in control muscles, but the order of recruitment was maintained. 4. The motor unit firing rate at recruitment was identical in control and disused muscles, but the maximal firing rate decreased in all motor units after immobilization. This decrease of the maximal firing rate was greater in motor units of lower threshold than in those of higher threshold. 5. The results further document motoneuronal plasticity in human muscles of different fibre type composition.