Twitch potentiation during fatiguing exercise in the elderly: the effects of training

Postactivation Potentiation and the Asynchronous Action of Muscular and Neural Responses

PURPOSE

This study examined the underlying mechanisms of postactivation potentiation and the time course of muscular- and neural-related variables.

METHODS

Fourteen trained males executed 4 sets of six 6-second maximum isometric conditioning plantar flexions, with 15 seconds and 2 minutes of interval between the contractions and sets, respectively. Peak twitch torque (TT), rate of torque development, time to peak torque, half relaxation time, and the neural-related variables of H-reflex and electromyogram, normalized to the maximum M-wave (H/M and RMS/M, respectively), were evaluated, as well as the level of the voluntary activation, assessed by the twitch interpolation technique. All neural-related variables were analyzed for the trial within each set when TT was maximal and for the trial within each set when the neural-related variable itself was maximal.

RESULTS

Compared with the baseline measures, TT and rate of torque development significantly increased in all sets (P < .001), whereas time to peak torque and half relaxation time significantly decreased in sets 1 to 4 and 2 to 4, respectively (P < .001). However, H/M and the RMS/M did not change for the repetition of each set for which the TT was maximal (P > .05). Interestingly, the within-set maximum H/M ratio of the lateral gastrocnemius muscle revealed a significant increase in all sets (P < .05), compared with the baseline measures.

CONCLUSION

One set of 4 contractions with 6-second duration is sufficient to cause postactivation potentiation for most participants, whereas peak TT augmentation does not coincide with changes in the examined neural-related variables. Further experiments should consider the time lag on their maximal values and their inherent between-participants variability.

Understanding altered contractile properties in advanced age: insights from a systematic muscle modelling approach

Age-related alterations of skeletal muscle are numerous and present inconsistently, and the effect of their interaction on contractile performance can be nonintuitive. Hill-type muscle models predict muscle force according to well-characterised contractile phenomena. Coupled with simple, yet reasonably realistic activation dynamics, such models consist of parameters that are meaningfully linked to fundamental aspects of muscle excitation and contraction. We aimed to illustrate the utility of a muscle model for elucidating relevant mechanisms and predicting changes in output by simulating the individual and combined effects on isometric force of several known ageing-related adaptations. Simulating literature-informed reductions in free Ca2+ concentration and Ca2+ sensitivity generated predictions at odds qualitatively with the characteristic slowing of contraction speed. Conversely, incorporating slower Ca2+ removal or a fractional increase in type I fibre area emulated expected changes; the former was required to simulate slowing of the twitch measured experimentally. Slower Ca2+ removal more than compensated for force loss arising from a large reduction in Ca2+ sensitivity or moderate reduction in Ca2+ release, producing realistic age-related shifts in the force-frequency relationship. Consistent with empirical data, reductions in free Ca2+ concentration and Ca2+ sensitivity reduced maximum tetanic force only slightly, even when acting in concert, suggesting a modest contribution to lower specific force. Lower tendon stiffness and slower intrinsic shortening speed slowed and prolonged force development in a compliance-dependent manner without affecting force decay. This work demonstrates the advantages of muscle modelling for exploring sources of variation and identifying mechanisms underpinning the altered contractile properties of aged muscle.

Determining the cortical, spinal and muscular adaptations to strength-training in older adults: A systematic review and meta-analysis

There are observable decreases in muscle strength as a result of ageing that occur from the age of 40, which are thought to occur as a result of changes within the neuromuscular system. Strength-training in older adults is a suitable intervention that may counteract the age-related loss in force production. The neuromuscular adaptations (i.e., cortical, spinal and muscular) to strength-training in older adults are largely equivocal and a systematic review with meta-analysis will serve to clarify the present circumstances regarding the benefits of strength-training in older adults. 20 studies entered the meta-analysis and were analysed using a random-effects model. A best evidence synthesis that included 36 studies was performed for variables that had insufficient data for meta-analysis. One study entered both. There was strong evidence that strength-training increases maximal force production, rate of force development and muscle activation in older adults. There was limited evidence for strength-training to improve voluntary-activation, the volitional-wave and spinal excitability, but strong evidence for increased muscle mass. The findings suggest that strength-training performed between 2 and 12 weeks increases strength, rate of force development and muscle activation, which likely improves motoneurone excitability by increased motor unit recruitment and improved discharge rates.

Utilisation of Post-Activation Performance Enhancement in Elderly Adults

With age, many physiological changes occur in the human body, leading to a decline in biological functions, and those related to the locomotor system are some of the most visible. Hence, there is a particular need to provide simple and safe exercises for the comprehensive development of physical fitness among elderly adults. The latest recommendations for the elderly suggest that the main goal of training should be to increase muscle power. The post-activation performance enhancement effect underpinning complex training might be an approach that will allow for the development of both muscle strength and velocity of movement, which will result in an increase in muscle power and improve the ability to perform daily activities and decrease injury risk. This article briefly introduces a complex training model adapted to the elderly with its potential benefits and proposes a direction for further studies.

Postactivation performance enhancement: Does conditioning one arm augment performance in the other?

The purpose was to determine whether postactivation performance enhancement is specific to the muscle being conditioned or if it is also observed within the homologous muscles of the contralateral limb (after accounting for the warm-up and random error). We also investigated whether this differed based on training status or muscle size. One hundred seven participants (75 untrained; 32 trained) participated in four sessions. Visit 1 included baseline measurements and familiarization. Visits 2-4 included the completion of one of the three experimental conditions: (a) control, (b) same side and (c) crossover completed in a randomized order. The control condition completed all testing except for the conditioning contraction. The same side condition completed the conditioning contraction on the same side as the strength test. The crossover condition completed the conditioning on the arm opposite to the strength test. The variable of interest was the change from baseline in isokinetic strength. Our analysis indicated that of the hypotheses compared, the posterior probabilities (posterior probability of 0.506) favoured the hypothesis that the effect was local and greatest in those who were resistance trained [mean (SD) of 1.4 (2.2) Nm over the control in those resistance trained]. We found no relationship between muscle size and postactivation performance enhancement. In conclusion, there is an influence of training status pertaining to the postactivation performance enhancement effect but no influence from baseline muscle size. It appears unlikely that the effect is due to a systemic mechanism.

Muscle Twitch Torque During Two Different in Volume Isometric Exercise Protocols: Fatigue Effects on Postactivation Potentiation

Xenofondos, A, Bassa, E, Vrabas, IS, Kotzamanidis, C, and Patikas, D. Muscle twitch torque during two different in volume isometric exercise protocols: fatigue effects on postactivation potentiation. J Strength Cond Res 32(2): 578-586, 2018-The purpose of this study was to quantify the effect of the contraction duration of 2 isometric exercise protocols on the postactivation potentiation of 14 well-trained men (age: 22.6 ± 2.8 years, height: 180.3 ± 5.9 cm, and body mass: 72.3 ± 37.9 kg). The protocols consisted of 4 × 6 maximal plantar flexions, of 3-second (P3) or 6-second (P6) duration, performed in random order, with a 2-minute and 15-second intervals between the sets and repetitions, respectively. The torque during maximal isometric voluntary contraction (MIVC), the peak twitch torque (TT), and the rate of torque development (RTD) after each MIVC were analyzed for the first and the last trial of each set, the average of all trials of each set, and the trials within each set that had the highest peak TT. The MIVC had an overall greater reduction during P6 compared with P3 (P3: -4.6 ± 2.3 vs. P6: -16.0 ± 1.9%). P6 showed higher potentiation in TT during the initial repetitions of the first 2 sets (p < 0.05) in contrast to the P3, which revealed a lower potentiation but for a longer period along the exercise session. However, both protocols had on average the same potential for potentiation (P3: 81.6 ± 6.1 vs. P6: 79.8 ± 6.3%). The twitch RTD presented no systematic difference between the 2 protocols (p > 0.05). These data demonstrate the dependence of the TT potentiation on the conditioning stimulus and verify the cumulative effect of potentiation, suggesting the implementation of longer contractions to achieve maximal but temporal TT potentiation and shorter contractions for less variable but prolonged potentiation.

Time-related changes in firing rates are influenced by recruitment threshold and twitch force potentiation in the first dorsal interosseous

NEW FINDINGS

What is the central question of this study? The influences of motor unit recruitment threshold and twitch force potentiation on the changes in firing rates during steady-force muscular contractions are not well understood. What is the main finding and its importance? The behaviour of motor units during steady force was influenced by recruitment threshold, such that firing rates decreased for lower-threshold motor units but increased for higher-threshold motor units. In addition, individuals with greater changes in firing rates possessed greater twitch force potentiation. There are contradictory reports regarding changes in motor unit firing rates during steady-force contractions. Inconsistencies are likely to be the result of previous studies disregarding motor unit recruitment thresholds and not examining firing rates on a subject-by-subject basis. It is hypothesized that firing rates are manipulated by twitch force potentiation during contractions. Therefore, in this study we examined time-related changes in firing rates at steady force in relationship to motor unit recruitment threshold in the first dorsal interosseous and the influence of twitch force potentiation on such changes in young versus aged individuals. Subjects performed a 12 s steady-force contraction at 50% maximal voluntary contraction, with evoked twitches before and after the contraction to quantify potentiation. Firing rates, in relationship to recruitment thresholds, were determined at the beginning, middle and end of the steady force. There were no firing rate changes for aged individuals. For the young, firing rates decreased slightly for lower-threshold motor units but increased for higher-threshold motor units. Twitch force potentiation was greater for young than aged subjects, and changes in firing rates were correlated with twitch force potentiation. Thus, individuals with greater increases in firing rates of higher-threshold motor units and decreases in lower-threshold motor units possessed greater twitch force potentiation. Overall, changes in firing rates during brief steady-force contractions are dependent on recruitment threshold and explained in part by twitch force potentiation. Given that firing rate changes were measured in relationship to recruitment threshold, this study illustrates a more complete view of firing rate changes during steady-force contractions.

Age-related differences in twitch properties and muscle activation of the first dorsal interosseous

OBJECTIVE

To examine twitch force potentiation and twitch contraction duration, as well as electromyographic amplitude (EMG_RMS) and motor unit mean firing rates (MFR) at targeted forces between young and old individuals in the first dorsal interosseous (FDI). Ultrasonography was used to assess muscle quality.

METHODS

Twenty-two young (YG) (age=22.6±2.7years) and 14 older (OD) (age=62.1±4.7years) individuals completed conditioning contractions at 10% and 50% maximal voluntary contraction, (MVC) during which EMG_RMS and MFRs were assessed. Evoked twitches preceded and followed the conditioning contractions. Ultrasound images were taken to quantify muscle quality (cross-sectional area [CSA] and echo intensity [EI]).

RESULTS

No differences were found between young and old for CSA, pre-conditioning contraction twitch force, or MFRs (P>0.05). However, OD individuals exhibited greater EI and contraction duration (P<0.05), and EMG_RMS (YG=35.4±8.7%, OD=43.4±13.2%; P=0.034). Twitch force potentiation was lower for OD (0.311±0.15N) than YG (0.619±0.26N) from pre- to post-50% conditioning contraction (P<0.001).

CONCLUSIONS

Lower levels of potentiation with elongated contraction durations likely contributed to greater muscle activation during the conditioning contractions in the OD rather than altered MFRs. Ultrasonography suggested age-related changes in muscle structure contributed to altered contractile properties in the OD.

SIGNIFICANCE

Greater muscle activation requirements can have negative implications on fatigue resistance at low to moderate intensities in older individuals.

The age related slow and fast contributions to the overall changes in tibialis anterior contractile features disclosed by maximal single twitch scan

This work aimed to verify if maximal electrically evoked single twitch (STmax) scan discloses the relative functional weight of fast and slow small bundles of fibres (SBF) in determining the contractile features of tibialis anterior (TA) with ageing. SBFs were recruited by TA main motor point stimulation through 60 increasing levels of stimulation (LS): 20 stimuli at 2Hz for each LS. The lowest and highest LS provided the least ST and STmax, respectively. The scanned STmax was decomposed into individual SBF STs. They were identified when twitches from adjacent LS were significantly different and then subtracted from each other. Nine young (Y) and eleven old (O) subjects were investigated. Contraction time (CT) and STarea/STpeak (A/PT) were calculated per each SBF ST. 143 and 155 SBF STs were obtained in Y and O, respectively. Y: CT and A/PT range: 45-105ms and 67-183mNs/mN, respectively. Literature data set TA fast fibres at 34% so, from the arrays of CT and A/PT, 65ms and 100mNs/mN were identified as the upper limit for SBF fast ST classification. O: no SBF ST could be classified as fast.

CONCLUSIONS

STmax scan reveals age-related changes in the relative contribution of fast and slow SBFs to the overall muscle mechanics.

Postactivation potentiation and muscular endurance training

INTRODUCTION

The aim of this study was to investigate muscle twitch force potentiation after voluntary conditioning contractions (CC) of various intensities and the CC duration necessary to achieve maximal potentiation before and after muscular endurance training.

METHODS

Fourteen healthy men and women (23.6 ± 0.96 years of age) performed repeated CCs of 25%, 50%, and 100% maximal voluntary contraction of the adductor pollicis muscle until maximal potentiation. CCs were followed by electrically evoked twitches. The training group performed a fatigue task and endurance trained for 8 weeks.

RESULTS

Endurance time increased by 79.8 ± 22.5% posttraining. Potentiation occurred after all CC intensities and was greater after training. The CC duration needed to achieve maximal potentiation decreased as CC intensity increased. Potentiation was greater during the fatigue task after compared to before training and was correlated with endurance time.

CONCLUSION

An increase in muscle force potentiation may function as a mechanism to prolong muscular endurance.

Cellular and whole muscle studies of activity dependent potentiation

With a single activation, a skeletal muscle fiber, motor unit or whole muscle will yield a twitch contraction. The twitch is not an "all-or-none" response, but a submaximal response that can vary from one time to another. Prior activation causes myosin regulatory light chain (RLC) phosphorylation, by an enzyme called myosin light chain kinase. This phosphorylation dissipates slowly over the next several minutes due to a slow activity of light chain phosphatase. Phosphorylation of the RLC increases the mobility of the S1 head of myosin, bringing the S1 head in closer proximity to the myosin binding sites on actin. This increased mobility increases the rate of engagement of cross-bridges and increases the rate of force development and contraction magnitude on subsequent twitch or other submaximal contractions. We call this increased contractile response "activity dependent potentiation". With sequential twitches or incompletely fused tetanic contractions, the term staircase is used to describe the progressive increase in amplitude of contraction. If a twitch is elicited after a tetanic contraction, we call the enhanced response posttetanic potentiation. Stretching a muscle fiber to a longer length will also bring the actin filaments close to the myosin heads. This increases the Ca²(+) sensitivity, independent of RLC phosphorylation. At long sarcomere lengths, the impact of RLC phosphorylation is diminished, because Ca²(+) sensitivity is already increased. Similarly, lowering the temperature at which the muscle is tested increases Ca²(+) sensitivity. At low temperatures, staircase and posttetanic potentiation are diminished, but RLC phosphorylation still occurs. Activity dependent potentiation is an important functional modulator of contractile response.

Postactivation potentiation of short tetanic contractions is differently influenced by stimulation frequency in young and elderly adults

The purpose of this study was to examine the effects of postactivation potentiation (PAP) on the torque and rate of torque development for contractions evoked by short trains of stimuli at different frequencies, in young and elderly adults. Individual mechanical contributions to each electrical stimulus within trains were also analysed. Single pulse, and two- (PT2) and three-pulse trains (PT3) delivered at 20, 50, 80 and 100 Hz were evoked before and during a 10 min period after a 6-s conditioning MVC. The results show that PAP of the torque for PT2 decreased with the increase of the stimulation frequency for young (from 184.5 to 140.4% of control values) and elderly (from 140.5 to 109.6%). Regardless of the stimulation frequency, the peak of PAP was greater in young than in elderly and occurred immediately after the conditioning MVC but was delayed for the 100 Hz condition in elderly adults. For PT3, the results were similar although the extent of PAP was less. The PAP of the mechanical contributions within the trains also decreased with the augmentation of the stimulation frequency. For most of the frequencies above 20 Hz, the peak of PAP for each mechanical contribution was delayed by 1 min after the conditioning MVC. These results indicate an age- and frequency-related PAP saturation of the successive mechanical contributions within a train of stimuli that decrease with time. The functional implication of the findings is that PAP effect is lower and delayed at high compared with low activation rate.

Age-related muscle fatigue after a low-force fatiguing contraction is explained by central fatigue

The contribution of central fatigue during and after low- and high-force isometric contractions sustained until failure with age is not established. We compared the time to failure and changes in voluntary activation measured using motor point stimulation of 15 young and 15 old adults for an isometric contraction sustained with the elbow flexor muscles at 20% and 80% of maximal voluntary contraction (MVC) force. Young adults had a briefer time to task failure than old adults for the 20% MVC fatiguing contraction, but a similar duration for the 80% task. Voluntary activation was reduced at the end of the 20% MVC task, but by greater magnitudes for old than young adults. The reduction in MVC torque after the low-force task was associated with the reduction in voluntary activation. After the 80% task, voluntary activation declined to similar levels for the young and old adults. Electromyographic activity levels (% MVC) of the biceps brachii and brachioradialis muscles during the fatiguing contraction were greater for the old than young for the 20% MVC task, but similar with age for the 80% MVC task. Our findings indicate that intensity and duration of contraction can be manipulated in young and old adults to induce varying magnitudes of fatigue within the central nervous system. Aging increases: (1) fatigue within the central nervous system immediately after a low-force fatiguing contraction, and (2) the potential for large neural adaptations during neuromuscular rehabilitation in old adults.

Postactivation potentiation of knee extensor muscles in power- and endurance-trained, and untrained women

This study compared postactivation potentiation (PAP) in knee extensor muscles after a 10 s conditioning isometric maximal voluntary contraction (MVC) in female power- (PT, n=12) and endurance-trained (ET, n=12) athletes, and untrained (UT, n=12) women aged 20-24 years. Isometric twitch characteristics of the knee extensor muscles were assessed in pre-MVC condition and during 15 min post-MVC period using supramaximal electrical stimulation of the femoral nerve by rectangular pulses of 1 ms duration. A significant (P<0.05) potentiation of twitch peak torque (Pt, 30-51% in different groups), maximal rates of torque development (50-125%) and relaxation (76-124%) occurred immediately (2 s) post-MVC. PAP declined sharply at 1-3 min of recovery, whereas a significant potentiation of twitch Pt was still present for ET athletes at 1 min, and for UT women and PT athletes at 5 min of recovery, respectively. There were no significant (P>0.05) changes in twitch contraction and half-relaxation times after a 10 s conditioning MVC. We concluded that PAP in knee extensor muscles is enhanced in PT but not in ET female athletes. The magnitude of PAP was greater when measured immediately after the conditioning MVC and its decline was slower in PT compared with ET athletes. Immediately post-MVC, twitch speed-related characteristics were potentiated to a greater extent than twitch Pt. The time-course of isometric twitch was not significantly altered by conditioning MVC.

Mechanomyographic and electromyographic responses to stimulated and voluntary contractions in the dorsiflexors of young and old men

The effect of age on mechanomyography (MMG) has not been examined for electrically evoked contractions. Similar to torque, we expected that postactivation potentiation of the MMG would differ between young and old subjects. Additionally, under voluntary conditions, we compared normalized MMG and electromyographic (EMG) signals in relation to torque, and expected that MMG, unlike EMG, would be affected by age. In 10 young and 10 old men, electrical stimulation was delivered before and after a 10-s maximal voluntary contraction (MVC) to assess potentiation of contractile (twitch torque; Pt), electrical (M-wave amplitude), and mechanical (MMG amplitude) properties of the dorsiflexors. Subsequently, subjects performed voluntary contractions at 20%, 40%, 60%, 80%, and 100% MVC for calculation of normalized MMG-torque and EMG-torque relationships. Following the MVC, Pt and evoked MMG were larger than at rest in both groups, but M-wave amplitude was unchanged. Twitch potentiation was greater in young than old, whereas evoked MMG was unaffected by age. Under voluntary conditions, values for MMG and EMG were similar between groups, except for greater MMG at MVC in young men. The shape of MMG and EMG relationships to torque was similar only in young men. Using the aging model, our results indicate that potentiation of the mechanical components (MMG) differ from those of twitch torque. Furthermore, the comparison of normalized voluntary MMG with age provides additional support for the concept of age-related motor unit remodeling.

Postactivation potentiation in a human muscle: effect on the rate of torque development of tetanic and voluntary isometric contractions

Postactivation potentiation (PAP), a mechanism by which the torque of a muscle twitch is increased following a conditioning contraction, is well documented in muscular physiology, but little is known about its effect on the maximal rate of torque development and functional significance during voluntary movements. The objective of this study was to investigate the PAP effect on the rate of isometric torque development of electrically induced and voluntary contractions. To that purpose, the electromechanical responses of the thumb adductor muscles to a single electrical stimulus (twitch), a train of 15 pulses at 250 Hz (HFT(250)), and during ballistic (i.e., rapid torque development) voluntary contractions at torque levels ranging from 10 to 75% of maximal voluntary contraction (MVC) were recorded before and after a conditioning 6-s MVC. The results showed that the rate of torque development was significantly (P < 0.001) increased after the conditioning MVC, but the effect was greater for the twitch ( approximately 200%) compared with the HFT(250) ( approximately 17%) or ballistic contractions (range: 9-24%). Although twitch potentiation was maximal immediately after the conditioning MVC, maximal potentiation for HFT(250) and ballistic contractions was delayed to 1 min after the 6-s MVC. Furthermore, the similar degree of potentiation for the rate of isometric torque development between tetanic and voluntary ballistic contractions indicates that PAP is not related to the modality of muscle activation. These observations suggest that PAP may be considered as a mechanism that can influence our contractions during daily tasks and can be utilized to improve muscle performance in explosive sports.

Muscular Performances at the Ankle Joint in Young and Elderly Men

The effect of aging on mechanical and electromyographic characteristics of ankle joint muscles was investigated in 11 young (mean age 24 years) and 12 elderly (mean age 77 years) males. Maximal and submaximal isometric voluntary torques were measured during ankle plantarflexion and dorsiflexion. Electromyographic activities of triceps surae and tibialis anterior muscles were recorded. The elderly group developed equal maximal dorsiflexion torques (42 vs 45 N.m, p >.05), but in plantarflexion, the elderly group was weaker (80 vs 132 N.m, p <.001) and presented a decreased twitch amplitude (11 vs 16 N.m) and lower coactivation (8% vs 15%) than that of the young adults. We established a linear relationship between the percentage of coactivation and developed resultant torque. Our results showed that dorsiflexor muscles were not affected by aging, contrary to plantarflexors, in which the decline in torque was partly explained by changes intervening at the peripheral level.

Training-Related Adaptations in Motor Unit Discharge Rate in Young and Older Adults

BACKGROUND

This study evaluated changes in motor unit (MU) firing rates in young and older adults during exercise training.

METHODS

Vastus lateralis MU discharge rates were measured in 8 young and 7 older adults. Each participant performed isometric knee extension contractions at 10%, 50%, and 100% of maximal voluntary contraction or effort (MVC) on two separate occasions. Participants then completed a 6-week resistance exercise training protocol.

RESULTS

Significant increases in maximal force were observed as early as 1 week after the first baseline testing session, and these were accompanied by increases in the MU discharge rate. Exercise training increased maximal voluntary force by 29% and 36% in the young and older adults, respectively. Motor unit discharge rates at 100% of maximal effort were significantly greater in the young (24.7 pps) than in the older adults (17.8 pps). Furthermore, the young adults also exhibited significantly greater discharge rates at 50% MVC, but there were no differences at the 10% force level. Maximal MU discharge rates increased during the 1-week period separating the two initial testing sessions. After the 6-week training period, maximal MU discharge rates were 15% higher for the young adults and 49% higher for the older adults. No changes in discharge rates were observed for either group at 10% or 50% MVC after exercise training.

CONCLUSIONS

The early increase in maximal MU discharge rate with repeated maximal force assessment may comprise an important neural mechanism mediating early, rapid gains in muscular force capability.

Postactivation potentiation influences differently the nonlinear summation of contractions in young and elderly adults

The force enhancement of a twitch after a maximal conditioning muscle contraction [i.e., postactivation potentiation (PAP)] is reduced with aging, but its influence on the summation of force in response to repetitive stimulation at different frequencies is not known. The purpose of this work was to compare the electrically evoked mechanical responses of the tibialis anterior muscle between young and elderly adults after a 6-s maximal voluntary contraction (MVC). The results showed that, immediately after the conditioning MVC, twitch torque and its maximal rate of development and relaxation were significantly enhanced in both groups, but the magnitude of potentiation was greater in young (148.0 +/- 14.2, 123.7 +/- 16.5, and 185.4 +/- 36.5%, respectively) compared with elderly adults (87.4 +/- 15.2, 63.8 +/- 9.9, and 62.9 +/- 11.0%, respectively). This age-related difference in potentiation of the twitch disappeared completely 1 min after the conditioning MVC. The potentiation of torque and speed-related parameters in response to two- and three-pulse trains, delivered at a constant interval of 10 ms (100 Hz), was less than for a single pulse for both groups. In young adults, the magnitude of PAP on the successive individual mechanical contributions within a train of stimuli declined progressively such that the third contribution did not differ significantly from the same contribution before the conditioning MVC. In contrast, the second and third contributions did not potentiate (P > 0.05) in elderly adults. Although these contributions did potentiate significantly at a lower frequency of stimulation (20 Hz) in the two groups, the difference in PAP between young and elderly adults still persisted. This overall attenuation of potentiation with aging, however, appears to have a moderate influence on the decrement of the muscular performance.

Postactivation potentiation in human muscle is not related to the type of maximal conditioning contraction

The mechanical performance of a muscle can be enhanced by preceding contractile activity, such as occurs with postactivation potentiation. To investigate whether the type of contraction influences the extent of potentiation, the effects of 6-s maximal isometric (ISO), concentric (CON), and eccentric (ECC) maximal voluntary contractions (MVC) on the muscle twitch were compared in the tibialis anterior of nine subjects. The study also examined the effect of postactivation potentiation on the force evoked by the second (C2) and third (C3) responses of two-pulse (PT2) and three-pulse (PT3) trains that were delivered at a 10-ms interpulse interval. The results showed that immediately after the conditioning MVC, twitch torque (Pt) and its maximal rate of torque development (+dPt/dt) and relaxation (-dPt/dt) were significantly enhanced, without any change in contraction time (CT), half-relaxation time ((1/2)RT), and compound muscle action potential (M wave). The extent of Pt potentiation was similar for all MVC modalities, and the mean maximal values ranged from 150% to 180%. Furthermore, postactivation potentiation was greater for the single pulse compared with PT2 and PT3 responses. All parameters returned to initial values within 7-10 min. Although Pt (or C1) was potentiated more than was C2 and C3, its decline over time was proportionally more rapid than those for C2 and C3. We conclude that postactivation potentiation was not related to the type of conditioning MVC under these experimental conditions. The observation that postactivation potentiation increased C1 more than C2 and C3 indicates that a saturation process limits the extent of potentiation during the summation of successive responses to a train of stimuli. These results have practical application in the design of functional electrical stimulation protocols.

Aptidão física relacionada à saúde de idosos: influência da hidroginástica

FUNDAMENTOS E OBJETIVOS: A prática de exercício físico, além de combater o sedentarismo, contribui de maneira significativa para a manutenção da aptidão física do idoso. O objetivo do estudo foi verificar o efeito da prática de hidroginástica sobre a aptidão física do idoso associada à saúde. METODOLOGIA: Foi realizado um ensaio controlado em 74 mulheres idosas, sem atividade física regular. Um grupo de 37 mulheres recebeu duas aulas semanais de hidroginástica durante três meses e outras 37 mulheres serviram como controle. A aptidão física foi avaliada através da bateria de testes de Rikli e Jones (1999), com avaliações de força e resistência de membros inferiores (levantar e sentar na cadeira), força e resistência de membros superiores (flexão do antebraço), flexão de membros inferiores (sentado, alcançar os membros inferiores com as mãos), mobilidade física - velocidade, agilidade e equilíbrio (levanta, caminha 2,44m e volta a sentar), flexibilidade dos membros superiores (alcançar atrás das costas com as mãos) e resistência aeróbica (andar seis minutos). A bateria de testes foi aplicada antes do início das aulas e no fim do programa após três meses. Os grupos foram semelhantes em relação a idade, IMC, renda familiar e anos de escolaridade. RESULTADOS: Ao longo dos três meses, foram acompanhadas 30 mulheres em cada grupo; perda amostral de 18,9%. Observou-se no grupo da hidroginástica um melhor desempenho em todos os pós-testes, quando comparados com os resultados do próprio grupo no pré-teste e com o controle no pós-teste (p < 0,05). CONCLUSÃO: Concluiu-se que a prática de hidroginástica para mulheres idosas sem exercícios físicos regulares contribuiu para a melhoria da aptidão física relacionada à saúde.

Interaction of fibre type, potentiation and fatigue in human knee extensor muscles

AIM

To examine the effect of fibre type on potentiation and fatigue.

METHODS

Young men (n = 4 per group) with a predominance of type I [61.4 +/- 6.9% (SD), group I (GI)] or type II [71.8 +/- 9.2%, group II (GII)] fibres in vastus lateralis, performed a fatigue protocol of sixteen 5-s maximal voluntary isometric contractions (MVCs) of the right knee extensors. Maximal twitches and corresponding muscle action potentials (M-waves) were evoked before the first MVC, during the 3-s rest period after each MVC and at intervals during the 5-min recovery period after the last MVC.

RESULTS

Group II [49.3 +/- 2.6% (SE)] had a greater decrease in MVC force than GI (22.8 +/- 6.2%) during the fatigue protocol. Group II (126.4 +/- 13.6%) showed greater twitch force potentiation early in the fatigue protocol than GI (38.2 +/- 2.3%), but greater depression at the end (33.7 +/- 13.7% vs.17.4 +/- 3.4%). Twitch time-to-peak torque (TPT) and half relaxation time (HRT) initially decreased but then increased as the fatigue protocol progressed; GII had a greater increase in HRT. During a 5-min recovery period twitch force increased above the prefatigue level and remained so until the end of the recovery period; the pattern was similar in GI and GII. Twitch TPT and HRT remained elevated during recovery. M-wave area increased throughout the fatigue protocol and the first part of recovery before returning to baseline values in GII, whereas there were no significant changes in GI. The interaction between potentiation and fatigue was amplified in GII early in the fatigue protocol with concurrently greater twitch and M-wave potentiation, and greater MVC force decrease and HRT increase. Late in the protocol, GII had a greater decrease in twitch and MVC force combined with greater M-wave potentiation.

CONCLUSION

It is concluded that fibre type distribution influences potentiation and fatigue of the twitch, and potentiation of the M-wave during fatiguing exercise.

Changes in motor unit discharge rate are not associated with the amount of twitch potentiation in old men

This study examined, in nine old men (82 +/- 4 yr), whether there is an association between the magnitude of change in motor unit discharge rate and the amount of twitch potentiation after a conditioning contraction (CC). The evoked twitch force and motor unit discharge rate during isometric ramp-and-hold contractions (10-18 s) of the triceps brachii muscle at 10, 20, and 30% of the maximal voluntary contraction were determined before and 10 s, 2 min, 6 min, and 11 min after a 5-s CC at 75% maximal voluntary contraction. After the CC, there was a potentiation of twitch force (approximately twofold), and the discharge rate of the 47 sampled motor units declined (P < 0.05) an average of 1 Hz 10 s after the CC, compared with the control condition. The CC had no effect on the variability (coefficient of variation) of both force and discharge rate, as well as the electromyographic activity recorded over the triceps brachii and biceps brachii muscles. In contrast to our earlier study of young men (Klein CS, Ivanova TD, Rice CL, and Garland SJ, Neurosci Lett 316: 153-156, 2001), the magnitude of the reduction in discharge rate after the CC was not associated (r = 0.06) with the amount of twitch potentiation. These findings suggest an age-related alteration in the neural strategy for adjusting motor output to a muscle after a CC.

Postactivation potentiation: role in human performance

Postactivation potentiation (PAP) is the transient increase in muscle contractile performance after previous contractile activity. This review describes the features and mechanism of PAP, assesses its potential role in endurance and strength/speed performance, considers strategies for exploiting PAP, and outlines how PAP might be affected by training.

Incomplete recovery of voluntary isometric force after fatigue is not affected by old age

The 60-min recovery profiles of voluntary and electrically stimulated force, contractile speed, surface electromyography, muscle activation via twitch interpolation, and muscle compound action potentials (M-waves) in the elbow flexors of seven young men (24 +/- 2 years) and seven men over 80 years of age (84 +/- 2 years) were compared following intermittent (3 s on, 2 s off) contractions at 60% of each subject's maximum voluntary contraction (MVC) force. There was no age-related difference between groups in the average time to fatigue or the rate of voluntary force loss; both groups lost 40% of their force within approximately 5 min. Despite a rapid increase to approximately 83% of the prefatigue MVC by the third minute of recovery for both groups, MVC force did not return to the prefatigue value within 60 min (94 +/- 4% young, 91 +/- 3% old). These results suggest that the incomplete recovery of voluntary force was likely due to a peripheral limitation in the muscle at the level of excitation--contraction coupling, and was not affected by age. Delayed recovery of voluntary force and a greater degree of low-frequency fatigue in the old men were not observed and there were no age-related impairments in any parameter normalized to the prefatigue value during fatigue or recovery. We suggest that the specific fatigue task may be more important to recovery than proposed alterations in the aged neuromuscular system when normalization and matching of the fatigue task criteria occurs.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Postactivation potentiation in endurance-trained male athletes

PURPOSE

The purpose of the study was to determine whether postactivation potentiation (PAP) was enhanced in the trained muscles of male endurance athletes.

METHODS

Triathletes (TRI), distance runners (RUN), active controls (AC), and sedentary control subjects (SED) (N = 10 per group) performed 10-s maximal isometric contractions (MVC) of the elbow extensor and ankle plantarflexor muscles. Maximal twitch contractions were evoked (percutaneous stimulation) before and during a 5-min period after the MVC. PAP was measured as the percentage change in peak twitch torque post-MVC.

RESULTS

TRI, who train both upper and lower limb muscles, had enhanced (relative to SED) PAP in both elbow extensor and plantarflexor muscles. In RUN, who train only the lower limbs, enhanced PAP was restricted to the plantarflexors. AC, whose main activity was upper and lower limb weight training, also had enhanced PAP in both muscle groups, although the enhancement in the plantarflexors was not as great as in TRI and RUN.

CONCLUSION

PAP is enhanced in endurance athletes. Enhanced PAP may counteract fatigue during endurance exercise. The mechanism(s) responsible for the enhanced PAP remain to be determined.

Effects of aging on motor-unit control properties

It was hypothesized that the age-related alterations in the morphological properties of a motor unit would be accompanied by modifications in the control aspects of the motor unit, as either an adaptive or compensatory mechanism to preserve smooth force production. In specific, the objective of the study was to investigate the age-related alterations in the concurrent firing behavior of multiple motor units in the first dorsal interosseous (FDI) muscle in isometric contractions at 20 and 50% of the subject's voluntary contraction level. Analysis of the data collected from 10 young (24-37 yr of age) and 10 elderly (65-88 yr of age) subjects led to three novel observations regarding the firing behavior of aged motor units. 1) Among elderly subjects, there is a decrease in the common fluctuations that are observed among the firing rates of motor units in the young. 2) The relationship observed between the firing rate and recruitment threshold of young subjects is disturbed in the elderly. Although in young subjects, at any point in a given submaximal contraction, earlier recruited motor units have higher firing rates than later-recruited units; in aged subjects this dependency of firing rate on recruitment rank is compromised. 3) The progressive decrease observed in the firing rates of concurrently active motor units in constant-force contractions in the young is not seen in the aged. In addition to these original findings, this study provided support for earlier reports of 1) decreased average firing rates probably reflecting the slowing of the muscle, 2) a shift in recruitment thresholds toward lower force levels in line with the shift toward type I fibers, and 3) multiphasic action potential shapes indicative of the reinnervation process that takes place during aging. Taken as a whole, these findings indicate significant age-related modifications in the control properties of human motor units.

Age-related changes in motor unit function

This review focuses on the functional relationship between age-related morphological and physiological changes at the level of the motor unit (MU). It is well established that older humans are weaker than younger people, exhibit reduced force control, and have slower neuromuscular contractile properties. Older people may also exhibit a decrease in MU discharge rate, and an increase in variability of MU discharge at high force levels. The matching of MU discharge and contractile properties may be an age-related neurophysiological strategy adopted to optimize motor control, similar to that observed in acute conditions such as fatigue. Because muscle force output is modulated partially by MU discharge behavior, the study of these properties may offer insights into the physiology of muscular weakness and motor function in older people. In turn, this will allow the implementation of optimal exercise and rehabilitation programs to reduce the degree of dependence associated with aging.

Gender differences in isometric contractile properties and fatigability in elderly human muscle

This study had two purposes: to compare the isometric contractile characteristics and fatigability in the elbow flexors (EF) and ankle dorsiflexors (DF) in older males and females (60-80 years), and to determine the effects of almost 2 years of twice weekly dynamic weight-lifting training on these properties One hundred nine male and female subjects completed the 22-month intervention, 54 in the exercise group and 55 in the control group. Isometric contractile properties and fatigue characteristics were assessed at baseline, 10 months, and 22 months. At baseline, males were significantly stronger (p < .05) than females in evoked twitch torque (Pt) and maximal voluntary strength (MVC) in both EF and DF. Females exhibited significantly less fatigue (expressed either absolutely or relatively) than males in both Pt and MVC during a 3-min protocol of intermittent MVCs (5 s contraction; 2 s rest). There were no significant effects of the 22 months of dynamic training on either the isometric contractile properties or the fatigue characteristics, suggesting a very strong and persistent specificity of training effect.

Isometric intermittent endurance of four muscle groups in men aged 20-74 yr

Muscular endurance of upper and lower extremities may provide a more practical measure of muscle function related to normal daily activity than measures of strength, especially with the elderly. Maximal isometric intermittent endurance of the finger flexors, thumb abductors, dorsiflexors, and plantar flexors were characterized by peak force, impulse, percent total impulse, and percent force decrement in men aged 20-74 yr. Volunteers (N = 153) were placed into appropriate 5-yr age groups: 20-24, 25-29, ..., 70-74 yr. The intermittent endurance task consisted of 11 consecutive, 2-s maximal contractions, each separated by 3-s rest. Age group differences within each 2-s interval as well as differences in endurance were determined by ANOVA, and after adjusting for differences in body composition, by ANCOVA. The lower extremities had the greatest declines in absolute measures of force within each 2-s interval, and declines occurred earlier than previously reported. When values were expressed as percent change over the entire endurance task, there was generally no age group difference in force or impulse; however, there were significant muscle group differences. In conclusion, declines in absolute measures of force occurred at different ages depending on the muscle group; however, relative measures of muscular endurance were maintained for all age groups but varied by muscle group location.

Heavy-resistance training in older Scandinavian men and women: short- and long-term effects on arm and leg muscles

The short- and long-term effects of heavy-resistance training (85% of one-repetition maximum (RM)) on elbow flexion and knee extension dynamic and isokinetic strength and on morphology in the biceps brachii and vastus lateralis muscles were evaluated during 1 year in 35 Scandinavian men and women, aged 70-77 years, 12 of whom formed a control group. After the first 11 weeks of training (n = 23; 3 times/week) elbow flexion and knee extension dynamic strength (1 RM) had increased [mean +/- SD] 49% +/- 16 and 163% +/- 75, respectively, with no significant difference between men and women. For the following 27 weeks, strength was maintained with one training session per week (n = 12) but dropped without training (n = 11). After the final 11 weeks of training (n = 11; 3 times/week), strength had further increased 32% +/- 16 in both the arm and the leg. Isokinetic strength measurements (Cybex II; 30 degrees/s) revealed similar but smaller gains than for dynamic strength. Muscle biopsies (n = 20) taken at the start and after the first 11 weeks of training showed a significant increase in the area of both type 1 and type 2 fibers in the biceps brachii muscle and a positive significant correlation between the percentage increase in the proportional area of type 2 fibers in the vastus lateralis muscle and the percentage increase in knee extension dynamic muscle strength. In conclusion, older Scandinavian men and women have a high capacity both to improve and to maintain muscle strength, some of which is mediated through an adaptation in the muscle fiber type population.

Aging of human muscle: structure, function and adaptability

With increasing age, human skeletal muscles gradually decrease in volume, mainly due to a reduced number of motor units and muscle fibers, and a reduced size of type 2 fibers. As a result, progressive weakening and impaired mobility occur. High-resistance strength training is beneficial, even in the very old, and could possibly reverse some of the detrimental effects of age-related weakness. The importance of exercise for older people affords an excellent opportunity for the medicine community as a major source of information and promotion of physical activity for this rapidly growing segment of the population. In this review, we summarize the current knowledge of the effects of aging on the human neuromuscular system, describe some of the major underlying mechanisms of the aging atrophy and focus on the importance of strength training to improve muscle function in older people.