An age-related shift in the force-frequency relationship affects quadriceps fatigability in old adults

Differential effects of stimulation frequency on isometric and concentric isotonic contractile function in human quadriceps

Electrically evoked contractions are used to assess the relationship between frequency input and contractile output to characterize inherent muscle function, and these have been done mostly with isometric contractions (i.e., no joint rotation). The purpose was to compare the electrically stimulated frequency and contractile function relationship during isometric (i.e., torque) with isotonic (i.e., concentric torque, angular velocity, and mechanical power) contractions. The knee extensors of 16 (5 female) young recreationally active participants were stimulated (∼1-2.5 s) at 14 frequencies from 1 to 100 Hz. This was done during four conditions, which were isometric and isotonic at loads of 0 (unloaded), 7.5%, and 15% isometric maximal voluntary contraction (MVC), and repeated on separate days. Comparisons across contractile parameters were made as a % of 100 Hz. Independent of the load, the mechanical power-frequency relationship was rightward shifted compared with isometric torque-frequency, concentric torque-frequency, and velocity-frequency relationships (all P ≤ 0.04). With increasing load (0%-15% MVC), the isotonic concentric torque-frequency relationship was shifted leftward systematically from 15 to 30 Hz (all P ≤ 0.04). Conversely, the same changes in load caused a rightward shift in the velocity-frequency relationship from 1 to 40 Hz (all P ≤ 0.03). Velocity was leftward shifted of concentric torque in the unloaded isotonic condition from 10 to 25 Hz (all P ≤ 0.03), but concentric torque was leftward shifted of velocity at 15% MVC isotonic condition from 10 to 50 Hz (all P ≤ 0.03). Therefore, isometric torque is not a surrogate to evaluate dynamic contractile function. Interpretations of evoked contractile function differ depending on contraction type, load, and frequency, which should be considered relative to the specific task.NEW & NOTEWORTHY In whole human muscle, we showed that the electrically stimulated power-frequency relationship was rightward shifted of the stimulated isometric torque-frequency relationship independent of isotonic load, indicating that higher stimulation frequencies are needed to achieve tetanus. Therefore, interpretations of evoked contractile function differ depending on contraction type (isometric vs. dynamic), load, and frequency. And thus, isometric measures may not be appropriate as a surrogate assessment when evaluating dynamic isotonic contractile function.

Time-of-day effects on motor unit firing and muscle contractile properties in humans

Intrinsic factors related to neuromuscular function are time-of-day dependent, but diurnal rhythms in neural and muscular components of the human neuromuscular system remain unclear. The present study aimed to investigate the time-of-day effects on neural excitability and muscle contractile properties by assessing the firing properties of tracked motor units and electrically evoked twitch muscle contraction. In 15 young adults (22.9 ± 4.7 yr), neuromuscular function was measured in the morning (10:00), at noon (13:30), in the evening (17:00), and at night (20:30). Four measurements were completed within 24 h. The measurements consisted of maximal voluntary contraction (MVC) strength of knee extension, recording of high-density surface electromyography (HDsEMG) from the vastus lateralis during ramp-up contraction to 50% of MVC, and evoked twitch torque of knee extensors by electrical stimulation. Recorded HDsEMG signals were decomposed to individual motor unit firing behaviors and the same motor units were tracked among the times of day, and recruitment thresholds and firing rates were calculated. The number of detected and tracked motor units was 127. Motor unit firing rates significantly increased from morning to noon, evening, and night (P < 0.01), but there were no significant differences in recruitment thresholds among the times of day (P > 0.05). Also, there were no significant effects of time of day on evoked twitch torque (P > 0.05). Changes in the motor unit firing rate and evoked twitch torque were not significantly correlated (P > 0.05). These findings suggest that neural excitability may be affected by the time of day, but it did not accompany changes in peripheral contractile properties in a diurnal manner.NEW & NOTEWORTHY We investigated the variations of tracked motor unit firing properties and electrically evoked twitch contraction during the day within 24 h. The variation of motor unit firing rate was observed, and tracked motor unit firing rate increased at noon, in the evening, and at night compared with that in the morning. The variation in motor unit firing rate was independent of changes in twitch contraction. Motor unit firing rate may be affected by diurnal rhythms.

Age-related differences in the neuromuscular performance of fatigue-provoking exercise under severe whole-body hyperthermia conditions

PURPOSE

The purpose of this study was to determine if aging would lead to greater decline in neuromuscular function during a fatiguing task under severe whole-body hyperthermia conditions.

METHODS

Twelve young (aged 19-21 years) and 11 older (aged 65-80 years) males were enrolled in the study, which comprised a randomized control trial under a thermoneutral condition at an ambient temperature of 23°C (CON) and an experimental trial with passive lower body heating in 43°C water (HWI-43°C). Changes in neuromuscular function and fatigability, and physical performance-influencing factors such as psychological, thermoregulatory, neuroendocrine, and immune responses to whole-body hyperthermia were measured.

RESULTS

A slower increase in rectal temperature, and a lower heart rate, thermal sensation, and sweating rate were observed in older males than young males in response to HWI-43°C trial (p < 0.05). Nevertheless, prolactin increased more in response to hyperthermia in young males, while interleukin-6 and cortisol levels increased more in older males (p < 0.05). Peripheral dopamine levels decreased in older males and increased in young males in response to hyperthermia (p < 0.05). Surprisingly, older males demonstrated greater neuromuscular fatigability resistance and faster maximal voluntary contraction (MVC) torque recovery after a 2-min sustained isometric MVC task under thermoneutral and severe hyperthermic conditions (p < 0.05).

CONCLUSION

Neuromuscular performance during fatigue-provoking sustained isometric exercise under severe whole-body hyperthermia conditions appears to decline in both age groups, but a lower relative decline in torque production for older males may relate to lower psychological and thermophysiological strain along with a diminished dopamine response and prolactin release.

Electrical stimulation for investigating and improving neuromuscular function in vivo: Historical perspective and major advances

This historical review summarizes the major advances - particularly from the last 50 years - in transcutaneous motor-level electrical stimulation, which can be used either as a tool to investigate neuromuscular function and its determinants (electrical stimulation for testing; EST) or as a therapeutic/training modality to improve neuromuscular and physical function (neuromuscular electrical stimulation; NMES). We focus on some of the most important applications of electrical stimulation in research and clinical settings, such as the investigation of acute changes, chronic adaptations and pathological alterations of neuromuscular function with EST, as well as the enhancement, preservation and restoration of muscle strength and mass with NMES treatment programs in various populations. For both EST and NMES, several major advances converge around understanding and optimizing motor unit recruitment during electrically-evoked contractions, also taking into account the influence of stimulation site (e.g., muscle belly vs nerve trunk) and type (e.g., pulse duration, frequency, and intensity). This information is equally important both in the context of mechanistic research of neuromuscular function as well as for clinicians who believe that improvements in neuromuscular function are required to provide health-related benefits to their patients.

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.

Intrinsic Neuromuscular Fatigability in Humans: The Critical Role of Stimulus Frequency

Electrically evoked contractions provide insight into intrinsic neuromuscular fatigability and also represent a valuable technique to maintain muscle mass in a clinical setting. To appropriately investigate intrinsic fatigability and design optimal stimulation protocols, it would seem to be crucial to stimulate the muscle at a frequency equivalent to the mean motor unit discharge rate expected at the target force level.

Anconeus motor unit firing rates during isometric and muscle-shortening contractions comparing young and very old adults

With effects of aging, voluntary neural drive to the muscle, measured as motor unit (MU) firing rate, is lower in older adults during sustained isometric contractions compared with young adults, but differences remain unknown during limb movements. Therefore, our purpose was to compare MU firing rates during both isometric and shortening contractions between two adult age groups. We analyzed intramuscular electromyography of single-MU recordings in the anconeus muscle of young (n = 8, 19-33 yr) and very old (n = 13, 78-93 yr) male adults during maximal voluntary contractions (MVCs). In sustained isometric and muscle-shortening contractions during limb movement, MU trains were linked with elbow joint kinematic parameters throughout the contraction time course. The older group was 33% weaker and 10% slower during movements than the young group (P < 0.01). In isometric contractions, median firing rates were 42% lower (P < 0.01) in the older group (18 Hz) compared with the young group (31 Hz), but during shortening contractions firing rates were higher for both age groups and not statistically different between groups. As a function of contraction time, firing rates at MU recruitment threshold were 39% lower in the older group, but the firing rate decrease was attenuated threefold throughout shortening contraction compared with the young group. At the single-MU level, age-related differences during isometric contractions (i.e., pre-movement initiation) do not remain constant throughout movement that comprises greater effects of muscle shortening. Results indicate that neural drive is task dependent and during movement in older adults it is decreased minimally.NEW & NOTEWORTHY Changes of neural drive to the muscle with adult aging, measured as motor unit firing rates during limb movements, are unknown. Throughout maximal voluntary efforts we found that, in comparison with young adults, firing rates were lower during isometric contraction in older adults but not different during elbow extension movements. Despite the older group being ∼33% weaker across contractions, their muscles can receive neural drive during movements that are similar to that of younger adults.

Effects of old age and contraction mode on knee extensor muscle ATP flux and metabolic economy in vivo

KEY POINTS

We used 31-phosphorus magnetic resonance spectroscopy to quantify in vivo skeletal muscle metabolic economy (ME; mass-normalized torque or power produced per ATP consumed) during three 24 s maximal-effort contraction protocols: (1) sustained isometric (MVIC), (2) intermittent isokinetic (MVDC_IsoK ), and (3) intermittent isotonic (MVDC_IsoT ) in the knee extensor muscles of young and older adults. ME was not different between groups during the MVIC but was lower in older than young adults during both dynamic contraction protocols. These results are consistent with an increased energy cost of locomotion, but not postural support, with age. The effects of old age on ME were not due to age-related changes in muscle oxidative capacity or ATP flux. Specific power was lower in older than young adults, despite similar total ATP synthesis between groups. Together, this suggests a dissociation between cross-bridge activity and ATP utilization with age.

ABSTRACT

Muscle metabolic economy (ME; mass-normalized torque or power produced per ATP consumed) is similar in young and older adults during some isometric contractions, but less is known about potential age-related differences in ME during dynamic contractions. We hypothesized that age-related differences in ME would exist only during dynamic contractions, due to the increased energetic demand of dynamic versus isometric contractions. Ten young (Y; 27.5 ± 3.9 years, 6 men) and 10 older (O; 71 ± 5 years, 5 men) healthy adults performed three 24 s bouts of maximal contractions: (1) sustained isometric (MVIC), (2) isokinetic (120°·s^-1 , MVDC_IsoK ; 0.5 Hz), and (3) isotonic (load = 20% MVIC, MVDC_IsoT ; 0.5 Hz). Phosphorus magnetic resonance spectroscopy of the vastus lateralis muscle was used to calculate ATP flux (mM ATP·s^-1 ) through the creatine kinase reaction, glycolysis and oxidative phosphorylation. Quadriceps contractile volume (cm³ ) was measured by MRI. ME was calculated using the torque-time integral (MVIC) or power-time integral (MVDC_IsoK and MVDC_IsoT ), total ATP synthesis and contractile volume. As hypothesized, ME was not different between Y and O during the MVIC (0.12 ± 0.03 vs. 0.12 ± 0.02 Nm^. s^. cm^{-3 .} mM ATP^-1 , mean ± SD, respectively; P = 0.847). However, during both MVDC_IsoK and MVDC_IsoT , ME was lower in O than Y adults (MVDC_IsoK : 0.011 ± 0.003 vs. 0.007 ± 0.002 J^. cm^{-3 .} mM ATP^-1 ; P < 0.001; MVDC_IsoT : 0.011 ± 0.002 vs. 0.008 ± 0.002; P = 0.037, respectively), despite similar muscle oxidative capacity, oxidative and total ATP flux in both groups. The lower specific power in older than young adults, despite similar total ATP synthesis between groups, suggests there is a dissociation between cross-bridge activity and ATP utilization with age.

Motor unit firing rates during constant isometric contraction: establishing and comparing an age-related pattern among muscles

Motor unit (MU) firing rates (FRs) are lower in aged adults, compared with young, at relative voluntary contraction intensities. However, from a variety of independent studies of disparate muscles, the age-related degree of difference in FR among muscles is unclear. Using a standardized statistical approach with data derived from primary studies, we quantified differences in FRs across several muscles between younger and older adults. The data set included 12 different muscles in young (18-35 yr) and older adults (62-93 yr) from 18 published and one unpublished study. Experiments recorded single MU activity from intramuscular electromyography during constant isometric contraction at different (step-like) voluntary intensities. For each muscle, FR ranges and FR variance explained by voluntary contraction intensity were determined using bootstrapping. Dissimilarity of FR variance among muscles was calculated by Euclidean distances. There were threefold differences in the absolute frequency of FR ranges across muscles in the young (soleus 8-16 and superior trapezius 20-49 Hz), but in the old, FR ranges were more similar and lower for nine out of 12 muscles. In contrast, the explained FR variance from voluntary contraction intensity in the older group had 1.6-fold greater dissimilarity among muscles than the young (P < 0.001), with FR variance differences being muscle dependent. Therefore, differences between muscle FR ranges were not explained by how FRs scale to changes in voluntary contraction intensity within each muscle. Instead, FRs were muscle dependent but were more dissimilar among muscles in the older group in their responsiveness to voluntary contraction intensity.NEW & NOTEWORTHY The mean frequency of motor unit firing rates were compared systematically among several muscles and between young and older adults from new and published data sets. Firing rates among muscles were lower and more similar during voluntary isometric contraction in older than younger adults. Firing rate responses from voluntary contraction intensity were muscle dependent and more dissimilar among muscles in the older than young adults.

Muscle architecture, voluntary activation, and low-frequency fatigue do not explain the greater fatigue of older compared with young women during high-velocity contractions

Although high-velocity contractions elicit greater muscle fatigue in older than young adults, the cause of this difference is unclear. We examined the potential roles of resting muscle architecture and baseline contractile properties, as well as changes in voluntary activation and low-frequency fatigue in response to high-velocity knee extensor work. Vastus lateralis muscle architecture was determined in quiescent muscle by ultrasonography in 8 young (23.4±1.8 yrs) and 8 older women (69.6±1.1). Maximal voluntary dynamic (MVDC) and isometric (MVIC), and stimulated (80Hz and 10Hz, 500ms) isometric contractions were performed before and immediately after 120 MVDCs (240°^.s^-1, one every 2s). Architecture variables did not differ between groups (p≥0.209), but the half-time of torque relaxation (T_1/2) was longer in older than young women at baseline (151.9±6.0 vs. 118.8±4.4 ms, respectively, p = 0.001). Older women fatigued more than young (to 33.6±4.7% vs. 55.2±4.2% initial torque, respectively; p = 0.004), with no evidence of voluntary activation failure (ΔMVIC:80Hz torque) in either group (p≥0.317). Low-frequency fatigue (Δ10:80Hz torque) occurred in both groups (p<0.001), as did slowing of T_1/2 (p = 0.001), with no differences between groups. Baseline T_1/2 was inversely associated with fatigue in older (r² = 0.584, p = 0.045), but not young women (r² = 0.147, p = 0.348). These results indicate that differences in muscle architecture, voluntary activation, and low-frequency fatigue do not explain the greater fatigue of older compared with young women during high-velocity contractions. The inverse association between baseline T_1/2 and fatigue in older women suggests that factors related to slower muscle contractile properties may be protective against fatigue during fast, repetitive contractions in aging.

Electrically evoked force loss of the knee extensors is equivalent for young and old females and males

Data are scant on sex-related differences for electrically evoked contractions, which assess intrinsic contractile properties while limiting spinal and supraspinal adaptations to mitigate fatigue. Furthermore, the few studies that exist use stimulus frequencies considerably higher than the natural motor unit discharge rate for the target force. The purpose of this study was to compare force loss to electrically evoked contractions at a physiological stimulus frequency among young females (n = 12), young males (n = 12), old females (n = 11), and old males (n = 11). The quadriceps of the dominant leg were fatigued by 3 min of intermittent transcutaneous muscle belly stimulation (15 Hz stimulus train to initially evoke 25% of maximal voluntary force). Impairment of tetanic contractile impulse (area under the curve) did not differ between sexes for young or old adults or between age groups, with a pooled value of 55.2% ± 12.4% control at the end of fatigue. These data contrast with previous findings at 30 Hz, when the quadriceps of females had greater fatigue resistance than males for young and old adults. The present results highlight the impact stimulus frequency has on intrinsic fatigability of muscle; the findings have implications for future fatigue paradigms and treatment approaches when utilizing electrical stimulation for rehabilitation.

Novelty Fatigue was not different between sexes with a stimulation frequency comparable to discharge rates during voluntary contractions. These results highlight that stimulus frequency not only influences fatigue development but also between-group differences.

Novel perspective on contractile properties and intensity-dependent verification of force-frequency relationship during neuromuscular electrical stimulation

PURPOSE

The aims of the present study were: (a) to examine the effect of the stimulus intensity on force-frequency and torque fluctuation-frequency relationships during Neuromuscular electrical stimulation; and (b) to identify a novel parameter that can be used to evaluate muscle contractile properties.

METHODS

Electrically elicited joint torque involving the quadriceps femoris muscle was recorded during neuromuscular electrical stimulation at two different stimulus intensities in 19 healthy men. Stimulation frequencies were set at 5-40 Hz with a duration of 10 s. Evoked joint torque was compared among all stimulation frequencies between the two stimulus intensities (68 and 113 V). The torque fluctuation at each stimulation frequency as the change in the contraction pattern was also compared between the intensities. Torque and torque fluctuation were normalized at each frequency by the largest torque or torque fluctuation, respectively. We extracted a novel parameter: the arrival point of tetanic contraction based on force-frequency and torque fluctuation-frequency curves.

RESULTS

There were significant differences in normalized torque at 5-25 and 40 Hz and in normalized torque fluctuation at 15-30 and 40 Hz between the two stimulus intensities. Extracted parameters showed no significant difference between the intensities.

CONCLUSION

The results suggest that force-frequency relationships during neuromuscular electrical stimulation are influenced by the intensity of stimulation applied to the quadriceps femoris muscle. However, we consider that it is possible to simultaneously evaluate contractile properties using the novel parameter.

Compensatory adjustments in motor unit behavior during fatigue differ for younger versus older men

BACKGROUND

The ability to maintain a submaximal force as a muscle fatigues is supplemented by compensatory adjustments in the nervous system's control of motor units.

AIM

We sought to compare vastus lateralis motor unit recruitment and firing rate data for younger versus older men during isometric fatigue.

METHODS

Twelve younger (age = 25 ± 3 years) and 12 older (75 ± 8 years) men performed contractions of the knee extensors at 50% of maximal voluntary contraction force until exhaustion. Surface electromyographic (sEMG) signals were detected from the vastus lateralis. A sEMG signal decomposition algorithm was used to quantify the motor unit action potential (MUAP) amplitude, mean firing rates, and recruitment threshold of each motor unit. For the latter two variables, our analyses only included motor units that featured similar action potential amplitude throughout the protocol.

RESULTS

There was no group difference for time to task failure (p = 0.362, d = 0.381). Both groups showed increases in MUAP amplitude [younger and older slopes = 0.0174 ± 0.0123 and 0.0073 ± 0.0123 mV/contraction, respectively (p = 0.082, d = 0.710)], but the change was more linear for the younger men (mean r² values = 0.565 and 0.455). Mean firing rates increased over time for the younger (p < 0.001), but not the older (p = 0.579), men. Similarly, recruitment thresholds decreased for younger men (p = 0.001).

CONCLUSION

We propose that aging results in neuromuscular impairments that hinder older adults' ability to make compensatory adjustments in motor unit control during fatigue.

High-Altitude Acclimatization Improves Recovery from Muscle Fatigue

PURPOSE

We investigated the effect of high-altitude acclimatization on peripheral fatigue compared with sea level and acute hypoxia.

METHODS

At sea level (350 m), acute hypoxia (environmental chamber), and chronic hypoxia (5050 m, 5-9 d) (partial pressure of inspired oxygen = 140, 74 and 76 mm Hg, respectively), 12 participants (11 in chronic hypoxia) had the quadriceps of their dominant leg fatigued by three bouts of 75 intermittent electrically evoked contractions (12 pulses at 15 Hz, 1.6 s between train onsets, and 15 s between bouts). The initial peak force was ~30% of maximal voluntary force. Recovery was assessed by single trains at 1, 2, and 3 min postprotocol. Tissue oxygenation of rectus femoris was recorded by near-infrared spectroscopy.

RESULTS

At the end of the fatigue protocol, the impairments of peak force and peak rates of force development and relaxation were greater (all P < 0.05) in acute hypoxia (~51%, 53%, and 64%, respectively) than sea level (~43%, 43%, and 52%) and chronic hypoxia (~38%, 35%, and 48%). Peak force and rate of force development recovered faster (P < 0.05) in chronic hypoxia (pooled data for 1-3 min: ~84% and 74% baseline, respectively) compared with sea level (~73% and 63% baseline) and acute hypoxia (~70% and 55% baseline). Tissue oxygenation did not differ among conditions for fatigue or recovery (P > 0.05).

CONCLUSIONS

Muscle adaptations occurring with chronic hypoxia, independent of other adaptations, positively influence muscle contractility during and after repeated contractions at high altitude.

UBC-Nepal expedition: peripheral fatigue recovers faster in Sherpa than lowlanders at high altitude

KEY POINTS

The reduced oxygen tension of high altitude compromises performance in lowlanders. In this environment, Sherpa display superior performance, but little is known on this issue. Sherpa present unique genotypic and phenotypic characteristics at the muscular level, which may enhance resistance to peripheral fatigue at high altitude compared to lowlanders. We studied the impact of gradual ascent and exposure to high altitude (5050 m) on peripheral fatigue in age-matched lowlanders and Sherpa, using intermittent electrically-evoked contractions of the knee extensors. Peripheral fatigue (force loss) was lower in Sherpa during the first part of the protocol. Post-protocol, the rate of force development and contractile impulse recovered faster in Sherpa than in lowlanders. At any time, indices of muscle oxygenation were not different between groups. Muscle contractile properties in Sherpa, independent of muscle oxygenation, were less perturbed by non-volitional fatigue. Hence, elements within the contractile machinery contribute to the superior physical performance of Sherpa at high altitude.

ABSTRACT

Altitude-related acclimatisation is characterised by marked muscular adaptations. Lowlanders and Sherpa differ in their muscular genotypic and phenotypic characteristics, which may influence peripheral fatigability at altitude. After gradual ascent to 5050 m, 12 lowlanders and 10 age-matched Sherpa (32 ± 10 vs. 31 ± 11 years, respectively) underwent three bouts (separated by 15 s rest) of 75 intermittent electrically-evoked contractions (12 pulses at 15 Hz, 1.6 s between train onsets) of the dominant leg quadriceps, at the intensity which initially evoked 30% of maximal voluntary force. Trains were also delivered at minutes 1, 2 and 3 after the protocol to measure recovery. Tissue oxygenation index (TOI) and total haemoglobin (tHb) were quantified by a near-infrared spectroscopy probe secured over rectus femoris. Superficial femoral artery blood flow was recorded using ultrasonography, and delivery of oxygen was estimated (eDO₂ ). At the end of bout 1, peak force was greater in Sherpa than in lowlanders (91.5% vs. 84.5% baseline, respectively; P < 0.05). Peak rate of force development (pRFD), the first 200 ms of the contractile impulse (CI₂₀₀ ), and half-relaxation time (HRT) recovered faster in Sherpa than in lowlanders (percentage of baseline at 1 min: pRFD: 89% vs. 74%; CI₂₀₀ : 91% vs. 80%; HRT: 113% vs. 123%, respectively; P < 0.05). Vascular measures were pooled for lowlanders and Sherpa as they did not differ during fatigue or recovery (P < 0.05). Mid bout 3, TOI was decreased (90% baseline) whereas tHb was increased (109% baseline). After bout 3, eDO₂ was markedly increased (1266% baseline). The skeletal muscle of Sherpa seemingly favours repeated force production at altitude for similar oxygen delivery compared to lowlanders.

Comparison of the effects of kilohertz- and low-frequency electric stimulations: A systematic review with meta-analysis

OBJECTIVE

This study aimed to determine whether kilohertz-frequency alternating current (KFAC) is superior to low-frequency pulsed current (PC) in increasing muscle-evoked torque and lessening discomfort.

DATA SOURCES

The electronic databases PubMed, PEDro, CINAHL, and CENTRAL were searched for related articles, published before August 2017. Furthermore, citation search was performed on the original record using Web of Science.

REVIEW METHODS

Randomized controlled trials, quasi-experimental studies, and within-subject repeated studies evaluating and comparing KFAC and PC treatments were included. The pooled standardized mean differences (SMDs) of KFAC and PC treatments, with 95% confidence intervals (CIs), were calculated using the random effects model.

RESULTS

In total, 1148 potentially relevant articles were selected, of which 14 articles with within-subject repeated designs (271 participants, mean age: 26.4 years) met the inclusion criteria. KFAC did not significantly increase muscle-evoked torque, compared to PC (pooled SMD: -0.25; 95% CI: -0.53, 0.06; P = 0.120). KFAC had comparable discomfort compared to that experienced using PC (pooled SMD: -0.06; 95% CI: -0.50, 0.38; P = 0.800). These estimates of the effects had a high risk of bias, as assessed using the Downs and Black scale, and were highly heterogeneous studies.

CONCLUSIONS

This meta-analysis does not establish that KFAC is superior to PC in increasing muscle-evoked torque and lessening discomfort level. However, no strong conclusion could be drawn because of a high risk of bias and a large amount of heterogeneity. High quality studies comparing the efficacy between PC and KFAC treatments with consideration of potential confounders is warranted to facilitate the development of effective treatment.

Fatigue reduction during aggregated and distributed sequential stimulation

INTRODUCTION

Transcutaneous neuromuscular electrical stimulation (NMES) can generate muscle contractions for rehabilitation and exercise. However, NMES-evoked contractions are limited by fatigue when they are delivered "conventionally" (CONV) using a single active electrode. Researchers have developed "sequential" (SEQ) stimulation, involving rotation of pulses between multiple "aggregated" (AGGR-SEQ) or "distributed" (DISTR-SEQ) active electrodes, to reduce fatigue (torque-decline) by reducing motor unit discharge rates. The primary objective was to compare fatigue-related outcomes, "potentiation," "variability," and "efficiency" between CONV, AGGR-SEQ, and DISTR-SEQ stimulation of knee extensors in healthy participants.

METHODS

Torque and current were recorded during testing with fatiguing trains using each NMES type under isometric and isokinetic (180°/s) conditions.

RESULTS

Compared with CONV stimulation, SEQ techniques reduced fatigue-related outcomes, increased potentiation, did not affect variability, and reduced efficiency.

CONCLUSIONS

SEQ techniques hold promise for reducing fatigue during NMES-based rehabilitation and exercise; however, optimization is required to improve efficiency. Muscle Nerve 56: 271-281, 2017.

Original Research: Central and peripheral quadriceps fatigue in young and middle-aged untrained and endurance-trained men: A comparative study

This study aimed to compare quadriceps function (i.e. strength, endurance, central, and peripheral fatigue) of young (Young-UnTr) and middle-aged (MidAge-UnTr) untrained men and young endurance-trained men (Young-Tr). Twenty-four male subjects (eight Young-UnTr (26 ± 4 yr), eight Young-Tr (29 ± 3 yr), and eight MidAge-UnTr (56 ± 4 yr) performed a maximal cycling test to assess their fitness level. On a separate visit, subjects performed sets of 10 intermittent (5-s on/5-s off) isometric contractions starting at 10% maximum voluntary contraction (MVC), with 10% MVC increments from one set to another until exhaustion. Electrophysiological and mechanical (e.g. twitch) evoked responses elicited with magnetic femoral nerve stimulation in the relaxed muscle and during MVC (i.e. estimation of voluntary activation using the interpolated twitch technique) were measured at baseline and after each set to assess peripheral and central fatigue, respectively. Endurance (= total number of contractions) was also evaluated. Young-UnTr exhibited larger reductions in evoked quadriceps mechanical responses than MidAge-UnTr and Young-Tr after identical standardized muscle loading (e.g. after the 50% MVC set, reduction in single potentiated twitch was -36 ± 9%, -21±16%, and -2 ± 4%, respectively). At both 50% MVC set and exhaustion, MidAge-UnTr exhibited similar reduction in maximal voluntary activation and displayed similar endurance compared to Young-UnTr. Young-Tr exhibited greater endurance than Young-UnTr without significant changes in maximal voluntary activation throughout the test. This study provides robust comparative data regarding the influence of chronic exposure to endurance training and middle-aged on central and peripheral quadriceps fatigability and endurance. Endurance-trained subjects showed smaller level of peripheral fatigue and displayed no significant central fatigue, even at exhaustion and despite greater endurance performance. Our findings also demonstrate that men in the sixth decade exhibit significant alterations in quadriceps function typically observed in much older subjects. These data emphasize the need for developing normative data for both central and peripheral quadriceps fatigability.

Effects of age and hindlimb immobilization and remobilization on fast troponin T precursor mRNA alternative splicing in rat gastrocnemius muscle

Fast skeletal muscle troponin T (TNNT3) is an important component of the skeletal muscle contractile machinery. The precursor mRNA (pre-mRNA) encoding TNNT3 is alternatively spliced, and changes in the pattern of TNNT3 splice form expression are associated with alterations in thin-filament calcium sensitivity and force production during muscle contraction and thereby regulate muscle function. Interestingly, during aging, the muscle force/cross-sectional area is reduced, suggesting that loss of mass does not completely account for the impaired muscle function that develops during the aging process. Therefore, in this study, we tested the hypothesis that age and changes in muscle loading are associated with alterations in Tnnt3 alternative splicing in the rat gastrocnemius muscle. We found that the relative abundance of several Tnnt3 splice forms varied significantly with age among 2-, 9-, and 18-month-old rats and that the pattern correlated with changes in body mass rather than muscle mass. Hindlimb immobilization for 7 days resulted in dramatic alterations in splice form relative abundance such that the pattern was similar to that observed in lighter animals. Remobilization for 7 days restored the splicing pattern toward that observed in the nonimmobilized limb, even though muscle mass had not yet begun to recover. In conclusion, the results suggest that Tnnt3 pre-mRNA alternative splicing is modulated rapidly (i.e., within days) in response to changes in the load placed on the muscle. Moreover, the results show that restoration of Tnnt3 alternative splicing to control patterns is initiated prior to an increase in muscle mass.

Influence of ageing and essential amino acids on quantitative patterns of troponin T alternative splicing in human skeletal muscle

Ageing is associated with a loss of skeletal muscle performance, a condition referred to as sarcopenia. In part, the age-related reduction in performance is due to a selective loss of muscle fiber mass, but mass-independent effects have also been demonstrated. An important mass-independent determinant of muscle performance is the pattern of expression of isoforms of proteins that participate in muscle contraction (e.g., the troponins). In the present study, we tested the hypothesis that ageing impairs alternative splicing of the pre-mRNA encoding fast skeletal muscle troponin T (TNNT3) in human vastus lateralis muscle. Furthermore, we hypothesized that resistance exercise alone or in combination with consumption of essential amino acids would attenuate age-associated effects on TNNT3 alternative splicing. Our results indicate that ageing negatively affects the pattern of TNNT3 alternative splicing in a manner that correlates quantitatively with age-associated reductions in muscle performance. Interestingly, whereas vastus lateralis TNNT3 alternative splicing was unaffected by a bout of resistance exercise 24 h prior to muscle biopsy, ingestion of a mixture of essential amino acids after resistance exercise resulted in a significant shift in the pattern of TNNT3 splice form expression in both age groups to one predicted to promote greater muscle performance. We conclude that essential amino acid supplementation after resistance exercise may provide a means to reduce impairments in skeletal muscle quality during ageing in humans.

β-Alanine supplementation enhances human skeletal muscle relaxation speed but not force production capacity

β-Alanine (BA) supplementation improves human exercise performance. One possible explanation for this is an enhancement of muscle contractile properties, occurring via elevated intramuscular carnosine resulting in improved calcium sensitivity and handling. This study investigated the effect of BA supplementation on in vivo contractile properties and voluntary neuromuscular performance. Twenty-three men completed two experimental sessions, pre- and post-28 days supplementation with 6.4 g/day of BA ( n = 12) or placebo (PLA; n = 11). During each session, force was recorded during a series of knee extensor contractions: resting and potentiated twitches and octet (8 pulses, 300 Hz) contractions elicited via femoral nerve stimulation; tetanic contractions (1 s, 1–100 Hz) via superficial muscle stimulation; and maximum and explosive voluntary contractions. BA supplementation had no effect on the force-frequency relationship, or the force responses (force at 25 and 50 ms from onset, peak force) of resting or potentiated twitches, and octet contractions ( P > 0.05). Resting and potentiated twitch electromechanical delay and time-to-peak tension were unaffected by BA supplementation ( P > 0.05), although half-relaxation time declined by 7–12% ( P < 0.05). Maximum and explosive voluntary forces were unchanged after BA supplementation. BA supplementation had no effect on evoked force responses, implying that altered calcium sensitivity and/or release are not the mechanisms by which BA supplementation influences exercise performance. The reduced half-relaxation time with BA supplementation might, however, be explained by enhanced reuptake of calcium, which has implications for the efficiency of muscle contraction following BA supplementation.

Human neuromuscular structure and function in old age: A brief review

Natural adult aging is associated with many functional impairments of the human neuromuscular system. One of the more observable alterations is the loss of contractile muscle mass, termed sarcopenia. The loss of muscle mass occurs primarily due to a progressive loss of viable motor units, and accompanying atrophy of remaining muscle fibers. Not only does the loss of muscle mass contribute to impaired function in old age, but alterations in fiber type and myosin heavy chain isoform expression also contribute to weaker, slower, and less powerful contracting muscles. This review will focus on motor unit loss associated with natural adult aging, age-related fatigability, and the age-related differences in strength across contractile muscle actions.

Skeletal muscle fatigue

Skeletal muscle fatigue is defined as the fall of force or power in response to contractile activity. Both the mechanisms of fatigue and the modes used to elicit it vary tremendously. Conceptual and technological advances allow the examination of fatigue from the level of the single molecule to the intact organism. Evaluation of muscle fatigue in a wide range of disease states builds on our understanding of basic function by revealing the sources of dysfunction in response to disease.

Potentiation of maximal voluntary concentric torque in human quadriceps femoris

PURPOSE

Previous studies have shown that one bout of 6-s maximal voluntary contraction (MVC) can enhance subsequent dynamic joint performance with maximal voluntary effort. However, the conditioning contraction also induces central fatigue, which attenuates the enhancement of the subsequent voluntary joint performance. Here, as a modality for minimizing the fatigue while enhancing joint performance, we examined the effects of one bout of a short-duration isometric MVC and neuromuscular electrical stimulation (NMES).

METHODS

In the first experiment, the optimal duration of NMES for inducing twitch potentiation of the quadriceps femoris muscle was determined, which was found to be 5 s. Then in the second experiment, 13 subjects performed maximal voluntary isokinetic concentric knee extensions at 210° · s(-1) in the following sequence: before and immediately after each of the 3-, 5-, and 10-s MVCs and 5-s NMES of knee extension and 1, 3, and 5 min thereafter.

RESULTS

When the isometric MVC was used as a conditioning contraction, the maximal voluntary concentric torque was significantly enhanced at 1 and 3 min after MVC only in the 5-s MVC trial (106.6% ± 2.3% and 107.2% ± 2.6% of the initial value, respectively) but not in the 3- and 10-s MVC trials. In the 5-s NMES trial, the voluntary concentric torque was significantly increased immediately after the conditioning contraction (105.1% ± 2.2%) as well as 1 and 3 min thereafter (107.5% ± 3.3% and 107.8% ± 2.7%, respectively).

CONCLUSIONS

These results suggest that conditioning contraction of around 5 s, performed with isometric MVC or NMES, can be a modality to enhance dynamic voluntary joint performance, with the latter having a more immediate effect.

Fatigability and recovery of arm muscles with advanced age for dynamic and isometric contractions

This study determined whether age-related mechanisms can increase fatigue of arm muscles during maximal velocity dynamic contractions, as it occurs in the lower limb. We compared elbow flexor fatigue of young (n=10, 20.8±2.7 years) and old men (n=16, 73.8±6.1 years) during and in recovery from a dynamic and an isometric postural fatiguing task. Each task was maintained until failure while supporting a load equivalent to 20% of maximal voluntary isometric contraction (MVIC) torque. Transcranial magnetic stimulation (TMS) was used to assess supraspinal fatigue (superimposed twitch, SIT) and muscle relaxation. Time to failure was longer for the old men than for the young men for the isometric task (9.5±3.1 vs. 17.2±7.0 min, P=0.01) but similar for the dynamic task (6.3±2.4 min vs. 6.0±2.0 min, P=0.73). Initial peak rate of relaxation was slower for the old men than for the young men, and was associated with a longer time to failure for both tasks (P<0.05). Low initial power during elbow flexion was associated with the greatest difference (reduction) in time to failure between the isometric task and the dynamic task (r=-0.54, P=0.015). SIT declined after both fatigue tasks similarly with age, although the recovery of SIT was associated with MVIC recovery for the old (both sessions) but not for the young men. Biceps brachii and brachioradialis EMG activity (% MVIC) of the old men were greater than that of the young men during the dynamic fatiguing task (P<0.05), but were similar during the isometric task. Muscular mechanisms and greater relative muscle activity (EMG activity) explain the greater fatigue during the dynamic task for the old men compared with the young men in the elbow flexor muscles. Recovery of MVC torque however relies more on the recovery of supraspinal fatigue among the old men than among the young men.

Effects of age and localized muscle fatigue on ankle plantar flexor torque development

BACKGROUND AND PURPOSE

Older adults often experience age-related declines in strength, which contribute to fall risk. Such age-related levels of fall risk may be compounded by further declines in strength caused by acute muscle fatigue. Both age- and fatigue-related strength reductions likely impact the ability to quickly develop joint torques needed to arrest falls. Therefore, the purpose of this study was to investigate the combined effects of age and localized muscle fatigue on lower extremity joint torque development.

METHODS

Young (mean age, 26 (2.5) years) and older (mean age, 71 (2.8) years) healthy male adults performed an isometric ankle plantar flexion force control task before and after an ankle plantar flexor fatiguing exercise. Force control performance was quantified using onset time, settling time, and rate of torque development.

RESULTS

Age-related increases and decreases were observed for onset time and rate of torque development, respectively. A fatigue-related decrease in rate of torque development was observed in young, but not older adults.

DISCUSSION

The results suggest performance declines that may relate to older adults' reduced ability to prevent falls. A fatigue-related performance decline was observed among young adults, but not older, suggesting the presence of age-related factors such as motor unit remodeling and alterations in perceived exertion.

CONCLUSIONS

Older adults demonstrated an overall reduction in the ability to quickly produce ankle torque, which may have implications for balance recovery and fall risk among older adults.

Power loss is greater following lengthening contractions in old versus young women

Compared with isometric and dynamic velocity-constrained (isokinetic) tasks, less is known regarding velocity-dependent (isotonic) muscle power and recovery in older adults following repeated fatiguing lengthening contractions. We investigated voluntary and evoked neuromuscular properties of the dorsiflexors in nine old (68.3 ± 6.1 years) and nine young women (25.1 ± 1.3 years) during and following 150 lengthening contractions for up to 30 min of recovery. At baseline, the old were ~21% weaker for maximum isometric voluntary contraction (MVC) torque (P < 0.05), ~21% slower for peak loaded shortening velocity (P < 0.05), and ~39% less powerful compared with the young (P < 0.05). Following the task, MVC torque was depressed equally (~28%) for both groups (P < 0.05), but power was reduced ~19% in the old and only ~8% in the young (P < 0.05). Both measures remained depressed during the 30-min recovery period. Peak twitch torque (P (t)) was ~50% lower in the old at task termination, whereas the young were unchanged. However, by 5 min of recovery, P (t) was reduced similarly (~50%) in both groups, and neither recovered by 30 min. The old were affected more by low-frequency torque depression than the young, as shown by the ~40% and ~20% decreases in the stimulated 10:50 Hz ratio at task termination respectively, whereas both groups were affected similarly (~50%) 5 min into recovery, and neither recovered by 30 min. Thus, the coexistence of fatigue and muscle damage induced by the repetitive lengthening contractions impaired excitation-contraction coupling and cross-bridge function to a greater extent in the old, leading to a more pronounced initial loss of power than the young for up to 10 min following the exercise However, power remained blunted in both groups during the 30-min recovery period. These results indicate that older women are more susceptible to power loss than young following lengthening contractions, likely owing to a greater impairment in calcium kinetics.

Variable stimulation patterns in younger and older thenar muscle

Neuromuscular electrical stimulation (NMES) is typically used with older adults receiving rehabilitation therapies, but little is known about the stimulation patterns that maximize force output and minimize fatigue in this population. The purpose of this study was to apply variable patterns of stimulation to the thenar muscles of the hand in younger and older adults to determine if force production and neuromuscular fatigue effects were similar. Three submaximal stimulation patterns were administered: A 20Hz constant frequency pattern, a pattern that increased from 20 to 40Hz, and a pattern that incorporated two closely spaced (5ms) doublet pulses. The doublet stimulation produced significantly higher average forces and force-time integrals (FTIs) than the constant frequency and increasing frequency patterns in both age groups. Additionally, older adults showed less fatigue than the younger group during isometric contractions performed after the fatiguing stimulation patterns. These results suggest that variable pulse NMES patterns enhance force production in the hand in both younger and older individuals better than constant frequency patterns, which are typically used in clinical applications. Also, greater fatigue resistance to electrical stimulation protocols may exist in the older population; this is critical information for the design and application of NMES rehabilitation regimens used with older adults.

Age-related differences in muscle fatigue vary by contraction type: a meta-analysis

BACKGROUND

During senescence, despite the loss of strength (force-generating capability) associated with sarcopenia, muscle endurance may improve for isometric contractions.

PURPOSE

The purpose of this study was to perform a systematic meta-analysis of young versus older adults, considering likely moderators (ie, contraction type, joint, sex, activity level, and task intensity).

DATA SOURCES

A 2-stage systematic review identified potential studies from PubMed, CINAHL, PEDro, EBSCOhost: ERIC, EBSCOhost: Sportdiscus, and The Cochrane Library.

STUDY SELECTION

Studies reporting fatigue tasks (voluntary activation) performed at a relative intensity in both young (18-45 years of age) and old (≥ 55 years of age) adults who were healthy were considered.

DATA EXTRACTION

Sample size, mean and variance outcome data (ie, fatigue index or endurance time), joint, contraction type, task intensity (percentage of maximum), sex, and activity levels were extracted.

DATA SYNTHESIS

Effect sizes were (1) computed for all data points; (2) subgrouped by contraction type, sex, joint or muscle group, intensity, or activity level; and (3) further subgrouped between contraction type and the remaining moderators. Out of 3,457 potential studies, 46 publications (with 78 distinct effect size data points) met all inclusion criteria.

LIMITATIONS

A lack of available data limited subgroup analyses (ie, sex, intensity, joint), as did a disproportionate spread of data (most intensities ≥ 50% of maximum voluntary contraction).

CONCLUSIONS

Overall, older adults were able to sustain relative-intensity tasks significantly longer or with less force decay than younger adults (effect size=0.49). However, this age-related difference was present only for sustained and intermittent isometric contractions, whereas this age-related advantage was lost for dynamic tasks. When controlling for contraction type, the additional modifiers played minor roles. Identifying muscle endurance capabilities in the older adult may provide an avenue to improve functional capabilities, despite a clearly established decrement in peak torque.

Systematic review and meta-analysis of skeletal muscle fatigue in old age

Despite intense interest in understanding how old age may alter skeletal muscle fatigability, a quantitative examination of the impact of study design on age-related differences in muscle fatigue does not exist.

PURPOSE

The purpose of this study was to conduct a systematic review of the differences in muscle fatigue between young and older adults, with specific examination of moderator variables suggested to contribute to discrepancies across studies: contraction intensity, contraction mode, duty cycle, fatigue index, sex, muscle group, and contraction type.

METHODS

The standardized effect of age on muscle fatigue was computed for 37 studies (60 standardized effects). Standardized effects were coded as positive when less fatigue was reported in older individuals compared with young individuals.

RESULTS

The overall standardized effect of age on muscle fatigue was positive (0.56). In studies using dynamic contractions or using muscle power as the index of fatigue, the standardized effect was negative (-0.12 and -2.5, respectively). The standardized effect for all other moderator categories was positive (range = 0.09-0.90), indicating less fatigue in older adults under all other methodological conditions.

CONCLUSION

This review provides the first quantitative analysis of the effect of study design on age-related differences in muscle fatigue. The results indicate that older individuals develop less muscle fatigue than young individuals, particularly during isometric contractions of the elbow flexor and knee extensor muscles. However, the results also suggest that older adults develop greater fatigue during dynamic contractions, particularly when the decline in power is assessed. Studies that verify this latter outcome are needed, as are studies designed to elucidate the mechanisms of fatigue.

Changes of the force-frequency relationship in the rat medial gastrocnemius muscle after total transection and hemisection of the spinal cord

The relationships between the stimulation frequency and the force developed by motor units (MUs) of the medial gastrocnemius muscle were compared between intact rats and animals after total transection or hemisection of the spinal cord at the low thoracic level. The experiments on functionally isolated MUs were carried out 14, 30, 90, and 180 days after the spinal cord injury. Axons of investigated MUs were stimulated with trains of pulses at 10 progressively increased frequencies (from 1 to 150 Hz), and the force-frequency curves were plotted. Spinal cord hemisection resulted in a considerable leftward shift of force-frequency curves in all types of MUs. After the total transection, a leftward shift of the curve was observed in fast MUs, whereas there was a rightward shift in slow MUs. These changes coincided with a decrease of stimulation frequencies necessary to evoke 60% of maximal force. Moreover, the linear correlation between these stimulation frequencies and the twitch contraction time observed in intact rats was disrupted in all groups of animals with spinal cord injury. The majority of the observed changes reached the maximum 1 mo after injury, whereas the effects evoked by spinal cord hemisection were significantly smaller and nearly constant in the studied period. The results of this study can be important for the prediction of changes in force regulation in human muscles after various extends of spinal cord injury and in evaluation of the frequency of functional electrical stimulation used for training of impaired muscles.

Paradoxical contractile properties in the knee extensors of HIV-infected men treated with antiretroviral therapy

We assessed contractile properties in the knee extensors of 18 men with the human immunodeficiency virus (HIV+) being treated with antiretroviral therapy (ART), and 9 healthy controls matched for age and body mass index. We found significant, divergent differences between groups with regard to force relaxation. Half-times of twitch relaxation were shorter (62.6 ± 5.4 ms vs. 48.9 ± 3.0 ms; p = 0.045) and maximum rates of torque relaxation were slower (0.47% ± 0.04%(s-)1 vs. 1.32% ± 0.10%(s-)1; p < 0.001) in HIV+ individuals. These preliminary findings suggest potential intramuscular impairments in HIV+ individuals on ART, perhaps because of interactions between calcium handling and mitochondrial dysfunction.

Power loss is greater in old men than young men during fast plantar flexion contractions

It is unclear during human aging whether healthy older adults (>70 yr old) experience greater, lesser, or the same fatigability compared with younger adults. The reported disparate findings may be related to the task-dependent nature of fatigue and the limited number of studies exploring nonisometric contractile function and aging. The purpose here was to determine the effects of fast shortening contractions on the fatigability of the triceps surae in 10 young (~24 yr old) and 10 old (~78 yr old) men using isometric and dynamic measures. Participants performed 50 maximal velocity-dependent plantar flexions at a constant load of 20% maximal voluntary isometric contraction (MVC). Isometric twitch properties and MVCs were tested at baseline and during and following the fatigue task. Voluntary activation was similar between the old and young (~98%) and was unaltered with fatigue. The old had 26% lower (P < 0.01) isometric MVC torque and 18% slower (P < 0.01) maximal shortening velocity than the young. Hence, peak power was 38% lower in the old (P < 0.01). At task termination, MVC torque was maintained in the old (P = 0.15) but decreased by 21% in the young (P < 0.01). Twitch half-relaxation time was lengthened in the old at task termination by 26% (P < 0.01) but unchanged in the young (P = 0.10). Peak power was reduced by 24% and 17% at task termination in the old and young, respectively (P < 0.01). Despite a better maintenance in isometric MVC torque production, the weaker and slower contracting triceps surae of the old was more fatigable than the young during fast dynamic efforts with an unconstrained velocity.

Differential age-related changes in motor unit properties between elbow flexors and extensors

AIM

Healthy adult ageing of the human neuromuscular system is comprised of changes that include atrophy, weakness and slowed movements with reduced spinal motor neurone output expressed by lower motor unit discharge rates (MUDRs). The latter observation has been obtained mostly from hand and lower limb muscles. The purpose was to determine the extent to which elbow flexor and extensor contractile properties, and MUDRs in six old (83 +/- 4 years) and six young (24 +/- 1 years) men were affected by age, and whether any adaptations were similar for both muscle groups.

METHODS

Maximal isometric voluntary contraction (MVC), voluntary activation, twitch contractile properties, force-frequency relationship and MUDRs from sub-maximal to maximal intensities were assessed in the elbow flexors and extensors.

RESULTS

Both flexor and extensor MVCs were significantly (P < 0.05) less (approximately 42% and approximately 46% respectively) in the old than in the young. Contractile speeds and the force-frequency relationship did not show any age-related differences (P > 0.05). For the elbow flexors contraction duration was approximately 139 ms and for the extensors it was approximately 127 ms for both age groups (P > 0.05). The mean MUDRs from 25% MVC to maximum were lower (approximately 10% to approximately 36%) in the old than in the young (P < 0.01). These age-related differences were larger for biceps (Cohen's d = 8.25) than triceps (Cohen's d = 4.79) brachii.

CONCLUSION

Thus, at least for proximal upper limb muscles, mean maximal MUDR reductions with healthy adult ageing are muscle specific and not strongly related to contractile speed.

Consequences of lower extremity and trunk muscle fatigue on balance and functional tasks in older people: a systematic literature review

Background

Muscle fatigue reduces muscle strength and balance control in young people. It is not clear whether fatigue resistance seen in older persons leads to different effects. In order to understand whether muscle fatigue may increase fall risk in older persons, a systematic literature review aimed to summarize knowledge on the effects of lower extremity and trunk muscle fatigue on balance and functional tasks in older people was performed.

Methods

Studies were identified with searches of the PUBMED and SCOPUS data bases.

Papers describing effects of lower extremity or trunk muscle fatigue protocols on balance or functional tasks in older people were included. Studies were compared with regards to study population characteristics, fatigue protocol, and balance and functional task outcomes.

Results

Seven out of 266 studies met the inclusion criteria. Primary findings were: fatigue via resistance exercises to lower limb and trunk muscles induces postural instability during quiet standing; induced hip, knee and ankle muscle fatigue impairs functional reach, reduces the speed and power of sit-to-stand repetitions, and produces less stable and more variable walking patterns; effects of age on degree of fatigue and rate of recovery from fatigue are inconsistent across studies, with these disparities likely due to differences in the fatigue protocols, study populations and outcome measures.

Conclusion

Taken together, the findings suggest that balance and functional task performance are impaired with fatigue. Future studies should assess whether fatigue is related to increased risk of falling and whether exercise interventions may decrease fatigue effects.

Lower energy cost of skeletal muscle contractions in older humans

Recent studies suggest that the cost of muscle contraction may be reduced in old age, which could be an important mediator of age-related differences in muscle fatigue under some circumstances. We used phosphorus magnetic resonance spectroscopy and electrically elicited contractions to examine the energetic cost of ankle dorsiflexion in 9 young (Y; 26 +/- 3.8 yr; mean +/- SD) and 9 older healthy men (O; 72 +/- 4.6). We hypothesized that the energy cost of twitch and tetanic contractions would be lower in O and that this difference would be greater during tetanic contractions at f(50) (frequency at 50% of peak force from force-frequency relationship) than at 25 Hz. The energy costs of a twitch (O = 0.13 +/- 0.04 mM ATP/twitch, Y = 0.18 +/- 0.06; P = 0.045) and a 60-s tetanus at 25 Hz (O = 1.5 +/- 0.4 mM ATP/s, Y = 2.0 +/- 0.2; P = 0.01) were 27% and 26% lower in O, respectively, while the respective force.time integrals were not different. In contrast, energy cost during a 90-s tetanus at f(50) (O = 10.9 +/- 2.0 Hz, Y = 14.8 +/- 2.1 Hz; P = 0.002) was 49% lower in O (1.0 +/- 0.2 mM ATP/s) compared with Y (1.9 +/- 0.2; P < 0.001). Y had greater force potentiation during the f(50) protocol, which accounted for the greater age difference in energy cost at f(50) compared with 25 Hz. These results provide novel evidence of an age-related difference in human contractile energy cost in vivo and suggest that intramuscular changes contribute to the lower cost of contraction in older muscle. This difference in energetics may provide an important mechanism for the enhanced fatigue resistance often observed in older individuals.

Modeling age-related neuromuscular changes in humans

With aging, motoneurons and muscle tissue undergo significant changes, which influence function in terms of strength, mobility, and overall independence. Mathematical modeling provides a practical method of studying the relationships among recruitment, rate-coding, and force output in motor units, and may be used to predict functional neuromuscular changes related to aging. For this study, the Heckman-Binder model was used to examine changes in human quadriceps motor units. Relationships among current input, firing frequency, and force output were defined for both a younger and an older individual. Included in the model were age-related effects associated with reduced muscle contractile speed; reduced muscle-fibre number, size, and specific tension; reduced gain of the frequency-current relationship; decreased size of motoneurons; and altered motor unit remodeling. Adjustment of this model to reflect age-related changes resulted in a leftward shift of the force-frequency function, lower firing frequency for any given current injected into the motoneuron, and a reduction in maximal force output. The model suggests that older individuals are capable of reaching force levels up to approximately 50% of those attained by younger individuals, with relatively similar or even slightly lower levels of current input. This could mean that the sense of effort and the contribution of factors other than degree of effort from afferent inputs to the pool, including conscious supraspinal centres, might be different in the older adult.

Effects of electrical stimulation parameters on fatigue in skeletal muscle

STUDY DESIGN

Experimental laboratory study.

OBJECTIVES

The primary purpose was to investigate the independent effects of current amplitude, pulse duration, and current frequency on muscle fatigue during neuromuscular electrical stimulation (NMES). A second purpose was to determine if the ratio of the evoked torque to the activated area could explain muscle fatigue.

BACKGROUND

Parameters of NMES have been shown to differently affect the evoked torque and the activated area. The efficacy of NMES is limited by the rapid onset of muscle fatigue.

METHODS AND MEASURES

Seven healthy participants underwent 4 NMES protocols that were randomly applied to the knee extensor muscle group. The NMES protocols were as follows: standard protocol (Std), defined as 100-Hz, 450-micros pulses and amplitude set to evoke 75% of maximal voluntary isometric torque (MVIT); short pulse duration protocol (SP), defined as 100-Hz, 150-micros pulses and amplitude set to evoke 75% of MVIT; low-frequency protocol (LF), defined as 25-Hz, 450-micros pulses and amplitude set to evoke 75% of MVIT; and low-amplitude protocol (LA), defined as 100-Hz, 450-micros pulses and amplitude set to evoke 45% of MVIT. The peak torque was measured at the start and at the end of the 4 protocols, and percent fatigue was calculated. The outcomes of the 4 NMES protocols on the initial peak torque and activated cross-sectional area were recalculated from a companion study to measure torque per active area.

RESULTS

Decreasing frequency from 100 to 25 Hz decreased fatigue from 76% to 39%. Decreasing the amplitude and pulse duration resulted in no change of muscle fatigue. Torque per active area accounted for 57% of the variability in percent fatigue between Std and LF protocols.

CONCLUSIONS

Altering the amplitude of the current and pulse duration does not appear to influence the percent fatigue in NMES. Lowering the stimulation frequency results in less fatigue, by possibly reducing the evoked torque relative to the activated muscle area.

Variable stimulation patterns for poststroke hemiplegia

Neuromuscular electrical stimulation can improve motor function in those affected by paralysis, but its use is limited by a high rate of muscular fatigue. Variable stimulation patterns have been examined in young adults with and without spinal cord injury, but much less investigation has been devoted to studying the effects of variable stimulation patterns administered to older adults or those paralyzed by stroke. Significant changes occur in the neuromuscular system with age that may affect the response to variable stimulation patterns. We administered three, 3-min intermittent stimulation patterns to the median nerves of 10 individuals with hemiplegia from stroke and 10 age-matched able-bodied adults: (1) constant 20 HZ, (2) a pattern that began at 20 HZ and progressively increased to 40 HZ in the latter half of the task, and (3) a 20-HZ pattern that switched to a 20-HZ doublet pattern after 90 s. In the able-bodied group the doublet pattern produced significantly higher force time integrals (FTI) (1409.72 +/- 3.15 N s) than the 20-40-HZ pattern (1067.46 +/- 1.15 N s) or the 20-HZ pattern (831 +/- 1.87 N s). In the poststroke individuals the doublet pattern also produced the highest FTI (724.04 +/- 2.02 N s), and there was no significant difference between the 20-40-HZ (636.42 +/- 1.65 N s) and 20-HZ (583.64 +/- 3.02 N s) patterns. These results indicate that protocols that incorporate doublets in the later stages of fatigue are effective in older adults and in older adults with paralysis from stroke.

An exploration of the association between frailty and muscle fatigue

Frailty is increasingly recognized as a geriatric syndrome that shares common biomedical determinants with rapid muscle fatigue: aging, disease, inflammation, physical inactivity, malnutrition, hormone deficiencies, subjective fatigue, and changes in neuromuscular function and structure. In addition, there is an established relationship between muscle fatigue and core elements of the cycle of frailty as proposed by Fried and colleagues (sarcopenia, neuroendocrine dysregulation and immunologic dysfunction, muscle weakness, subjective fatigue, reduced physical activity, low gait speed, and weight loss). These relationships suggest that frailty and muscle fatigue are closely related and that low tolerance for muscular work may be an indicator of frailty phenotype.

Contrasting influences of age and sex on muscle fatigue

PURPOSE

Greater resistance to muscle fatigue has been observed in women versus men and in older versus young individuals. As suggested mechanisms for these differences include task intensity and duty cycle, the purpose of this study was to evaluate fatigue in healthy young and older men and women during maximum-effort isometric contractions with a 70% duty cycle (7 s of contraction, 3 s of rest). We hypothesized that no differences in fatigue would be observed across age or sex, in contrast to studies incorporating lower duty cycles.

METHODS

The protocol was carried out on ankle dorsiflexors of older (73 +/- 1 yr) and younger (25 +/- 1 yr) men and women. Volitional and stimulated force, compound muscle action potential, and muscle contractile responses were collected before, during, and immediately after the fatigue protocol. These measurements allowed for assessment of fatigue as well as central and peripheral activation.

RESULTS

At baseline, older subjects had longer force half-relaxation times and less twitch potentiation than younger subjects, consistent with a slower muscle phenotype. During contractions, younger subjects fatigued more than older subjects did, with no differences between men and women. Central activation decreased similarly in all groups with fatigue. There were no fatigue-related differences in peripheral excitation or contractile properties attributable to age or sex.

CONCLUSIONS

These data indicate that age-related differences in fatigue are observed even during intermittent MVC with a high duty cycle, and that these differences are independent of central and peripheral activation. Further, it seems that sex-based differences in both fatigue and central activation failure were abolished with this duty cycle. Overall, these results suggest that age- and sex-based differences in fatigue arise from distinct mechanisms.

Effect of ageing on the regulation of motor unit force in rat medial gastrocnemius muscle

The influence of ageing on the regulation of force through the firing rate (force-frequency relationship) and motor unit contractile output were investigated in three types of motor unit (MU): FF, FR and S, in the medial gastrocnemius muscle. A control group of young (5-10 months) Wistar rats was compared to three groups of older (20-21, 24-25 and 28-30 months) animals. The optimal tetanus characterized by the maximum contractile output (force-time area - FTA - per single pulse) was determined. During ageing, the steep part of the force-frequency relationship of medial gastrocnemius MUs shifted towards lower stimulation rates. However, in all MU types of the oldest rats, the opposite shift (towards higher rates) was observed. Ageing induced a substantial increase in the maximal FTA per pulse, particularly in S and FF units, but only subtly altered the fusion index of the optimal tetanus of MUs. Moreover, a transient increase in the mean forces of FF MUs was revealed in the groups of 20-21 and 24-25 months rats, and a significant decrease in the fatigue resistance of FR MUs accompanied ageing. These findings increase our understanding of the functional mechanisms responsible for changes in rate coding and alterations in muscle fatigability during ageing.

Fatigability is increased with age during velocity-dependent contractions of the dorsiflexors

BACKGROUND

Muscle power is more relevant to the activities of daily living than is isometric strength. However, dynamic contractions have received little attention as they relate to the effect of age on muscle fatigue, particularly in very old persons. Thus, the purpose of this study was to investigate fatigue of the dorsiflexors during a velocity-dependent (isotonic) power task in 12 young (26 years), 12 old (64 years), and 12 very old (84 years) men.

METHODS

The fatigue protocol involved 25 maximal (as fast as possible) contractions at a load of 20% maximum isometric strength through a 25 degrees range of motion. Electromyographic signals of the tibialis anterior and soleus muscles were recorded to assess agonist activation and antagonist coactivation, respectively.

RESULTS

Fatigability increased progressively with age as muscle power decreased by 13% in young men, 19% in old men, and 24% in very old men. In contrast, agonist activation and antagonist coactivation were unaffected by age. During the fatigue protocol, agonist activity decreased by 3%, 5%, and 4%, whereas antagonist activity increased by 11%, 13%, and 13% in young, old, and very old men, respectively.

CONCLUSION

These results demonstrate that older adults are more fatigable than young adults during a velocity-dependent power task. This finding is in contrast to the majority of fatigue data previously reported from less functionally relevant isometric or isokinetic tasks.

Age-related resistance to skeletal muscle fatigue is preserved during ischemia

During voluntary contractions, the skeletal muscle of healthy older adults often fatigues less than that of young adults, a result that has been explained by relatively greater reliance on muscle oxidative metabolism in the elderly. Our aim was to investigate whether this age-related fatigue resistance was eliminated when oxidative metabolism was minimized via ischemia induced by cuff (220 mmHg). We hypothesized that 1) older men (n = 12) would fatigue less than young men (n = 12) during free-flow (FF) contractions; 2) both groups would fatigue similarly during ischemia; and 3) reperfusion would reestablish the fatigue resistance of the old. Subjects performed 6 min of intermittent, maximal voluntary isometric contractions of the ankle dorsiflexors under FF and ischemia-reperfusion (IR) conditions. Ischemia was maintained for the first 3 min of contractions, followed by rapid cuff deflation and reperfusion for 3 additional minutes of contractions. Central activation, peripheral activation, and muscle contractile properties were measured at 3 and 6 min of contractions. Older men fatigued less than young men during FF (P </= 0.02), ischemia (P < 0.001), and reperfusion (P < 0.001). During FF, activation and contractile properties changed similarly across age groups. At the end of ischemia, central (P = 0.02) and peripheral (P </= 0.03) activation declined more in the young, with no effect of age on the changes in contractile properties. Thus age-related fatigue resistance was evident during FF and IR, indicating that differences in blood flow and oxidative metabolism do not explain the fatigue resistance of old age.