Reliability of Corticospinal and Motoneuronal Excitability Evaluation During Unfatiguing and Fatiguing Cycling Exercise.

INTRODUCTION: Central nervous system excitability depends on the task performed, muscle group solicited, and contraction type. However, little is known on corticospinal and motoneuronal excitability measured during locomotor exercise. This study aimed at determining the reliability of motor-evoked potentials (MEP) and thoracic motor-evoked potentials (TMEP) in dynamic mode during unfatiguing and fatiguing cycling exercise. METHODS: Twenty-two participants completed four visits. Visit 1 comprised familiarization and a maximal incremental test to determine maximal power output (Wmax). The remaining visits encompassed unfatiguing evaluations, which included 8 brief bouts of moderate (50% Wmax) and high intensity cycling (80% Wmax). In each bout, a set of two TMEPs, five MEPs and one M-max were obtained. Subsequently, a fatiguing exercise to exhaustion at 80% Wmax was performed, with four sets of measurements 3 min through the exercise and 4 additional sets at exhaustion, both measured at 50% Wmax. RESULTS: Intraclass correlation coefficients (ICCs) for 5, 10, 15 and 20 MEP·Mmax-1 revealed excellent reliability at both intensities and during fatiguing exercise (ICC ≥ 0.92). TMEP·Mmax-1 showed ICCs ≥0.82 for moderate and high intensity, and it was not affected by fatigability. Overall standard error of measurement was 0.090 [0.083, 0.097] for MEP·Mmax-1 and 0.114 [0.105, 0.125] for TMEP·Mmax-1. A systematic bias associated to the number of stimulations, especially at high intensity, suggested the evaluation itself may be influenced by fatigability. A mean reduction of 8% was detected in TMEP·Mmax-1 at exhaustion. CONCLUSIONS: Motoneuronal and corticospinal excitability measured in dynamic mode presented good to excellent reliability in unfatiguing and fatiguing exercise. Further studies inducing greater fatigability must be conducted to assess the sensitivity of central nervous system excitability during cycling.

Downhill running increases markers of muscle damage and impairs the maximal voluntary force production as well as the late phase of the rate of voluntary force development.

PURPOSE: To examined the time-course of the early and late phase of the rate of voluntary force development (RVFD) and muscle damage markers after downhill running. METHODS: Ten recreational runners performed a 30-min downhill run at 10 km h-1 and -20% (-11.3°) on a motorized treadmill. At baseline and each day up to 4 days RVFD, knee extensors maximum voluntary isometric force (MVIC), serum creatine kinase (CK) concentration, quadriceps swelling, and soreness were assessed. The early (0-50 ms) and late (100-200 ms) phase of the RVFD, as well as the force developed at 50 and 200 ms, were also determined. RESULTS: MVIC showed moderate decrements (p < 0.05) and recovered after 4 days (p > 0.05). Force at 50 ms and the early phase were not impaired (p > 0.05). Conversely, force at 200 ms and the late phase showed moderate decrements (p < 0.05) and recovered after 3 and 4 days, respectively (p > 0.05). CK concentration, quadriceps swelling, and soreness increased (p < 0.05) were overall fully resolved after 4 days (p > 0.05). CONCLUSION: Downhill running affected the knee extensors RVFD late but not early phase. The RVFD late phase may be used as an additional marker of muscle damage in trail running.

Age-Related Differences between Old and Very Old Men in Performance and Fatigability Are Evident after Cycling but Not Isometric or Concentric Single-Limb Tasks.

PURPOSE: This study aimed to compare performance and fatigability between young (n = 13; 18-30 yr), old (n = 13; 60-80 yr), and very old (n = 12; >80 yr) men during a single-joint isometric (ISO) and concentric (CON) task performed on an isokinetic dynamometer and a cycling (BIKE) task. METHODS: Participants randomly performed incremental tasks consisting of stages of 75 contractions (i.e., 120 s, 0.8 s on/0.8 s off) for ISO and CON and 120 s at 37.5 rpm (similar duty cycle) for BIKE. Increments were set as a percentage of body weight. Knee extensor maximal force, voluntary activation, and twitch amplitude were measured at baseline, after each stage, and at task failure (five out of eight contractions below the target force or 6 s in a row at a cadence <37.5 rpm). RESULTS: Compared with young men, performance (number of stages) was 24% and 40% lower in old and very old men in ISO, 54% and 59% lower in CON, and 36% and 60% lower in BIKE (all P < 0.05). Performance of old and very old differed only in BIKE (P < 0.01). For the last common stages performed, compared with young, force loss was greater for very old men in ISO and for old and very old men in BIKE (all P < 0.05). Overall, for the last common stage performed and task failure, old and very old men presented similar force loss, alterations in voluntary activation, and twitch amplitude. CONCLUSIONS: Our findings reveal that, with workloads relative to body weight, differences in performance between old and very old men could only be observed during BIKE (i.e., the more ecologically valid task). Results from isometric or concentric conditions might not be transferable to dynamic exercise with large muscle masses.

Fatigue in elite fencing: Effects of a simulated competition.

The fatigue induced by fencing remains scarcely investigated. We aimed to investigate both objective (neuromuscular performance fatigability) and subjective (perceived fatigue, effort, and workload) manifestations of fatigue in elite fencers following a five-bout simulated competition. Changes in countermovement jump height, knee extensors maximal isometric torque, rate of torque development, voluntary activation, and contractile response to muscular electrical stimulation were measured in 29 elite fencers [12 epee (6 women), 11 saber (5 women), and 6 foil]. Perceived fatigue and effort were evaluated with visual analog scales, and the perceived workload with the NASA Task Load Index scale. During the competition, maximal torque and rate of torque development decreased by 1.6% (p = 0.017) and 2.4% (p < 0.001) per bout, respectively. Perceived fatigue before each bout increased (12% per bout), with similar values observed at the end of all bouts (bout × period interaction: p < 0.001). Perceived effort increased during the bouts (10% per period, p < 0.001) and during the competition (3% per bout, p = 0.011). Perceived mental demand increased during the competition (2% per bout, p = 0.024). These results suggest that elite fencers needed to increase the allocation of mental rather than physical resources to the task to counterbalance the deleterious effect of fatigue on performance.

Elite vs. Experienced Male and Female Trail Runners: Comparing Running Economy, Biomechanics, Strength, and Power.

ABSTRACT: Besson, T, Pastor, FS, Varesco, G, Berthet, M, Kennouche, D, Dandrieux, P-E, Rossi, J, and Millet, GY. Elite vs. experienced male and female trail runners: comparing running economy, biomechanics, strength, and power. J Strength Cond Res 37(7): 1470-1478, 2023-The increased participation in trail running (TR) races and the emergence of official international races have increased the performance level of the world best trail runners. The aim of this study was to compare cost of running (Cr) and biomechanical and neuromuscular characteristics of elite trail runners with their lower level counterparts. Twenty elite (10 females; ELITE) and 21 experienced (10 females; EXP) trail runners participated in the study. Cr and running biomechanics were measured at 10 and 14 km·h-1 on flat and at 10 km·h-1 with 10% uphill incline. Subjects also performed maximal isometric voluntary contractions of knee and hip extensors and knee flexors and maximal sprints on a cycle ergometer to assess the power-torque-velocity profile (PTVP). Athletes also reported their training volume during the previous year. Despite no differences in biomechanics, ELITE had a lower Cr than EXP (p < 0.05). Despite nonsignificant difference in maximal lower-limb power between groups, ELITE displayed a greater relative torque (p < 0.01) and lower maximal velocity (p < 0.01) in the PTVP. Females displayed shorter contact times (p < 0.01) compared with males, but no sex differences were observed in Cr (p > 0.05). No sex differences existed for the PTVP slope, whereas females exhibited lower relative torque (p < 0.01) and velocity capacities (p < 0.01) compared with males. Although not comprehensively assessing all determining factors of TR performance, those data evidenced level and sex specificities of trail runners in some factors of performance. Strength training can be suggested to lower level trail runners to improve Cr and thus TR performance.

Elite Road vs. Trail Runners: Comparing Economy, Biomechanics, Strength, and Power.

ABSTRACT: Sabater Pastor, FS, Besson, T, Berthet, M, Varesco, G, Kennouche, D, Dandrieux, P-E, Rossi, J, and Millet, GY. Elite road vs. trail runners: comparing economy, biomechanics, strength, and power. J Strength Cond Res 37(1): 181-186, 2023-The purpose of this study was to determine the differences between road (ROAD) vs. trail (TRAIL) elite runners in terms of force-velocity profile (FVP), running biomechanics, lower-limb maximal isometric strength, cost of running (Cr), and training. Seventeen male elite athletes (10 TRAIL and 7 ROAD) participated in this study. Force-velocity profile was measured using a 2-sprint test on a cycle ergometer. Strength was assessed with a dynamometer measuring isometric maximum voluntary torque of the knee extensors and knee flexors. Biomechanics parameters (running kinematics and stiffness) were measured, and Cr was calculated at 10 and 14 km·h-1 at 0% slope and at 10 km·h-1 on a 10% slope on a treadmill. Athletes also reported their training duration during the previous year. Theoretical maximal torque (F0) and maximal power (Pmax) in the FVP were higher for TRAIL vs. ROAD (122 ± 13 vs. 99 ± 7 N·m, p = 0.001; and 726 ± 89 vs. 626 ± 44 W; p = 0.016). Cost of running was higher for TRAIL compared with ROAD on flat at 14 km·h-1 (4.32 ± 0.22 vs. 4.06 ± 0.29 J·kg-1·m-1; p = 0.047) but similar at 10 km·h-1 and uphill. No differences were found in maximal isometric strength or running biomechanics. ROAD spent 81% more time training than TRAIL (p = 0.0003). The specific training (i.e., "natural" resistance training) performed during graded running in trail runners and training on level surface at high speed may explain our results. Alternatively, it is possible that trail running selects stronger athletes because of the greater strength requirements of graded running.

Performance fatigability during isometric vs. concentric quadriceps fatiguing tasks in men and women.

In the present study, we aimed to provide a robust comparison of the fatigability of the knee extensors following isometric (ISO) and concentric (CON) tasks. Twenty young adults (25 ± 4 yr, 10 women) randomly performed the ISO and CON quadriceps intermittent fatigue test, consisting of ten (5 s on/5-s off, ISO) or one-hundred (0.5-s on/0.5-s off, CON) contractions with 10 % increments per stage until exhaustion. Performance fatigability was quantified as maximal isometric (MVIC) and concentric (MVCC) torque loss. Voluntary activation and contractile function (peak-twitch) were investigated using peripheral nerve stimulation. Number of stages (6.2 ± 0.7 vs. 4.9 ± 0.8; P < 0.001) and torque-time integral (20,166 ± 7,821 vs. 11,285 ± 4,933 Nm.s; P < 0.001) were greater for ISO than CON. MVIC, MVCC and voluntary activation decreased similarly between sessions (P > 0.05) whereas peak-twitch amplitude decreased more for CON (P < 0.001). The number of contractions was similar across sexes (ISO: men = 62 ± 8, women = 61 ± 5; CON: men = 521 ± 67, women = 458 ± 76, P > 0.05). MVCC was more reduced in women for both sessions (all P < 0.05), while MVIC loss was similar between sexes. We concluded that, despite greater torque-time integral and duration for ISO, both sessions induced a similar performance fatigability at exhaustion. Contractile function was more altered in CON. Finally, sex-related difference in fatigability depends on the contraction mode used during testing.

Acute effects of quadriceps muscle versus tendon prolonged local vibration on force production capacities and central nervous system excitability.

PURPOSE: The present study aimed to directly compare the effects of 30 min muscle (VIBmuscle) vs. tendon (VIBtendon) local vibration (LV) to the quadriceps on maximal voluntary isometric contraction (MVIC) and rate of torque development (RTD) as well as on central nervous system excitability (i.e. motoneuron and cortical excitability). METHODS: Before (PRE) and immediately after (POST) LV applied to the quadriceps muscle or its tendon, we investigated MVIC and RTD (STUDY #1; n = 20) or vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) electromyography responses to thoracic electrical stimulation (TMEPs; motoneuron excitability) and transcranial magnetic stimulation (MEPs; corticospinal excitability) (STUDY #2; n = 17). MEP/TMEP ratios were further calculated to quantify changes in cortical excitability. RESULTS: MVIC decreased at POST (P = 0.017) without any difference between VIBtendon and VIBmuscle, while RTD decreased for VIBtendon (P = 0.013) but not VIBmuscle. TMEP amplitudes were significantly decreased for all muscles (P = 0.014, P < 0.001 and P = 0.004 for VL, VM and RF, respectively) for both LV sites. While no changes were observed for MEP amplitude, MEP/TMEP ratios increased at POST for VM and RF muscles (P = 0.009 and P = 0.013, respectively) for both VIBtendon and VIBmuscle. CONCLUSION: The present results suggest that prolonged muscle and tendon LV are similarly effective in modulating central nervous system excitability and decreasing maximal force. Yet, altered explosive performance after tendon but not muscle LV suggests greater neural alterations when tendons are vibrated.

Downhill running affects the late but not the early phase of the rate of force development.

PURPOSE: This study aimed to evaluate the acute changes in the knee extensors maximum voluntary isometric contraction force (MVIC), rate of force development (RFD), and rate of EMG rise (RER) following a bout of downhill running. METHODS: MVIC and RFD at 0-50, 50-100, 100-200, and 0-200 ms were determined in thirteen men (22 ± 2 yr) before and after 30 min of downhill running (speed: 10 km h-1; slope: - 20%). Vastus lateralis maximum EMG (EMGmax) and RER at 0-30, 0-50, and 0-75 ms were also recorded. RESULTS: MVIC, RFD0-200, and EMGmax decreased by ~ 25% [Cohen's d = - 1.09 (95% confidence interval: - 1.88/- 0.24)], ~ 15% [d = - 0.50 (- 1.26/0.30)], and ~ 22% [d = - 0.37 (- 1.13/0.42)] (all P < 0.05), respectively. RFD100-200 was also reduced [- 25%; d = - 0.70 (- 1.47/0.11); P < 0.001]. No change was observed at 0-50 ms and 50-100 ms (P ≥ 0.05). RER values were similar at each time interval (all P > 0.05). CONCLUSION: Downhill running impairs the muscle capacity to produce maximum force and the overall ability to rapidly develop force. No change was observed for the early phase of the RFD and the absolute RER, suggesting no alterations in the neural mechanisms underlying RFD. RFD100-200 reduction suggests that impairments in the rapid force-generating capacity are located within the skeletal muscle, likely due to a reduction in muscle-tendon stiffness and/or impairments in the muscle contractile apparatus. These findings may help explain evidence of neuromuscular alterations in trail runners and following prolonged duration races wherein cumulative eccentric loading is high.

Very old adults show impaired fatigue resistance compared to old adults independently of sex during a knee-extensors isometric test.

The present study aimed to investigate age-related differences in fatigability induced by an isometric quadriceps intermittent fatiguing test in 30 young (<35 years old, 15 females), 19 old (>60 years old, 9 females) and 30 very old (>80 years old, 15 females) adults. Maximal force loss, contractile function and voluntary activation of the knee extensors were evaluated throughout an isometric fatiguing test using femoral nerve magnetic stimulations. Older adults performed more contractions (index of relative performance) than young (P = 0.046) and very old adults (P = 0.007), without differences between young and very old adults (P = 1.000). Force-time integral (absolute performance) was greater for young and old adults compared to very old adults (P < 0.001), without differences between young and old adults (P = 0.124). At exhaustion, force loss was greater for young (-28 ± 9%) compared to old adults (-19 ± 8%; P = 0.002), but not very old adults (-23 ± 8%; P = 0.106), and no difference between old and very old adults (P = 0.414). Data from the interpolated twitch technique suggested similar alteration in contractile function with age, with no impairments in voluntary activation. Impairments were similar across sexes. This study showed that older adults were less fatigable than young adults during an isometric intermittent fatiguing task of the knee extensors. This greater fatigue resistance was not maintained in very old adults independent of sex. Fatigability at exhaustion was likely due to impairments in contractile function for the three age groups.

Performance Determinants in Trail-Running Races of Different Distances.

PURPOSE: While the physiological determinants of road running have been widely studied, there is a lack of research in trail-running racing performance. The aim of our study was to determine the physiological predictors of trail-running performance in races of different distances in similar terrain and weather conditions. METHODS: Seventy-five trail runners participating in one of the races of the Ultra-Trail du Mont-Blanc were recruited. Previous to the race, each runner was evaluated with (1) an incremental treadmill test to determine maximal oxygen uptake, ventilatory thresholds, cost of running, and substrate utilization; (2) a power-force-velocity profile on a cycle ergometer; (3) maximal voluntary contractions of the knee extensors and plantar flexors; and (4) anthropometric characteristics. Neuromuscular fatigue was evaluated after the races. Twenty-four runners finished a SHORT (<55 km), 16 finished a MEDIUM (101 km), and 14 finished a LONG (>145 km) race. Correlations and multiple linear regressions were used to find the determinants of performance in each race distance. RESULTS: Performance in SHORT was explained by maximal oxygen uptake and lipid utilization at 10 km/h (r2 = .825, P < .001). Performance in MEDIUM was determined by maximal oxygen uptake, maximal isometric strength, and body fat percentage (r2 = .917, P < .001). A linear model could not be applied in LONG, but performance was correlated to peak velocity during the incremental test. CONCLUSIONS: Performance in trail running is mainly predicted by aerobic capacity, while lipid utilization also influences performance in races <60 km and performance in approximately 100 km is influenced by muscle strength and body composition.

Association Between Physical Activity, Quadriceps Muscle Performance, and Biological Characteristics of Very Old Men and Women.

The aim of the study was to evaluate the association between physical activity, knee extensors (KE) performance (ie, isometric strength and fatigability), and biological parameters (ie, muscle structural, microvascular, and metabolic properties) in healthy very old men and women. Thirty very old adults (82 ± 1 years, 15 women) performed an isometric Quadriceps Intermittent Fatigue (QIF) test for the assessment of KE maximal force, total work (index of absolute performance), and fatigability. Muscle biopsies from the vastus lateralis muscle were collected to assess muscle fibers type and morphology, microvasculature, and enzymes activity. Correlation analyses were used to investigate the relationships between physical activity (steps/day, actimetry), KE performance, and biological data for each sex separately. Men, compared to women, showed greater total work at the QIF test (44 497 ± 8 629 Ns vs 26 946 ± 4 707 Ns; p < .001). Steps per day were correlated with total work only for women (r = 0.73, p = .011). In men, steps per day were correlated with the percentage (r = 0.57, p = .033), shape factor (r = 0.75, p = .002), and capillary tortuosity of type IIX fibers (r = 0.59, p = .035). No other relevant correlations were observed for men or women between steps per day and biological parameters. Physical activity level was positively associated with the capacity of very old women to perform a fatiguing test, but not maximal force production capacity of the KE. Physical activity of very old men was not correlated with muscle performance. We suggest that very old women could be at higher risk of autonomy loss and increasing the steps per day count could provide a sufficient stimulus for adaptations in less active women.

Sex Differences in Neuromuscular Fatigue and Changes in Cost of Running after Mountain Trail Races of Various Distances.

INTRODUCTION: Women have been shown to experience less neuromuscular fatigue than men in knee extensors (KE) and less peripheral fatigue in plantar flexors (PF) after ultratrail running, but it is unknown if these differences exist for shorter trail running races and whether this may impact running economy. The purpose of this study was to characterize sex differences in fatigability over a range of running distances and to examine possible differences in the postrace alteration of the cost of running (Cr). METHODS: Eighteen pairs of men and women were matched by performance after completing different races ranging from 40 to 171 km, divided into SHORT versus LONG races (<60 and >100 km, respectively). Neuromuscular function and Cr were tested before and after each race. Neuromuscular function was evaluated on both KE and PF with voluntary and evoked contractions using electrical nerve (KE and PF) and transcranial magnetic (KE) stimulation. Oxygen uptake, respiratory exchange ratio, and ventilation were measured on a treadmill and used to calculate Cr. RESULTS: Compared with men, women displayed a smaller decrease in maximal strength in KE (-36% vs -27%, respectively, P < 0.01), independent of race distance. In SHORT only, women displayed less peripheral fatigue in PF compared with men (Δ peak twitch: -10% vs -24%, respectively, P < 0.05). Cr increased similarly in men and women. CONCLUSIONS: Women experience less neuromuscular fatigue than men after both "classic" and "extreme" prolonged running exercises but this does not impact the degradation of the energy Cr.

Reliability and agreement of a dynamic quadriceps incremental test for the assessment of neuromuscular function.

The quadriceps-intermittent-fatigue (QIF) test assesses knee extensors strength, endurance and performance fatigability in isometric condition. We aimed to assess reliability and agreement for this test in dynamic conditions and with the use of transcranial magnetic stimulation. On two separate sessions, 20 young adults (25 ± 4 yr, 10 women) performed stages of 100 knee extensors concentric contractions at 120°/s (60° range-of-motion) with 10% increments of the initial maximal concentric torque until exhaustion. Performance fatigability across the test was quantified as maximal isometric and concentric torque loss, and its mechanisms were investigated through the responses to transcranial magnetic and electrical stimulations. Reliability and agreement were assessed using ANOVAs, coefficients of variation (CVs) and intra-class correlation coefficients (ICCs) with 95% CI. Good inter-session reliability and high agreement were found for number of contractions [489 ± 75 vs. 503 ± 95; P = 0.20; ICC = 0.85 (0.66; 0.94); CV = 5% (3; 7)] and total work [11,285 ± 4,932 vs. 11,792 ± 5838 Nm.s; P = 0.20; ICC = 0.95 (0.87; 0.98); CV = 8% (5; 11)]. Poor reliability but high agreement were observed for isometric [-33 ± 6 vs. -31 ± 7%; P = 0.13; ICC = 0.47 (0.05; 0.75); CV = 6% (4;8)] and concentric [-20 ± 11% vs. -19 ± 9%; P = 0.82; ICC = 0.26 (-0.22; 0.63); CV = 9% (6; 12)] torque loss. The dynamic QIF test represents a promising tool for neuromuscular evaluation in isokinetic mode.

Effects of pre-induced fatigue vs. concurrent pain on exercise tolerance, neuromuscular performance and corticospinal responses of locomotor muscles.

KEY POINTS: Fatigue and muscle pain induced in a remote muscle group has been shown to alter neuromuscular performance in exercising muscles. Inhibitory neural feedback associated with activation of mechano- and metabo-sensitive muscle afferents has been implicated in this phenomenon. The present study aimed to quantify and compare the effects of pre-induced fatigue and concurrent rising pain (evoked by muscle ischaemia) on the contralateral leg exercise capacity, neuromuscular performance, and corticomotor excitability and inhibition of knee extensor muscles. Pre-induced fatigue in one leg had a greater detrimental effect than the concurrent rising pain on the contralateral limb cycling capacity. Furthermore, pre-induced fatigue, but not concurrent rising pain, reduced corticospinal inhibition recorded from tested contralateral muscles. Regardless of the origin or mechanisms modulating sensory afferents during single-leg cycling exercise (i.e. pre-induced fatigue vs. concurrent rising pain), the limit of exercise tolerance remained the same and exercise was terminated upon achievement of a sensory tolerance limit. ABSTRACT: Individuals often need to maintain voluntary contractions during high intensity exercise in the presence of fatigue and pain. This investigation examined the effects of pre-induced fatigue and concurrent rising pain (evoked by muscle ischaemia) in one leg on motor fatigability and corticospinal excitability/inhibition of the contralateral limb. Twelve healthy males undertook four experimental protocols including unilateral cycling to task failure at 80% of peak power output with: (i) the right-leg (RL); (ii) the left-leg (LL); (iii) RL immediately preceded by LL protocol (FAT-RL); and (iv) RL when blood flow was occluded in the contralateral (left) leg (PAIN-RL). Participants performed maximal and submaximal 5 s right-leg knee extensions during which transcranial magnetic and femoral nerve electrical stimuli were delivered to elicit motor-evoked and compound muscle action potentials, respectively. The pre-induced fatigue reduced the right leg cycling time-to-task failure (mean ± SD; 332 ± 137 s) to a greater extent than concurrent pain (460 ± 158 s), compared to RL (580 ± 226 s) (P < 0.001). The maximum voluntary contraction force declined less following FAT-RL (P < 0.019) and PAIN-RL (P < 0.032) compared to RL. Voluntary activation declined and the corticospinal excitability recorded from knee extensors increased similarly after the three conditions (P < 0.05). However, the pre-induced fatigue, but not concurrent pain, reduced corticospinal inhibition compared to RL (P < 0.05). These findings suggest that regardless of the origin and/or mechanisms modulating sensory afferent feedback during single-leg cycling (e.g. pre-induced fatigue vs. concurrent rising pain), the limit of exercise tolerance remains the same, suggesting that exercise will be terminated upon achievement of sensory tolerance limit.

Rate of force development and rapid muscle activation characteristics of knee extensors in very old men.

The age-related decrease in neuromuscular performance is usually accentuated in very old age (>80 yr) as evidenced by a marked reduction in maximal force production. However, little is known about the ability to rapidly produce force, which limits daily activities and increase the risk of falling. We aimed to assess rapid force production characteristics and rate of muscle activation of the knee extensors in 15 very old (82 ±â€¯1 yr) vs 12 young (24 ±â€¯4 yr) men. Maximal force (Fmax) and maximal rate of force development (RFDmax) were determined during separated specific isometric contractions. EMG from the vastus lateralis was analyzed to assess the rate of EMG rise (RER). Finally, RFD and RER were examined at time intervals of 0-50, 50-100, 100-200, 0-200 ms and 0-30, 0-50, 0-75 ms, respectively. We reported lower Fmax (414 ±â€¯91 N vs. 661 ±â€¯139 N) and absolute RFDmax (8720 ±â€¯2056 N*s-1vs. 5700 ±â€¯2474 N*s-1) in the very old men compared to young men (P < 0.01). When normalized to Fmax, RFDmax was similar between groups. Normalized RFD at 0-50 and 0-200 ms were lower (-34% and - 46%, P = 0.04) for the very old men, while no difference was observed at 50-100 ms and 100-200 ms. RER values were higher (~346%, P < 0.01) for the young men at every time interval. These data suggest that the decline in RFDmax is associated with the decrease in Fmax. Impairments in RFD were accentuated in the first phase of the contraction, which has been associated in the literature with impairments in neural factors associated with aging.