Acute and prolonged competing effects of activation history on human motor unit firing rates during contractile impairment and recovery.

The purpose of this study was to investigate the effect of inducing post-activation potentiation (PAP) during prolonged low-frequency force depression (PLFFD) on motor unit (MU) firing rates. In 10 participants, grouped firing rates of 3027 MUs from the tibialis anterior were recorded with tungsten microelectrodes. Baseline MU firing rates at 25% isometric maximal voluntary contraction (MVC) were ∼14 Hz. A 1 min dorsiflexion MVC reduced torque and maximal MU firing rates (36 Hz) by 49% and 52%, respectively. Following task completion, firing rates at 25% of baseline MVC torque and torque in response to electrically evoked (single twitch, 10 Hz and 50 Hz) stimulation were assessed before and after a 5 s MVC (to induce PAP) every 10 min for 60 min. From 10 to 60 min after task completion, the torque ratios (twitch:50 Hz and 10:50 Hz) were depressed (∼30%) relative to baseline (P < 0.001), indicating PLFFD; and firing rates were higher by ∼15% relative to baseline (P < 0.001). This occurred despite recovery of MVC rates (∼99%) and torque (∼95%) by 10 min (P > 0.3). Inducing PAP during PLFFD increased both low to high torque ratios (twitch and 10:50 Hz) by ∼200% and ∼135%, respectively (P < 0.001) and firing rates were ∼18% lower relative to PLFFD rates (P < 0.001), despite a speeding of evoked contractile properties (P = 0.001). Thus, firing rates appear strongly matched to alterations in torque, rather than contractile speed when modified by contractile history, and lower rates during PAP may be a mechanism to mitigate effects of PLFFD. The effect of activation history on contractile function demonstrates acute compensatory responses of motoneuron output. KEY POINTS: Prolonged low frequency force depression (PLFFD) following a sustained 1 min isometric maximal voluntary contraction causes an increase in submaximal mean motor unit (MU) firing rates. Inducing post-activation potentiation (PAP) during PLFFD, however, causes a reduction in mean submaximal MU firing rates to a level below those at baseline. The mean firing rate reduction during PAP occurs despite a speeding of evoked contractile properties and thus firing rates are more strongly matched to alterations in torque, rather than contractile speed when modified by various contractile histories. The reductions in firing rates during PAP may mitigate the effects of PLFFD during voluntary contractions. These results demonstrate that firing rates are highly responsive to opposing influences on the contractile state and can make rapid compensatory rate adjustments dependent on the active state of the muscle.

Contractile properties are less affected at long than short muscle length after eccentric exercise.

PURPOSE: The aim of the present study was to investigate whether the electrically evoked muscle responses are differently affected over time by the knee joint angle after an exercise-induced muscle damage (EIMD). We hypothesized that low-frequency-evoked responses would be less affected at long than short muscle length, and that mechanisms located within the muscle and tendinous tissues would be involved. METHODS: Fifteen males performed 45 min loaded downhill walking (DW) exercise. Maximal voluntary contraction torque (MVC), optimal angle for torque production, voluntary activation level (VAL), twitch, doublet at 10 and 100 Hz (Db10 and Db100, respectively), rate of torque development (RTD), post-activation potentiation (PAP), muscle shear elastic modulus (µ) and aponeurosis stiffness were assessed before, after, and 4, 24, 48, 72 and 168 h after the exercise at a knee angle of 40°, 90° and 120° (0°: full extension). RESULTS: MVC, VAL and Db100 were similarly decreased across joint angles after the DW and optimal angle was not affected. Twitch, Db10, Db10/Db100, PAP and RTD were less affected and muscle µ more increased at long than short muscle lengths (p < 0.05), especially during the first 24 h after the DW exercise. CONCLUSION: Low-frequency-evoked responses were more preserved at long than short muscle length the first 24 h after the DW exercise, suggesting that joint angle should be taken into account to assess muscular alterations after EIMD. This length-dependence could be associated to the higher sensitivity to Ca2+ and the higher increase in muscle stiffness at long than short muscle length.

Comparison of prolonged low-frequency force depression assessed using isometric torque and isotonic power following a dynamic fatiguing task.

PURPOSE: Prolonged low-frequency force depression (PLFFD) occurs following both dynamic and static fatiguing tasks, but it has been assessed predominately using measures of isometric torque. However, it is unknown whether PLFFD induced during dynamic tasks is adequately characterized by isometric torque, which excludes velocity and power. The purpose of this study was to compare PLFFD assessed using isometric torque and isotonic power following a concentric fatiguing task. METHODS: Young (18-31 years) males (n = 9) and females (n = 4) performed isotonic plantar flexion contractions until a ~ 75% reduction in peak power. Isotonic and isometric contractions were electrically evoked at 10 Hz and 50 Hz via tibial nerve stimulation. Isotonic and isometric PLFFD was assessed as the ratio of 10 to 50 Hz for power and torque, respectively. Recovery was assessed immediately, and at 2.5, 5, 10, 20, and 30 min after task termination. RESULTS: Relative to baseline, 10:50 Hz ratio assessed using isotonic power was reduced more than isometric torque (30 min 41 ± 17 vs. 25 ± 12% reduction, p = 0.001); however, both contraction modes displayed similar trajectories throughout recovery (p = 0.906). The larger reduction in isotonic 10:50 Hz ratio was due to greater impairments in 10 Hz power compared to 10 Hz isometric torque (30 min 38 ± 20 vs. 21 ± 11% reduction, p < 0.001). CONCLUSION: The similar trajectories of 10:50 Hz ratios throughout recovery indicate that PLFFD can be adequately characterized using either isometric torque or isotonic power.

Increasing the resting time between drop jumps lessens delayed-onset muscle soreness and limits the extent of prolonged low-frequency force depression in human knee extensor muscles.

PURPOSE: Unaccustomed eccentric contractions generally result in a long-lasting contractile impairment, referred to as prolonged low-frequency force depression (PLFFD), and delayed-onset muscle soreness (DOMS). We here used repeated drop jumps (DJs) as an eccentric contraction model and studied the effects of increasing the time between DJs from 20 s to 5 min. We hypothesized that both PLFFD and DOMS would be less marked at the longer DJ interval due to the longer time to restore structural elements between DJs. METHODS: Young men (n = 12) randomly performed 50 DJs with either 20-s (DJ-20 s) or 5-min (DJ-5 min) rest between DJs. Voluntary, 20 Hz and 100 Hz electrically stimulated isometric knee extension torques and muscle soreness were monitored before and for 7 days after DJs; serum CK activity was measured to assess muscle fibre protein leakage. In additional experiments, changes in mRNA levels were assessed in muscle biopsies collected before and 1 h after exercise. RESULTS: A marked PLFFD was observed with both protocols and the extent of 20 Hz torque depression was smaller immediately and 1 day after DJ-5 min than after DJ-20 s (p < 0.05), whereas the MVC and 100 Hz torques were similarly decreased with the two protocols. Markedly larger differences between the two protocols were observed for the muscle soreness score, which 1-4 days after exercise was about two times larger with DJ-20 s than with DJ-5 min (p < 0.01). CONCLUSIONS: The larger protective effect of the longer DJ interval against DOMS than against PLFFD indicates that their underlying mechanisms involve different structural elements.

Exercising muscle mass influences neuromuscular, cardiorespiratory, and perceptual responses during and following ramp-incremental cycling to task failure.

Neuromuscular (NM), cardiorespiratory, and perceptual responses to maximal-graded exercise using different amounts of active muscle mass remain unclear. We hypothesized that during dynamic exercise, peripheral NM fatigue (declined twitch force) and muscle pain would be greater using smaller muscle mass, whereas central fatigue (declined voluntary activation) and ventilatory variables would be greater using larger muscle mass. Twelve males (29.8 ± 4.7 years) performed two ramp-incremental cycling tests until task failure: 1) single-leg (SL) with 10 W·min-1 ramp and 2) double-leg (DL) with 20 W·min-1 ramp. NM fatigue was assessed at baseline, task failure (post), and after 1, 4, and 8 min of recovery. Cardiorespiratory and perceptual variables [i.e., ratings of perceived exertion (RPE), pain, and dyspnea] were measured throughout cycling. Exercise duration was similar between sessions (SL: 857.7 ± 263.6 s; DL: 855.0 ± 218.8 s; P = 0.923), and higher absolute peak power output was attained in DL (SL: 163.2 ± 43.8 W; DL: 307.0 ± 72.0 W; P < 0.001). Although central fatigue did not differ between conditions (SL: -6.6 ± 6.5%; DL: -3.5 ± 4.8%; P = 0.091), maximal voluntary contraction (SL: -41.6 ± 10.9%; DL: -33.7 ± 8.5%; P = 0.032) and single twitch forces (SL: -59.4 ± 18.8%; DL: -46.2 ± 16.2%; P = 0.003) declined more following SL. DL elicited higher peak oxygen uptake (SL: 42.1 ± 10.0 mL·kg-1·min-1; DL: 50.3 ± 9.3 mL·kg-1·min-1; P < 0.001), ventilation (SL: 137.1 ± 38.1 L·min-1; DL: 171.5 ± 33.2 L·min-1; P < 0.001), and heart rate (SL: 167 ± 21 bpm; DL: 187 ± 8 bpm; P = 0.005). Dyspnea (P = 0.025) was higher in DL; however, RPE (P = 0.005) and pain (P < 0.001) were higher in SL. These results suggest that interplay between NM, cardiorespiratory, and perceptual determinants of exercise performance during ramp-incremental cycling to task failure is muscle mass dependent.

Low frequency fatigue and changes in muscle fascicle length following eccentric exercise of the knee extensors.

NEW FINDING: What is the central question of this study? Does low frequency muscle fatigue indicate a failure of excitation-contraction coupling after eccentric exercise, or is it simply due to a change in muscle length? What is the main finding and its importance? The low to high frequency muscle fatigue ratio was relatively insensitive to changes in muscle length, and any changes in length following eccentric exercise were far too small to account for the high degree of low frequency fatigue. The results strengthen the suggestion that the early loss of force following eccentric exercise is due to a deficit of excitation-contraction coupling. ABSTRACT: Development of long lasting fatigue (low frequency fatigue; LFF), assessed as the ratio of forces at 20 and 100 Hz stimulation, suggests the early phase of muscle damage caused by eccentric exercise is due to a deficit of excitation-contraction coupling. However, this could be caused by a change of muscle length. Eleven men (21.3 ± 2.0 years) performed 200 maximum eccentric knee extensions (30-110 deg flexion). Force generated by 20 and 100 Hz stimulation and maximum isometric force (MIF) were determined at knee angles 50, 70 and 90 deg before and immediately after the exercise. Vastus lateralis fascicle length (FL) was measured by ultrasound of resting and contracting muscle. Peak MIF (829 ± 119 N) was at 70 deg knee flexion, falling to 486 ± 180 N (P < 0.001) after exercise, but with no change in optimum angle. FLs at rest were unaffected by eccentric exercise, but during contraction they were on average 8.8% (95% CI: 4.1, 13.5%, P = 0.002) longer after exercise. Before exercise, the 20/100 ratio increased with muscle length, from 0.69 ± 0.09 at 50 deg, 0.72 ± 0.05 at 70 deg and 0.80 ± 0.08 at knee angle 90 deg (P < 0.001). After eccentric exercise the 20/100 ratio was reduced to 0.29 ± 0.08 at 50 deg, 0.27 ± 0.04 at 70 deg and 0.34 ± 0.04 at 90 deg (P < 0.001). The 20/100 ratio was relatively insensitive to changes in muscle length and the decrease following eccentric exercise was far greater than might be caused by any changes in muscle length after eccentric exercise. The results show that LFF following eccentric exercise is not due to change in muscle length and strengthen the suggestion that it represents a deficit in excitation-contraction coupling.

The contribution of low-frequency fatigue to the loss of quadriceps contractile function following repeated drop jumps.

NEW FINDINGS: What is the central question of this study? Why do some subjects recover slowly following a bout of eccentric exercise and why is recovery faster following a repeated bout? What is the main finding and its importance? The results are consistent with two major causes of the reduction of quadriceps torque, the onset of low-frequency fatigue which recovered relatively fast and a second, delayed form of damage. Differences in the delayed damage process largely accounted for the differences in the rate of torque recovery between subjects after a first bout and it was suppression of the delayed damage which accounted for the faster recovery following a repeated bout of eccentric exercise. ABSTRACT: The purpose of this study was to determine the extent to which low-frequency fatigue (LFF) accounts for the loss of quadriceps strength and time course of recovery following a series of drop jumps (DJs). Seventeen female subjects (20.8 ± 1.4 years) undertook 100 DJs, which were repeated 4 weeks later. Maximum isometric torque (MIT) and the ratio of torque generated by 20 and 100 Hz electrical stimulation (20/100), as a measure of LFF, were measured over 7 days following each series of DJs. After the first series the 20/100 ratio fell to a greater extent than MIT (to 35 ± 8.7% and 69 ± 11%, respectively) but recovered over 2-3 days, while MIT showed little recovery over this time. Changes of the 20/100 ratio were similar between subjects with fast or slow MIT recovery. Following the second series of DJs, changes in the 20/100 ratio were similar to those of the first bout and there were no differences between fast and slow recovering subjects. MIT, however, recovered more rapidly than after the first bout; the faster recovery was confined to the subjects who recovered slowly following the first bout. The results are consistent with two major causes of the reduction of quadriceps torque, the onset of low-frequency fatigue which recovered relatively fast and a second, delayed, form of damage. The latter largely accounted for the differences in MIT recovery between subjects after the first bout, while suppression of the delayed damage accounted for the faster recovery following the repeated bout.

Acute effects of very low-volume high-intensity interval training on muscular fatigue and serum testosterone level vary according to age and training status.

PURPOSE: To compare the acute physiological responses of three different very low-volume cycling sessions (6 × 5 s, 3 × 30 s, and 3 × 60 s) and their dependence on age and training status. METHODS: Subjects were untrained young men (mean ± SD; age 22.3 ± 4.6 years, VO2peak 42.4 ± 5.5 ml/kg/min, n = 10), older untrained men (69.9 ± 6.3 years, 26.5 ± 7.6 ml/kg/min, n = 11), and endurance-trained cyclists (26.4 ± 9.4 years, 55.4 ± 6.6 ml/kg/min, n = 10). Maximal voluntary contraction (MVC) and electrically stimulated knee extension torque, and low-frequency fatigue, as ratio of stimulation torques at 20-100 Hz (P20/100), were measured only 24 h after exercise. Serum testosterone (Te) and blood lactate concentrations were measured only 1 h after exercise. RESULTS: All protocols increased the blood lactate concentration and decreased MVC and P20/100 in young men, but especially young untrained men. In old untrained men, 6 × 5 s decreased P20/100 but not MVC. Te increased after 3 × 30 s and 3 × 60 s in young untrained men and after 3 × 60 s in older untrained men. The increase in Te correlated with responses of blood lactate concentration, MVC, and P20/100 only in old untrained men. CONCLUSIONS: As little as 6 × 5 s all-out cycling induced fatigue in young and old untrained and endurance-trained cyclists. Slightly higher-volume sessions with longer intervals, however, suppressed contractile function more markedly and also transiently increased serum testosterone concentration in untrained men.

The effect of blood flow occlusion during acute low-intensity isometric elbow flexion exercise.

PURPOSE: Blood flow restriction (BFR) with low-intensity (< 30% of 1 repetition maximum strength) muscle contraction has been used chronically (> 4 weeks) to enhance resistance training. However, mechanisms underlying muscle adaptations following BFR are not well understood. To explore changes related to chronic BFR adaptations, the current study used blood flow occlusion (BFO) during an acute bout of low-intensity isometric fatiguing contractions to assess peripheral (muscle) factors affected. METHODS: Ten males completed separate fatiguing elbow flexor protocols to failure; one with BFO and one with un-restricted blood flow (FF). Baseline, post-task failure, and 30 min of recovery measures of voluntary and involuntary contractile properties were compared. RESULTS: BFO had greater impairment of intrinsic measures compared with FF, despite FF lasting 80% longer. Following task failure, maximal voluntary contraction and 50 Hz torque decreased in both protocols (~ 60% from baseline). Voluntary activation decreased ~ 11% from baseline at failure following both protocols, but recovered at a faster rate following BFO, whereas MVC recovered to ~ 90% of baseline in both protocols. The 10/50 Hz torque ratio was decreased by ~ 68% and ~ 21% from baseline, for BFO and FF, respectively (P < 0.01). 50 Hz half-relaxation-time (HRT) was significantly longer immediately following BFO (~ 107% greater than baseline), with no change following FF. CONCLUSIONS: Thus, greater peripheral fatigue that recovers at a similar rate compared to conventional exercise is likely driving muscle adaptations observed with chronic BFR exercise. Likely BFO alters energy demand and supply of working muscle similar to chronic BFR, but is exaggerated in this paradigm.

What are the best isometric exercises of muscle potentiation?

PURPOSE: The aim of this study was to follow post-activation potentiation (PAP), low-frequency fatigue (LFF), metabolic-induced fatigue and post-contractile depression (PCD) in response to different isometric muscle contraction modalities. METHODS: Young healthy men (N = 120) were randomly assigned to one of ten exercise modality groups which differed in contraction duration (5-60 s), activation pattern (intermittent or continuous contractions), activation mode (voluntary or stimulated), and intensity [maximal or submaximal (50%)]. Isometric maximal voluntary contraction (MVC), and electrically induced knee extension torque were measured at baseline and at regular intervals for 60 min after exercise. RESULTS: Muscle contraction modalities involving 5 s MVC were the most effective for PAP, whereas the lowest PAP effectiveness was found after the 12 × 5-MVC modality. After all of the 5-15 s MVC and 6 × 5-MVC protocols, the potentiation of the twitch rate was significantly higher than that recorded after continuous 30-60 s protocols (P < 0.001). Tetanic maximal torque (100 Hz) potentiation occurred 5 min after 15-30 s repetitive MVC modalities and after modality involving 15 electrical stimuli (P < 0.05). CONCLUSIONS: The findings demonstrate that post-activation potentiation was most effective after brief duration continuous and repetitive MVC protocols. To understand the resultant warm-up of motor performance, it is necessary to recognize the coexistence of muscle PAP, tetanic maximal force potentiation, rapid recovery of metabolic muscle, and central muscle activation processes, as well as prolonged LFF and prolonged PCD.

Adjustments in the force-frequency relationship during passive and exercise-induced hyperthermia.

INTRODUCTION: We examined the extent to which fatiguing cycling exercise in the heat influences contractile function in modulating the force-frequency relationship. METHODS: Before (∽37.0 °C) and after (∽38.5 °C) exercise (ExH) and passive (PaH) hyperthermia, an 8-s train of stimulation at 10, 20, 50, and 100 Hz (2 s per frequency) and a potentiated twitch were evoked on the relaxed knee extensors using percutaneous stimulation. RESULTS: ExH and PaH produced a decrease in the 20:50 Hz force ratio, indicative of low-frequency fatigue (P < 0.01). This adjustment was more pronounced after ExH than PaH (P < 0.01). A rightward displacement in the force-frequency relationship occurred after ExH and PaH (P < 0.05) and was exacerbated by ExH (P < 0.05). Peak twitch force also decreased after ExH (P < 0.05). CONCLUSIONS: ExH reduces force summation due to development of skeletal muscle fatigue, exacerbating the shift in force-frequency to the right relative to PaH.

Fatigability of the thenar muscles using electrical nerve stimulation with fixed stimuli count, while varying the frequency and duty cycle.

Our aim was to compare three electrical stimulation protocols (P20, P30 and P40), with the same number of stimuli, but different stimulation frequencies (20, 30 and 40 Hz, respectively) and duty cycles [1.2:1.2 s (continuous), 0.8:1.2 s (intermittent) and 0.6:1.2 s (intermittent), respectively). Twitch force and the peak-to-peak M-wave amplitude of the thenar muscles were measured before, during and after each protocol at 1-40 Hz in random order. Twelve healthy adults (23-41 years old) were examined for each protocol in random order and in separate sessions. P20 elicited the highest mean force, and P40 the lowest decrease in percent force at the end of the protocol. Force evoked at 1 and 10 Hz decreased less after P40, compared with P20 and P30. The M-wave amplitude was significantly reduced throughout all protocols, with the largest decrease observed during P30. Although an increase in frequency typically induced earlier and greater decrement in force, this was compensated or even reversed by increasing the interval between each stimulation train, while keeping the number of pulses per stimulation cycle constant. The lesser decrease in M-wave amplitude during P40 compared with P20 indicates that longer between-train intervals may help maintaining the integrity of neuromuscular propagation.

Maximal results with minimal stimuli: the fewest high-frequency pulses needed to measure or model prolonged low-frequency force depression in the dorsiflexors.

Quantifying prolonged low-frequency force depression (PLFFD) with the gold-standard 1-s trains presents challenges, so paired pulses have been used. Owing to greater impairment of high-frequency doublet than tetanic torque, paired pulses underestimate PLFFD. This study aimed to approximate the minimum number of high-frequency pulses needed to avoid such underestimation and assess the feasibility of modeling PLFFD from a limited number of experimental pulses. In 13 participants, a 1-s 10-Hz train and 100-Hz trains with 2, 4, 7, 12, 15, 25, 50, or 100 pulses were evoked before and after (15 min, 2, 4, and 7 days) eccentric exercise of the dorsiflexors. With ≤12 pulses, impairment of 100-Hz torque was greater than the 1-s train (P ≤ 0.05; e.g., 12 vs. 100 pulses at 4 days: 97.8 ± 8.5% vs. 100.5 ± 8.2% baseline). Consequently, with ≤12 pulses, PLFFD was underestimated compared with the gold-standard measure (P ≤ 0.05; e.g., 12 vs. 100 pulse 10:100-Hz torque ratio at 4 days: 86.8 ± 12.8% vs. 84.6 ± 13.5% baseline). Modeling reproduced 10:100-Hz ratios (PLFFD) with 95% limits of agreement of -13.6% to 16.7% of experimental values with ≥12 pulses. Our results indicate that a minimum of 13-25 pulses of 100 Hz are needed to accurately quantify PLFFD in the dorsiflexors. Although this may not be the minimum range for other muscles, a similar relationship with pulse number likely exists. Modeling may eventually provide an option to estimate PLFFD from experimental trains with relatively few pulses; however, further development is imperative to reduce variability.NEW & NOTEWORTHY Ideally, prolonged low-frequency force depression (PLFFD) is measured with 1-s trains of supramaximal stimuli; however, this induces considerable discomfort. We tested briefer trains to approximate the minimum number of high-frequency pulses needed to accurately determine PLFFD and the feasibility of modeling 1-s tetani with relatively few pulses. After eccentric exercise, 13-25 high-frequency pulses were needed to accurately measure PLFFD. Modeling reproduced mean experimental values but had considerable variability.

Effects of whole-body vibrations on neuromuscular fatigue: a study with sets of different durations.

BACKGROUND: Whole body vibrations have been used as an exercise modality or as a tool to study neuromuscular integration. There is increasing evidence that longer WBV exposures (up to 10 minutes) induce an acute impairment in neuromuscular function. However, the magnitude and origin of WBV induced fatigue is poorly understood. PURPOSE: The study aimed to investigate the magnitude and origin of neuromuscular fatigue induced by half-squat long-exposure whole-body vibration intervention (WBV) with sets of different duration and compare it to non-vibration (SHAM) conditions. METHODS: Ten young, recreationally trained adults participated in six fatiguing trials, each consisting of maintaining a squatting position for several sets of the duration of 30, 60 or 180 seconds. The static squatting was superimposed with vibrations (WBV30, WBV60, WBV180) or without vibrations (SHAM30, SHAM60, SHAM180) for a total exercise exposure of 9-minutes in each trial. Maximum voluntary contraction (MVC), level of voluntary activation (%VA), low- (T20) and high-frequency (T100) doublets, low-to-high-frequency fatigue ratio (T20/100) and single twitch peak torque (TWPT) were assessed before, immediately after, then 15 and 30 minutes after each fatiguing protocol. RESULT: Inferential statistics using RM ANOVA and post hoc tests revealed statistically significant declines from baseline values in MVC, T20, T100, T20/100 and TWPT in all trials, but not in %VA. No significant differences were found between WBV and SHAM conditions. CONCLUSION: Our findings suggest that the origin of fatigue induced by WBV is not significantly different compared to control conditions without vibrations. The lack of significant differences in %VA and the significant decline in other assessed parameters suggest that fatiguing protocols used in this study induced peripheral fatigue of a similar magnitude in all trials.

Prolonged low-frequency force depression is underestimated when assessed with doublets compared with tetani in the dorsiflexors.

Prolonged low-frequency force depression (PLFFD) after damaging eccentric exercise may last for several days. Historically, PLFFD has been calculated from the tetanic force responses to trains of supramaximal stimuli. More recently, for methodological reasons, stimulation has been reduced to two pulses. However, it is unknown whether doublet responses provide a valid measure of PLFFD in the days after eccentric exercise. In 12 participants, doublets and tetani were elicited at 10 and 100 Hz before and after (2, 3, 5 min, 48 and 96 h) 200 eccentric maximal voluntary contractions of the dorsiflexors. Doublet and tetanic torque responses at 10 Hz were similarly depressed throughout recovery (P > 0.05; e.g., 2 min: 58.9 ± 12.8% vs. 57.1 ± 14.5% baseline; 96 h: 85.6 ± 11.04% vs. 85.1 ± 10.8% baseline). At 100 Hz, doublet torque was impaired more than tetanic torque at all time points (P < 0.05; e.g., 2 min: 70.5 ± 14.2% vs. 88.1 ± 11.7% baseline; 96 h: 83.0 ± 14.2% vs. 98.7 ± 9.5% baseline). As a result, the postfatigue reduction of the 10 Hz-to-100 Hz ratio (PLFFD) was markedly greater for tetani than for doublets (P < 0.05; e.g., 2 min: 64.3 ± 15.1% vs. 83.0 ± 5.8% baseline). In addition, the doublet ratio recovered by 48 h (99.2 ± 5.0% baseline), whereas the tetanic ratio was still impaired at 96 h (88.2 ± 9.7% baseline). Our results indicate that doublets are not a valid measure of PLFFD in the minutes and days after eccentric exercise. If study design favors the use of paired stimuli, it should be acknowledged that the true magnitude and duration of PLFFD are likely underestimated. NEW & NOTEWORTHY Prolonged low-frequency force depression (PLFFD) will result from damaging exercise and may last for several days. After 200 eccentric maximal dorsiflexor contractions, we compared the gold-standard measure of PLFFD (calculated using trains of supramaximal stimulation) to the value obtained from an alternative technique that is becoming increasingly common (paired supramaximal stimuli). Doublets underestimated the magnitude and duration of PLFFD compared with tetani, so caution must be used when reporting PLFFD derived from paired stimuli.

Concentric strength training at optimal or short muscle length improves strength equally but does not reduce fatigability of hamstring muscles.

The purpose of this study was to compare the effect of a 6-week period of knee flexion strength training at either optimal or short muscle length, on length-specific muscle strength and fatigue. Twelve healthy volunteers performed dynamic (isokinetic concentric) training with one leg at short and the contralateral leg at optimal muscle length for 6 weeks. Knee flexor muscle strength was assessed before and after training, comprising maximal voluntary isometric and dynamic contractions at short, intermediate and near optimal muscle length and electrically evoked, contractions at near optimal length only. Fatigability was tested by performing 60 maximal concentric contractions at either short or optimal muscle length. Isometric torque at all muscle lengths improved equally by training at short and optimal muscle length, for example, tested at short 18 (17) versus 21 (17) % (CI) and at optimal 14 (8) versus 17 (16) % muscle length, respectively. Likewise, equal improvements were observed for dynamic contractions in both groups. Prior to training, fatigue induced at optimal muscle length tended to be more pronounced than at short muscle length (fatigue-indexes -41 (6) vs. -34 (7) %, respectively, P = 0.05). However, training at either length did not reduce fatigability. Training with maximal concentric contractions at either short or optimal muscle length for 6 weeks improved isometric and dynamic muscle strength in the entire range of motion without inducing any discernible length-specific adaptations. However, strength training at restricted muscle length did not reduce relative fatigue when induced at either short or optimal muscle length.

Force-velocity relationship during isometric and isotonic fatiguing contractions.

Fatiguing contractions change the force-velocity relationship, but assessment of this relationship in fatigue has usually been obtained after isometric contractions. We studied fatigue caused by isometric or isotonic contractions, by assessment of the force-velocity relationship while the contractions maintaining fatigue were continued. This approach allowed determination of the force-velocity relationship during a steady condition of fatigue. We used the in situ rat medial gastrocnemius muscle, a physiologically relevant preparation. Intermittent (1/s) stimulation at 170 Hz for 100 ms resulted in decreased isometric force to ~35% of initial or decreased peak velocity of shortening in dynamic contractions to ~45% of initial. Dynamic contractions resulted in a transient initial increase in velocity, followed by a rapid decline until a reasonably steady level was maintained. Data were fit to the classic Hill equation for determination of the force-velocity relationship. Isometric and dynamic contractions resulted in similar decreases in maximal isometric force and peak power. Only Vmax was different between the types of contraction ( P < 0.005) with greater decrease in Vmax during isotonic contractions to 171.7 ± 7.3 mm/s than during isometric contractions to 208.8 mm/s. Curvature indicated by a/Po (constants from fit to Hill equation) changed from 0.45 ± 0.04 to 0.71 ± 0.11 during isometric contractions and from 0.51 ± 0.04 to 0.85 ± 0.18 during isotonic contractions. Recovery was incomplete 45 min after stopping the intermittent contractions. At this time, recovery of low-frequency isometric force was substantially less after isometric contractions, implicating force during intermittent contractions as a determining factor with this measure of fatigue. NEW & NOTEWORTHY The force-velocity relationship was captured while fatigue was maintained at a constant level during isometric and dynamic contractions. The curvature of the force-velocity relationship was less curved during fatigue than prefatigued, but within 45 min this recovered. Low-frequency fatigue persisted with greater depression of low-frequency force after isometric contractions, possibly because of higher force contractions during intermittent contractions.

The Time-Course of Voluntary and Electrically Evoked Muscle Performance During and After Stretch-Shortening Exercise is Different.

The aim of the study was to establish the dynamics of maximal voluntary contraction force (MVCF), height of drop jump (DJ) and electrically evoked quadriceps muscle force at different stimulation frequencies during and after 100 DJs (stretch-shortening exercise, SSE). Healthy untrained men (n = 11; age = 21.8 ± 1.7 years) participated in the study. DJs were performed with 30 s intervals between jumps from the height of 0.5 m with counter-movement to 90 degrees angle in the knee and immediate maximal rebound. The force of the quadriceps muscle, evoked by electrical stimulation at 1 Hz (Pt), 20 Hz (P20) and 100 Hz (P100) frequencies (electrically evoked performance, EEP), MVCF and height of DJ (voluntary evoked performance, VEP) were established during SSE (after 10, 50, 100 DJ) as well as at 1, 4, 8, 24, 48 and 72 h after SSE. Time-course of P20 and P100 during and after SSE was time (ANOVA: p < 0.001) and frequency dependent (ANOVA: p < 0.001) The Pt, P20 and P100 decreased significantly (p < 0.01) more than MVCF and H of DJ during SSE. At the beginning of SSE (during 1-10 DJs) P20 and P100 decreased significantly (p < 0.001) more than during 11-50 and 51-100 DJs. There was a significant (p < 0.05) increase in Pt, P20 and P100 from 8 h to 48 h, whereas height of DJ and MVCF significantly decreased at that time. In conclusion, the differences in time course of VEP and EEP are most evident at beginning of SSE, where VEP does not change as EEP decreases, and within 8-48 hours after SSE, where VEP decreases as EEP increases. Key pointsThere was no change in voluntary muscle performance while electrically evoked performance decreased significantly during first 10 drop jumps.There was a significant increase in electrically evoked muscle performance from 8 h to 48 h after 100 drop jumps, whereas voluntary contraction force, decreased significantly.The secondary decrease in the height of drop jump as well as in maximal voluntary contraction force correlated significantly with muscle soreness within 24-48 h after exercise.

Effects of age and muscle action type on acute strength and power recovery following fatigue of the leg flexors.

Short-term strength and power recovery patterns following fatigue have received little research attention, particularly as they pertain to age-specific responses, and the leg flexors (i.e., hamstrings) muscle group. Thus, research is warranted addressing these issues because both age-related alterations in the neuromuscular system and mode of muscle action (e.g., eccentric, concentric, isometric) may differentially influence recovery responses from fatigue. The aim of this study was to investigate the strength and power recovery responses for eccentric, concentric, and isometric muscle actions of the leg flexors in young and older men following an isometric, intermittent fatigue-inducing protocol. Nineteen young (age = 25 ± 3 years) and nineteen older (71 ± 4) men performed maximal voluntary contractions (MVCs) for eccentric, concentric, and isometric muscle actions followed by a fatigue protocol of intermittent (0.6 duty cycle) isometric contractions of the leg flexors at 60% of isometric MVC. MVCs of each muscle action were performed at 0, 7, 15, and 30 min following fatigue. Peak torque (PT) and mean power values were calculated from the MVCs and the eccentric/concentric ratio (ECR) was derived. For PT and mean power, young men showed incomplete recovery at all time phases, whereas the older men had recovered by 7 min. Eccentric and isometric muscle actions showed incomplete recovery at all time phases, but concentric recovered by 7 min, independent of age. The ECR was depressed for up to 30 min following fatigue. More rapid and pronounced recovery in older men and concentric contractions may be related to physiological differences specific to aging and muscle action motor unit patterns. Individuals and clinicians may use these time course responses as a guide for recovery following activity-induced fatigue.

Direct and indirect measurement of neuromuscular fatigue in Canadian football players.

This study assessed the effects of a fatiguing game simulation (G-Sim) on the balance of collegiate Canadian football players. The purpose of the study was to evaluate postural control as a potential tool for monitoring neuromuscular fatigue (NMF) in collision-based team sports. Fifteen male Canadian football players were recruited (mean±SD: age 21.8±1.6 years, weight 97.6±14.7 kg). Indirect NMF measures (postural sway and countermovement jump (CMJ)) were performed 24 h before (TBase), immediately before (TPre) and after (TPost), and 24 h (T24) and 48 h after (T48) a Canadian football G-Sim. Peak isometric knee extensor torque of a maximal voluntary contraction (MVC) and electrically evoked tetani at 20 Hz (P20) and 80 Hz (P80) were also recorded as direct NMF measures at TBase, TPre, TPost, and T48. At TPost, we observed significant declines in MVC, P20, and the MVC/P80 ratio (-15.3%, -15.7%, and -12.1%, respectively; n=12) along with reductions in CMJ takeoff velocity and peak power (-6.9% and -6.5%, respectively; n=12) and larger area of the center of pressure trajectory (95.2%; n=10) during a 60-s postural sway task. All variables were no longer different than baseline by T48. Acute neuromuscular impairment in this cohort is likely attributable to alterations in excitation-contraction coupling due to structural damage and central activation failure. Congruency between the direct and indirect measures of NMF suggests monitoring postural sway has the potential to identify both neuromuscular and somatosensory alterations induced by acute game-induced fatigue in collision-based team sports players.