Prolonged Low-frequency Force Depression Research Articles

Prolonged Low-frequency Force Depression Of Intact Single Mice Myofibers Is Partly Recovered With Post-Tetanic Potentiation

Prolonged Low-frequency Force Depression Of Intact Single Mice Myofibers Is Partly Recovered With Post-Tetanic Potentiation

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

AbstractThe 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. imageKey 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.

Eccentric exercise-induced muscle weakness amplifies the history dependence of force.

Following active lengthening or shortening contractions, isometric steady-state torque is increased (residual force enhancement; rFE) or decreased (residual force depression; rFD), respectively, compared to fixed-end isometric contractions at the same muscle length and level of activation. Though the mechanisms underlying this history dependence of force have been investigated extensively, little is known about the influence of exercise-induced muscle weakness on rFE and rFD. Assess rFE and rFD in the dorsiflexors at 20%, 60%, and 100% maximal voluntary torque (MVC) and activation matching, and electrically stimulated at 20% MVC, prior to, 1h following, and 24h following 150 maximal eccentric dorsiflexion contractions. Twenty-six participants (13 male, 24.7 ± 2.0y; 13 female, 22.5 ± 3.6y) were seated in a dynamometer with their right hip and knee angle set to 110° and 140°, respectively, with an ankle excursion set between 0° and 40° plantar flexion (PF). MVC torque, peak twitch torque, and prolonged low frequency force depression were used to assess eccentric exercise-induced neuromuscular impairments. History-dependent contractions consisted of a 1s isometric (40°PF or 0°PF) phase, a 1s shortening or lengthening phase (40°/s), and an 8s isometric (0°PF or 40°PF) phase. Following eccentric exercise; MVC torque was decreased, prolonged low frequency force depression was present, and both rFE and rFD increased for all maximal and submaximal conditions. The history dependence of force during voluntary torque and activation matching, and electrically stimulated contractions is amplified following eccentric exercise. It appears that a weakened neuromuscular system amplifies the magnitude of the history-dependence of force.

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

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. Young (18-31years) 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 10Hz and 50Hz via tibial nerve stimulation. Isotonic and isometric PLFFD was assessed as the ratio of 10 to 50Hz for power and torque, respectively. Recovery was assessed immediately, and at 2.5, 5, 10, 20, and 30min after task termination. Relative to baseline, 10:50Hz ratio assessed using isotonic power was reduced more than isometric torque (30min 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 10:50Hz ratio was due to greater impairments in 10Hz power compared to 10Hz isometric torque (30min 38 ± 20 vs. 21 ± 11% reduction, p < 0.001). The similar trajectories of 10:50Hz ratios throughout recovery indicate that PLFFD can be adequately characterized using either isometric torque or isotonic power.

Frequency-dependent coexistence of muscle fatigue and potentiation assessed by concentric isotonic contractions in human plantar flexors.

The purpose was to investigate whether postactivation potentiation (PAP) mitigates power (i.e., torque × angular velocity) loss during dynamic fatiguing contractions and subsequent recovery by enhancing either muscle torque or angular velocity in human plantar flexors. In 12 participants, electrically stimulated (1, 10, and 50 Hz) dynamic contractions were done during a voluntary isotonic fatiguing protocol until a 75% loss in voluntary peak power, and throughout 30 min of recovery. At the initial portion of fatigue (20% decrease), power responses of evoked low frequencies (1 and 10 Hz) were enhanced due to PAP (156% and 137%, respectively, P < 0.001), whereas voluntary maximal efforts were depressed due to fatiguing mechanisms. Following the fatiguing task, prolonged low-frequency force depression (PLFFD) was evident by reduced 10:50 Hz peak power ratios (21%-24%) from 3 min onward during the 30-min recovery (P < 0.005). Inducing PAP with maximal voluntary dynamic contractions during PLFFD enhanced the peak power responses of low frequencies (1 and 10 Hz) by 128%-160%, P < 0.01. This PAP response mitigated the effects of PLFFD as the 1:50 (P < 0.05) and 10:50 (P > 0.4) Hz peak power ratios were greater or not different from the prefatigue (baseline) values. In addition, PAP enhanced peak torque more than peak angular velocity during both baseline and fatigue measurements (P < 0.03). These results indicate that PAP can ameliorate PLFFD acutely when evaluated during concentric isotonic contractions and that peak torque is enhanced to a greater degree compared with peak angular velocity at baseline and in a fatigued state.NEW & NOTEWORTHY Postactivation potentiation (PAP) enhanced stimulated low frequencies (1 and 10 Hz) during muscle fatigue development when assessed with power (torque × angular velocity) in a voluntary isotonic fatiguing task. Following the task during 30 min of recovery, prolonged low-frequency force depression (PLFFD) was evident, and inducing PAP with brief maximal contractions during this state ameliorated the effects of PLFFD. PAP enhanced peak torque more than peak angular velocity during both baseline and fatiguing conditions.

Prolonged Low Frequency Force Depression Assessed Using Isometric and Isotonic Contractions Following a Dynamic Fatiguing Task

BackgroundNeuromuscular fatigue induced by contractile activity can lead to prolonged depression of muscle forces evoked through low frequency electrical stimulations, termed prolonged low‐frequency force depression (PLFFD). PLFFD is assessed as the ratio of forces evoked from low (e.g., 10 Hz) and high (e.g., 50 Hz) frequency stimulations, which has been examined most often using isometric contractions. However, fatigue is task specific, and it is unknown if PLFFD induced via dynamic contractions is accurately characterized using isometric torque, as it is not influenced by the velocity or power components of dynamic movements.Objectives and HypothesesOur primary objective was to compare isometric and dynamic PLFFD following a concentric fatiguing task in healthy young adults. We hypothesized that both isometric and dynamic PLFFD will be impaired following the task but dynamic PLFFD will be reduced to a greater extent.MethodsYoung (18‐35 years) males (n=5) and females (n=1) performed a prone, knee straight (180°) concentric plantar flexion fatiguing task (Humac Norm). The task involved maximal velocity isotonic‐like contractions at a resistance of 15% maximal voluntary contraction (MVC) through a 30° ankle range of motion (90° to 120° plantar flexion) until a 75% reduction in peak power. To assess PLFFD, contractions were electrically evoked at 10 Hz and 50 Hz via tibial nerve stimulation using a current that elicited maximal isometric 50 Hz torque. Isometric (joint ankle 90°) and isotonic (5% MVC load) PLFFD were assessed as the ratio of 10 to 50 Hz for peak torque and peak power, respectively. Recovery of isotonic and isometric PLFFD (Figure 1), MVC, and voluntary peak power (5% MVC load) was evaluated at 0‐, 2.5‐, 5‐, 10‐, 20‐, and 30‐minutes post task failure.ResultsOn average (±SD), participants performed 151 ±47 contractions, which resulted in an ~74% reduction in peak power. Isometric (~6%) and isotonic (~8%) 10:50 Hz ratios were similarly reduced following task failure (Figure 2). However, at 2.5‐ and 5‐minutes, isotonic PLFFD was further reduced (~15% and ~21%, respectively), but isometric PLFFD remained relatively unchanged (~0% and ~5%, respectively). By 10 minutes of recovery, isotonic and isometric PLFFD were reduced compared to baseline, which remained until the 30 minute time point (~31% and ~20%, respectively). Voluntary peak power was reduced more than isometric MVC at task failure (~44% vs. ~29%), but both recovered similarly by 30 minutes (~8% vs. ~13%).ConclusionsThis preliminary analysis observed that PLFFD assessed using isometric and isotonic contractions are similarly reduced following task failure; however, differences in the degree of depression may exist throughout 30 minutes of recovery, particularly within the early (&lt;10 minutes) phase. These results demonstrate that PLFFD can be assessed using isotonic contractions, which may have implications for better understanding fatigue responses during dynamic tasks.

Dynamic Muscle Fatigue and Potentiation in Human Plantar Flexors

Activation history can enhance or impair subsequent contractile responses. Following a brief high‐intensity voluntary contraction, the contractile response for the same submaximal input stimulus will be enhanced (post‐activation potentiation; PAP). However, prolonged voluntary activation can impair contractile responses due to neuromuscular fatigue. Because both effects are dependent on immediate prior activation, a co‐existence between these two competing processes is observed. Specifically, submaximal frequencies are enhanced (PAP) while maximal frequencies are simultaneously depressed (fatigue). Due to preferential enhancement of low frequency activation, PAP can mitigate the effects of prolonged low‐frequency force depression (PLFFD), ‐ a long‐lasting reduction of force evoked at low‐frequencies following fatiguing exercise. Usually, this relationship has been evaluated using isometric contractions. The purpose was to assess the relationship between PAP, muscle fatigue and recovery during and following an isotonic (dynamic) fatiguing protocol through which power (force and velocity) are assessed. It is hypothesized that PAP will attenuate PLFFD during the fatiguing protocol and throughout 30 minutes of recovery.The plantar flexors of 5 healthy young adult males (n=4) and females (n=1) to date have been tested while lying prone in a Cybex dynamometer. The ankle was at 90° and knee joint at 180° (straight leg). Inelastic large Velcro straps firmly secured the hips, shoulders and thighs to the dynamometer. Participants performed isotonic plantar flexion contractions over a 20° range of motion (90° to 110°) of the ankle joint with a load of 20% of maximal voluntary isometric contraction (MVC) until peak power was reduced by 20%, 50% and 75% of baseline values. At these intervals of power loss, electrical stimulation of the tibial nerve in the popliteal fossa was done to record unloaded power responses at 10 Hz and 50 Hz. Following task failure (75% power loss), recovery was assessed immediately and at 3, 5, 10, 20 and 30 minutes. For these, voluntary contractions (10 concentric at 20% MVC) were done to induce PAP. Evoked unloaded dynamic measurements were done prior to and following the 10 concentric contractions to assess any effect of PAP.PLFFD is a preferential loss of low frequency force assessed by comparing a stimulated force ratio between responses at 10 Hz to 50 Hz. During dynamic contractions we used a peak power 10:50 Hz ratio to incorporate the factor of velocity. During fatigue the peak power 10:50 Hz ratio was enhanced by ~35% at 20% power loss, by ~15% at 50% power loss, and minimally enhanced (~9%) at task failure (75% power loss). During recovery this ratio was enhanced follow the potentiating contractions compared to non‐potentiated; task failure (~3%), 3 min (~26%), 5 min (~45%), 10 min (~20%), 20 min (~25%) and 30 min (~38%).These results indicate that during dynamic muscle fatigue and recovery, PAP may mitigate PLFFD to varying degrees.

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.

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 20s to 5min. 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. Young men (n = 12) randomly performed 50 DJs with either 20-s (DJ-20s) or 5-min (DJ-5min) rest between DJs. Voluntary, 20Hz and 100Hz electrically stimulated isometric knee extension torques and muscle soreness were monitored before and for 7days 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 1h after exercise. A marked PLFFD was observed with both protocols and the extent of 20Hz torque depression was smaller immediately and 1day after DJ-5min than after DJ-20s (p < 0.05), whereas the MVC and 100Hz torques were similarly decreased with the two protocols. Markedly larger differences between the two protocols were observed for the muscle soreness score, which 1-4days after exercise was about two times larger with DJ-20s than with DJ-5min (p < 0.01). The larger protective effect of the longer DJ interval against DOMS than against PLFFD indicates that their underlying mechanisms involve different structural elements.

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.

Recovery from Fatigue after Cycling Time Trials in Elite Endurance Athletes.

We determined the recovery from neuromuscular fatigue in six professional (PRO) and seven moderately trained (MOD) cyclists after repeated cycling time trials of various intensities/durations. Participants performed two 1-min (1minTT) or two 10-min (10minTT) self-paced cycling time trials with 5 min of recovery in between. Central and peripheral fatigue were quantified via preexercise to postexercise (15-s through 15-min recovery) changes in voluntary activation (VA) and potentiated twitch force. VA was measured using the interpolated twitch technique, and potentiated twitch force was evoked by single (QTsingle) and paired (10-Hz (QT10) and 100-Hz (QT100)) electrical stimulations of the femoral nerve. Mean power output was 32%-72% higher during all the time trials and decreased less (-10% vs -13%) from the first to second time trial in PRO compared with MOD (P < 0.05). Conversely, exercise-induced reduction in QTsingle and QT10/QT100 was significantly lower in PRO after every time trial (P < 0.05). Recovery from fatigue from 15 s to 2 min for QTsingle and QT10/QT100 was slower in PRO after every time trial (P < 0.05). In both groups, the reduction in QTsingle was lower after the 10minTTs compared with 1minTTs (P < 0.05). Conversely, VA decreased more after the 10minTTs compared with 1minTTs (P < 0.05). Our findings showed that excitation-contraction coupling was preserved after exercise in PRO compared with MOD. This likely contributed to the improved performance during repeated cycling time trials of various intensity/duration in PRO, despite a slower rate of recovery in its early phase. Finally, the time course of recovery from neuromuscular fatigue in PRO was dependent on the effects of prolonged low-frequency force depression.

Fast skeletal muscle troponin activator CK-2066260 mitigates skeletal muscle weakness independently of the underlying cause.

BackgroundMuscle weakness is a common symptom in numerous diseases and a regularly occurring problem associated with ageing. Prolonged low‐frequency force depression (PLFFD) is a form of exercise‐induced skeletal muscle weakness observed after exercise. Three different intramuscular mechanisms underlying PLFFD have been identified: decreased sarcoplasmic reticulum Ca2+ release, decreased myofibrillar Ca2+ sensitivity, and myofibrillar dysfunction. We here used these three forms of PLFFD as models to study the effectiveness of a fast skeletal muscle troponin activator, CK‐2066260, to mitigate muscle weakness.MethodsExperiments were performed on intact single muscle fibres or fibre bundles from mouse flexor digitorum brevis, which were stimulated with electrical current pulses, while force and the free cytosolic [Ca2+] ([Ca2+]i) were measured. PLFFD was induced by three different stimulation protocols: (i) repeated isometric contractions at low intensity (350 ms tetani given every 5 s for 100 contractions); (ii) repeated isometric contractions at high intensity (250 ms tetani given every 0.5 s for 300 contractions); and (iii) repeated eccentric contractions (350 ms tetani with 20% length increase given every 20 s for 10 contractions). The extent and cause of PLFFD were assessed by comparing the force–[Ca2+]i relationship at low (30 Hz) and high (120 Hz) stimulation frequencies before (control) and 30 min after induction of PLFFD, and after an additional 5 min of rest in the presence of CK‐2066260 (10 μM).ResultsProlonged low‐frequency force depression following low‐intensity and high‐intensity fatiguing contractions was predominantly due to decreased sarcoplasmic reticulum Ca2+ release and decreased myofibrillar Ca2+ sensitivity, respectively. CK‐2066260 exposure resulted in marked increases in 30 Hz force from 52 ± 16% to 151 ± 13% and from 6 ± 4% to 98 ± 40% of controls with low‐intensity and high‐intensity contractions, respectively. Following repeated eccentric contractions, PLFFD was mainly due to myofibrillar dysfunction, and it was not fully reversed by CK‐2066260 with 30 Hz force increasing from 48 ± 8% to 76 ± 6% of the control.ConclusionsThe fast skeletal muscle troponin activator CK‐2066260 effectively mitigates muscle weakness, especially when it is caused by impaired activation of the myofibrillar contractile machinery due to either decreased sarcoplasmic reticulum Ca2+ release or reduced myofibrillar Ca2+ sensitivity.

Females and males do not differ for fatigability, muscle damage and magnitude of the repeated bout effect following maximal eccentric contractions.

Unaccustomed eccentric (ECC) exercise induces muscle fatigue as well as damage and initiates a protective response to minimize impairments from a subsequent bout (i.e., repeated bout effect; RBE). It is uncertain if the sexes differ for neuromuscular responses to ECC exercise and the ensuing RBE. Twenty-six young adults (13 females) performed 2 bouts (4 weeks apart) of 200 ECC maximal voluntary contractions (MVCs) of the dorsiflexors. Isometric (ISO) MVC torque and the ratio of ISO torque in response to low- versus high-frequency stimulation (10:100 Hz) were compared before and after (2-10 min and 2, 4, and 7 days) exercise. The decline in ECC and ISO MVC torque and the 10:100 Hz ratio following bout 1 did not differ between sexes (P > 0.05), with reductions from baseline of 31.5% ± 12.3%, 24.1% ± 15.4%, and 51.3% ± 12.2%, respectively. After bout 2, the 10:100 Hz ratio declined less (45.0% ± 12.4% from baseline) and ISO MVC torque recovered sooner compared with bout 1 but no differences between sexes were evident for the magnitude of the RBE (P > 0.05). These data suggest that fatigability with ECC exercise does not differ for the sexes and adaptations that mitigate impairments to calcium handling are independent of sex. Novelty: One bout of 200 maximal eccentric dorsiflexor contractions caused equivalent muscle fatigue and damage for females and males. The repeated bout effect observed after a second bout 4 weeks later also had no sex-related differences. Prolonged low-frequency force depression is promoted as an indirect measure of muscle damage in humans.

Carbohydrates do not accelerate force recovery after glycogen-depleting followed by high-intensity exercise in humans.

Prolonged low-frequency force depression (PLFFD) induced by fatiguing exercise is characterized by a persistent depression in submaximal contractile force during the recovery period. Muscle glycogen depletion is known to limit physical performance during prolonged low- and moderate-intensity exercise, and accelerating glycogen resynthesis with post-exercise carbohydrate intake can facilitate recovery and improve repeated bout exercise performance. Short-term, high-intensity exercise, however, can cause PLFFD without any marked decrease in glycogen. Here, we studied whether recovery from PLFFD was accelerated by carbohydrate ingestion after 60minutes of moderate-intensity glycogen-depleting cycling exercise followed by six 30-seconds all-out cycling sprints. We used a randomized crossover study design where nine recreationally active males drank a beverage containing either carbohydrate or placebo after exercise. Blood glucose and muscle glycogen concentrations were determined at baseline, immediately post-exercise, and during the 3-hours recovery period. Transcutaneous electrical stimulation of the quadriceps muscle was performed to determine the extent of PLFFD by eliciting low-frequency (20Hz) and high-frequency (100Hz) stimulations. Muscle glycogen was severely depleted after exercise, with a significantly higher rate of muscle glycogen resynthesis during the 3-hours recovery period in the carbohydrate than in the placebo trials (13.7 and 5.4mmol glucosyl units/kg wet weight/h, respectively). Torque at 20Hz was significantly more depressed than 100Hz torque during the recovery period in both conditions, and the extent of PLFFD (20/100Hz ratio) was not different between the two trials. In conclusion, carbohydrate supplementation enhances glycogen resynthesis after glycogen-depleting exercise but does not improve force recovery when the exercise also involves all-out cycling sprints.

Mechanisms of prolonged low-frequency force depression: in vivo studies get us closer to the truth.

Mechanisms of prolonged low-frequency force depression : in-vivo studies get us closer to the truth

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.

Treatment with EUK-134 improves sarcoplasmic reticulum Ca2+ release but not myofibrillar Ca2+ sensitivity after fatiguing contraction of rat fast-twitch muscle.

Skeletal muscles undergoing vigorous activity can enter a state of prolonged low-frequency force depression (PLFFD). This study was conducted to examine whether antioxidant treatment is capable of accelerating the recovery from PLFFD, with a focus on the function of the sarcoplasmic reticulum (SR) and myofibril. One hour before fatiguing stimulation (FS) was administered, rats received an intraperitoneal injection of Eukarion (EUK-134), which mimics the activities of superoxide dismutase and catalase. Intact muscles of the hindlimbs were electrically stimulated via the sciatic nerve until the force was reduced to ~50% of the initial force (FS). Thirty minutes after cessation of FS, the superficial regions of gastrocnemius muscles were dissected and used for biochemical and skinned-fiber analyses. Whole muscle analyses revealed that antioxidant alleviated the FS-induced decrease in the reduced glutathione content. Skinned-fiber analyses showed that the antioxidant did not affect the FS-induced decrease in the ratio of force at 1 Hz to that at 50 Hz. However, the antioxidant partially inhibited the FS-mediated decrease in the ratio of depolarization-induced force to the maximum Ca2+-activated force. Furthermore, the antioxidant completely suppressed the FS-induced increase in myofibrillar Ca2+ sensitivity. These results suggest that antioxidant treatment is ineffective in facilitating the restoration of PLFFD, probably due to its negative effect on myofibrillar Ca2+ sensitivity, which supersedes its positive effect on SR Ca2+ release.

Thermal pretreatment facilitates recovery from prolonged low-frequency force depression in rat fast-twitch muscle.

The aim of this study was to examine whether thermal pretreatment can accelerate recovery from prolonged low‐frequency force depression. The hindlimbs of thermal treated (T‐treated) rats were immersed in water heated to 42.0°C for 20 min (thermal pretreatment). The thermal pretreatment was performed once a day for 5 days before fatiguing stimulation. Intact gastrocnemius muscles were electrically stimulated via the sciatic nerve until force was reduced to ~50% of the initial and dissected immediately [recovery 0 (REC0)] or 60 min [recovery 60 (REC60)] following the cessation of stimulation. Using skinned fiber prepared from the superficial region, the ratio of force at 1 Hz to that at 50 Hz (low‐to‐high force ratio), the ratio of depolarization (depol)‐induced force to maximum Ca2+‐activated force (depol/max Ca2+ force ratio), the steepness of force‐Ca2+ concentration curves, and myofibrillar Ca2+ sensitivity were measured. At REC0, the low‐to‐high force ratio and depol/max Ca2+ force ratio decreased in stimulated muscles from both non‐ and thermal‐treated rats. At REC60, these two parameters remained depressed in non‐treated rats, whereas they reverted to resting levels in T‐treated rats. Thermal pretreatment exerted no effect on myofibrillar Ca2+ sensitivity. The present results reveal that thermal pretreatment can facilitate recovery of submaximum force after vigorous contraction, which is mediated via a quick return of Ca2+ release from the sarcoplasmic reticulum to resting levels.

Prolonged Low‐frequency Force Depression is Underestimated When Assessed with Doublets Compared to Trains

Unaccustomed eccentric exercise induces muscle damage, which leads to an impairment of force or torque that can last for several days. A common indirect measure of muscle damage is a greater depression of the force response to low‐frequency (e.g., 10Hz) compared to high‐frequency (e.g., 100Hz) stimulation; i.e., a post‐exercise reduction in the 10:100Hz ratio. This phenomenon is termed, prolonged low‐frequency force depression (PLFFD). Historically, trains of stimuli (e.g., 1s) have been used to evoke tetanic responses for the ratio. More recently, some authors have used only paired stimuli to evoke doublets. However, it is unknown if the PLFFD indicated by doublet responses is equivalent to the value derived from the traditional tetanic responses. Hence, the purpose of this study was to compare the magnitude of PLFFD determined by doublet vs. tetanic responses. We hypothesized that the doublet ratio (DR) would indicate significantly less PLFFD than the train ratio (TR). Eight participants (four females) performed 200 eccentric maximal voluntary contractions (4 sets of 50 repetitions, 1s rest between repetitions and 1 min rest between sets) of the dorsiflexors to induce muscle damage. Eccentric contractions were performed at 60°/s, started at a neutral ankle position, and finished at 30° of plantar flexion. Before the fatiguing protocol, baseline neuromuscular function was measured by a sequence of isometric contractions evoked by supramaximal electrical stimulation of the fibular nerve (paired stimuli at 10 and 100Hz, and 1s trains at 10 and 100Hz, each separated by 1s rest). This neuromuscular function test was repeated three times prior to fatigue and 2 min, 3 min, 5 min, 10 min and 48h after the fatiguing protocol. Using the peak torque of each response, 10:100Hz ratios were calculated at all time points; post‐exercise values were expressed as a percentage of the respective baseline ratio. At baseline, the DR was greater than TR (0.81 ± 0.04 vs. 0.48 ± 0.09; P &lt; 0.01). PLFFD was observed by 2 min for TR (73 ± 12%) and 3 min for DR (83 ± 9%), continued to develop over the acute recovery phase (at 10 min, DR = 73 ± 9%; TR = 51 ± 11%), and recovered by 48h (DR = 98 ± 6%; TR = 91 ± 12%). The TR was reduced more than the DR at all acute recovery time points (P &lt; 0.05), but not 48h. With DR and TR revealing different levels of PLFFD following damaging eccentric contractions, our results indicate that these techniques should not be used interchangeably. Whenever possible, trains of stimuli should be used to indicate the true extent of PLFFD. In cases where the reproducibility of the tetanic force response may be problematic (e.g., femoral nerve stimulation) or a train is deemed prohibitively uncomfortable, it should be acknowledged that paired stimuli will underestimate PLFFD.Support or Funding InformationSupported by NSERC, CFI and BCKDFThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

A mitochondrial-targeted antioxidant improves myofilament Ca2+ sensitivity during prolonged low frequency force depression at low PO2.

Skeletal muscle contractile activity is associated with an enhanced reactive oxygen species (ROS) generation. At very low PO2, ROS generation by mitochondria can be elevated in intact cells. An elevated intracellular oxidant activity may affect muscle force development and recovery from fatigue. We treated intact single muscle fibres with a mitochondrial antioxidant and stimulated the fibres to contract at a low extracellular PO2 that is similar to the intracellular PO2 that is observed during moderate to intense exercise in vivo. The mitochondrial antioxidant prevented a sustained decrease in the myofibrillar Ca2+ sensitivity and improved muscle submaximal force development after fatigue at low extracellular PO2. Skeletal muscle can develop a prolonged low frequency-stimulation force depression (PLFFD) following fatigue-inducing contractions. Increased levels of reactive oxygen species (ROS) have been implicated in the development of PLFFD. During exercise the skeletal muscle intracellular PO2 decreases to relatively low levels, and can be further decreased when there is an impairment in O2 diffusion or availability, such as in certain chronic diseases and during exercise at high altitude. Since ROS generation by mitochondria is elevated at very low PO2 in cells, we tested the hypothesis that treatment of muscle fibres with a mitochondrial-targeted antioxidant at a very low, near hypoxic, PO2 can attenuate PLFFD. We treated intact single fibres from mice with the mitochondrial-specific antioxidant SS31, and measured force development and intracellular [Ca2+ ] 30min after fatigue at an extracellular PO2 of ∼5Torr. After 30min following the end of the fatiguing contractions, fibres treated with SS31 showed significantly less impairment in force development compared to untreated fibres at submaximal frequencies of stimulation. The cytosolic peak [Ca2+ ] transients (peak [Ca2+ ]c ) were equally decreased in both groups compared to pre-fatigue values. The combined force and peak [Ca2+ ]c data demonstrated that myofibrillar Ca2+ sensitivity was diminished in the untreated fibres 30min after fatigue compared to pre-fatigue values, but Ca2+ sensitivity was unaltered in the SS31 treated fibres. These results demonstrate that at a very low PO2, treatment of skeletal muscle fibres with a mitochondrial antioxidant prevents a decrease in the myofibrillar Ca2+ sensitivity, which alleviates the fatigue induced PLFFD.

Prolonged force depression after mechanically demanding contractions is largely independent of Ca2+ and reactive oxygen species.

Increased production of reactive oxygen/nitrogen species (ROS) and impaired cellular Ca2+ handling are implicated in the prolonged low-frequency force depression (PLFFD) observed in skeletal muscle after both metabolically and mechanically demanding exercise. Metabolically demanding high-intensity exercise can induce PLFFD accompanied by ROS-dependent fragmentation of the sarcoplasmic reticulum Ca2+ release channels, the ryanodine receptor 1s (RyR1s). We tested whether similar changes occur after mechanically demanding eccentric contractions. Human subjects performed 100 repeated drop jumps, which require eccentric knee extensor contractions upon landing. This exercise caused a major PLFFD, such that maximum voluntary and electrically evoked forces did not recover within 24 h. Drop jumps induced only minor signs of increased ROS, and RyR1 fragmentation was observed in only 3 of 7 elderly subjects. Also, isolated mouse muscle preparations exposed to drop-jump-mimicking eccentric contractions showed neither signs of increased ROS nor RyR1 fragmentation. Still, the free cytosolic [Ca2+] during tetanic contractions was decreased by ∼15% 1 h after contractions, which can explain the exaggerated force decrease at low-stimulation frequencies but not the major frequency-independent force depression. In conclusion, PLFFD caused by mechanically demanding eccentric contractions does not involve any major increase in ROS or RyR1 fragmentation.-Kamandulis, S., de Souza Leite, F., Hernandez, A., Katz, A., Brazaitis, M., Bruton, J. D., Venckunas, T., Masiulis, N., Mickeviciene, D., Eimantas, N., Subocius, A., Rassier, D. E., Skurvydas, A., Ivarsson, N., Westerblad, H. Prolonged force depression after mechanically demanding contractions is largely independent of Ca2+ and reactive oxygen species.