Piperine enhances contractile force in slow- and fast-twitch muscle.

Piperine has been shown to bind to myosin and shift the distribution of conformational states of myosin molecules from the super-relaxed state to the disordered relaxed state. However, little is known about the implications for muscle force production and potential underlying mechanisms. Muscle contractility experiments were performed using isolated muscles and single fibres from rats and mice. The dose-response effect of piperine on muscle force was assessed at several stimulation frequencies. The potentiation of muscle force was also tested in muscles fatigued by eccentric contractions. Potential mechanisms of force potentiation were assessed by measuring Ca2+ levels during stimulation in enzymatically dissociated muscle fibres, while myofibrillar Ca2+ sensitivity was assessed in chemically skinned muscle fibres. Piperine caused a dose-dependent increase in low-frequency force with no effect on high-frequency force in both slow- and fast-twitch muscle, with similar relative increases in twitch force, rate of force development and relaxation rate. The potentiating effect of piperine on low-frequency force was reversible, and piperine partially recovered low-frequency force in fatigued muscle. Piperine had no effect on myoplasmic free [Ca2+] levels in mouse muscle fibres, whereas piperine substantially augmented the force response to submaximal levels of [Ca2+] in rat MyHCII fibres and MyHCI fibres along with a minor increase in maximum Ca2+-activated force. Piperine enhances low-frequency force production in both fast- and slow-twitch muscle. The effects are reversible and can counteract muscle fatigue. The primary underlying mechanism appears to be an increase in Ca2+ sensitivity. KEY POINTS: Piperine is a plant alkaloid derived from black pepper. It is known to bind to skeletal muscle myosin and enhance resting ATP turnover but its effects on contractility are not well known. We showed for the first time a piperine-induced force potentiation that was pronounced during low-frequency electrical stimulation of isolated muscles. The effect of piperine was observed in both slow and fast muscle types, was reversible, and could counteract the force decrements observed after fatiguing muscle contractions. Piperine treatment caused an increase in myofibrillar Ca2+ sensitivity in chemically skinned muscle fibres, while we observed no effect on intracellular Ca2+ concentrations during electrical stimulation in enzymatically dissociated muscle fibres.

The effect of low-frequency fatigue on the torque-velocity relationship in human quadriceps.

Low-frequency fatigue (LFF) is usually defined as the decline in low:high-frequency force of electrically evoked isometric muscle contractions. The influence of LFF on dynamic muscle function is not well studied. Our aim was to assess the effect of LFF on the electrically evoked torque-velocity relationship in humans. Sixteen participants underwent a series of electrically evoked knee extensions in an isokinetic dynamometer to establish torque-velocity relationships at 15 and 50 Hz using isokinetic contractions. Hereafter, fatigue was induced by five sets of 10 repetitions of maximal voluntary dynamic knee extensions. After 30 min of rest, torque-velocity tests were repeated. Maximal torque (Fmax) was measured, whereas maximal contraction velocity (Vmax) and maximal power (Pmax) were estimated using Hill's force-velocity equation, 15:50 Hz ratios were calculated for Fmax, Vmax, and Pmax as markers of LFF. Fmax decreased by 40% at 15 Hz (P = 0.001) and by 15% at 50 Hz (P = 0.001) in the fatigued state. No significant change was detected for Vmax at 15 Hz [-2%, (P = 0.349)] or 50 Hz [+3% (P = 0.763)], whereas 15 and 50 Hz Pmax decreased by 30% (P = 0.004) and 10% (P = 0.008), respectively. Following the fatigue protocol, the 15:50 Hz Fmax ratio decreased by 31% (P < 0.001), indicating LFF. The 15:50 Hz Pmax ratio also decreased by 23% (P = 0.002), whereas the 15:50 Hz Vmax ratio was unchanged (P = 0.313). In conclusion, fatiguing contractions decreased Fmax and Pmax at both high and low stimulation frequencies, whereas Vmax appeared unaffected. Nevertheless, LFF influences power production during human dynamic contractions at a range of submaximal velocities.NEW & NOTEWORTHY Force-velocity relationships were established using either low- or high-frequency electrical stimulation before and after fatiguing voluntary eccentric/concentric contractions of the knee extensors. Low-frequency fatigue was assessed by the relative decrease in low- and high-frequency maximal torque, maximal shortening velocity, and maximal power estimated by the force-velocity relationship. Low-frequency fatigue manifests itself as a large decrease in low-frequency maximal force and power with a modest decrease in high-frequency maximal force and power. Contraction velocity does not seem to decrease in the same manner.

Applicability of the Load-Velocity Relationship to Predict 1-Repetition Maximum in the Half-Squat in High-Level Sprinters.

PURPOSE: To investigate the indirect measurement of 1-repetition-maximum (1RM) free-weight half-squat in high-level sprinters using the load-velocity relationship. METHODS: Half-squat load and velocity data from 11 elite sprinters were collected in 2 separate testing sessions. Approximately 24 hours prior to the first testing session, sprinters completed a fatiguing high-intensity training session consisting of running intervals, staircase exercises, and body-weight exercises. Prior to the second testing session, sprinters had rested at least 48 hours. Two different prediction models (multiple-point method, 2-point method) were used to estimate 1RM based on the load and either mean or peak concentric velocity data of submaximal lifts (40%-90% 1RM). The criterion validity of all methods was examined through intraclass correlation coefficients, coefficient of variation (CV%), Bland-Altman plots, and the SEM. RESULTS: None of the estimations were significantly different from the actual 1RM. The multiple-point method showed higher intraclass correlation coefficients (.91 to .97), with CVs from 3.6% to 11.7% and SEMs from 5.4% to 10.6%. The 2-point method showed slightly lower intraclass correlation coefficients (.76 to .95), with CVs 1.4% to 17.5% and SEMs from 9.8% to 26.1%. Bland-Altman plots revealed a mean random bias in estimation of 1RM for both methods (mean and peak velocity) ranging from 1.06 to 13.79 kg. CONCLUSION: Velocity-based methods can be used to roughly estimate 1RM in elite sprinters in the rested and fatigued conditions. However, all methods showed variations that limit their applicability for accurate load prescription for individual athletes.

Torque and Discomfort During Electrically Evoked Muscle Contractions in Healthy Young Adults: Influence of Stimulation Current and Pulse Frequency.

OBJECTIVE: To investigate (1) how current and pulse frequency of electrical stimulation (ES) as well as contraction mode (isometric, concentric, and eccentric) influence torque output and discomfort and (2) how familiarization by repeated ES sessions influences ratings of perceived discomfort. DESIGN: An experimental study, 3 sessions. SETTING: A university laboratory. PARTICIPANTS: Eight healthy participants (5 men, 3 women; mean age 25.2 years; N=8). INTERVENTIONS: Participants completed 3 trial days, each including 17 electrically evoked thigh muscle contractions. On each trial day, the first 6 contractions consisted of 2 isometric, 2 concentric, and 2 eccentric muscle contractions randomly ordered with a fixed stimulation current and pulse frequency (200 mA, 20 Hz), while the remaining 11 muscle contractions were all isometric with randomly ordered combinations of current (100-250 mA) and pulse frequency (20-100 Hz). MAIN OUTCOME MEASURES: Torque and perceived discomfort were measured for each ES-evoked contraction. RESULTS: Overall, the findings revealed that a higher stimulation frequency was associated with an increased torque without increased discomfort, while higher currents were associated with increases of both torque and discomfort. Contraction type did not influence level of discomfort, despite eccentric contractions eliciting higher torque compared with concentric and isometric contractions (P<.001). Finally, a significant familiarization to ES (P<.001) was observed after just 1 of 3 identical stimulation sessions. CONCLUSIONS: The outlined data suggest that to elicit high torque levels while minimizing levels of discomfort in young subjects, eccentric muscle contractions evoked with a low stimulation current, and a high pulse frequency are preferable. Furthermore, a single familiarization session significantly lowers rating of perceived discomfort during ES.

Prolonged loss of force and power following fatiguing contractions in rat soleus muscles. Is low-frequency fatigue an issue during dynamic contractions?

Low-frequency fatigue (LFF) is defined by a relatively larger deficit in isometric force elicited by low-frequency electrical stimulation compared with high-frequency stimulation. However, the effects of LFF on power during dynamic contractions elicited at low and high frequencies have not been thoroughly characterized. In the current study, rat soleus muscles underwent fatiguing either concentric, eccentric, or isometric contractions. Before and 1 h after the fatiguing contractions, a series of brief isometric and dynamic contractions elicited at 20 and 80 Hz stimulation to establish force-velocity relationships. Maximal force (Fmax), velocity (Vmax), and power (Pmax) were assessed for each frequency. Sarcoplasmic reticulum (SR) Ca2+ release and reuptake rates were assessed pre- and postfatigue. Prolonged fatigue was observed as a loss of Fmax and Pmax in muscles fatigued by concentric or eccentric, but not by isometric contractions. When quantified as a decrease in the ratio between 20 Hz and 80 Hz contractile output, LFF was more pronounced for isometric force than for power (-21% vs. -16% for concentrically fatigued muscles, P = 0.003; 29 vs. 13% for eccentrically fatigued muscles, P < 0.001). No changes in SR Ca2+ release or reuptake rates were observed. We conclude that LFF is less pronounced when expressed in terms of power deficits than when expressed in terms of force deficits, and that LFF, therefore, likely affects performance to a lesser degree during fast concentric contractions than during static or slow contractions.

The Role of Plasma Extracellular Vesicles in Remote Ischemic Conditioning and Exercise-Induced Ischemic Tolerance.

Ischemic conditioning and exercise have been suggested for protecting against brain ischemia-reperfusion injury. However, the endogenous protective mechanisms stimulated by these interventions remain unclear. Here, in a comprehensive translational study, we investigated the protective role of extracellular vesicles (EVs) released after remote ischemic conditioning (RIC), blood flow restricted resistance exercise (BFRRE), or high-load resistance exercise (HLRE). Blood samples were collected from human participants before and at serial time points after intervention. RIC and BFRRE plasma EVs released early after stimulation improved viability of endothelial cells subjected to oxygen-glucose deprivation. Furthermore, post-RIC EVs accumulated in the ischemic area of a stroke mouse model, and a mean decrease in infarct volume was observed for post-RIC EVs, although not reaching statistical significance. Thus, circulating EVs induced by RIC and BFRRE can mediate protection, but the in vivo and translational effects of conditioned EVs require further experimental verification.

Potassium-induced potentiation of subtetanic force in rat skeletal muscles: influences of ß2-activation, lactic acid, and temperature.

Moderate elevations of extracellular K+ concentration ([K+]o) occur during exercise and have been shown to potentiate force during contractions elicited with subtetanic frequencies. Here, we investigated whether lactic acid (reduced chloride conductance), ß2-adrenoceptor activation, and increased temperature would influence the potentiating effect of potassium in slow- and fast-twitch muscles. Isometric contractions were elicited by electrical stimulation at various frequencies in isolated rat soleus and extensor digitorum longus (EDL) muscles incubated at normal (4 mM) or elevated K+, in combination with salbutamol (5 µM), lactic acid (18.1 mM), 9-anthracene-carboxylic acid (9-AC; 25 µM), or increased temperature (30-35°C). Elevating [K+]o from 4 mM to 7 mM (soleus) and 10 mM (EDL) potentiated isometric twitch and subtetanic force while slightly reducing tetanic force. In EDL, salbutamol further augmented twitch force (+27 ± 3%, P < 0.001) and subtetanic force (+22 ± 4%, P < 0.001). In contrast, salbutamol reduced subtetanic force (-28 ± 6%, P < 0.001) in soleus muscles. Lactic acid and 9-AC had no significant effects on isometric force of muscles already exposed to moderate elevations of [K+]o. The potentiating effect of elevated [K+]o was still well maintained at 35°C. Addition of salbutamol exerts a further force-potentiating effect in fast-twitch but not in slow-twitch muscles already potentiated by moderately elevated [K+]o, whereas lactic acid, 9-AC, or increased temperature does not exert any further augmentation. However, the potentiating effect of elevated [K+]o was still maintained in the presence of these, thus emphasizing the positive influence of moderately elevated [K+]o for contractile performance during exercise.

Concomitant excitation and tension development are required for myocellular gene expression and protein synthesis in rat skeletal muscle.

AIM: Loading-induced tension development is often assumed to constitute an independent cue to initiate muscle protein synthesis following resistance exercise. However, with traditional physiological models of resistance exercise, changes in loading-induced tension development also reflect changes in neural activation patterns, and direct evidence for a mechanosensitive mechanism is therefore limited. Here, we sought to examine the importance of excitation and tension development per se on initiation of signalling, gene transcription and protein synthesis in rat skeletal muscle. METHODS: Isolated rat extensor digitorum longus muscles were allocated to the following interventions: (a) Excitation-induced eccentric contractions (ECC); (b) Passive stretching without excitation (PAS); (c) Excitation with inhibition of contractions (STIM + IMA ) and; (d) Excitation in combination with both inhibition of contractions and PAS (STIM + IMA  + PAS). Assessment of transcriptional and translational signalling, gene transcription and acute muscle protein synthesis was compared in stimulated vs contra-lateral non-stimulated control muscle. RESULTS: Protein synthesis increased solely in muscles subjected to a combination of excitation and tension development (ECC and STIM + IMA  + PAS). The same pattern was true for p38 mitogen-activated protein kinase signalling for gene transcription as well as for gene transcription of immediate early genes FOS and JUN. In contrast, mechanistic target of rapamycin Complex 1 signalling for translation initiation increased in all muscles subjected to increased tension development (ECC and STIM + IMA  + PAS as well as PAS). CONCLUSIONS: The current study suggests that exercise-induced increases in protein synthesis as well as transcriptional signalling is dependent on the concomitant effect of excitation and tension development, whereas signalling for translation initiation is only dependent of tension development per se.

Six Weeks of Low-Load Blood Flow Restricted and High-Load Resistance Exercise Training Produce Similar Increases in Cumulative Myofibrillar Protein Synthesis and Ribosomal Biogenesis in Healthy Males.

Purpose: High-load resistance exercise contributes to maintenance of muscle mass, muscle protein quality, and contractile function by stimulation of muscle protein synthesis (MPS), hypertrophy, and strength gains. However, high loading may not be feasible in several clinical populations. Low-load blood flow restricted resistance exercise (BFRRE) may provide an alternative approach. However, the long-term protein synthetic response to BFRRE is unknown and the myocellular adaptations to prolonged BFRRE are not well described. Methods: To investigate this, 34 healthy young subjects were randomized to 6 weeks of low-load BFRRE, HLRE, or non-exercise control (CON). Deuterium oxide (D2O) was orally administered throughout the intervention period. Muscle biopsies from m. vastus lateralis were collected before and after the 6-week intervention period to assess long-term myofibrillar MPS and RNA synthesis as well as muscle fiber-type-specific cross-sectional area (CSA), satellite cell content, and myonuclei content. Muscle biopsies were also collected in the immediate hours following single-bout exercise to assess signaling for muscle protein degradation. Isometric and dynamic quadriceps muscle strength was evaluated before and after the intervention. Results: Myofibrillar MPS was higher in BFRRE (1.34%/day, p < 0.01) and HLRE (1.12%/day, p < 0.05) compared to CON (0.96%/day) with no significant differences between exercise groups. Muscle RNA synthesis was higher in BFRRE (0.65%/day, p < 0.001) and HLRE (0.55%/day, p < 0.01) compared to CON (0.38%/day) and both training groups increased RNA content, indicating ribosomal biogenesis in response to exercise. BFRRE and HLRE both activated muscle degradation signaling. Muscle strength increased 6-10% in BFRRE (p < 0.05) and 13-23% in HLRE (p < 0.01). Dynamic muscle strength increased to a greater extent in HLRE (p < 0.05). No changes in type I and type II muscle fiber-type-specific CSA, satellite cell content, or myonuclei content were observed. Conclusions: These results demonstrate that BFRRE increases long-term muscle protein turnover, ribosomal biogenesis, and muscle strength to a similar degree as HLRE. These findings emphasize the potential application of low-load BFRRE to stimulate muscle protein turnover and increase muscle function in clinical populations where high loading is untenable.