Mechanisms underlying the slow recovery of force after fatigue: importance of intracellular calcium.
ABSTRACT Recovery of force production after an intense bout of activity may sometimes take several days, especially at low activation frequencies ('low frequency fatigue'). This slow recovery can also be observed in isolated muscle and single muscle fibres. The origin of the force deficit is failure of excitation-contraction coupling at the level of the triads. The most likely cause of the failure is an elevated intracellular Ca2+ level, but the site of action of Ca2+ is unclear. Available evidence does not support the involvement of Ca2+-activated proteases. Ca2+-induced damage to mitochondria or swelling of t-tubules do not seem to be causative factors. Other mechanisms are discussed, including possible detrimental effects of Ca2+-activated lipases, calmodulin, and reactive oxygen species.
Article: Intense Resistance Exercise Induces Early and Transient Increases in Ryanodine Receptor 1 Phosphorylation in Human Skeletal Muscle.[show abstract] [hide abstract]
ABSTRACT: BACKGROUND: While ryanodine receptor 1 (RyR1) critically contributes to skeletal muscle contraction abilities by mediating Ca(2+)ion oscillation between sarcoplasmatic and myofibrillar compartments, AMP-activated protein kinase (AMPK) senses contraction-induced energetic stress by phosphorylation at Thr(172). Phosphorylation of RyR1 at serine(2843) (pRyR1Ser(2843)) results in leaky RyR1 channels and impaired Ca(2+)homeostasis. Because acute resistance exercise exerts decreased contraction performance in skeletal muscle, preceded by high rates of Ca(2+)-oscillation and energetic stress, intense myofiber contractions may induce increased RyR1 and AMPK phosphorylation. However, no data are available regarding the time-course and magnitude of early RyR1 and AMPK phosphorylation in human myofibers in response to acute resistance exercise. PURPOSE: Determine the effects and early time-course of resistance exercise on pRyR1Ser(2843) and pAMPKThr(172) in type I and II myofibers. METHODS: 7 male subjects (age 23±2 years, height: 185±7 cm, weight: 82±5 kg) performed 3 sets of 8 repetitions of maximum eccentric knee extensions. Muscle biopsies were taken at rest, 15, 30 and 60 min post exercise. pRyR1Ser(2843) and pAMPKThr(172) levels were determined by western blot and semi-quantitative immunohistochemistry techniques. RESULTS: While total RyR1 and total AMPK levels remained unchanged, RyR1 was significantly more abundant in type II than type I myofibers. pRyR1Ser(2843) increased 15 min and peaked 30 min (p<0.01) post exercise in both myofiber types. Type I fibers showed relatively higher increases in pRyR1Ser(2843) levels than type II myofibers and remained elevated up to 60 min post resistance exercise (p<0.05). pAMPKThr(172) also increased 15 to 30 min post exercise (p<0.01) in type I and II myofibers and in whole skeletal muscle. CONCLUSION: Resistance exercise induces acutely increased pRyR1Ser(2843) and concomitantly pAMPKThr(172) levels for up to 30 min in resistance exercised myofibers. This provides a time-course by which pRyR1Ser(2843) can mechanistically impact Ca(2+)handling properties and consequently induce reduced myofiber contractility beyond immediate fatiguing mechanisms.PLoS ONE 01/2012; 7(11):e49326. · 4.09 Impact Factor
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ABSTRACT: Eccentric contractions are thought to induce greater low-frequency fatigue (LFF) and delayed-onset muscle soreness (DOMS) than concentric contractions. In this study we induced a similar amount of eccentric quadriceps muscle fatigue during either a concentric or eccentric fatigue task to compare LFF and DOMS. Subjects (n = 22) performed concentric or eccentric fatigue tasks using 75% of the pre-fatigue maximal voluntary contraction (MVC) torque, and both tasks ended when the MVC eccentric torque decreased by 25% pre-fatigue. When subjects reached the failure criterion during the eccentric and concentric tasks, the concentric MVC was 78 +/- 9.8% and 64 +/- 8.4% of initial, respectively. LFF was greater after the concentric than the eccentric protocols (22 +/- 12.4% and 15 +/- 7.6% increase, respectively; P < 0.01). DOMS was over 100% greater for the eccentric protocol. These results indicate that DOMS is not dependent on the events that contribute to LFF.Muscle & Nerve 08/2010; 42(2):230-8. · 2.37 Impact Factor
Article: Eccentric exercise and delayed onset muscle soreness of the quadriceps induce adjustments in agonist-antagonist activity, which are dependent on the motor task.[show abstract] [hide abstract]
ABSTRACT: This study investigates the effects of eccentric exercise and delayed onset muscle soreness (DOMS) of the quadriceps on agonist-antagonist activity during a range of motor tasks. Ten healthy volunteers (age, mean ± SD, 24.9 ± 3.2 years) performed maximum voluntary contractions (MVC) and explosive isometric contractions of the knee extensors followed by isometric contractions at 2.5, 5, 10, 15, 20, and 30% MVC at baseline, immediately after and 24 h after eccentric exercise of the quadriceps. During each task, force of the knee extensors and surface EMG of the vasti and hamstrings muscles were recorded concurrently. Rate of force development (RFD) was computed from the explosive isometric contraction, and the coefficient of variation of the force (CoV) signal was estimated from the submaximal contractions. Twenty-four hours after exercise, the subjects rated their perceived pain intensity as 4.1 ± 1.2 (score out of 10). The maximum RFD and MVC of the knee extensors was reduced immediately post- and 24 h after eccentric exercise compared to baseline (average across both time points: 19.1 ± 17.1% and 11.9 ± 9.8% lower, respectively, P < 0.05). The CoV for force during the submaximal contractions was greater immediately after eccentric exercise (up to 66% higher than baseline, P < 0.001) and remained higher 24 h post-exercise during the presence of DOMS (P < 0.01). For the explosive and MVC tasks, the EMG amplitude of the vasti muscles decreased immediately after exercise and was accompanied by increased antagonist EMG for the explosive contraction only. On the contrary, reduced force steadiness was accompanied by a general increase in EMG amplitude of the vasti muscles and was accompanied by increased antagonist activity, but only at higher force levels (>15% MVC). This study shows that eccentric exercise and subsequent DOMS of the quadriceps reduce the maximal force, rate of force development and force steadiness of the knee extensors, and is accompanied by different adjustments of agonist and antagonist muscle activities.Experimental Brain Research 11/2011; 216(3):385-95. · 2.39 Impact Factor