Abstract
This study was undertaken to investigate whether sarcolemmal excitability is impaired during a sustained low-force contraction [10% maximal voluntary contraction (MVC)] by assessing muscle conduction velocity and also by analyzing separately the first and second phases of the muscle compound action potential (M wave). Twenty-one participants sustained an isometric knee extension of 10% MVC for 3min. M waves were evoked by supramaximal single shocks to the femoral nerve given at 10-s intervals. The amplitude, duration, and area of the first and second M-wave phases were computed. Muscle fiber conduction velocity, voluntary surface electromyographic (EMG), perceived effort, MVC force, peak twitch force, and temperature were also recorded. The main findings were: (1) During the sustained contraction, conduction velocity remained unchanged. (2) The amplitude of the M-wave first phase decreased for the first ~30s (−7%, p<0.05) and stabilized thereafter, whereas the second phase amplitude increased for the initial ~30s (+7%, p<0.05), before stabilizing. (3) Both duration and area decreased steeply during the first ~30s, and then more gradually for the rest of the contraction. (4) During the sustained contraction, perceived effort increased fivefold, whereas knee extension EMG increased by ~10%. (5) Maximal voluntary force and peak twitch force decreased (respectively, −9% and −10%, p<0.05) after the low-force contraction. Collectively, the present results indicate that sarcolemmal excitability is well preserved during a sustained 10% MVC task. A depression of the M-wave first phase during a low-force contraction can occur even in the absence of changes in membrane excitability. The development of fatigue during a low-force contraction can occur without alteration of membrane excitability.
Highlights
IntroductionThe excitability of the muscle fiber membrane (i.e., the ability to generate and propagate transmembrane action potentials) requires the maintenance of steep chemical gradients for Na+ and K+ across the sarcolemma (Nielsen and Clausen, 2000)
The excitability of the muscle fiber membrane requires the maintenance of steep chemical gradients for Na+ and K+ across the sarcolemma (Nielsen and Clausen, 2000)
This study investigated three research questions: (1) Is sarcolemmal excitability preserved during prolonged low-force contractions? (2) How do the amplitudes of the first and second M-wave phases vary during such contractions? (3) Does sarcolemmal excitability have any role in the development of fatigue in such contractions? These questions were addressed by examining separately the first and second phases of the M wave during the course of a 3-min contraction sustained at 10% maximal voluntary contraction (MVC) in the quadriceps, and by assessing the changes in muscle fiber conduction velocity
Summary
The excitability of the muscle fiber membrane (i.e., the ability to generate and propagate transmembrane action potentials) requires the maintenance of steep chemical gradients for Na+ and K+ across the sarcolemma (Nielsen and Clausen, 2000). It has been suggested that contractions below 15% MVC might be sustained “indefinitely” (beyond 45 min; Bigland-Ritchie and Woods, 1984; Burnley et al, 2012). This prolonged endurance time may be due to the fact that, at such low contraction levels, blood flow is sufficiently high to maintain K+ homeostasis, as previously suggested (Sjøgaard et al, 1988). Despite the evidence suggesting that K+ homeostasis is maintained during a sustained low-force contraction, it is not clear whether membrane excitability is preserved during such contractions
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