Influence of posture and training on the endurance time of a low-level isometric contraction

Classically, the critical force of a muscle (the relative force below which an isometric contraction can be maintained for a very long time without fatigue) is comprised of between 15 and 20% of its maximum voluntary contraction (MVC). However, some authors believe that the value is below 10% MVC. If such is the case, signs that accompany the establishment of muscle fatigue (EMG changes, continuous increase in systolic blood pressure [SBP] and heart rate [HR]) would have to appear more rapidly and with a higher intensity if the muscle is already partially fatigued at the start of maintaining a contraction at 10% MVC. Twelve healthy untrained participants carried out two isometric contractions with the digit flexors: one (test A) began with a maximum contraction sustained for 4 min followed without interruption by a contraction at 10% MVC for 61 min; the other (test B) was a contraction maintained at 10% MVC for 65 min. For test B, after an initial increase of 4 bpm with respect to at rest, HR remained stable until the end of contraction, SBP progressively increased by 24 mm Hg in 28 min, then remained unchanged until the end, and there were no significant changes in EMG (absence of spectral deviation towards low frequencies). For test A, in spite of the initial maximum contraction, changes in the parameters being studied (total maintenance time, HR, SBP, EMG) during maintenance at 10% MVC were identical to those for test B. The results show that (1) when the number and intensity of the co-contractions are minimized by applying an appropriate posture, it is possible to sustain an isometric contraction at 10% MVC for at least 65 min without the appearance of signs of muscle fatigue; (2) the critical force of the digit flexors is higher than 10% MVC.