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 vol untary contraction (MVC). However, some authors believe that the value is below 10% MVC. If such is the case, signs that accompany the estab lishment of muscle fatigue (EMG changes, continuous increase in systol ic blood pressure [SEP] and heart rate [HR]) would have to appear more rapidly and with a higher intensity if the muscle is already partiall y fatigued at the start of maintaining a contraction at 10% MVC. Twelv e healthy untrained participants carried out two isometric contraction s with the digit flexors: one (test A) began with a maximum contractio n sustained for 4 min followed without interruption by a contraction a t 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 contracti on, SEP progressively increased by 24 mm HE in 28 min, then remained u nchanged 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 b eing studied (total maintenance time, HR, SEP, 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 isom etric contraction at 10% MVC for at least 65 min without the appearanc e of signs of muscle fatigue; (2) the critical force of the digit flex ors is higher than 10% MVC.