Variation of the maximum maintenance time of an isometric contraction in the presence of a second contraction

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Abstract
During maintenance of an isometric, contraction, an increase in perfusion of the contracting muscle would cause a prolongation in the maximum maintenance time (limit-time). This improvement in blood perfusion to an upper limb muscle can easily be obtained if the subject is asked to perform simultaneously a second isometric contraction of another muscle group at a slightly higher relative force than the first; it happens involuntarily through the involvement of different muscle groups (fixator muscles of the body trunk, limbs, neck and head) at the same time as the muscle under study. Seventeen subjects (nine women and eight men) carried out simultaneously sustained isometric contractions until exhaustion of the right (RA) and left elbow flexors (LA). The relative forces were 40% of the maximum voluntary contraction (MVC) for RA and 50% of MVC for LA. Contraction of LA occurred after a relative time interval (RTI) corresponding to 0, 25, 50, 75, and 100% of the limit-time of RA alone which was predetermined. Under these conditions, simultaneous contraction of LA was responsible both for a more important increase in heart rate and hence an improvement of blood perfusion to RA, and also for an increase in the level of central activation. RA limit-time was unaffected at RTI = 0 and 25% in the female subjects, and at RTI =0,25 and 50% in the male subjects. In contrast for RTI = 50% and 75% in the female group and RTI =75 and 100% in the male group, RA limit-time was significantly increased (respectively: + 15%, p < 0.02, and +25%, ppp<0.02). In the female subjects, for RTI = 100%, the increase in RA limit-time was +11%, but insignificant. These results suggest that it is probably the increase in the frequency of discharge of the motor units belonging to the type I fibres together with an improved muscle perfusion at a moment when the spatial recruitment of type I fibres is at its maximum that is responsible for the increase in RA limit-time.