The factors of mechanical power in slow twitch muscle

Maréchal, Georges [UCL] Beckers-Bleukx, G.

(eng) The mechanical power exerted by the contracting muscle must be matched to the power required to move the load. The maximal mechanical power, Pmax, sets the maximal physiological performance of muscle, liberated if all its fibres are stimulated. In most physiological contexts, animals modulate the number of fibres activated and thus are able to obtain the desired match. In muscle artificially stimulated for cardiac assistance, the situation is however different, since the number of fibres stimulated is presumably maximal. No provi sion is made to match the muscle power to the power requirement of the failing heart. There is thus a large probability that power mismatch occurs, entailing low efficiency. Soleus muscle of mouse is a slow-twitch muscle fatigue resistant and therefore a good model for the analysis of the contribution of the factors of power. Two strains were studied, NMRI and C57. The two strains develop the same maximum power at velocities of shortening VPmax noticeably different, about 50 % higher for C57 mouse. Thus C57 mouse will tend to shorten its soleus muscle at higher velocities, in order to obtain the best power match. We are thus led to the concept that the velocity of shortening VPmax required to produce Pmax can be adjusted by suitable modifications of the factors of Pmax in such way that Pmax itself does not change. It is possible to control the velocity of shortening VPmax at which maximum power is produced by changing the proportions of slow type isomyosin SM2 and IM in the muscle. It may be possible to induce transformation of fibres into the slow form of Type IIAs fibres and thus control not only maximal power but also the corresponding velocity of shortening. Whether this aim could be obtained by a suitable pattern of artificial stimulation is an objective for future research.