Factors limiting force during slow, shortening actions of the quadriceps femoris muscle group in vivo

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Abstract
Speed-torque relations of the quadriceps femors muscle group were determined using eight healthy subjects. Isometric or isovelocity, shortening muscle actions were performed at 15 speeds (0-6.28 rad s-1) and torque was measured 0.78 and 0.52 rad below horizontal. Unilateral actions were evoked by surface electrical stimulation or by maximal voluntary effort. Stimulation current evoked a torque equal to approx. 70% of maximal voluntary isometric and was held constant across speeds. For the voluntary or stimulation tests, torque decreased (P < 0.05) with increasing speed. This response in relative terms was greater (P < 0.05) for the stimulation than for the voluntary tests. The difference in the decline in relative torque for the simulation vs. voluntary tests was not influenced by the angle at which torque was measured, and thereby muscle length. Speed-torque data for the two stimulation tests fit a linearized plot of a hyperbolic relation for the higher tests speeds. When torque was measured at the greater joint angle (0.78 rad), and thereby longer muscle length, the equation was Po-P/V = Po 0.724-1.078 (P < 0.05, r2 = 0.95) for speeds greater than 0.70 rad s-1. For the shorter muscle length, the equation for data collected at speeds greater than 1.13 rad s-1 was Po-P/V = Po 0.467 + 4.61 (P < 0.05, r2 = 0.91). Inclusion of data for the next slower speed markedly compromised the 'fit' for either linear relation.(ABSTRACT TRUNCATED AT 250 WORDS)