Effects of muscle fiber distribution on the mechanical efficiency of human locomotion.

To study the effects of muscle fiber distribution (m. vastus lateralis) on the mechanical efficiency of human locomotion, 12 subjects were observed, six in the slow-twitch (ST) group (38.0% +/- 6.1% FT) and six in the fast-twitch (FT) group (63.8% +/- 5.9% FT). Pure positive, pure negative, and a combination of negative and positive work were performed with a special "sledge ergometer." The mechanical efficiency of the pure positive work (eta) was on the average 16.4% +/- 2.5% and 17.1% +/- 2.1% for the ST and FT groups, respectively. The mechanical efficiency of the pure negative work (eta -) was slightly greater in the ST group (94.0% +/- 30.2% vs 79.6% +/- 32.7%, P = NS). The mechanical efficiency of the positive work in the combined negative/positive work (eta +) was on the average in the ST group 36.4% +/- 5.9% and in the FT group 33.2% +/- 5.5% (t = 2.02, P less than 0.05). The eta + and the calculated elastic parameters (work due to elasticity Wel, utilization of the prestretch %El, and relative Wel) together with the EMG analysis demonstrated that the ST group had a better stiffness regulation and elastic performance. It is suggested that the basic differences in the reflex control between the two types of muscles and functional differences between the respective fiber types could be the possible reasons for the results observed under the conditions of the present slow-type stretch-shortening cycle exercises.