HUMAN SKELETAL-MUSCLE FIBER TYPES AND FORCE - VELOCITY PROPERTIES

It has been reported that there is a relationship between power output and fibre type distribution in mixed muscle. The strength of this rel ationship is greater in the range of 3-8 rad.s(-1) during knee extensi on compared to slower or faster angular knee extensor speeds. A mathem atical model of the force:velocity properties of muscle with various c ombinations of fast- and slow-twitch fibres may provide insight into w hy specific velocities may give better predictions of fibre type distr ibution. In this paper, a mathematical model of the force:velocity rel ationship for mixed muscle is presented. This model demonstrates that peak power and optimal velocity should be predictive of fibre distribu tion and that the greatest fibre type discrimination in human knee ext ensor muscles should occur with measurement of power output at an angu lar velocity just greater than 7 rad.s(-1). Measurements of torque:ang ular velocity relationships for knee extension on an isokinetic dynamo meter and fibre type distribution in biopsies of vastus lateralis musc les were made on 31 subjects. Peak power and optimal velocity were det ermined in three ways: (1) direct measurement, (2) linear regression, and (3) fitting to the Hill equation. Estimation of peak power and opt imal velocity using the Hill equation gave the best correlation with f ibre type distribution (r>0.5 for peak power or optimal velocity and p ercentage of fast-twitch fibres). The results of this study confirm th at prediction of fibre type distribution is facilitated by measurement of peak power at optimal velocity and that fitting of the data to the Hill equation is a suitable method for evaluation of these parameters .