Force-Velocity Properties

Evaluating Force-Velocity Properties in Relation to Motor Unit Recruitment

With post-doc Natalie Holt (now at UC Irvine), we have recently evaluated the force-velocity properties of the rat plantaris muscle under differing stimulation conditions to test how motor unit recruitment affects a muscle's F-V properties.  In general, vertebrate muscles exhibit orderly recruitment  from slow (small) to fast (large) motor units, according to a well-known 'size principle'. The size-principle is demonstrated in most conditions of neuromuscular recruitment patterns for a variety of tasks (such as changing gait from walking to running, or jumping).

Rat Plantaris Velocity vs Isotonic Force
Nevertheless, recruitment of slow units together with fast motor units during more forceful, rapid tasks has been considered a potential liability to force development and power output of the muscle. By stimulating the rat plantaris to recruit preferentially slow (block) or fast (sub-max) motor units, versus all (supra-max) motor units (as is most commonly done in whole muscle studies), we found that recruiting all of the muscle fibers resulted in the greatest shortening velocities.  Recruitment of either slow or fast portions of the muscle resulted in slower shortening velocities, as well as reduced F-V curvature.  These results indicate that inertial, viscous and/or series-elastic effects are important in considering whole muscle force, velocity and power output.  Recruitment of only a portion of the muscle results in a slowing of whole muscle shortening.  Based on these results, the size principle does not incur a penalty to muscle shortening and power output, as has been previously suggested.  (Holt, Wakeling & Biewener, Proc. Roy. Soc. B. 2014)