#  Force-Velocity Properties 

 



 ##  

  expand\_more  

 
  

 

## 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](/sites/g/files/omnuum6301/files/styles/hwp_1_1__720x720_scale/public/biewener/files/ratplantarisf-v.jpg?itok=EBGa-hBH) 

 

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)