MOTOR UNIT RECRUITMENT IN HUMAN MEDIAL GASTROCNEMIUS-MUSCLE DURING COMBINED KNEE FLEXION AND PLANTAR FLEXION ISOMETRIC CONTRACTIONS

Previous work on multifunctional muscle has suggested that motor unit recruitment during a combined force task is the result of an interacti ve effect of weighted inputs acting simultaneously on the-motoneuron p ool. The present study shows that a similar effect describes motor uni t activation in a two-joint muscle as forces are combined at both prox imal and distal attachments. The recruitment thresholds of single moto r units in medial gastrocnemius muscle were determined during combined knee flexion and plantarflexion isometric contractions. Slow isometri c ramp contractions in knee flexion were produced while maintaining va rious background levels of plantarflexion force. The combination of kn ee flexion and plantarflexion forces at which a motor unit initially d ischarged was used to characterize recruitment as represented by the s lope of the regression line fit to the individual data points. Each su bject completed two experiments; one at each of two knee joint angles, with the ankle joint fixed at 90-degrees. The effect of knee angle wa s assessed by comparing the slopes of the regression lines that charac terized motor unit recruitment at each knee angle. Motor units in medi al gastrocnemius were recruited when the linear sum of the forces exer ted in plantarflexion and knee flexion exceeded a certain threshold of combined force. Specifically, the apparent force threshold of recruit ment in knee flexion decreased as the level of force maintained in pla ntarflexion increased. Further, evidence is provided indicating that t he linear relationship describing recruitment in two-joint muscle is d ependent upon joint angle. The basis for the alteration in force thres hold is thought to be related to changes in muscle length and mechanic al advantage which might adjust the relative weighting of inputs that determine muscle activation patterns. These results indicate a possibl e common strategy employed by the nervous system in coordinating the a ctivation of motor units to perform a specific task.