SUMMATION OF MOTOR UNIT TENSIONS IN THE TIBIALIS POSTERIOR MUSCLE OF THE CAT UNDER ISOMETRIC AND NONISOMETRIC CONDITIONS

1. The tension produced by the combined stimulation of two to four single motor units of the cat tibialis posterior muscle was compared with the algebraic sum of the tensions produced by each individual motor unit. Comparisons were made under isometric conditions and during imposed changes in muscle length. 2. Under isometric conditions, the tension resulting from combined stimulation of units displayed marked nonlinear summation, as previously reported in other cat hindlimb muscles. On average, the measured tension was approximately 20% greater than the algebraic sum of the individual unit tensions. However, small trapezoidal movements imposed on the muscle during stimulation significantly reduced the degree of nonlinear summation both during and after the movement. This effect was seen with imposed movements as small as 50-mu-m. 3. The degree of nonlinear summation was not dependent on motor unit size or on stimulus frequency. The effect was also unrelated to tendon compliance because the degree of nonlinear summation of motor unit forces was unaffected by the inclusion of different amounts of the external tendon between the muscle and the force transducer. 4. Our results support previous suggestions that the force measured when individual motor units are stimulated under isometric conditions is reduced by friction between the active muscle fibers and adjacent passive fibers. These frictional effects are likely to originate in the connective tissue matrix connecting adjacent muscle fibers. However, because these effects are virtually eliminated by small movements, linear summation of motor unit tensions should occur at low force levels under nonisometric conditions. 5. Small trapezoidal movements imposed in the midst of a contraction led to a postmovement increase in isometric tension after release-first movements and a decrease in tension after stretch-first movements. The threshold movement amplitude for postmovement tension enhancement was approximately 50-mu-m. The reduction of internal friction and the enhancement of tension by a small release followed by restretch may underlie the phenomenon of force assistance reported by Clamann and Schelhorn, whereby the tension produced by a single motor unit was enhanced after the superimposed stimulation of another motor unit.