Summation of Forces From Multiple Motor Units in the Cat Soleus Muscle

Abstract

Nearly all muscle models and most motor control concepts assume that forces from individual muscle fibers and motor units sum in an additive manner once effects of in-series tendon compliance are taken into account. Due to the numerous mechanical linkages between individual fibers, though, it is unclear whether this assumption is warranted. This work examined motor unit force summation over a wide range of muscle forces in the cat soleus. Nonadditive summation implies a nonlinear summation of motor unit forces. Summation nonlinearities were quantified during interactions of 10 individual motor units and 4 motor unit bundles containing approximately 10 units each. These protocols allowed motor unit force summation to be examined from approximately 0 to 25% of tetanic muscle force. Nonlinear summation was assessed by comparing the actual forces to the algebraic sum of individual units and bundles stimulated in isolation. Superadditive summation meant that the actual force exceeded the algebraic sum, whereas subadditive summation meant that the actual force was smaller than the algebraic sum. Experiments tested the hypothesis that superadditive summation occurs at low force levels when few motor units are recruited, whereas subadditive summation prevails above 10% of tetanic force. Results were consistent with this hypothesis. As in previous studies, nonlinear summation in the soleus was modest, but a clear transition from predominately superadditive to predominantly subadditive summation occurred in the range of 6–8% of tetanic force. The largest nonlinearities were transient and appeared at the onset of recruitment and derecruitment of groups of motor units. The results are discussed in terms of the mechanical properties of the connective tissue forming the tendon and linking muscle fibers.

Effects of series elastic compliance on muscle force summation and the rate of force rise

“…We concluded that force summation was limited, at least in part, by high series elastic compliance affording significant internal shortening and, therefore, a progressive loss in force-generating capacity according to the known force-length (Gordon et al, 1966) and force-velocity (Fenn and Marsh, 1935) properties of muscle. A similar reduction in force summation has been demonstrated during tetanic contractions when individual parts of whole muscle are stimulated simultaneously, rather than individually, and is likely owing to the common in-series elastic compliance allowing a higher amplitude and velocity of shortening (Sandercock, 2000;Perreault et al, 2003). Despite numerous examples of human and animal lower limb muscles experiencing significant internal shortening under constant-length conditions (see Roberts, 2002), the role the history dependence of force generation (Abbott and Aubert, 1952) plays in reduced force summation has received relatively little attention.…”

Model Based Sensitivity Analysis of EMG–Force Relation with Respect to Motor Unit Properties: Applications to Muscle Paresis in Stroke

“…It should be acknowledged that nonadditive (nonlinear) summation has been reported in the cat soleus muscles. 24 This nonlinear effect may influence the resultant EMGforce relation.…”

Effect of temperature on spike-triggered average torque and electrophysiological properties of low-threshold motor units

“…This effect is independent of the modifications of electrophysiological fiber properties. Moreover, the twitch forces summate nonlinearly (37,38), depending on the number of active motor units (37). A superadditive summation occurs with a small number of recruited motor units (37).…”

“…Moreover, the twitch forces summate nonlinearly (37,38), depending on the number of active motor units (37). A superadditive summation occurs with a small number of recruited motor units (37). Although unlikely, since the joint force did not change in the different contractions, it cannot be excluded that the change in temperature affected the nonlinearity in twitch summation.…”