S1-5.9 Jan. 6 Unraveling the Link between Muscle Activation Timing and Force BIEWENER, A.A.; Harvard University email@example.com
The temporal relationship between muscle activation and tension development is critical for studies of muscle recruitment using electromyography (EMG), as well as for how EMGs may be used to interpret muscle function more generally. However, several factors influence the timing of neuromotor activation relative to force production. These include (1) the intrinsic rate of force development, which depends on the rates of excitation-contraction coupling and acto-myosin cross-bridge cycling, (2) the amount of series-elastic compliance (SEC) in the muscle or muscle-tendon unit, and (3) the nature of external loading transmitted to the muscle via the skeletal system it attaches to and moves. Higher operating frequencies require faster intrinsic rates of force development and reduced SEC, and are facilitated by active stretch of a muscle before it shortens or by isometric contraction. These differences in contractile function and the timing between EMG and force development are influenced by animal size and locomotor mode, or biomechanical context. Relative timing is compared and evaluated for various muscle systems in which in vivo force and EMG data are available based on the authorís and othersí work. Whereas the phase advance and EMG duration relative to force output is 6% and 63% for the cockatiel pectoralis during flight and 5% and 43% for the plantaris longus of Xenopus during swimming, it is 8% and 68% for the lateral gastocnemius (LG) of goats during trotting. This means that force development continues beyond EMG activation for 20% of the locomotor cycle in the cockatiel pectoralis, 38% in Xenopus plantaris longus but only 9% in the goat LG. It is likely that phase delay and relative duration of EMG versus force are less for faster muscles and smaller animals than for slower muscles and larger animals. Consequently, interpretations of muscle function based on EMG timing can be unraveled by neuromotor linkages.