Journal of Neurophysiology vol:107 issue:12 pages:3385-3396
Leg kinematics during backward walking (BW) is very similar to the time-reversed kinematics during forward walking (FW). This suggested that the underlying muscle activation pattern could originate from a simple time-reversal as well. Experimental EMG studies have confirmed that this is the case for some muscles. Furthermore, it was hypothesized that muscles showing a time-reversal, should also exhibit a reversal in function (from accelerating the body center of mass (COM) to decelerating). However, this has not been verified yet in simulation studies. In the present study, forward simulations have been used to study the effects of muscles on the acceleration of COM in FW and BW. It was found that a reversal in function was indeed present in the muscle control of the horizontal movement of COM (e.g. tibialis anterior and gastrocnemius). In contrast, muscles antigravity contributions maintained their function for both directions of movement. An important outcome of the present study is therefore that similar muscles can be used to achieve opposite functional demands at the level of control of the COM when walking direction is reversed. However, some muscles showed direction-specific contributions (i.e. dorsiflexors). It was concluded that the changes in muscle contributions imply that a simple time-reversal would be insufficient to produce BW from FW. It is therefore proposed that BW utilizes extra elements, presumably supraspinal, in addition to a common spinal drive. These additions are needed for propulsion and require a partial reconfiguration of lower level common networks.