Synchronization of lower limb motor unit activity during walking in human subjects

Synchronization of motor unit activity was investigated during treadmill wa lking (speed: 3-4 km/h) in 25 healthy human subjects. Recordings were made by pairs of wire electrodes inserted into the tibialis anterior (TA) muscle and by pairs of surface electrodes placed over Us muscle and a number of o ther lower limb muscles (soleus, gastrocnemius lateralis, gastrocnemius med ialis, biceps femoris, vastus lateralis, and vastus medialis). Short-lastin g synchronization (average duration: 9.6 +/- 1.1 ms) was observed between s pike trains generated from multiunit electromyographic (EMG) signals record ed by the wire electrodes in TA in eight of nine subjects. Synchronization with a slightly longer duration (12.8 +/- 1.2 ms) was also found in 13 of 1 4 subjects for paired TA surface EMG recordings. The duration and size of t his synchronization was within the same range as that observed during tonic dorsiflexion in sitting subjects. There was no relationship between the am ount of synchronization and the speed of walking. Synchronization was also observed for pairs of surface EMG recordings from different ankle plantarfl exors (soleus, medial gastrocnemius, and lateral gastrocnemius) and knee ex tensors (vastus lateralis lis and medialis of quadriceps), but not or rarel y for paired recordings from ankle and knee muscles. The data demonstrate t hat human motor units within a muscle as well as synergistic muscles acting on the same joint receive a common synaptic drive during human gait. It is speculated that the common drive responsible for the motor unit synchroniz ation during gait may be similar to that responsible for short-term synchro nization during tonic voluntary contraction.