Evaluation of Lumbar Muscle Synergy in Flexion Movement Using Time-Varying Muscle Synergies

According to the literature, changes in muscle activity patterns are considered as one of the causes of non-specific chronic low back pain. Recent studies have introduced muscle synergy as a valuable tool for analyzing how muscles work in body movements. In this way, a new study method is proposed for modeling the upper body and extracting time-varying muscle synergies in a flexural motion of the waist. In this way, a new study method is proposed for modeling trunk and extracting time-varying muscle synergies in-plane bending movements of lumbar flexion. Considering 18 effective muscles and function of the combined cost of the minimum jerk - energy, 24 different movements, and their corresponding muscle patterns have been simulated. To evaluate the role of velocity, the pattern of muscle activity was divided into two parts: tonic, to overcome the gravity force and phasic, proportional to the trunk movement velocity. In the following, three fast-reaching times of 0.75, 1, and 2 seconds were considered for each direction. The results showed that 77% of the lumbar muscle pattern of movement was achieved by four phasic synergies and four tonic synergies. The resulting synergies are quite influenced by the movement direction and velocity so that each pair of phasic and tonic synergy is most effective in one of the main directions. On the other hand, the increase in velocity causes elevated amplitude coefficient and accelerated the activation of phasic synergies compared to normal mode. Considering the 45 ° flexion combination with 30 ° left lateral bending, 77.2% of the muscle pattern of movement has been reconstructed using time-varying synergies. It can be argued that the use of muscle synergies expresses a good explanation for how muscles work in the movement in different directions and velocities.