10Determination of muscle forces during motion can help to understand motor control, assess 11 pathological movement , diagnose neuromuscular disorders, or estimate joint loads. Difficulty of in 12 vivo measurement made computational analysis become a common alternative in which, as several 13 muscles serve each degree of freedom, the muscle redundancy problem must be solved. Unlike static 14 optimization (SO), synergy optimization (SynO) couples muscle activations across all time frames, 15 thereby altering estimated muscle co-contraction. This study explores whether the use of a muscle 16 synergy structure within a static optimization framework improves prediction of muscle activations 17 during walking. A motion/force/EMG gait analysis was performed on five healthy subjects. A 18 musculoskeletal model of the right leg actuated by 43 Hill-type muscles was scaled to each subject 19 and used to calculate joint moments, muscle-tendon kinematics and moment arms. Muscle activations 20 were then estimated using SynO with two to six synergies and traditional SO, and these estimates 21were compared with EMG measurements. SynO neither improved SO prediction of experimental 22 activation patterns nor provided SO exact matching of joint moments. Finally, synergy analysis was 23 performed on SO estimated activations, being found that the reconstructed activations produced poor 24 matching of experimental activations and joint moments. As conclusion, it can be said that, although 25SynO did not improve prediction of muscle activations during gait, its reduced dimensional control 26 space could be beneficial for applications such as functional electrical stimulation (FES) or motion 27 control and prediction. 28