Quantification of Task-Dependent Leg Muscle Synergy Patterns During Isometric Force Generation Tasks in a Seated Position Among Healthy Older Individuals

The generation of isometric forces in the legs, which require the activation of combined muscle groups as muscle synergy, is essential during various daily activities. However, there is a lack of information on whether task-dependent leg muscle synergy patterns exist in a seated position. Thus, this study aimed to investigate whether task-dependent leg muscle synergy patterns exist in simplified direction-specific force generation tasks (FGT) on the sagittal and frontal planes in healthy older individuals. Thirteen healthy old individuals were seated with their hips and knees flexed at 90° on a customized lower limb force-measuring device. They generated isometric force in response to visual cues presented in anterior, posterior, lateral, and medial directions. Simultaneously, muscle activities from nine muscles in the same leg were obtained. Peak forces significantly differed across the four directions (p<0.05). Three muscle synergies accounted for 95% of the variance in electromyographic (EMG) data during the FGTs. Muscle synergies exhibited high reliability across subjects, with an average intraclass correlation coefficient (ICC) of 0.91 and a 95% confidence interval of 0.8-0.98 (p<0.001). Each muscle synergy had a significantly different preferred direction (PD) angle in degrees (p<0.01). Peak EMG activity of the quadriceps, hamstrings, and adductor muscles also exhibited significant differences across directions (p<0.05, p<0.01, and p<0.001, respectively). Our findings and approach may provide the foundation for developing an evaluation tool to plan targeted rehabilitation for individuals with neuromuscular disorders during the early stage of their rehabilitation or for older individuals when walking is not feasible.