Influence of Knee Monoarticular Muscle Strength on Lower Limb Joint Moments and Muscle Synergies During Walking

Human locomotion involves intricate interactions between mono-articular and bi-articular muscles, each playing distinct roles in joint mechanics. Mono-articular muscles, which cross a single joint, typically exhibit straightforward flexor or extensor actions, whereas bi-articular muscles, spanning two joints, demonstrate complex activation patterns essential for precise force distribution. Despite extensive research on bi-articular muscle functions, the impact of knee mono-articular muscle strength-the biceps femoris short head (BFSH) and vasti muscles-on joint moments at the hip, knee, and ankle during walking remains underexplored. This study investigates how variations in the strength ($F_{0}$) of knee mono-articular muscles affect the activation patterns of knee bi-articular muscles and the resulting sagittal joint moments during walking. Utilizing a comprehensive musculoskeletal model developed by Rajagopal et al., this study analyzed how ±30% variations in $F_{0}$ for knee mono-articular muscles influenced knee muscle activations and lower limb joint moments. The results revealed that changes in $F_{0}$ led to significant variations in knee flexor moments and associated hip and ankle moments, especially during stance. For instance, increased quadriceps strength correlated with higher knee flexion moments and compensatory adjustments in the hip and ankle joints. Conversely, weakened vasti muscles resulted in increased activation amplitude, while reduced BFSH strength caused compensatory activation in the gastrocnemius medialis to maintain joint stability. These findings underscore the complex interplay between mono-articular and bi-articular muscles, highlighting their critical roles in dynamic joint stabilization and movement coordination. Understanding these interactions is vital for accurate biomechanical modeling and improving interventions for gait-related disorders. Future research will explore additional muscle parameters and activities to further elucidate the knee monoarticular and biarticular muscle relationships.