Characterizing the relationship between isolated extrinsic lower limb muscle activity and Talocrural, Subtalar, and...

Extrinsic lower limb muscle activity is often studied using simplified segmental models that overlook distinct kinematics of individual joints within the ankle joint complex and its associated articulations. This study provides a joint-level characterization of how individual muscle groups influence talocrural, subtalar, and midtarsal joint kinematics under an unweighted condition. Six fresh-frozen cadaveric specimens were mounted to a tendon force actuator system to independently actuate six functional muscle groups: tibialis anterior (TA), extensors (EXT), peroneals (PER), flexors (FLX), tibialis posterior (TP), and Achilles. Individual stepwise loading trials were performed for the five muscle groups other than the Achilles. Each muscle group was tested in 20 % force increments while the Achilles was held at a constant 10 % baseline tension. Three-dimensional rotations at the tibiotalar (TT), talofibular, tibiofibular, subtalar (ST), talonavicular (TN), and calcaneocuboid joints were measured. Parallel coordinate plots revealed distinct muscle-specific and shared joint coordination patterns, with consistent ST/TN coronal and transverse coupled motions across muscle groups. Principal component analysis showed that the first principal component strongly reflected force-dependent motion (R² = 0.75-0.90). TA primarily drove TT dorsiflexion and ST/TN inversion and internal rotation. EXT produced ST/TN eversion and external rotation. PER and FLX contributed greater midtarsal motions in the coronal and transverse planes, consistent with common hindfoot-midfoot stabilization strategies. FLX and TP did not generate strong TT plantarflexion, likely due to the initial plantarflexed alignment and baseline Achilles activation, but TP showed strong ST/TN inversion and internal rotation. Findings highlight the influence of initial foot posture, hindfoot-midfoot coupling, and the value of joint-level analysis for understanding muscle function, refining musculoskeletal models, and evaluating surgical interventions.