Introduction: Ankle Foot Orthoses (AFO) is an assistive device commonly used to improve gait after a wide range of musculoskeletal problems, with many designs available to provide support and alignment, compensate for muscle weakness, and help prevent secondary musculoskeletal deformities. The big challenge is what is the effect of using ankle-foot orthosis (AFO) on lower limb muscles and joint biomechanics. Therefore, the main aim of this study was to evaluate the kinematics and kinetics of the lower limb during running by musculoskeletal modeling in OpenSim software.

Methods: Twenty-nine subjects participated in this study (68.4±5.7 kg, 173±8.7 cm, 28.5±7.5 years). Each subject ran with and without AFO in the motion analysis lab. Twelve Vicon motion captures (Vicon MX, Oxford, UK, 200 Hz) and two force plates (Kistler Instrumente AG, Winterthur, Switzerland, sampling rate 2000 Hz) were used. The full-body model of OpenSim consists of joints with 23 degrees of freedom and 92 muscle-tendon actuators to represent 76 muscles in the lower extremities and the torso was selected as a generic model. The generic model was scaled by mass, height, isometric strength, and markers data in the static position. The inverse kinematics, inverse dynamics, static optimization, and joint reaction analysis were used to evaluate the relationship between wearing the AFO and lower limb biomechanics. The t-test was used to compare the two groups (barefoot and AFO) in MATLAB software.

Results: The kinematics and kinetics results of the lower limb showed there are significant differences between wearing AFO and barefoot conditions (P<0.05). By wearing the AFO muscle (knee flexors and extensors and ankle plantar flexors and dorsi flexors) activity patterns changed and consequently, the joint reaction analysis of the ankle, knee, and hip joints completely changed. The ankle plantar flexor muscles were most heavily impacted by the AFOs, with gastrocnemius recruitment decreasing 19–81% and correlating with increasing knee flexor moments (r2 = 0.26–0.87).

Discussion: These findings support the potential use of AFOs for athletes and highlight how subject-specific kinematics and kinetics may influence muscle demand and recruitment to inform AFO design. This valuable information may be used by sports engineers, coaches, and athletes.