Controlling Negative and Positive Power for Efficiency Enhancement and Muscle Strain Mitigation During Squatting with a Portable Knee Exoskeleton

Purpose: Workers face a notable risk of musculoskeletal injuries when performing squatting tasks. Knee exoskeletons offer a promising solution to mitigate muscle strain through squat assistance. However, existing studies on knee exoskeletons lack a comprehensive study that meets the multifaceted requirements of squatting assistance in terms of portability, efficiency, and muscle strain mitigation. Furthermore, another open research question pertains to the control strategy of squat assistance, which should be adaptable to various postures and cadences for different individuals. In particular, the effect of controlling negative power assistance during the squat-down phase is not studied. Methods: To fill these two gaps, first, we develop a simple (computationally efficient and implementable in a microcontroller) and generalizable (for different postures, cadences, and individuals) torque controller for portable knee exoskeletons that delivers both negative and positive power. Our portable knee exoskeleton can benefit users by enhancing efficiency (reducing metabolic cost, heart rate, breathing ventilation), mitigating muscle strain (reducing EMG), and reducing perceived exertion (reducing Borg 6–20 scale) during squatting. Second, we study the effect of three levels of negative power assistance during the squat-down phase. Results: This study integrates comprehensive biomechanics and physiology analyses that evaluate our exoskeleton's effectiveness using four objective and two subjective metrics with a group of able-bodied subjects (n = 7). The exoskeleton reduced metabolic cost by 12.8%, heart rate by 13.8%, breathing ventilation by 8.9%, and reduced extensor muscle activity by 39.4–43.2%, flexor muscle activity by 18.9–20.3%, and Borg perceived exertion rate by 1.8 during squatting compare with not wearing the robot. Conclusion: Different from the musculoskeletal model predictions that suggest increasing benefit with a higher level of negative power assistance, we find that the best performances were achieved with a moderate level of negative power assistance, followed by no assistance and then high assistance.