The human ankle varies impedance and delivers net positive work during the stance period of walking. In contrast, commercially available ankle-foot prostheses are passive during stance, causing many clinical problems for transtibial amputees, including non-symmetric gait patterns, higher gait metabolism, and poorer shock absorption. In this investigation, we develop and evaluate a myoelectric-driven, finite state controller for a powered ankle-foot prosthesis that modulates both impe-dance and power output during stance. The system employs both sensory inputs measured local to the external prosthesis, and myoelectric inputs measured from residual limb muscles. Using local prosthetic sensing, we first develop two finite state controllers to produce biomimetic movement patterns for level-ground and stair-descent gaits. We then employ myoelectric signals as control commands to manage the transition between these finite state controllers. To transition from level-ground to stairs, the amputee flexes the gastrocnemius muscle, triggering the prosthetic ankle to plantar flex at terminal swing, and initiating the stair-descent state machine algorithm. To transition back to level-ground walking, the amputee flexes the tibialis anterior muscle, triggering the ankle to remain dorsiflexed at terminal swing, and initiating the levelground state machine algorithm. As a preliminary evaluation of clinical efficacy, we test the device on a transtibial amputee with both the proposed controller and a conventional passive-elastic control. We find that the amputee can robustly transition between the finite state controllers through direct muscle activation, allowing rapid transitioning from level-ground to stair walking patterns. Additionally, we find that the proposed finite state controllers result in a more biomimetic ankle response, producing net propulsive work during level-ground walking and greater shock absorption during stair descent. The results of this study highlight the potential of prosthetic leg controllers that exploit neural signals to trigger terrain-appropriate, local prosthetic leg behaviors.

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Modelling of human motion is used in a wide range of applications. An important aspect of accurate representation of human movement is the ability to customize models to account for individual differences. The following work proposes a methodology using Hill-based candidate functions in the Fast Orthogonal Search (FOS) method to predict translational force at the wrist from flexion and extension torque at the elbow. Within this force estimation framework, it is possible to implicitly estimate subject-specific physiological parameters of Hill-based models of upper arm muscles. Surface EMG data from three muscles of the upper arm (biceps brachii, brachioradialis and triceps brachii) were recorded from 10 subjects as they performed isometric contractions at varying elbow joint angles. Estimated muscle activation level and joint kinematic data (joint angle and angular velocity) were utilized as inputs to the FOS model. The resulting wrist force estimations were found to be more accurate for models utilizing Hill-based candidate functions, than models utilizing candidate functions that were not physiologically relevant. Subject-speci¯c estimates of optimal joint angle were determined via frequency analysis of the selected FOS candidate functions. Subject-specfic optimal joint angle estimates demonstrated low variability and fell within the range of angles presented in the literature.

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Background: Isometric force measurements in the lower extremity are widely used in rehabilitation of subjects with neurological movement disorders (NMD) because walking ability has been shown to be rela-ted to muscle strength. Therefore muscle strength measurements can be used to monitor and control the ef-fects of training programs. A new method to assess isometric muscle force was implemented in the driven gait orthosis (DGO) Lokomat. To evaluate the capabilities of this new measurement method, inter- and intra-rater reliability were assessed. Methods: Reliability was assessed in subjects with and without NMD. Sub-jects were tested twice on the same day by two different therapists to test inter-rater reliability and on two separate days by the same therapist to test intra-rater reliability. Results: Results showed fair to good reliability for the new measurement method to assess isometric muscle force of lower extremities. In subjects without NMD, intraclass correlation coefficients (ICC) for inter-rater reliability ranged from 0.72 to 0.97 and intra-rater reliability from 0.71 to 0.90. In subjects with NMD, ICC ranged from 0.66 to 0.97 for inter-rater and from 0.50 to 0.96 for intra-rater reliability. Conclusion: Inter- and intra- rater reliability of an assessment method for measuring maximal voluntary isometric muscle force of lower extremities was demonstrated. We suggest that this method is a valuable tool for documentation and control-ling of the rehabilitation process in patients using a DGO.

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We examined healthy human subjects wearing robotic ankle exoskeletons to study the metabolic cost of ankle muscle–tendon work during uphill walking. The exoskeletons were powered by artificial pneumatic muscles and controlled by the user’s soleus electromyography. We hypothesized that as the demand for net positive external mechanical work increased with surface gradient, the positive work delivered by ankle exoskeletons would produce greater reductions in users’ metabolic cost. Nine human subjects walked at 1.25ms–1 on gradients of 0%, 5%, 10% and 15%. We compared rates of O2 consumption and CO2 production, exoskeleton mechanics, joint kinematics, and surface electromyography between unpowered and powered exoskeleton conditions. On steeper inclines, ankle exoskeletons delivered more average positive mechanical power (P<0.0001; +0.37±0.03Wkg–1 at 15% grade and +0.23±0.02Wkg–1 at 0% grade) and reduced subjects’ net metabolic power by more (P<0.0001; –0.98±0.12Wkg–1 at 15% grade and –0.45±0.07Wkg–1 at 0% grade). Soleus muscle activity was reduced by 16–25% when wearing powered exoskeletons on all surface gradients (P<0.0008). The ‘apparent efficiency’ of ankle muscle–tendon mechanical work decreased from 0.53 on level ground to 0.38 on 15% grade. This suggests a decreased contribution from previously stored Achilles’ tendon elastic energy and an increased contribution from actively shortening ankle plantar flexor muscle fibers to ankle muscle–tendon positive work during walking on steep uphill inclines. Although exoskeletons delivered 61% more mechanical work at the ankle up a 15% grade compared with level walking, relative reductions in net metabolic power were similar across surface gradients (10–13%). These results suggest a shift in the relative distribution of mechanical power output to more proximal (knee and hip) joints during inclined walking.

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We examined the metabolic cost of plantar flexor muscle–tendon mechanical work during human walking. Nine healthy subjects walked at constant step frequency on a motorized treadmill at speeds corresponding to 80% (1.00ms–1), 100% (1.25ms–1), 120% (1.50ms–1) and 140% (1.75ms–1) of their preferred step length (L*) at 1.25ms–1. In each condition subjects donned robotic ankle exoskeletons on both legs. The exoskeletons were powered by artificial pneumatic muscles and controlled using soleus electromyography (i.e. proportional myoelectric control). We measured subjects’ metabolic energy expenditure and exoskeleton mechanics during both unpowered and powered walking to test the hypothesis that ankle plantarflexion requires more net metabolic power (Wkg–1) at longer step lengths for a constant step frequency (i.e. preferred at 1.25ms–1). As step length increased from 0.8 L to 1.4 L, exoskeletons delivered ~25% more average positive mechanical power (P=0.01; +0.20±0.02Wkg–1 to +0.25±0.02Wkg–1, respectively). The exoskeletons reduced net metabolic power by more at longer step lengths (P=0.002; –0.21±0.06Wkg–1 at 0.8 L* and –0.70±0.12Wkg–1 at 1.4 L*). For every 1 J of exoskeleton positive mechanical work subjects saved 0.72 J of metabolic energy (‘apparent efficiency’=1.39) at 0.8 L and 2.6 J of metabolic energy (‘apparent efficiency’=0.38) at 1.4 L. Declining ankle muscle–tendon ‘apparent efficiency’ suggests an increase in ankle plantar flexor muscle work relative to Achilles’ tendon elastic energy recoil during walking with longer steps. However, previously stored elastic energy in Achilles’ tendon still probably contributes up to 34% of ankle muscle–tendon positive work even at the longest step lengths we tested. Across the range of step lengths we studied, the human ankle muscle–tendon system performed 34–40% of the total lower-limb positive mechanical work but accounted for only 7–26% of the net metabolic cost of walking.

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