Enhancing performance during inclined loaded walking with a powered ankle-foot exoskeleton

A simple ankle-foot exoskeleton that assists plantarflexion during push-off can reduce the metabolic power during walking. This suggests that walking performance during a maximal incremental exercise could be improved with an exoskeleton if the exoskeleton is still efficient during maximal exercise intensities. Therefore, we quantified the walking performance during a maximal incremental exercise test with a powered and unpowered exoskeleton: uphill walking with progressively higher weights. Nine female subjects performed two incremental exercise tests with an exoskeleton: 1 day with (powered condition) and another day without (unpowered condition) plantarflexion assistance. Subjects walked on an inclined treadmill (15 %) at 5 km h(-1) and 5 % of body weight was added every 3 min until exhaustion. At volitional termination no significant differences were found between the powered and unpowered condition for blood lactate concentration (respectively, 7.93 +/- A 2.49; 8.14 +/- A 2.24 mmol L-1), heart rate (respectively, 190.00 +/- A 6.50; 191.78 +/- A 6.50 bpm), Borg score (respectively, 18.57 +/- A 0.79; 18.93 +/- A 0.73) and peak (respectively, 40.55 +/- A 2.78; 40.55 +/- A 3.05 ml min(-1) kg(-1)). Thus, subjects were able to reach the same (near) maximal effort in both conditions. However, subjects continued the exercise test longer in the powered condition and carried 7.07 +/- A 3.34 kg more weight because of the assistance of the exoskeleton. Our results show that plantarflexion assistance during push-off can increase walking performance during a maximal exercise test as subjects were able to carry more weight. This emphasizes the importance of acting on the ankle joint in assistive devices and the potential of simple ankle-foot exoskeletons for reducing metabolic power and increasing weight carrying capability, even during maximal intensities

Transitions between symmetrical and asymmetrical gaits: a biomechanical analysis

INTRODUCTION: Gallop is a skipping gait in which one leg (the leading leg) is continuously kept in front of the other (3). This type of locomotion occurs spontaneously in the development of locomotion in children (1) and occurs sometimes in adults when descending stairs or a slope at high speed (2). Although gallop is a naturally occurring human locomotion pattern, research on human gallop is limited. METHODS: Fifteen female subjects with homogeneous stature were selected. They were asked to walk, run and gallop at preferred speed and to perform multiple transitions from walking to running (WRT), galloping to running (GRT) and walking to galloping (WGT). Subjects were equipped with 59 reflective markers and performed the trials on an overground walkway with 6 built‐in forceplates and 12 infrared cameras (Pro Reflex, Qualisys). Kinematics and kinetics were calculated using commercial software (Visual 3D, C‐motion). RESULTS: Subjects consistently used the same leading leg during gallop. Joint kinematics and kinetics showed differences between the leading and trailing leg in gallop. Transition speed of GRT (3.83±0.34ms‐1) was significantly higher than transition speed of WGT (2.66±0.24ms‐1)(p<0.01) and WRT (2.82±0.26ms‐1)(p<0.01) but no statistical difference was found between WGT and WRT (p=0.410)(fig.1). A clear transition step was seen in the WRT and the GRT based on joint kinematics, kinetics, and patterns of mechanical energy. In the WRT, GRT and WGT the swing phase prior to the transition step showed greater (dorsi)flexion in the ankle, knee and hip in comparison with previous walking/galloping steps. In the WGT (2.66±0.24 ms‐1) also the stance phase in the step before transition showed more (dorsi)flexion in the ankle, knee and hip. When subjects initiated the WGT when the leading leg was not in front, they showed some inconsistent intermediate running/skipping steps before they started galloping. Fig.1: transition speed for WGT, WRT and GRT. *and* are significant differences between transitions (p<0,01) CONCLUSIONS: Gallop is appropriately called an asymmetrical gait pattern as the leading and trailing leg execute a different movement. Adults seldom switch spontaneously from walking to galloping so the WGT is supposed to be planned. Still the initiation of transition seems to occur spontaneously because transition sometimes initiated when the leading leg was not in front. If the transition would occur intentionally, one would expect that transition only initiates when the leading leg is in front. Transition speed is very similar for WGT and WRT so it could be that a similar mechanism (arising in the acceleration from walking) determines when the transition occurs. Transition is prepared in the same way in the WRT and the GRT. There is a limited preparation in the swing phase preceding the actual transition step. In the WGT two transition steps were seen. As both legs carry out a different movement in gallop, it seems like each leg needs a transition step to alter the new gait configuration. The transition from an asymmetrical gait pattern to a symmetrical gait pattern (GRT) seems easier to perform than a transition from a symmetrical gait pattern to an asymmetrical gait pattern (WGT) as at least two steps were necessary to make the transition in the WGT in comparison with one step in the GRT. The gait pattern before the transitions, seems to determine the instant of transition initiation (similarity between WGT and WRT). The gait pattern after transition seems to be important in the way the transition is prepared (similarity between WRT and GRT). REFERENCES 1.Clark JE, Whitall J. Changing patterns of locomotion: from walking to skipping. In: Woollacott M, Shumway‐Cook A, editors. Development of posture and gait across the life span. Columbia: University of South Carolina Press; 1989.p. 128‐51. 2.Getchell N, Whitall J. Transitions to and from asymmetrical gait patterns. Journal of Motor Behaviour. 2004; 36 (1):13‐27. 3.Minetti AE. The biomechanics of skipping gaits: a third locomotion paradigm? Proceedings of the Royal Society B: Biological Sciences. 1998;265:1227‐35

Analysis of walking with unilateral exoskeleton assistance compared to bilateral assistance with matched work

The finding of the highest negative metabolic rate versus mechanical work ratio in the Bilateral Matched Total Work condition means that if a constrained amount of mechanical work is available (e.g. from a battery) it is more advanta- geous to distribute this work evenly over both legs. The EMG reductions in the unassisted leg also suggest that if the goal is to maximize assistance to one (impaired) leg it might still be advantageous to use a bilateral exoskeleton, perhaps with a different actuation pattern for each leg that is specifically optimized such that each exoskeleton side assists specific phases in the impaired leg

Bi-articular Knee-Ankle-Foot Exoskeleton Produces Higher Metabolic Cost Reduction than Weight-Matched Mono-articular Exoskeleton

The bi-articular m. gastrocnemius and the mono-articular m. soleus have different and complementary functions during walking. Several groups are starting to use these biological functions as inspiration to design prostheses with bi-articular actuation components to replace the function of the m. gastrocnemius. Simulation studies indicate that a bi-articular configuration and spring that mimic the m. gastrocnemius could be beneficial for orthoses or exoskeletons. Our aim was to test the effect of a bi-articular and spring configuration that mimics the m. gastrocnemius and compare this to a no- spring and mono-articular configuration. We tested nine participants during walking with knee-ankle-foot exoskeletons with dorsally mounted pneumatic muscle actuators. In the bi-articular plus spring condition the pneumatic muscles were attached to the thigh segment with an elastic cord. In the bi-articular no-spring condition the pneumatic muscles were also attached to the thigh segment but with a non-elastic cord. In the mono-articular condition the pneumatic muscles were attached to the shank segment. We found the highest reduction in metabolic cost of 13% compared to walking with the exoskeleton powered-off in the bi-articular plus spring condition. Possible explanations for this could be that the exoskeleton delivered the highest total positive work in this condition at the ankle and the knee and provided more assistance during the isometric phase of the biological plantarflexors. As expected we found that the bi-articular conditions reduced m. gastrocnemius EMG more than the mono-articular condition but this difference was not significant. We did not find that the mono-articular condition reduces the m. soleus EMG more than the bi-articular conditions. Knowledge of specific effects of different exoskeleton configurations on metabolic cost and muscle activation could be useful for providing customized assistance for specific gait impairments

Bi-articular knee-ankle-foot exoskeleton produces higher metabolic cost reduction than weight-matched mono-articular exoskeleton

The bi-articular m. gastrocnemius and the mono-articular m. soleus have different and complementary functions during walking. Several groups are starting to use these biological functions as inspiration to design prostheses with bi-articular actuation components to replace the function of the m. gastrocnemius. Simulation studies indicate that a bi-articular configuration and spring that mimic the m. gastrocnemius could be beneficial for orthoses or exoskeletons. Our aim was to test the effect of a bi-articular and spring configuration that mimics the m. gastrocnemius and compare this to a no-spring and mono-articular configuration. We tested nine participants during walking with knee-ankle-foot exoskeletons with dorsally mounted pneumatic muscle actuators. In the bi-articular plus spring condition the pneumatic muscles were attached to the thigh segment with an elastic cord. In the bi-articular no-spring condition the pneumatic muscles were also attached to the thigh segment but with a non-elastic cord. In the mono-articular condition the pneumatic muscles were attached to the shank segment. We found the highest reduction in metabolic cost of 13% compared to walking with the exoskeleton powered-off in the bi-articular plus spring condition. Possible explanations for this could be that the exoskeleton delivered the highest total positive work in this condition at the ankle and the knee and provided more assistance during the isometric phase of the biological plantarflexors. As expected we found that the bi-articular conditions reduced m. gastrocnemius EMG more than the mono-articular condition but this difference was not significant. We did not find that the mono-articular condition reduces the m. soleus EMG more than the bi-articular conditions. Knowledge of specific effects of different exoskeleton configurations on metabolic cost and muscle activation could be useful for providing customized assistance for specific gait impairments