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 ± 2.49; 8.14 ± 2.24 mmol L(-1)), heart rate (respectively, 190.00 ± 6.50; 191.78 ± 6.50 bpm), Borg score (respectively, 18.57 ± 0.79; 18.93 ± 0.73) and VO₂ peak (respectively, 40.55 ± 2.78; 40.55 ± 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 ± 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.