## 4 Discussion
[3] on a population of children with cerebral palsy estimated the net work of a flexible AFO featuring a stiffness in plantarflexion similar to that of the present custom PD-AFO (0.11 \(\pm\) 0.13 N*m/deg), albeit more rigid in dorsiflexion (0.7 \(\pm\) 0.2 N*m/deg) by about 0.03 J/kg. Although not directly comparable to what measured here, [PERSON] et al. reported a positive work of 0.06 W/kg for AFOs with stiffness ranging from 1.4 to 23.5 N*m/deg [12]. Despite the relatively low stiffness of the present PD-AFO, the ankle plantarflexion angle was significantly lower (\(p\) < 0.05) in the swing phase when wearing the custom PD-AFO with respect to the shed condition [17]. All participants in this study were affected by a foot drop condition, but only some of them suffered also from mild plantarflexion deficit. These individuals are likely to benefit the most by using low-stiffness PD-AFOs, as excessive rigidity of the calf-shell in plantarflexion would hinder the ankle joint physiological mobility in the push-off phase. Although further validation through a larger clinical trial is warranted, preliminary evidence seems to suggest that individuals with foot drop exhibiting mild plantarflexion weakness may benefit from PD-AFOs characterized by minimal plantarflexion stiffness. Consequently, a subsequent study will be designed to quantify the minimum plantarflexion stiffness of a PD-AFO necessary to provide adequate foot and footwear support during the swing phase of gait. As far as the outcome of the present energetic analysis, the temporal profiles of the work calculated over the normalized stance phase duration were rather consistent across PD-AFOs (Figure 4). At foot contact, all PD-AFOs absorbed some energy by plantarflexing the calf-shell, which was released to the ankle up to midstance. Subsequently, energy was absorbed to dorsiflex the calf-shell up to about 90% of the stance phase. This energy was released to the ankle at push-off. It is therefore plausible that this, albeit small, amount of energy may be used to improve foot and ankle propulsion at late stance or to decrease the amount of ankle net work required to walk at the same walking speed. Indeed, while the participants walked faster when wearing the PD-AFO with respect to the shed condition, the power released at the ankle was lower, albeit not statistically significant, in the PD-AFO condition. Although a larger dataset is needed to establish a correlation between varying AFO stiffness and gait expenditure relative to ankle plantar flexor deficits, this study provides a foundation for individualized prescription guidelines for AFOs in drop foot conditions. These results should be critically assessed with respect to some limitations. Since no direct comparison with other custom AFO designs was performed in the same testing conditions, it is difficult to assess how the present low-stiffness design performs with respect to other--more rigid--custom dynamic AFOs. While the outcome of the energetic analysis was rather consistent across participants, the limited sample size prevents the present energetic data to be representative of the varying nature of the foot drop condition. Indeed, a larger foot drop population would allow us to include different walking patterns that may affect the present outcome. In addition, due to its flexibility, the AFO foot plate can also store and release some energy in the push-off phase and during metatarsopha-langeal joint dorsiflexion and plantar flexion. Thus, it can be hypothesized that the actual positive work released to the ankle could be larger than what reported in this study by analyzing the energetics of the calf-shell only. Furthermore, no follow-up evaluations were conducted thus the long-term energetics of the PD-AFO, its effect on ankle power, and the biomechanical adaptations or changes in performance following familiarization.