77 Trunk movements in people with HSP mass (CoM) movements13, 16, 18, 19; and iii) the hip strategy, where upper body segments are rotated around the CoM. 13, 14 When all three strategies are available for balance recovery, foot placement adjustment strategies and ankle strategies are preferred, while hip strategies are generally recruited when both foot placement and ankle strategies are insufficient; for instance, when walking on a narrow beam14, when performing under time pressure (i.e., responding to a perturbation that occurs just prior to foot contact, leaving insufficient time to perform an adequate foot placement adjustment20-22), or when affected by a neurological disease15). People with HSP may be limited in using the ankle strategy for balance corrections due to lower-extremity spasticity (e.g., of the calf muscles), muscle weakness (e.g. of the ankle dorsiflexors), and/or ankle-foot deformities (e.g., pes equinus or pes equinovarus), or even sensory or cerebellar ataxia. 4 To compensate, patients become more dependent on the foot placement strategy. 17 When adjustments in the mediolateral direction are needed, for example, step width must be increased. In more severely affected individuals with HSP, adjustments in foot placement themselves may be impaired due to proximal lower-extremity spasticity (e.g. of the hip adductors), ataxia, and/or apparent slowness of postural responses. 4 In these patients, the recruitment of hip strategies may become particularly important to maintain balance during gait, which would explain the presently reported increased trunk movements. Although less likely, we cannot rule out truncal ataxia as an alternative explanation for increase trunk movements as we did not assess its presence at the time of the instrumented gait analysis. In support of this line of reasoning, a previous study reported that after restoring the prerequisites for recruitment of ankle strategies and foot placement strategies in a patient with HSP and bilateral structural pes equinus (through bilateral Achilles tendon lengthening), a clear decrease in trunk movements was observed, suggesting less reliance on hip strategies to maintain balance during gait. 15 In addition, studies in other populations have demonstrated increased trunk movements when ankle strategy recruitment was artificially hampered. For example, in nine healthy participants, bilateral foot- and ankle-immobilizing casts were used to limit ankle strategy recruitment. During gait, trunk lateroflexion increased in the castcondition compared to walking with lightweight sneakers. 23 Comparably, in children with cerebral palsy, rigid ankle-foot orthoses hampering the recruitment of ankle strategies were found to increase lateral trunk movements compared to barefoot walking 24, 25 and compared to walking with shoes without orthoses. 26 As plantigrade foot contact is a prerequisite for effective use of ankle strategies, 4 the loss of ankle strategies likely explains the large proportion of toe walkers in our patient category with markedly increased trunk movements. 5
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