76 Chapter 5 Table 1. Spatiotemporal gait parameters for each level of trunk movements in participants with HSP. Trunk movements during gait Normal (n=35) Moderately increased (n=38) Markedly increased (n=13) Walking speed (m/s) 0.93 [0.72 - 1.35] 0.86 [0.26 - 1.26]* 0.87 [0.53 - 1.04]*** Cadence (steps/min) 103.8 ± 10.0 97.9 ± 15.6 90,8 ± 12.3 *** Step length (m) 0.55 ± 0.07 0.50 ± 0.10* 0.53 ± 0.07 Step width (m) 0.19 [0.13 - 0.38] 0.19 [0.13 - 0.37]** 0.15 [0.09 - 0.42]*** Toe-walkers (% of subgroup) 0% 6%** 69% Values displayed are means (± standard deviation) or median [range]. * indicates significant differences between patients with normal trunk movements and moderately increased trunk movements. ** indicates significant differences between patients with moderately increased trunk movements and markedly increased trunk movements. *** indicates significant differences between patients with normal trunk movements and markedly increased trunk movements. Discussion In this historic cohort study, we investigated whether increased trunk movements during gait in people with HSP were associated with reduced balance capacity. Previous studies already reported increased trunk movements during gait in people with HSP in comparison to healthy controls 7-9, 15. The current study has added value by exploring the potential association between increased trunk movements and clinical balance performance. Although our retrospective and cross-sectional study design does not allow inferences about causality, we found an association between increased trunk movements and reduced balance capacity. This association may have several – not mutually exclusive – explanations. One of these explanations is that trunk movements, at least partly, reflect balance correcting strategies. Balance perturbations can be due to extrinsic factors (such as an icy pavement) or intrinsic factors (such as an impaired anticipatory postural adjustment to self-initiated movement). In people with HSP, intrinsic factors appear to play an important part. For instance, calf muscle spasticity may result in a sudden knee extension during the single-stance phase of gait, or cause retropulsion in a sit-tostance transfer, jeopardizing postural stability. 4 Compensatory trunk movements for enlarging step length and foot clearance during gait8, 11, 12 may also act as balance perturbations. Following an intrinsic balance perturbation, balance needs to be restored reactively. In general, humans have three strategies to restore balance during gait i): the foot placement strategy, where people alter foot placement of the swing leg to adjust the base of support13, 14, 16-18; ii) the ankle strategy, where ankle moments of the stance leg are modulated to make (minor) adjustments to center-of-
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