Movement strategies for maintaining standing balance during arm tracking in people with multiple sclerosis

Abstract

The purpose of this study was to quantify hip and ankle movement strategies during a standing arm tracking task in people with multiple sclerosis (MS). Full-body kinematics and kinetics were assessed with motion analysis cameras and force plates in nine MS and nine age-matched control subjects. While standing, participants used their dominant hand to track a target moving around a large horizontal or vertical figure eight on a screen in front of them. The target moved at constant speed, or linearly increasing speeds, with a frequency between 0.05 Hz and 0.35 Hz. Hip and ankle moments and angles during tracking were calculated from kinematic and kinetic measurements. Ratios of peak-to-peak (PP) hip/ankle moments (kinetics) and angles (kinematics) were calculated to determine the strategies of the hips and ankles used to maintain balance during arm movements. Center of mass (CoM) root mean square (RMS) acceleration was calculated as a measure of overall balance performance. The MS group produced larger PP hip/ankle moments at all speeds compared with the control group (P < 0.05). The CoM RMS acceleration increased with tracking speed for both groups but was not significantly different between groups. Additionally, the ratios of hip to ankle moments were highly correlated with the Berg Balance Scale during horizontal steady-speed tracking in MS. These results suggest that people with MS increase the use of the hip during standing arm tracking compared with age-matched control subjects. This adapted strategy might allow people with MS to achieve balance performance similar to control subjects, possibly increasing the importance of the hip in maintaining balance during voluntary movements.

Keywords: balance strategy; perturbation; posture; tracking.

Fig. 1.

Representative single participant data: representative kinematic, kinetic, and EMG data from a representative control and a representative multiple sclerosis (MS) participant during a vertical 0.25-Hz tracking trial. At the hip, flexion angles and external moments are positive and extension angles and external moments are negative. At the ankle, dorsiflexion angles and external moments are positive and plantarflexion angles and external moments are negative. MG, medial head of the gastrocnemius; TA, tibialis anterior.

Fig. 2.

Group average kinematic and kinetic plots for figure eight tracking: MS and control group averages of hip and ankle sagittal plane moments and sagittal plane angles during a 0.20-Hz vertical steady-speed tracking task. Each plot represents a full cycle of tracking (0–100% cycle) around the figure eight trajectory. Positive moments indicate hip extension or ankle plantarflexion moments, and negative moments indicate hip flexion or ankle dorsiflexion moments. Positive angles indicate hip flexion or ankle dorsiflexion, and negative angles indicate hip extension or ankle plantarflexion. The MS group was observed to have increased hip moment and angular range of motion and reduced ankle moment and angular range of motion compared with the control group.

Fig. 3.

Group average EMG Plots. MS and control group averages of the left and right leg MG and TA muscles during a 0.35-Hz vertical and horizontal steady-speed (SS) tracking task. Each plot represents a full cycle of tracking (0–100% cycle) around the figure eight trajectory. The MS group was observed to have reduced MG and TA activity compared with the control group.

Fig. 4.

Peak-to-peak (PP) hip-to-ankle ratio of joint kinematics and kinetics for each speed: group averages of PP hip-to-ankle ratio of joint moments and joint angles for each steady-speed and ramp-speed vertical and horizontal tracking. The MS group showed an increased PP hip-to-ankle ratio for joint moments and angles across all tracking speeds, compared with the control group. *Significance across pairwise comparisons between same speeds (P < 0.05). Standard deviation bars are indicated for each speed (negative bars for control group and positive bars for MS group).

Fig. 5.

Steady-speed PP hip-to-ankle ratios averaged across all speeds compared with the Berg Balance Scale (BBS). The participants with MS showed a strong regression between PP hip-to-ankle moment ratio and BBS during steady-speed horizontal tracking, demonstrating that the PP hip-to-ankle moment ratio increased with decreasing BBS. *Regression significance (P < 0.05).