Ankle resistance with a unilateral soft exosuit increases plantarflexor effort during pushoff in unimpaired individuals

Abstract

Background: Ankle-targeting resistance training for improving plantarflexion function during walking increases rehabilitation intensity, an important factor for motor recovery after stroke. However, understanding of the effects of resisting plantarflexion during stance on joint kinetics and muscle activity-key outcomes in evaluating its potential value in rehabilitation-remains limited. This initial study uses a unilateral exosuit that resists plantarflexion during mid-late stance in unimpaired individuals to test the hypotheses that when plantarflexion is resisted, individuals would (1) increase plantarflexor ankle torque and muscle activity locally at the resisted ipsilateral ankle, but (2) at higher forces, exhibit a generalized response that also uses the unresisted joints and limb. Further, we expected (3) short-term retention into gait immediately after removal of resistance.

Methods: Ten healthy young adults walked at 1.25 m s^-1 for four 10-min discrete bouts, each comprising baseline, exposure to active exosuit-applied resistance, and post-active sections. In each bout, a different force magnitude was applied based on individual baseline ankle torques. The peak resistance torque applied by the exosuit was 0.13 ± 0.01, 0.19 ± 0.01, 0.26 ± 0.02, and 0.32 ± 0.02 N m kg^-1, in the LOW, MED, HIGH, and MAX bouts, respectively.

Results: (1) Across all bouts, participants increased peak ipsilateral biological ankle torque by 0.13-0.25 N m kg^-1 (p < 0.001) during exosuit-applied resistance compared to corresponding baselines. Additionally, ipsilateral soleus activity during stance increased by 5.4-11.3% (p < 0.05) in all but the LOW bout. (2) In the HIGH and MAX bouts, vertical ground reaction force decreased on the ipsilateral limb while increasing on the contralateral limb (p < 0.01). Secondary analysis found that the force magnitude that maximized increases in biological ankle torque without significant changes in limb loading varied by subject. (3) Finally, peak ipsilateral plantarflexion angle increased significantly during post-exposure in the intermediate HIGH resistance bout (p < 0.05), which corresponded to the greatest average increase in soleus activity (p > 0.10).

Conclusions: Targeted resistance of ankle plantarflexion during stance by an exosuit consistently increased local ipsilateral plantarflexor effort during active resistance, but force magnitude will be an important parameter to tune for minimizing the involvement of the unresisted joints and limb during training.

Keywords: Gait biomechanics; Locomotor adaptation; Resistance training; Soft exosuit.

Fig. 1

Experimental setup and protocol overview. A Unilateral soft ankle exosuit. B Experimental conditions for each subject and force application schedule for each condition (top). Applied peak torque for each condition across subjects normalized by peak biological ankle torque from an initial biomechanics collection without any device (bottom left). Applied force profile for each experimental condition across the gait cycle (0% is heel strike) for a sample subject (bottom right)

Fig. 2

Ipsilateral plantarflexor effort across resistance magnitudes. A Change in peak net and biological ankle torque, and normalized soleus activity during stance during EXP and POST relative to BASE across all subjects. B Average ankle torque profiles during EXP across all subjects for each condition, normalized by body mass (left). Exosuit torque magnitude is plotted here for figure space efficiency but is negative for all conditions and subjects (see Additional file 1: Fig. S2C). Average normalized soleus activation profiles across all subjects for each condition during BASE, EXP, and POST (right). ^†One subject did not complete the MAX condition, and one subject did not have usable EMG data from the MED and HIGH conditions (N = 9). All error bars are s.e.m

Fig. 3

Ipsilateral joint work across resistance magnitudes. A Changes in magnitude of positive biological ankle work during stance (left), and magnitudes of negative knee and hip work during stance (center, right) in EXP and POST relative to BASE. B Average ankle, knee, and hip joint power profiles across the gait cycle in the MAX condition across all subjects. ^†One subject did not complete the MAX condition (N = 9). All error bars are s.e.m

Fig. 4

Average vertical ground reaction forces across resistance force magnitudes. A Change in resisted ipsilateral (left) and unresisted contralateral (right) average vertical ground reaction force during stance in EXP and POST relative to BASE across all subjects. B Vertical ground reaction force during BASE and EXP for a single subject at the HIGH condition segmented by ipsilateral heel strikes. †One subject did not complete the MAX condition (N = 9). All error bars are s.e.m

Fig. 5

After-effects upon removal of resistance at each force magnitude. A Change in peak plantarflexion angle during POST compared to BASE. B Change in mean soleus (SOL) activity during stance during POST compared to BASE. ^†One subject did not complete the MAX condition and one subject did not have usable SOL data for MED and HIGH (N = 9). All error bars are s.e.m

Fig. 6

Effects of resistance magnitude on specificity during EXP at an individual level (Table 1). Condition that maximizes increase in peak biological ankle torque without changes in bilateral limb loading for each subject. A shift in limb loading is considered significant if both the left and right limbs change significantly. A Representative changes in peak biological ankle torque data from two subjects. B Changes in average stance vertical ground reaction force on both limbs. ^‡The resistance magnitude that maximally targets the ankle may lie outside the explored range for these subjects. All error bars are s.e.m

Fig. 7

Comparison with conventional resistive band training during EXP. A Participant walking with passive resistance from a sensorized resistance band (top) and with exosuit-applied plantarflexion resistance (bottom). B Changes in peak biological ankle torque, peak torso angle, and peak hip torque in early stance with respect to BASE for both subjects (S1 and S4). C Changes in average biological torque during the early (0–20 %GC), mid (20–40 %GC), and late (40–65 %GC) stance phase with the band and exosuit applied resistance conditions

Fig. 8

Effects of explicit instructions during EXP. A Change in peak biological ankle torque compared to peak applied exosuit resistive torque (left). Change in average soleus (SOL) activity (right). B Change in average vertical ground reaction force (VGRF) on ipsilateral (left) and contralateral (right) limbs with implicit and explicit instructions for both subjects