EMG-informed neuromuscular model assesses the effects of varied bodyweight support on muscles during overground walking

Abstract

Bodyweight supported walking is a common gait rehabilitation method that can be used as an experimental approach to better understand walking biomechanics. Neuromuscular modeling can provide an analytical means to gain insight into how muscles coordinate to produce walking and other movements. To better understand how muscle length and velocity affect muscle force during overground walking with bodyweight support, we used an electromyography (EMG)-informed neuromuscular model to investigate changes in muscle parameters (muscle force, activation and fiber length) at varying bodyweight support levels: 0%, 24%, 45% and 69% bodyweight. Coupled constant force springs provided a vertical support force while we collected biomechanical data (EMG, motion capture and ground reaction forces) from healthy, neurologically intact participants walking at 1.20 ± 0.06 m/s. The lateral and medial gastrocnemius demonstrated a significant decrease in muscle force (lateral: p = 0.002 and medial: p < 0.001) and activation (lateral: p = 0.007 and medial: p < 0.001) through push-off at higher levels of support. The soleus, in contrast, had no significant change in muscle activation through push-off (p = 0.652) regardless of bodyweight support level even though soleus muscle force decreased with increasing support (p < 0.001). During push-off, the soleus had shorter muscle fiber lengths and faster shortening velocities as bodyweight support levels increased. These results provide insight into how muscle force can be decoupled from effective bodyweight during bodyweight supported walking due to changes in muscle fiber dynamics. The findings contribute evidence that clinicians and biomechanists should not expect a reduction in muscle activation and force when using bodyweight support to assist gait during rehabilitation.

Keywords: Bodyweight support; CEINMS; Electromyography; Neuromuscular modeling; OpenSim.

Fig 1:

Experimental design protocol, characterized. RF-Rectus Femoris, VL/VM-Vastus Lateralis/Vastus Medialis, BF-Bicep Femoris, LG/MG-Lateral Gastrocnemius/Medial Gastrocnemius, TA-Tibialis Anterior, SOL-Soleus, MOCAP- Motion Capture, EMG-Electromyogram

Fig 2:

Stance phase was separated into three sections: loading response, mid-stance and push-off. These sections are separated by the first local maxima (F1) and minima (F2) in the subject’s vertical ground reaction forces.

Fig 3: (

Left) Ankle and knee external joint moment shown at 0% BW, 24% BW, 45% BW and 69% BW, standard deviation shown as shaded area. (Right) Ankle and knee angle shown at 0% BW, 24% BW, 45% BW and 69% BW, standard deviation shown as shaded area. Grey lines indicate F1 and F2 which split the gait cycle into loading response, mid-stance, and push-off.

Fig 4:

Multivariate statistics repeated measures MANOVA (reporting Wilk’s Lambda), how bodyweight condition affects each muscle and phase for all response variables. LR is loading response, MS is mid-stance, and PS is push-off.

Fig 5:

Univariate statistics, reporting the Greenhouse-Geisser sig. values. (Top) Muscle force, univariate statistics for how bodyweight condition affects each muscle and phase. (Middle) Fiber length range, univariate statistics for how bodyweight condition affects each muscle and phase. (Bottom) Muscle activation, univariate statistics for how bodyweight condition affects each muscle and phase.

Fig 6:

(Left) Soleus normalized force, normalized activation and normalized fiber length. (Middle) Gastrocnemius lateralis normalized force, normalized activation and normalized fiber length. (Right) Gastrocnemius medialis normalized force, normalized activation and normalized fiber length. Means shown with standard deviation as shaded error.

Fig 7:

Upper leg muscles, vastus lateralis, vastus medialis, bicep femoris and rectus femoris shown as normalized muscle force mean±s.d. (error bars) for loading response, mid-stance and push-off. Lower leg muscles, lateral gastrocnemius, medial gastrocnemius, soleus and tibialis anterior shown as normalized muscle force mean±s.d. (error bars) for loading response, mid-stance and push-off.