Kinematic and Neuromuscular Adaptations in Incomplete Spinal Cord Injury after High- versus Low-Intensity Locomotor Training

Abstract

Recent data demonstrate improved locomotion with high-intensity locomotor training (LT) in individuals with incomplete spinal cord injury (iSCI), although concerns remain regarding reinforcement of abnormal motor strategies. The present study evaluated the effects of LT intensity on kinematic and neuromuscular coordination in individuals with iSCI. Using a randomized, crossover design, participants with iSCI received up to 20 sessions of high-intensity LT, with attempts to achieve 70-85% of age-predicted maximum heart rate (HRmax), or low-intensity LT (50-65% HRmax), following which the other intervention was performed. Specific measures included spatiotemporal variables, sagittal-plane gait kinematics, and neuromuscular synergies from electromyographic (EMG) recordings. Correlation analyses were conducted to evaluate associations between variables. Significant improvements in sagittal-plane joint excursions and intralimb hip-knee coordination were observed following high- but not low-intensity LT when comparing peak treadmill (TM) speed before and after LT. Neuromuscular complexity (i.e., number of synergies to explain >90% of EMG variance) was also increased following high- but not low-intensity LT. Comparison of speed-matched trials confirmed significant improvements in the knee excursion of the less impaired limb and intralimb hip-knee coordination, as well as improvements in neuromuscular complexity following high-intensity LT. These findings suggest greater neuromuscular complexity may be due to LT and not necessarily differences in speeds. Only selected kinematic changes (i.e., weak hip excursion) was correlated to improvements in treadmill speed. In conclusion, LT intensity can facilitate gains in kinematic variables and neuromuscular synergies in individuals with iSCI.

Keywords: high-intensity exercise; locomotor training; muscle synergy; non-negative matrix factorization.

FIG. 1.

Schematic representation of neuromuscular synergy analysis. Six normalized electromyographic (EMG) envelopes were arranged in a matrix (100 × 6) and imported into non-negative matrix factorization (NNMF). This iterative optimization algorithm seeks the smallest number of activation profiles (Synergy activation: H), which suffice to regenerate the original EMG signals with at least 90% accuracy. Synergy activation profiles (H) and synergy coordination (W) are then determined so as to minimize the error between original EMG measurements and reconstructed EMG signals (W × H). In this schematic diagram, the optimization algorithm of NNMF has been solved for four synergies leading to H (4 × 100) and W (6 × 4). MG, medial gastrocnemius; MH, hamstring; RF, rectus femoris; SOL, soleus; TA, tibialis anterior; VL, vastus lateralis. Color image is available online.

FIG. 2.

Comparison of intralimb coordination before (BSL) and after (POST) high-intensity training. Angular component of the coefficient of correspondence (ACC) and skewness (SKW) were quantified from the hip-knee cyclogram. Color image is available online.

FIG. 3.

Two representative examples of additional synergy modules formed following high-intensity training. (A,B) The number of modules increased from three baseline to four post-training in participant 6, with led to increased differentiation between mid-stance and loading response (reduced co-activation between TA and RF muscles at module H3). Two additional synergy modules formed following high-intensity training, from three at baseline (C) to five at post-training (D). Specific changes include greater differentiation at terminal swing and initial contact/stance (reduced co-activation between MH and VL at module H1), greater differentiation of terminal stance and initial swing (reduced co-activation of SOL and TA at module H2), and greater differentiation between mid-stance and loading response (reduced co-activation between TA and RF muscles at module H3). MG, medial gastrocnemius; MH, medial hamstring; RF, rectus femoris; SOL, soleus; TA, tibialis anterior; VL, vastus lateralis. Color image is available online.