ABSTRACT
Following spinal cord injury (SCI) task-dependent modulation of spinal reflexes are often impaired. To gain insight into the state of the spinal interneuronal pathways following injury, we studied the amplitude modulation of triceps surae cutaneous reflexes to non-noxious stimuli during standing and early-to-mid stance phase of walking in participants with and without chronic incomplete SCI. Reflex eliciting nerve stimulation was delivered to the superficial peroneal, sural, and distal tibial nerves about the ankle. Reflexes were analyzed in the short (SLR, 50-80 ms post stimulation onset) and the medium (MLR, 80-120 ms) latency response windows. Further, the relation between cutaneous and H-reflexes was also examined during standing. In participants without injuries the soleus SLR was modulated task-dependently with nerve specificity, and the soleus and medial gastrocnemius MLRs were modulated task-dependently. In contrast, participants with SCI, no task-dependent or nerve-specific modulation of triceps cutaneous reflexes was observed. The triceps surae cutaneous and H-reflexes were not correlated in either group (r = 0.01-0.37). The presence of cutaneous reflexes but the absence of significant amplitude modulation may suggest impaired function of spinal interneuronal pathways in this population. The lack of correlation between the cutaneous and H-reflexes may suggest that interneurons that are involved in H-reflex modulation and cutaneous reflex modulation do not receive common input, or the impact of the common input is outweighed by other input. Present findings highlight the importance of examining multiple spinal reflexes to better understanding spinal interneuronal pathways that affect motor control in people after SCI.
ABSTRACT
Coordination between ipsilateral and contralateral muscles of the lower limbs has a critical role in movement control. However, the roles that aging and physical activity have in maintaining bilateral coordination are understudied. The aim of this study was to examine the roles of physical activity and age on pathways between the soleus and tibialis anterior muscles. Fourteen young and 14 older subjects (7 active, 7 sedentary) participated. A Hoffmann (H)-reflex was elicited in the ipsilateral soleus following a conditioning stimulus to the contralateral common peroneal nerve at differing intervals (25-300 ms). Significant H-reflex facilitation from the control value was observed for the sedentary group at the 50-ms (28.7 %), 75-ms (24.5 %), and 150-ms (34.0 %) intervals when compared with the physically active group. There were also significant differences between the young and older groups. Results demonstrated differences in soleus H-reflex excitability as a result of contralateral conditioning and highlighted the influence of age and physical activity in maintaining these neural pathways.
