Task Specific Locomotor Training Effects On Ventilatory Drive In Men With Incomplete Spinal Cord Injury

Abstract

Exercise hyperpnea transitions toward a steady state through a feed-forward coupling of locomotor activation and a cortical irradiation pathway, which occurs prior to blood metabolite stimulation of the respiratory center. During a constant work rate (CWR) the transition curve is characterized by a sudden exponential rise in Phase I and II expired minute ventilation (VE) resolving in a Phase III steady state. However, in adults with incomplete spinal cord injuries (iSCI) efferent and afferent neural coupling is often impaired, potentially altering ventilatory control. PURPOSE: This study aimed to characterize exercise hyperpnea during a rest to CWR transition and the effects of 15 weeks of task-specific locomotor training (LT) on CWR hyperpnea. Participants: Subjects were 4 adult males with iSCI (age 24.75±7.80 yrs; BMI 20.4±5.1 kg·m2) with C4/C5 lesions capable of step initiation and independent standing. METHODS: LT principles included: practice variability, progressive overload and task specificity. Individual sessions included 5 segments: joint mobility, volitional muscle activation, task-isolation, task-integration, activity rehearsal. Training occurred 2x/week for 90 minutes focusing on developing walking efficiency through mastering the specific components of the gait cycle. All activities were weight-bearing and under volitional control. Assistance was only given when needed to ensure safety. Six minutes of CWR treadmill walking was performed before and after the LT at self-selected pace (0.5 or 0.7mph), with pulmonary gas analysis throughout the tests. VE line of best fit was predicted with linear regression and compared to actual VE observed (VEOBS - VEPred=VE variability), with VE variability assessed via an f-statistic. RESULTS: Suitability of linear regression was checked through visual inspection of CWR VE data. Prediction error variability decreased on average by 69% (p<0.001) after LT in 3 of 4 participants. CONCLUSION: CWR VE from rest to work was linear throughout the transition with no phase III plateau. A significant level of VE variability was observed before LT. In 3 of the 4 participants, VE variability was reduced by 69% after 15 weeks of LT. In these subjects with iSCI, it appears 15 weeks of LT improves exercise hyperpnea by reducing the variability in VE.