Effects of short-term training on heart rate dynamics in individuals with spinal cord injury

Increased risk of cardiovascular diseases and autonomic dysregulation are common health concerns in individuals with spinal cord injury (SCI). Two therapies that may help improve cardiovascular control are body-weight supported treadmill training (BWSTT) and head-up tilt training (HUTT). The purpose of this study was to examine the effects of short-term BWSTT and HUTT on cardiac autonomic function. Seven participants (6 male, 37.1 ± 7.7 years) with SCI (C5-T10, ASIA A-C; 5.0 ± 4.4 years post-injury) completed the study protocol. In this randomized cross-over design, participants were required to complete 4 weeks of thrice-weekly BWSTT and HUTT (i.e. 12 sessions each), separated by a 4 week detraining period. Cardiac autonomic function was assessed at rest, before and after, each 4 week training period using linear and non-linear measures (sample entropy and detrended fluctuation analysis (α1)) of heart rate dynamics. Participants completed equivalent amounts of time performing BWSTT and HUTT (453.7 ± 27.3 min vs. 471.6 ± 19.7 min, p = 0.24). There were no significant differences in linear heart rate variability following BWSTT or HUTT (p > 0.05). In contrast, there was a significant change in sample entropy following BWSTT (1.05 ± 0.14 to 1.42 ± 0.12, p < 0.05). Due to the bi-directional pattern of α1 values, distance scores were calculated (│1 − α1│) and demonstrated a significant reduction following BWSTT (0.54 ± 0.06 to 0.26 ± 0.05, p = 0.001). In conclusion, 4 weeks of BWSTT but not HUTT training are sufficient to increase sample entropy and reduce the fractal scaling distance score in participants with SCI.