Respiratory muscle training in athletes with cervical spinal cord injury: effects on cardiopulmonary function and exercise capacity

Key points The effect of combined inspiratory and expiratory muscle training on resting and reflexive cardiac function, as well as exercise capacity, in individuals with cervical spinal cord injury (SCI) is presently unknown. Six weeks of combined inspiratory and expiratory muscle training enhances both inspiratory and expiratory muscle strength in highly-trained athletes with cervical SCI with no significant effect on lung function. There was a significant decrease in left-ventricular filling and stroke volume at rest in response to 45 degrees head-up tilt, which is irreversible by respiratory muscle training. Combined inspiratory and expiratory muscle training increased peak aerobic work rate and reduced end-expiratory lung volumes during exercise, which may have implications for left-ventricular filling during exercise. To investigate the pulmonary, cardiovascular and exercise responses to combined inspiratory and expiratory respiratory muscle training (RMT) in athletes with tetraplegia, six wheelchair rugby athletes (five males and one female, aged 33 +/- 5 years) completed 6 weeks of pressure threshold RMT, 2 sessions day(-1) on 5 days week(-1). Resting pulmonary and cardiac function, exercise capacity, exercising lung volumes and field-based exercise performance were assessed at pre-RMT, post-RMT and after a 6-week no RMT period. RMT enhanced maximal inspiratory (pre- vs. post-RMT: -76 +/- 15 to -106 +/- 23 cmH(2)O, P = 0.002) and expiratory (59 +/- 26 to 73 +/- 32 cmH(2)O, P = 0.007) mouth pressures, as well as peak expiratory flow (6.74 +/- 1.51 vs. 7.32 +/- 1.60 L/s, P < 0.04). Compared to pre-RMT, peak work rate was higher at post-RMT (60 +/- 23 to 68 +/- 22 W, P = 0.003), whereas exercising end-expiratory lung volumes were reduced (P < 0.017). Peak oxygen uptake increased in all athletes at post-RMT (1.24 +/- 0.40 vs. 1.40 +/- 0.50 l min(-1), P = 0.12). After 6 weeks of no RMT all indices returned towards baseline, with peak work rate (P = 0.037), peak oxygen uptake (P = 0.041) and end-expiratory lung volume (P < 0.034) being significantly lower at follow-up than at post-RMT. There was a significant decrease in left-ventricular end-diastolic volume and stroke volume in response to 45 degrees head-up tilt (P = 0.030 and 0.021, respectively); however, all cardiac indices in both supine and tilted positions were unchanged by RMT. Our findings demonstrate the efficacy of RMT with respect to enhancing respiratory muscle strength, lowering exercising lung volumes and increasing exercise capacity. Although the precise mechanisms by which RMT may enhance exercise capacity remain unclear, our data suggest that it is probably not the result of a direct cardiac adaptation associated with RMT.