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Objectives: The purpose of this investigation was to evaluate the physiologic and anatomic determinants of the exercise response in individuals with spinal cord injuries (SCI).
Clinical Relevance: Differences in physiologic and anatomic determinants of exercise due to habitual activity are indicative of effects that are seen in training programs. In the absence of organized training programs, SCI individuals may still have significant cardiovascular and structural benefits from increases in habitual activity alone. Health care providers should therefore encourage and facilitate persons with spinal cord injuries to increase habitual activities to improve cardiovascular responses.
Methods: Participants included fourteen males with SCI (8 tetraplegic, 6 paraplegic), aged 28-56 years (mean 45). Subjects were further categorized by habitual activity level: 7 sedentary and 7 active (aerobic activity >30 mm/day, at least 3x/wk). Individualized graded maximal exercise testing was performed using Monark arm crank ergometry. Cardio-respiratory responses including VO2, VE, and RER were assessed with a MedGraphics Cardio2 system. Cardiac structural analysis was determined via transthoracic M-mode echocardiography. Regional body composition was derived from whole body densitometry by use of Hologic QDR-1000/W. Relationships between variables were assessed by use of linear regression analysis. Analysis of variance with post hoc Scheffe testing was used to evaluate differences in physiologic and anatomic components due to level of lesion (tetraplegia vs. paraplegia) and activity status. Alpha levels of 0.05 were used to test for significance.
Results: Although VEmax was a moderately strong predictor of peak VO2 in spinal cord injured individuals (R2=0.85) when not separated by lesion level, there was a near perfect association in paraplegic individuals (R2=0.98) with a much weaker association in tetraplegic individuals (R2=0.49). While no differences were attributable to lesion level, active individuals showed significantly greater left ventricular diameter during diastole. However, there was not a significant relationship between diastolic dimensions and peak V02. Individuals with paraplegia had significantly greater percent upper extremity lean tissue than those with tetraplegia. Lean tissue mass was moderately predictive of peak VO2 (R2=0.59) and this relationship was not substantially different in those with paraplegia than tetraplegia.
Conclusions: The reduced association observed between maximal ventilation and peak VO2 in individuals with tetraplegia is attributed to decreased innervation of muscles of respiration in individuals with tetraplegia versus those with paraplegia. Inactive individuals have decreased left ventricular diastolic dimensions as compared with their active counterparts. Active individuals may maximize physiologic mechanisms to increase preload to the left ventricle thus increasing left ventricular diameter during diastole. As expected, the amount of metabolically active tissue is correlated with the amount of physiologic work a SCI individual is capable of performing. In tetraplegic individuals, there is no significant relationship between percentage of lean tissue and activity level. However, in paraplegic subjects, percent of lean tissue increases with higher activity level.
Acknowledgments: VA Rehabilitation R & D Merit Review Grant #B2110-RA.