VO 2 -PO Discordance In Paraplegia; Considerations For Using Power Output To Prescribe Exercise At Various Intensities

Abstract

Endurance exercise is often prescribed relative to peak power output (% POpeak) obtained during a graded exercise test (GXT). Persons with spinal cord injury (SCI) conducting arm cycle ergometry (ACE) display a unique VO2-PO relationship not yet quantified in the context of exercise prescription. Indirect evidence suggests that a relatively low % POpeak will be required to elicit a given % VO2peak during moderate intensity continuous exercise (MICE). Applying this concept of VO2-PO discordance to high intensity interval exercise (HIIE) prescription, one must consider the possibility for a seemingly moderate intensity % POpeak to elicit a physiological response indicative of HIIE. PURPOSE: To determine the % POpeak required to elicit a target VO2 during MICE, and to explore the use of % POpeak to prescribe HIIE in persons with SCI. METHODS: Ten adult men (39±10 yr) with chronic (13.2±8.8 yr) paraplegia (T2-T10) completed a GXT with 3 min stages where PO increased 20 W·stage-1 from a starting PO (10-40 W) estimated to elicit volitional exhaustion in 8-12 min. Then, in a randomized order, % POpeak was used to prescribe MICE and HIIE. The duration of each session was chosen so that the sessions were isocaloric. During MICE, ∆VO2/∆PO was used to estimate the power output that would elicit a steady state 50% VO2peak. HIIE was completed with 2 min duty cycles at 70:10% POpeak. For HIIE, the last minute of the work or recovery phase was used to calculate the mean VO2 of working and recovery phases. RESULTS: Mean cardiorespiratory fitness (19.2±5.2 ml·kg-1·min-1) classified participants as ‘good’ based on normative data. POpeak was a strong predictor of VO2peak (r=.960, p<.001) and VO2 gain (∆VO2/∆PO) was 10.3±1.8 ml·min-1·W-1. During MICE, 24.6±6.7% POpeak elicited a VO2 of 53.1±6.5% VO2peak (10.1±2.2 ml·kg-1·min-1). During HIIE, the work and recovery phases averaged 88.3±6.7 and 49.2±6.8% VO2peak (16.9±4.2 and 9.3±2.2 ml·kg-1·min-1, respectively), and 29.4±7.7% of the session was spent above 80% VO2peak. MICE and HIIE were isocaloric (115.9±21.8 and 116.6±35.0 kcal, respectively; p=.903). CONCLUSION: Both MICE and HIIE conditions demonstrated evidence of VO2-PO discordance. These findings emphasize the unique considerations pertaining to the use of % POpeak to prescribe ACE in SCI.