Mechanical efficiency during hand-rim wheelchair propulsion: effects of base-line subtraction and power output

The purpose of this study was to compare four methods of calculating the mechanical efficiency of hand-rim wheelchair propulsion.

After completing a maximal incremental test, 18 untrained able-bodied males (mean age 22 years) performed four 4-min bouts of exercise (0%, 40%, 55%, and 70% of peak power output) on a wheelchair-ergometer. The mechanical efficiency was calculated during each submaximal bout by the ratio of work accomplished to the amount of energy expended with four indices: gross (no base-line correction), net (resting metabolism as base-line correction), work (unloaded movement as base-line correction) and delta (measurable work as base-line correction) indices.

Work efficiency (16.8-13.8%) were significantly higher (P < 0.001) than gross (6.5-8.9%) or net efficiency (9.9-11.1%) at 40-70% peak power output, respectively. Net efficiency was also significantly lower than work efficiency (P < 0.001) at all intensity. Moreover, gross and work efficiency increased and decreases (P < 0.001) respectively with intensity. In contrast, net and delta efficiency remained unchanged.

Depending upon the base-line used, each efficiency index provided different values and were differently affected by the exercise intensity. Precautions should be taken on the conclusions about the mechanical efficiency of hand-rim wheelchair propulsion. Moreover, comparisons between the four efficiency indices among themselves allowed a better and complete description of wheelchair locomotion and wheelchair dependent person. Gross or net efficiencies better reflected the actual mechanical efficiency of the hand-rim wheelchair propulsion whereas work or delta efficiencies investigated the efficiency of muscular contraction during this movement.

Since the four mechanical efficiency indices measured different aspects of performance, it will have consequences when it comes to choosing an appropriate index. The better understanding of mechanical efficiency index may help future investigators to choose the most appropriate index for their purposes.