Energy cost of running uphill as compared to running on the level with impeding horizontal forces.

Purpose: We have previously shown that accelerated running on flat terrain is biomechanically equivalent to running uphill at a constant speed. This hypothesis was further investigated comparing the energy cost of running at a constant speed either uphill, or on flat terrain against an equivalent horizontal impeding force, mimicking acceleration.

Methods: Steady-state O₂ consumption and the corresponding energy cost (per unit body mass and distance) were determined on 12 male subjects during treadmill running at speeds between 2.11 and 2.89 m/s: (i) on the level, (ii) uphill at 10 or 20% incline ( $I$ ), or (iii) on the level against a horizontal traction force of 10 or 20% of the subject's body weight ( $\mathrm{TF}$ ). This allowed us to estimate the net efficiency ( $net\eta$ ) of running against horizontal or vertical forces, as given by the ratio between the additional mechanical work output under $\mathrm{TF}$ , or the corresponding $I$ condition, and the difference between the appropriate energy cost above that for running at constant speed on flat terrain.

Results: The $net\eta$ values when running uphill ( $I$ ) amount to 0.35-0.40, whereas those for running against an equivalent impeding force ( $\mathrm{TF}$ ) are about 10% greater (0.45-0.50), a fact that may be due to a greater recovery of elastic energy in the $\mathrm{TF}$ as compared to the $I$ condition.

Conclusion: Making allowance for these small differences, these data support the view of considering accelerated running on flat terrain biomechanically equivalent to running at a constant speed, up an equivalent slope.