Is increasing the effective leg length of a human runner metabolically beneficial?

Abstract

The purpose of this study was to understand if footwear midsole thickness can decrease the metabolic cost of running by increasing the effective leg length of a runner. We also sought to understand the role of midsole compliance on these measures. Participants (n = 16) ran on a treadmill at 14 km h-1 in four mass-matched shoe conditions: 30, 40, and 60 mm thickness of a firm midsole foam and 60 mm thickness of a compliant midsole foam. Over two testing sessions, we measured metabolic cost (two 5-minute trials in each condition) and biomechanical measures (one 2-minute trial in each condition). Increasing thickness in the firm midsole conditions increased effective leg length during early, mid, and late stance (all p < 0.001). However, metabolic cost increased (p = 0.031). Changing material from firm to compliant decreased effective leg length during midstance (p < 0.001), but not during early or late stance, and decreased metabolic cost (p < 0.001). Results suggest that increasing midsole thickness can increase the effective leg length of a runner, but this isolated effect leads to increasing in metabolic cost. On the other hand, increasing midsole compliance leads to reductions in metabolic cost, agreeing with previous literature. We suggest that the performance benefits seen in advanced footwear technology with increased midsole thickness are likely due to increased capacity to return mechanical energy, not due increases in effective leg length. Summary StatementIncreased midsole compliance, not increased effective leg length, is responsible for decreased metabolic cost in running.