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

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 increases in metabolic cost. On the other hand, increasing midsole compliance, while keeping midsole thickness constant, leads to reductions in metabolic cost, agreeing with previous literature. We suggest that the performance benefits seen in advanced footwear technology are likely due to increased resilience of the midsole, not due to increases in effective leg length from increased midsole thickness.