Correcting for heel lift reduces ankle joint operating range for the ankle plantar flexor strength curve

The ankle plantar flexor muscles are stronger in ankle dorsiflexion than plantar flexion. However, heel lift is commonly observed during ankle plantar flexor strength testing, a phenomenon that would alter ankle angle. The effect of heel lift on the relation between ankle angle and plantar flexor strength has largely been ignored. This research examined the ankle plantar flexor strength curve, correcting for ankle plantar flexion due to heel lift. Six males and six females performed maximal isometric ankle plantar flexor contractions at five ankle positions. The purpose-built dynamometer could measure both resting ankle angle and foot plantar flexion due to heel lift; the combination was used to calculate the ankle angle during contraction. A significant interaction between dynamometer position and heel lift correction was observed (P < 0.001). The interaction had a linear trend (adjusted R² = 0.94, P < 0.001) indicating heel lift was smallest in ankle plantar flexion and increased with greater ankle dorsiflexion. The difference in ankle angle following heel lift correction occurred with the ankle in dorsiflexion (P < 0.001, Cohen's d = 2.6) and near neutral (P < 0.001, Cohen's d = 2.2). After correcting for heel lift, the ankle plantar flexor strength curve was compressed to a smaller range of ankle angles, specifically, reducing the maximum dorsiflexion angle.