Effect of Metatarsophalangeal Joint Position on Maximal Voluntary Isometric Ankle Plantar‐Flexion Torque

The joint position at which maximal torque is produced indicates the optimal position for agonist muscles to function. It is well-documented that the maximal voluntary isometric contraction (MVIC) torque of ankle plantar-flexion varies with knee and ankle joint positions. However, metatarsophalangeal (MTP) joint position may also affect MVIC torque, reflecting force-length relationships of the toe flexor muscles. This study examined the effect of MTP joint position on MVIC torque of ankle plantar-flexion. Sixteen young males performed MVICs of ankle plantar-flexion with the MTP joint at 0°, 15°, 30°, and 45° of dorsiflexion. Ankle plantar-flexion torque, plantar pressure distribution, and surface electromyography (sEMG) were recorded. MVIC torque was significantly greater at 15° than at 0°, 30°, and 45° (+11.0%-21.4%). Similarly, forces applied under the toes were significantly greater at 15° than at the other conditions (+13.4%-37.5%), whereas forces applied under the proximal foot and sEMG amplitudes of the major ankle plantar-flexors remained comparable across MTP joint positions. At 45° MTP joint dorsiflexion, toe forces were 17.5% lower than at 30°, COP shifted 3-5 mm proximally, and sEMG amplitudes were 26.8% lower in abductor hallucis than at 30°, and 28.9% and 26.3% lower in flexor digitorum brevis than at 15° and 30°, respectively. These results demonstrate that MVIC torque of ankle plantar-flexion varies with MTP joint position in an inverted U-shaped manner and is maximized at 15° of MTP joint dorsiflexion, reflecting changes in toe flexor forces, proximodistal COP displacement, and sEMG amplitudes of the intrinsic foot muscles.