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.