Muscles of the Ankle and Foot

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• Strongest plantarflexor - Achilles tendon.
  • Inserts onto the posterior-medial calcaneus.
  • Fires during midstance to slow the forward motion of the tibia over the foot
• Strongest dorsiflexor - tibialis anterior.
  • Provides eccentric control during stance from heel-strike to foot flat.
  • Prevents foot slap, and during swing phase is allows clearance of the foot from the floor
• Strongest inverter - posterior tibialis muscle.
  • Dynamic supporter of the medial longitudinal arch. Inverts the subtalar joint during mid and late stance which results in locking of the transverse tarsal joint providing rigidity to the foot during toe-off.
  • Loss of this muscle results in acquired pes planus. The arch flattens, the forefoot abducts, and the heel everts. The patient is unable to actively invert their heel during toe rise.
• Primary everters - peroneus longus and peroneus brevis.
  • Peroneus longus
    • Inserts on the base of the first metatarsal and medial cuneiform.
    • Depresses (plantarflexes) the metatarsal head at the tarsometatarsal joint.
    • Loss of this muscle results in elevation of the first metatarsal head, decreasing loads on the first metatarsal, and results in dorsal bunion development.
  • Peroneus brevis - strongest everter
    • Stabilises the forefoot laterally by resisting inversion.
    • Loss of this muscle results in varus of the hindfoot.
• Interosseus muscles
  • Active during late stance, stabilises the forefoot during toe-off.
  • Imbalance between intrinsics and extrinsics leads to toe deformities such as hammer toes, claw toes, and mallet toes
• Intrinsic and extrinsic muscles
  • Flexor digitorum brevis - primary flexor of the PIPJ
  • Flexor digitorum longus - primary flexor of the DIPJ
  • Sesamoid bones within flexor hallucis brevis under the head of the first metatarsal increases lever arm distance of the muscle enabling greater flexion torque at the first MTPJ by moving the tendon away from the centre of the joint
  • Extrinsic extensors extend the MTPJ through the sagittal bands by lifting the proximal phalanges into extension.

References

• Basic Biomechanics of the Musculoskeletal System - Nordin 4th edition 2012