Third Layer of Intrinsic Muscles
The third layer consists of three primary muscles:
- Adductor hallucis
- Flexor hallucis brevis
- Flexor digiti minimi
These muscles function as a cohesive unit, originating from the cuboid, cuneiforms, proximal metatarsal bases, and adjacent connective tissues.
Adductor Hallucis
- Structurally similar to the adductor pollicis in the hand.
- Composed of two distinct heads:
- Oblique head
- Transverse head
- Both heads insert onto the lateral base of the proximal phalanx of the hallux, along with the adjacent lateral sesamoid bone.
- Primary functions:
- Adduction of the hallux at the metatarsophalangeal (MTP) joint.
- Assists in flexion of the MTP joint.
Flexor Hallucis Brevis (FHB)
- Has two separate heads that insert onto the medial and lateral base of the proximal phalanx of the hallux.
- Sesamoid bones embedded within the tendons optimize the mechanical leverage for hallux flexion.
- Primary function:
- Flexion of the MTP joint of the hallux.
Flexor Digiti Minimi
- Works in conjunction with the abductor digiti minimi.
- Inserts laterally onto the proximal phalanx of the fifth toe.
- Primary function:
- Flexion of the fifth toe at the MTP joint.
Fourth Layer of Intrinsic Muscles
The fourth and deepest layer consists of seven muscles:
- Three plantar interossei
- Four dorsal interossei
The arrangement and function of these interossei closely resemble those in the hand, with a key distinction:
- In the hand, the third digit (middle finger) serves as the reference point for abduction and adduction.
- In the foot, the second digit (second toe) is the reference point.
Dorsal Interossei (Dorsal Interosseous Muscles)
The dorsal interossei are bipennate muscles that play a crucial role in toe movement and stability.
- Unlike the other toes, the second toe has two dorsal interossei attached to it, allowing movement medially or laterally.
- The third and fourth toes each have one dorsal interosseous muscle that attaches laterally.
- All dorsal interossei insert on the proximal phalanges:
- First and second dorsal interossei attach to the medial and lateral sides of the second toe.
- Third and fourth dorsal interossei attach to the lateral sides of the third and fourth toes.
- Primary function:
- Abduction of the MTP joints (moving toes away from the second toe).
Plantar Interossei (Plantar Interosseous Muscles)
There are three plantar interossei, each associated with the third, fourth, and fifth toes.
- Unlike the bipennate dorsal interossei, plantar interossei are unipennate.
- They insert on the medial base of the proximal phalanges of the corresponding toes.
- Primary function:
- Adduction of the MTP joints (moving toes toward the second toe).
Functional Significance of Intrinsic Muscles
- Muscle action studies often assume a non-weight-bearing state, where free movement of the toes is unrestricted.
- However, in functional weight-bearing conditions, these muscles serve a different role.
- Intrinsic foot muscles do not function for fine manipulation like the hand.
- Instead, their primary roles are:
- Providing balance and stability while standing and walking.
- Enhancing foot structure during the push-off phase of gait.
- Supporting the medial longitudinal arch.
Electromyography (EMG) studies show that intrinsic foot muscles are most active during late stance phase (heel-off and toe-off) when the heel lifts from the ground.
Key Differences Between Intrinsic Muscles of the Hand and Foot
- The foot lacks intrinsic muscles responsible for opposition (unlike the thumb and pinky of the hand).
- These anatomical and functional differences highlight the foot's role in stability and propulsion, contrasting with the hand's role in dexterity and manipulation.