Tibialis anterior: dorsiflexion, inversion and foot drop
Tibialis anterior dorsiflexes the foot (lifts it towards the shin bone) and inverts it (turns the sole inwards); its loss causes foot drop, where the foot cannot be lifted properly.
Remember
Up and in: tibialis anterior lifts the foot and turns the sole inwards, and its loss causes foot drop.
Memory aid
tibialis anterior is the up-and-in muscle (dorsiflexion up, inversion in), while its partner tibialis posterior is the down-and-in muscle (plantarflexion down, inversion in)
Think up and in for tibialis anterior and down and in for tibialis posterior, so the two muscles differ only in the first direction.
Tibialis anterior lies along the outer surface of the shin bone (the tibia), at the front of the leg, in the anterior compartment (the front group of leg muscles). The deep fibular nerve supplies the whole compartment. Its tendon attaches to the medial cuneiform (a small bone on the inner side of the midfoot) and the base of the first metatarsal (the long bone behind the big toe).
The tendon attaches at the inner border of the foot, so as well as lifting the foot it turns the sole inwards. It is the strongest dorsiflexor of the foot, meaning it pulls the foot up towards the shin at the ankle. Its secondary action is inversion, which means turning the sole inwards. Think up and in. Tibialis posterior, its partner at the back of the leg, is down and in: it also inverts the foot, but it plantarflexes (points the foot downwards) instead.
In the swing phase (when the foot is off the ground and moving forwards), tibialis anterior lifts the front of the foot so the toes clear the floor. If the muscle or the deep fibular nerve is damaged, the foot cannot be lifted properly. This is foot drop: the toes catch the ground and patients trip. Weak dorsiflexors also let the front of the foot fall quickly after the heel strikes, so it slaps down, which is called foot slap. Foot slap shows up as the foot lands, whereas toe drag shows up while the foot swings through.
Key facts
- Main action
- dorsiflexion, pulling the foot up towards the shin at the ankle, and it is the strongest dorsiflexor of the foot.
- Second action
- inversion (turning the sole inwards), a secondary action that comes from the tendon attaching on the inner side of the foot.
- Nerve supply
- the deep fibular nerve, which supplies the whole anterior compartment.
- Contrast
- tibialis anterior is up and in (dorsiflexion up, inversion in), while tibialis posterior is down and in (plantarflexion down, inversion in).
- Foot drop
- weakness or inability to dorsiflex the foot, with toe drag in the swing phase and foot slap just after heel strike.
Exam traps
- Tibialis posterior also inverts the foot, which makes it tempting, but it plantarflexes (points the foot downwards) and does not dorsiflex, because it belongs to the deep posterior compartment at the back of the leg.
- Extensor digitorum longus is tempting because it shares the compartment and the deep fibular nerve and helps dorsiflexion, but its main job is extending the lateral four toes; inversion is not one of its actions.
- Eversion (turning the sole outwards) is not a tibialis anterior action, because tibialis anterior actually inverts the foot, and eversion would not lift the front of the foot.
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