The fibularis longus and fibularis brevis serve as the primary evertors of the foot, providing dynamic lateral stability to the ankle. Due to their critical stabilizing role, athletes who are prone to inversion sprains, such as basketball and volleyball players, often incorporate targeted strengthening, conditioning, and coordination exercises for these muscles in their training programs.
However, despite their effectiveness in resisting inversion forces, these muscles often fail to prevent injuries because their reflexive contraction response is typically too slow to counteract sudden and unexpected inversion movements. To enhance injury prevention, neuromuscular control strategies such as feed-forward mechanisms and anticipatory muscle activation are necessary.
Biomechanical Role of the Fibularis Muscles
Both fibularis longus and fibularis brevis have a substantial eversion moment arm (exceeding 2 cm) at the subtalar joint, allowing them to exert strong control over foot movement.
Their tendons pass posterior to the lateral malleolus, using it as a pulley, which enables them to cross behind the talocrural joint's axis of rotation. Due to this unique anatomical configuration, these muscles also contribute to plantarflexion at the talocrural joint. Additionally, they play a secondary role in abduction at both the subtalar and transverse tarsal joints.
The fibularis longus exhibits a distinct distal attachment, which allows it to generate an effective eversion torque at the forefoot. This function is particularly evident when attempting maximal supination in a non-weight-bearing position, where a subtle lateral rotation and plantarflexion at the first ray can be observed. Furthermore, the fibularis longus plays a crucial role in counteracting the medial pull of the tibialis anterior, thereby maintaining structural stability at the first tarsometatarsal joint. If this stabilizing function is compromised, the first ray may excessively deviate medially, increasing the risk of hallux valgus deformity.
Activation Patterns During Gait
The fibularis longus and brevis are most active during mid and late stance phases of the gait cycle.
Mid and Late Stance Phase
During this period, the dorsiflexed talocrural joint rapidly transitions into plantarflexion, while supination (inversion) occurs at the subtalar joint. The fibularis muscles help control the rate and magnitude of this motion through eccentric contractions.
Additionally, the active contraction of the fibularis longus stabilizes the first ray against the ground. If fibularis longus dysfunction, paralysis, or inhibition occurs, the posterior tibialis muscle exerts an unopposed supination force on the forefoot, leading to excessive medial deviation of the foot and significantly increasing the risk of inversion sprains.
Push-Off and Final Propulsion Phase
During the final push-off phase, the fibularis longus and brevis assist other plantarflexor muscles in producing plantarflexion torque at the talocrural joint.
Their lateral positioning plays a key role in counteracting the strong inversion (supination) tendency produced by the posterior tibialis, gastrocnemius, and other plantarflexors.
Toe-Off Stability and Muscle Coordination
This muscle balance is particularly important during toe-standing movements. As the heel lifts off the ground, the fibularis longus and posterior tibialis contract simultaneously, creating a "sling" mechanism that stabilizes both the transverse and medial longitudinal arches.
As a result of this coordination, the non-weight-bearing rearfoot naturally assumes a slightly supinated position, further enhancing the stability of the medial longitudinal arch and forefoot structures. This stability ensures that plantarflexion torque is efficiently transferred to the metatarsal heads, facilitating a strong and stable push-off.
Role in Weight Transfer and Step Transition
As heel lift occurs during the push-off phase, the fibularis longus plays a crucial role in smoothly transferring body weight from the lateral to the medial forefoot. This controlled weight shift ensures a seamless transition of body weight to the opposite foot, effectively initiating the next stance phase of the gait cycle.