28d • research at a glance
The Soleus Muscle: A Foundational Contributor to Locomotion and Running Efficiency
Among the triceps surae group, the soleus remains one of the most underappreciated muscles in clinical and performance discussions. While the gastrocnemius often receives primary focus for its visible contribution to plantarflexion and sprinting, the soleus — with its endurance-oriented fiber composition and unique mechanical role — is arguably the first major muscle group to engage in the kinetic chain during gait initiation and running.
Anatomical and Functional Overview
The soleus originates from the posterior aspect of the tibia and fibula and inserts into the calcaneus via the Achilles tendon. Unlike the gastrocnemius, it does not cross the knee joint, allowing for consistent activation regardless of knee angle. This monoarticular design enables the soleus to provide postural stability and generate sustained plantarflexion torque, particularly during stance and propulsion phases (Michaud, n.d.).
The muscle’s composition is predominantly Type I slow-twitch fibers, making it highly fatigue-resistant and capable of sustaining prolonged contraction without rapid decline in force output (Böhm et al., 2021). This structural adaptation is critical for its continuous role in stabilizing the tibia over the talus and maintaining center-of-mass control throughout gait.
Early Engagement and Mechanical Role in Gait
During the stance phase of both walking and running, the soleus is among the first lower-limb muscle groups to engage following heel contact. It eccentrically controls tibial advancement over the foot, then transitions concentrically to provide propulsive force. Research using ultrasound and EMG data confirms that the soleus fascicles remain active throughout stance, operating near optimal shortening velocities for mechanical efficiency (Böhm et al., 2021; Lai et al., 2015).
In running, this function becomes magnified. Sasaki and Neptune (2006) demonstrated that the soleus contributes substantially to both vertical support and forward propulsion, generating forces up to eight times body weight at push-off. These mechanical contributions occur with minimal metabolic cost due to its fiber type composition and tendon elasticity, highlighting its role in energy conservation during repetitive loading cycles.
Clinical Implications
From a rehabilitation and performance perspective, the soleus serves as a critical stabilizer of the lower kinetic chain. It decelerates tibial progression, protects the Achilles tendon from excessive strain, and supports efficient energy transfer between the foot and the proximal segments (Gordon et al., 2013). Insufficient soleus strength or endurance has been implicated in common overuse injuries including Achilles tendinopathy, medial tibial stress syndrome, and patellofemoral dysfunction due to altered load distribution and tibial control (Michaud, n.d.).
Clinically, the soleus should be specifically targeted in both strength and endurance training programs — not just grouped under general “calf” work. Since the gastrocnemius’ activation diminishes with knee flexion, bent-knee plantarflexion exercises (such as seated heel raises or isometric holds at 90° knee flexion) preferentially recruit the soleus. For runners and endurance athletes, programming should include high-repetition, moderate-load plantarflexion work and functional endurance drills to mirror its sustained role in locomotion.
Summary for Clinical Practice
- The soleus is the first major lower-leg muscle to engage during stance and continues to operate throughout gait and running cycles.
- It contributes substantially to postural control, shock absorption, and propulsion, making it vital for efficient locomotion.
- Deficits in soleus strength or endurance may predispose patients to a range of lower-limb pathologies.
- Rehabilitation and conditioning programs should incorporate soleus-specific loading strategies, particularly under conditions that mimic functional knee flexion.
Recognizing the soleus not merely as a supporting muscle but as a primary driver of stability and efficiency in gait allows clinicians to refine both prevention and performance strategies for athletes and general populations alike.
References
Böhm, S., Mersmann, F., Santuz, A., Schroll, A., & Arampatzis, A. (2021). Muscle-specific economy of force generation and efficiency of work production during human running. eLife, 10, e67182. https://doi.org/10.7554/eLife.67182
Gordon, K. E., Selgrade, B. P., Sawicki, G. S., & Ferris, D. P. (2013). Locomotor adaptation to a soleus EMG-controlled antagonistic plantar flexor simulation. Journal of Neurophysiology, 109(6), 1578–1586. https://doi.org/10.1152/jn.00637.2012
Lai, A. K. M., Lichtwark, G. A., Schache, A. G., Lin, Y. C., Brown, N. A., & Pandy, M. G. (2015). In vivo behavior of the human soleus muscle with changing speeds of locomotion. Journal of Applied Physiology, 118(5), 559–569. https://doi.org/10.1152/japplphysiol.00128.2015
Michaud, T. (n.d.). The Overlooked and Underappreciated Soleus Muscle. Human Locomotion. Retrieved from https://www.humanlocomotion.com/wp-content/uploads/2024/11/Soleus-new.pdf
Sasaki, K., & Neptune, R. R. (2006). Differences in muscle function during walking and running: An investigation using a three-dimensional muscle-actuated model. Journal of Biomechanics, 39(11), 2035–2044. https://doi.org/10.1016/j.jbiomech.2005.05.019
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The Soleus Muscle: A Foundational Contributor to Locomotion and Running Efficiency
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