The Functional Importance of Plantar Intrinsic Muscles for Locomotion

Study Title: 
The functional importance of human foot muscles for bipedal locomotion 

Authors: 
Farris, DJ. et al. 

Publication Information: 
PNAS (2019) Vol. 116: 5. 1645-1650.

Background Information: 
One of the most significant developments in the evolution of the human foot is replacement of an opposable first digit in favor of a longitudinal arch (LA). Research has shown that the purpose of this LA, via the restructuring of the bones within the foot, is to stiffen the foot to enable bipedalism by providing leverage for propulsion to the ground. It has also been demonstrated that the LA exhibits elastic mechanics and is able to act in a spring-like manner to minimize energy cost in running.

Recent research has since focused on the intrinsic muscles of the feet, investigating their role in supportive foot mechanics during static and dynamic weightbearing.

The purpose of this study was to test the importance of the plantar intrinsic muscles (PIMS) for stiffening of the foot, for providing LA support, and for propulsion generation during walking and running.
 
Study Methods:
Two experiments were performed in which posterior tibial nerve blocks were used to prevent PIM activation in each. The first experiment examined controlled loading of the lower leg via linear actuator with and without nerve block. The second experiment examined walking and running on a treadmill with and without nerve block. The effect of the nerve block on LA deformation was measured by changes in the Cal-Met angle formed between the calcaneus and metatarsal segments.
 
Results:
- There was a significant effect on LA deformation via nerve block, particularly during peak force.
- No significant changes in LA deformation were found between the conditions (with and without nerve block) during the initial loading of the midfoot.
- A significant effect of the nerve block in reducing angular impulse generated by the midfoot moment during arch recoil was present. Propulsive impulses during walking and running were also negatively affected by nerve block.
- The nerve block also significantly reduced stiffness of the MTP joint during late stance leading to a drop in vertical ground reaction forces and MTP dorsiflexion.
- The ability to generate power and work through the foot and ankle during late stance was significantly affected by the nerve block. 
- Interestingly, the ability to generate power about the hip during walking and faster running (not slow walking) was also negatively affected by the nerve block.
- In contrast to the “lengthening” of the plantar aponeurosis via the windlass mechanism, tension of the PIMs occurred both isometrically and concentrically.
 
Conclusions:
While it is known that the longitudinal arch of the human foot has evolved for the purposes of both energy absorption and force transfer, this study demonstrated that the contributions of the plantar intrinsic muscles are load and activity specific. LA absorption of energy was only minimally supported by PIM activity, particularly in midstance, yet stiffness of the foot during push-off was highly dependent for the purposes of propulsion. 

Personal Interpretation and Significance:
In simplest terms, strengthening of the plantar intrinsic muscles may do very little to improve the shape and height of the arch of the foot. Instead, the focus of plantar intrinsic muscle strength should be placed on its role in contributing to lever rigidity and dynamic foot stiffness for the purposes of propulsion in late stance of higher speed walking and running. 

For introductory ideas on how to strengthen the plantar intrinsic muscles, this post might help.

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