Increasing Intrinsic Foot Muscle Tension Seems To Decrease The Tensile Stress On Plantar Fascia During The Midstance Of The Gait Cycle

Recent studies have shown an association between lower volume of intrinsic foot muscles (IFM) and plantar fasciitis (PF) and a role of IFM exercises in rehabilitation for PF. However, no study has been conducted to evaluate whether increasing the tension of the IFM is associated with a decrease in tensile stress on the plantar fascia, a mechanism which may potentially explain the association between IFM and plantar fasciitis. PURPOSE: To investigate the relationship between IFM tension and plantar fascia tensile load (PFTL) using fresh frozen cadavers. METHODS: Ten fresh frozen lower limbs with normal range of motion and without definite arthritic changes in the foot and ankle were used for this experiment. Custom-made apparatus for experiments were applied to all specimens with uniform protocol. The tibia and fibular bones were cut at distal one-third and firmly attached to the apparatus. To simulate physiological tension of the IFM, the flexor digitorum brevis and abductor hallucis muscles were detached from their origin of the calcaneus and connected to a servomotor. To analyze tensile stress loaded on the plantar fascia, the plantar fascia was detached from its origin and connected to the servomotor with in-line load cells for direct real-time measurement of the tensile stress. To stimulate midstance of the gait cycle, vertical compression force of 300N was axially applied, and the PFTL at zero position was set at 50N. Then, the load was gradually applied from 0 to 210N to the IFM. Simultaneously, the changes of PFTL were measured with the load cells. This experiment was repeated for all ten cadaveric lower limbs. RESULTS: The Pearson’s correlation coefficient between IFM tension and PFTL was -0.980 (p < 0.001). The mean PFTL with maximal IFM tension decreased by 38.2% compared to the mean PFTL with no IFM tension (28.9N vs. 46.8N). An increase in IFM tension by 1N was associated with a decrease in the PFTL by 0.22%. CONCLUSIONS: The current experiment demonstrated that the IFM tension is negatively associated with the PFTL. The findings of our study suggest that lower IFM may be associated with the development of plantar fasciitis by increasing the tensile stress on plantar fascia. Furthermore, IFM exercises may be beneficial in treating plantar fasciitis as strengthening IFM may decrease the tensile stress on plantar fascia.