The unexpected influence of a stiff minimalist insole on the forefoot and ankle joint biomechanics during walking

The aim of this study was to explore the efficacy of a minimalist stiff insole in limiting/reducing first metatarsophalangeal joint (1MTP) range of motion and to explore the impact on the ankle joint during walking. It was hypothesized that the minimalist stiff insole would significantly reduce 1MTP kinematic and kinetic parameters whilst causing compensatory effects at the ankle joint.

Methods

Fourteen healthy participants were recruited. Three-dimensional lower limb and forefoot kinematics were measured during walking at a self-selected speed with a stiff and control insole. Discrete point statistical analysis and statistical parametric mapping (SPM) were used to evaluate the main effects of the insole conditions on kinematic and kinetic parameters.

Results

Significant differences were seen between insole conditions during the 3rd rocker phase during walking with the stiff insole surprisingly increasing the range of motion (p < 0.001), reducing MTP dorsiflexion angular velocity and increasing MTP plantarflexion angular velocity (p < 0.001), and increasing external MTP dorsiflexion moment and power generation (p < 0.001). At the ankle, the stiff insole increased ankle dorsiflexion angular velocity (p < 0.001), decreased ankle plantarflexion angular velocity (p < 0.001), and decreased ankle power absorption (p < 0.001) and power generation (p < 0.001).

Conclusion/clinical relevance

Clinicians and researchers should be cautious about the expected mechanical effects and the actual biomechanical impact of stiff insoles and keep in mind that acting on one joint can create proximal compensations.