Unpredictable shoe midsole perturbations provide an instability stimulus to train ankle posture and motion during forward and lateral gym lunges.

Unstable footwear may enhance training effects to the lower-limb musculature and sensorimotor system during dynamic gym movements. This study compared the instability of an unstable shoe with irregular midsole deformations (IM) and a control shoe (CS) during forward and lateral lunges. Seventeen female gym class participants completed two sets of ten forward and lateral lunges in CS and IM. Ground reaction forces, lower-limb kinematics and ankle muscle activations were recorded. Variables around initial ground contact, toe-off, descending and ascending lunge phases were compared statistically ( < .05). Responses to IM compared to CS were similar across lunge directions. The IM induced instability by increasing the vertical loading rate ( < .001, = .009) and variability of frontal ankle motion during descending ( = .001, < .001) and ascending phases ( = .150, = .003), in forward and lateral lunges, respectively. At initial ground contact, ankle adjustments enhanced postural stability in IM. Across muscles, there were no activation increases, although results indicate peroneus longus activations increased in IM during the ascending phase. As expected, IM provided a more demanding training stimulus during lunge exercises and has potential to reduce ankle injuries by training ankle positioning for unpredictable instability.