Effects of Modified Outsole Patterns in Tennis Shoes on Frictional Force and Biomechanical Variables of Lower Extremity Joints

(1) Background: This study aimed to examine the effects of four different outsole patterns on frictional force and lower extremities during tennis-specific movements. (2) Methods: Four tennis shoes with different widths of outsole treads were manufactured for this study (A: all thin, B: all thick, C: laterally thin and medially thick, D: medially thick and laterally thin). The frictional force was measured during a mechanical test. Eleven male recreational tennis players participated in the study. The data were collected using sixteen infrared cameras and a force plate for a biomechanical test. (3) Results: During the mechanical test, there was a significant difference between the shoes in the forward and sideward Coefficient of Translational Friction (CoF) and clockwise rotational friction (p < 0.05). There were significant differences in the maximum ankle internal rotation angle during braking motion (p < 0.05). In contrast, a significant difference in the maximum internal rotation moment of the ankle was found during forward braking motion (p < 0.05). (4) Conclusions: Based on these results, the difference in the outsole tread width (0.6 mm) affected mechanical frictional force, but this phenomenon was less influenced by the adaptation of the lower extremity joint’s movement in a tennis-specific motion. Finally, the difference in the outsole pattern affects the rotational movement and moment of the ankle joint. Thus, any slight change in friction by modified outsole patterns of tennis shoes requires close attention to develop the functional requirements for tennis performance.