Effects of squatting speed on lower extremity kinematics and kinetics during stable and unstable squat
The purpose of the study was to investigate the effects of surface instability and movement speed on key kinematic and kinetic factors (ground reaction force, moment arm [MA], resultant joint moments) in the lower extremity joints (ankles, knees, and hips) during squat. A total of 30 healthy college students (8 males and 22 females) performed six different squat conditions based on 2 surfaces (stable and unstable) and 3 speeds (slow, moderate, and fast). Normalized peak resultant joint moments (RJM) of the lower extremity joints (i.e., hips, knees, and ankles) were extracted from each trial. Two two-way repeated-measure MANOVAs (2 x 3) were performed. The first MANOVA test was to compare resultant joint moment variables, whereas, the second MANOVA test was conducted to compare ground reaction force and moment arm variables with the speed and surface condition being the factors in both. The first MANOVA with RJM variables revealed a significant speed * surface interaction (p < .001). The fast speed condition showed significantly larger RJMs of the lower extremity joints than the moderate and slow speed conditions and the moderate speed condition showed larger RJMs than the slow speed condition in both surface conditions. Significant larger hip and ankle RJMs and lower knee RJM observed in the unstable surface condition across speed conditions. The second MANOVA with GRF and MA variables revealed no significant speed * surface interaction (p = .055). However, significant main effects of speed factor (p < .001) was observed in the ground reaction force (GRF) and surface factor (p < .001) was observed in the moment arm (MA). Significant larger GRFs observed on the fast speed condition than the moderate and slow speeds and the moderate speed condition showed larger GRFs than the slow speed condition in all joints. The unstable surface condition revealed larger hip and ankle MAs and significantly lower knee MA than the stable surface condition. Ensemble average normalized RJM patterns were analyzed. The overall shapes of the hip and knee RJM patterns were similar to those of the MA patterns and the trends were similar in both surface conditions. The surface conditions generated very different ankle joint MA patterns, whereas, the ankle RJM patterns were similar to the MA patterns. The peaks RJM of the lower extremity joints were observed hovering around the maximum knee flexion (MKF) of squat. Based on the results of this study, the unstable surface condition would induce larger force acting on the hip and ankle joints and lower force acting on the knee joint compare to the stable surface condition.