Title: Acute effects of cyclic and sustained squat exposures on knee power in healthy young subjects
Abstract: Purpose: Previous research has shown that occupations that have high rates of high knee flexion activity, such as kneeling and squatting, are associated with increased rates of knee osteoarthritis. However, there is currently is no consensus on whether it is the frequency or duration of the movements that is more detrimental to knee joint health. The goal of the study was to directly compare how cyclic and sustained squatting exposures influence knee power. This outcome measure was chosen since decreases in power have important implications for mobility and quality of life. Additionally, decreases in power have been associated with the development and progression of knee osteoarthritis. We hypothesized that exposure to a sustained squat would result in greater decreases to lower-limb power than exposure to a cyclic squat. Methods: Thirty healthy young participants were included in the study (15 female/15 male). Participants completed two types of squatting exposures, each with the same total squat duration. A cyclic squat that consisted of six cycles of 40-second squats followed by 20 seconds of rest; and a sustained squat that was four minutes in length. Participants completed two countermovement jumps pre and post squatting exposures, for a total of eight countermovement jumps. A 4-minute standing recovery was completed between squat exposures. Motion capture data was collected using six 3D-motion capture cameras (Optotrak); marker clusters were affixed to the foot, shank and thigh of the dominant leg and the pelvis. Motion capture data on the dominant leg was collected at 100 Hz. All but three participants were right leg dominant. Kinetic data was measured using two embedded force plates (AMTI). Force plate data was measured at a sampling rate of 2000 Hz. Kinetic and kinematic data were both low-pass filtered at 10 Hz. Knee power was determined as the average peak power obtained during the takeoff phase of two countermovement jumps. Changes in knee power were reported as the difference between pre and post squat exposure values, with negative values indicating a decrease in knee power following squat exposure. Power was normalized to mass. A two-way mixed ANOVA was conducted to examine the influence of sex and squat exposure type on changes to peak knee power during takeoff of a countermovement jump. Results: There was a significant main effect for squat exposure type (Figure 1), such that there was a greater decrease in knee power following the sustained squat exposure than following the cyclic squat exposure (mean sustained: -.71 (± .71) W/kg vs. mean cyclic: -.33 (± .53) W/kg, (F(1,28) = 6.55, p < .05). Additionally, a main effect of sex was also found, with males (mean: -.73 (± .76) W/kg) having greater decreases in knee power after squatting exposures than females (mean: -.32 (± .43) W/kg, F (1,28) = 6.99, p < .001). There was not a significant interaction between sex and squat exposure type (F (1,28) = 2.68, p = .113). Conclusions: The study demonstrated that an acute exposure to a sustained squat causes greater decreases to peak countermovement jump knee power during takeoff than a cyclic exposure, when controlling for time in the high knee flexion posture in a healthy, young population. Additionally, male participants experienced greater reductions in knee power than female participants. This result supports the theory that the duration of the high knee flexion posture is potentially more detrimental to the development of knee power in subsequent tasks than the frequency of the movement. The finding that even acute exposures to high knee flexion postures can cause decreases in knee power in a healthy young population highlights the need to further examine the affect of occupational exposures as well as how they affect an older population. Previous studies have found increased deficits in knee power with progression of knee OA. Therefore, any decrease in knee power following high knee flexion movements may be especially detrimental to an OA population, where knee power is important for performance of activities of daily living and mobility.