Knee joint kinetics in relation to commonly prescribed squat loads and depths

Abstract

Controversy exists regarding the safety and performance benefits of performing the squat exercise to depths beyond 90° of knee flexion. Our aim was to compare the net peak external knee flexion moments (pEKFM) experienced over typical ranges of squat loads and depths. Sixteen recreationally trained men (n = 16; age, 22.7 ± 1.1 years; body mass, 85.4 ± 2.1 kg; height, 177.6 ± 0.96 cm; mean ± SEM) with no previous lower-limb surgeries or other orthopedic issues and at least 1 year of consistent resistance training experience while using the squat exercise performed single-repetition squat trials in a random order at squat depths of above parallel, parallel, and below parallel. Less than 1 week before testing, 1RM values were found for each squat depth. Subsequent testing required the subjects to perform squats at the 3 depths with 3 different loads: unloaded, 50% 1RM, and 85% 1RM (9 total trials). Force platform and kinematic data were collected to calculate pEKFM. To assess the differences among loads and depths, a 2-factor (load and depth) repeated measures analysis of variance with significance set at the p < 0.05 level was used. Squat 1RM significantly decreased 13.6% from the above-parallel to the parallel squat and another 3.6% from the parallel to the below-parallel squat (p < 0.05). Net peak external knee flexion moments significantly increased as both squat depth and load were increased (p ≤ 0.02). Slopes of pEKFM were greater from unloaded to 50% 1RM than when progressing from 50% to 85% 1RM (p < 0.001). The results suggest that typical decreases in squat loads used with increasing depths are not enough to offset increases in pEKFM.

Figure 1

Above parallel (A); parallel (B); and below parallel (C) back squat.

Figure 2

Light curtain setup. Sagittal view (A); posterior view (B).

Figure 3

Marker placement utilizing the Point-Cluster Technique.

Figure 4

Squat 1RM values. Data are presented as mean ± SE (n = 16). Significant difference (* P < 0.05; *** P < 0.001) between 1RM values.

Figure 5

Normalized peak external knee flexion moments (%BW*ht) vs. normalized weight on the bar (%1RM). Data are presented as mean ± SE (n = 16). All trials were significantly different (P ≤ 0.02).

Figure 6

Bar graphs of the slopes of the normalized peak external knee flexion moments (%BW*ht) vs. weight on the bar from Figure 4. Data are presented as mean ± SE (n = 16). Significant differences were observed in the average slopes across all depths (***, 0–50% vs. 50–85%, P < 0.001), between below parallel and the other depths when increasing from 0–50% (†† P < 0.01), and at each depth (0–50% vs. 50–85%, ‡‡ P < 0.01; ‡‡‡ P < 0.001).