# Patellofemoral Joint Stress During Front and Back Squats at Two Depths
## 4 Discussion  
Our study showed the location of applied load (FS vs. BS) did not change PFJS. However, a greater depth (60% leg length) increased the PFJS in both front and back squats. These data showed that as squat depth increases, peak PFJF, peak knee flexion angle, and peak quadriceps force increase that results in increases in peak PFJS. There were no interaction effects between the type of squat (front/back) and squat depth, indicating the pattern of changes were not dependent on the type of squat. In this study, the estimated patellofemoral joint stress (PFJS) ranged from approximately 14 to 24 MPa, which is consistent with previous literature using similar modeling techniques and weight-bearing squats (e.g., about 17 MPa [24] and about 25 to 35 MPa [12]). In contrast, the study by [PERSON] and [PERSON] (2018) [4] reported significantly lower PFJS values (~4 MPa) during unloaded squats, where participants kept their knees either behind or beyond the toes. The differences can be attributed to several factors, including variations in squat type, required knee flexion angles, and the presence or absence of external load. Squatting with added weight increases compressive forces at the joint, and likely alters the contact area, resulting in higher PFJS values [12]. Some studies align with our findings of PFJS not being different between front and back squats [9; 25]. [PERSON] et al. (2020) reported that knee moments were similar between BS and FS with a straight bar and transformer bar, with the exception of a greater knee moment during descending phase (the eccentric phase) of 70% of straight bar FS compared to BS [9]. In terms of muscle activity, some studies using lower loads (70% 1 RM and 50 pounds) did not detect any differences between FS/BS [7; 26], but higher vastus lateralis activation was reported during FS while carrying higher loads of 1 RM or 90% 1 RM [27; 28]. [PERSON] et al. (2009) and [PERSON] et al. (2016) both  
Figure 5: Ensemble average with standard deviation error bars for (**A**) patellofemoral joint stress (PFJS), (**B**) quadriceps force (QF), (**C**) knee flexion angle during a squatting cycle (front squat (FS) and back squat (BS)) at 60% and 80% leg length depths. reported greater knee joint loading when performing a load of 70% 1- repetition maximum (1 RM) BS compared to a FS [7; 8]. [PERSON] and [PERSON] (2020), reported a higher knee extensor moment during an 80% FS 1 RM than BS [10]. Specific to the patellofemoral joint, [PERSON] et al. (2016) reported a statistically significant patellofemoral contact force of 4.95 +/ \(-\) 2.16 times bodyweight during a BS versus 4.55 +/ \(-\) 1.83 during a FS while lifting 70% of their BS 1 RM [8]. The differences in patellofemoral contact force between our study and that of [PERSON] et al. (2015) [8] may be attributed to several factors, including differences in participant demographics (our study included female participants), variations in modeling techniques, the use of different joint contact area estimation algorithms, and discrepancies in squat depth between the two studies. The lack of difference between techniques in our study may be explained by applying a lighter weight (20.41 kg bar) and the compensations at other joints to maintain a consistent load on the patellofemoral joint. As was expected, when the barbell is placed anterior to the neck (FS), the horizontal position of the barbell anterior to the knee had a larger magnitude compared to a BS. One might expect that the larger external moment arm during FS would require the muscles, particularly the quadriceps, to generate greater force to produce the necessary extension torque for the squat. This increased muscular force would, in turn, be expected to result in higher compressive forces and greater stress on the patellofemoral joint. However, this was not observed in our findings. [PERSON] and [PERSON] (2020) showed that the knee extensor net joint moment difference between FS and BS is higher and most apparent with greater external load (60-80% of 1 RM) [10].