# Patellofemoral Joint Stress During Front and Back Squats at Two Depths
## 4 Discussion
Our data showed that the maximum trunk flexion angle decreased during a FS. This showed a balancing effect between the horizontal position of the bar (external moment arm) and the trunk flexion angle, meaning that an increase in one variable showed a decrease in the other. [PERSON] et al. (2013) examined the influence of trunk flexion angle on knee joint loads and demonstrated that decreasing the forward inclination of a Smith Squat Rack shift larger moments toward the hip rather than the knee. That study concluded that a decrease in trunk flexion angle could decrease patellar tendon force, knee torque, and compressive forces at the tibeforemal and patellofemoral joints [29]. Other authors have reported that bar position determines trunk angle in order to balance the line of gravity over the base of support during squatting that may impact the knee external moment arm [25]. So, the implications of decreased trunk flexion angle align with our findings may show that compensatory motions at the trunk may help maintain a relatively constant PFJS. This strategy seems to compensate the technique variation and would show a consistence PFJS during carrying lower external loads [10]. Although the current study utilized a constant external load, the findings that trunk flexion angles may change with alterations to the applied load, whether it is an increased load or an increased external moment arm, support the notion that kinematic characteristics of other joints likely need to be considered to fully understand the mechanics occurring at the knee. Increasing the depth of squat from 80% leg length to 60% of leg length resulted in an approximate 62% increase in PFJS. The location of applied load (BS vs. FS) only showed a 7% increase in PFJS. These data appear to indicate that squat depth may be the more influential factor in altering PFJS when compared to the location of applied load. Our finding is aligned with the majority of prior publications [12; 14; 15]. The most obvious explanation is that higher knee flexion during a deeper squat may influence PFJS. [PERSON] et al. (2002) and [PERSON] et al. (2014) both showed a higher PFJS when the depth of squat is increased to 90 degrees of flexion in healthy subjects with low loading [12] or with just bodyweight [21]. [PERSON] et al. (2009) reported that as relative knee flexion angle increased from 60\({}^{\circ}\) to 90\({}^{\circ}\) during a wall squat, PFJS progressively increased from approximately 4 MPa PFJS to 8 MPa [30]. Results in the present study also showed increases in PFJS (approximately7 MPa) as squat depth increased. Other studies reported similar results of increased PFJS with increasing knee flexion angles, inferring that depth of squat appears to be directly associated with PFJS. Both [PERSON], [PERSON], and [PERSON] (2015) and [PERSON] and [PERSON] (2018) showed similar findings of increased PFJS and quadriceps force as the knee moved into greater flexion when analyzed using same musculoskeletal model used in our study [4,24]. It was shown that higher knee flexion during a deeper squat causes a higher PFJRF [18,19]. [PERSON] et al. (2012) reported an interaction between squat depth and the amount of knee extensor (quadriceps) muscular effort. Their findings showed that decreased squat depths required less muscular effort than increased squat depths, but there was no difference at intermediate squat depths [17]. Peak PFJRF was also found to increase with peak external knee flexion moments when the knee flexion and squat depth increased [12,14,16]. There may be an interaction effect on joint loading between the magnitude of the applied load and squat depth [12,13,14]. [PERSON] et al. (2013) reported an increase in net peak external flexion moments from squatting to a higher depth or using a higher external load. [16]. [PERSON] and colleagues (2021) showed that the effects of depth (above parallel, parallel, and below parallel) on peak PFJS were different between unloaded, 50%, and 85% of depth-specific one repetition maximum conditions.