# Patellofemoral Joint Stress During Front and Back Squats at Two Depths
## 1 Introduction  
Patellofemoral joint pain is a common diagnosis for knee pain, affecting individuals of all ages and activity levels. One theory of patellofemoral pain proposes that increased patellofemoral joint stress (PFJS) creates increased pressure on the subchondral tissues of the patellofemoral joint, resulting in painful symptoms [1]. PFJS is largely influenced by the magnitude of quadriceps force and knee flexion angle, which interact to produce large patellofemoral joint reaction forces (PFJRF) [2]. These forces combined with changes in patellar contact area as the knee flexes have been shown to generate considerable PFJS during common weight-bearing activities. Squatting has been implicated in patellofemoral pain symptoms, possibly because of how knee flexion angle and quadriceps force influence patellofemoral joint loading. [PERSON] al. (2009) reported that barbell squats allow the ability to modify the amount and location of the external load and the depth of a squat, which may be important modifications for changing the magnitude of PFJRF and subsequent PFJS in symptomatic individuals [3]. To assess the characteristics of patellofemoral joint loads during exercise, musculoskeletal modeling has been used to estimate the quantities of forces within and around the joint [1]. Using this model, [PERSON] et al. (2018) and [PERSON] et al. (2018) reported that anterior displacement of the knee beyond the toes influenced PFJS during a squat exercise [4,5]. [PERSON] et al. (2003) in a two-dimensional study reported that maintaining knee position posterior to the toes while performing a barbell squat increased hip torque about 10 times and decreased knee torque about 0.78 times [6]. Based on these investigations, it appears that placing the applied load more posterior to the knee may result in decreased patellofemoral joint loading while placing the applied load more anterior to the knee may result in increased joint loading. The squat is a closed kinetic chain exercise that requires sequential joint movements and coordination. Different types of squatting require different trunk, knee, and ankle positioning that affects the placement of the line of gravity relative to the knee and influences PFJS. The influence of bar location relative to the body during front and back squat (FS and BS) has been discussed frequently [7, 8, 9, 10]. Kinematically, FS results in a more upright torso position and less hip flexion than BS [7]. Knee joint kinematics between the two types of squats are inconsistent [7,10,11]. There are some discrepancies in terms of how FS and BS require different joint moments and muscle activities [7,8,10,11]. It seems that the inconsistency between studies could be related to the different relative magnitude, methods, or even population [9,10]. The magnitude of patellofemoral joint load also appears to be influenced by squat depth. The effect of squat depth as a main factor on knee loading during squatting with or without resistance has been discussed frequently [12, 13, 14, 15, 16, 17]. Previous studies showed an increase in peak PFJRF as knee flexion angle increased [16,18,19]. Most studies indicate that greater knee flexion is associated with increased knee extensor (quadriceps) muscular effort during parallel/full back squats at 50% and 90% one-repetition-maximum (1 RM) [17], higher peak external knee flexion moments (during parallel, above-parallel, and below-parallel back squats at 0%, 50%, and 90% 1 RM) [16], and greater peak PFJS (during no load and 35% Body weight load back squats with 110 degrees of knee flexion [12] or during parallel, above parallel, and below-parallel back squats at 0%, 50%, and 90% 1 RM) [14]. [PERSON] and [PERSON] (2001) reported little effect on patellofemoral joint loading during squatting from 70 degrees to 110 degrees of knee flexion using a standard barbell (22 kg) in five female athlete participants [13].