# Estimation of Tibiofemoral and Patellofemoral Joint Forces during Squatting and Kneeling
## 1 Introduction  
Kneeling and deep squatting are common activities in daily living and typical postures in certain occupations, such as carpet layer or tile setter, and in social-cultural and religious contexts, specifically in Asia and the Middle East. Nevertheless, despite this common practice the knowledge about the biomechanics of squatting and kneeling, in particular with regards to the associated knee joint forces, is still low.  
A couple of epidemiological studies suggest that prolonged squatting and kneeling is a risk factor for the development of osteoarthritis [1, 2, 3, 4, 5, 6]. In recent years, some European countries (such as Denmark NBoII [7] and Germany BMAS [8]) have recognized osteoarthritis of the knee joint as an occupational disease caused by kneeling and squatting over a longer term. The mechanical stress of the knee joint cartilage during squatting and kneeling is under consideration as a potential pathomechanism [9, 10, 11, 12, 13]. The reasoning for this is based on the basic principle that knee bending in a standing position causes a strong increase in the tibiofemoral and patellofemoral compressive joint forces.  
Based on inverse dynamics model approaches, several studies calculated peak tibiofemoral joint compressive forces from 4 to 7 body weight (BW) during lowering to or rising from squat [14, 15, 16, 17, 18]. Associated patellofemoral compressive peak forces ranging from 4 to 9 BW were reported [15, 19]. Adopting these results to the final static postures of deep squatting and kneeling is critical because most of the studies focused on peak forces during the rising and lowering motion, which may entail a considerable inertial force component due to acceleration and deceleration. Further, peak loads may be associated with positions distinct from the final postures of static squatting and kneeling [14, 15, 17, 18]. The major experimental issue, however, is the disregarding of the soft-tissue contact force of thigh and calf at full kneeflexion [14; 15; 17; 19; 20; 21]. Using a pressure-sensitive map, [PERSON] et al. [22] reported average thigh-calf contact forces of 31 \(\pm\) 9.3 to 34 \(\pm\) 9.7% BW per leg during deep kneeling and squatting respectively. Based on their results from 2007, [PERSON] et al. [23] calculated compressive knee joint forces at full flexion as 3.07 BW during a theoretical estimated downward squatting movement. Using a similar experimental setup and pressure-sensitive map [PERSON] et al. [24] found overall higher soft-tissue contact forces. The thigh-calf contact forces observed were 39 \(\pm\) 14% BW for squatting and 36 \(\pm\) 10% BW for deep kneeling-including a possible gluteus-heel contact of 11 \(\pm\) 6% BW. Overall, the soft-tissue contact force on the dorsal side of the leg decreased the knee flexion moment by an average of 48%. However, [PERSON] et al. [24] did not calculate any muscle or articular knee joint forces.  
In conclusion, the actual magnitudes of knee joint forces in final squatting and kneeling postures still seem unclear (Figure 1). The purpose of this study was to examine the differences of the knee joint forces between lowering to squat, or rising from squat, deep kneeling, and typical final postures of squatting and kneeling. Additionally, the differences within the final postures of squatting and kneeling regarding the knee joint forces were examined.