# The Use of Free Weight Squats in Sports: A Narrative Review--Terminology and Biomechanics
## 1 Introduction  
In the past and presently, most strength and conditioning coaches and sport scientists have considered the squat to be an essential and primary tool for the complete development of athletic potential [1; 2; 3; 4; 5; 6; 7]. The overall importance that the squat plays in strength and conditioning programs can be ascertained from the fact that this category of exercise is commonly identified as one of the most important and frequently prescribed exercises used by strength and conditioning coaches in many sports, reflecting its importance in strength and conditioning programs. For example, the squat and weightlifting movements have been reported to be the most commonly used exercises in professional sports including Major League Baseball (MLB) [8], the National Basketball Association (NBA) [9], the National Football League (NFL) [10], the National Hockey League (NHL) [11] and Major League Soccer (MLS) [12; 13]. Additionally, the squat exercise has been considered to be a very important training exercise used in United States high school strength and conditioning programs [14]. Outside of the United States, the squat is one of the most frequently used exercises in the preparation of athletes in Spanish handball, basketball, volleyball, and football [15], Argentinian rugby [16], and for rowers in the United Kingdom [3]. Ultimately, when examining coaching and scientific literature, squats are widely regarded as a critical exercise group for developing lower body strength in athletic populations [17]. While the squat is often considered a primary exercise in the development of athletes [1; 2; 4; 5], several authors have suggested that squats, especially deep squatting, may increase the risk of knee injury [18; 19; 20; 21]. Deep squats are those in which during the descent, the top of the thigh reaches parallel or a below parallel position, i.e., parallel and full squats. Typical rationales for the avoidance of squatting movements primarily deal with degenerative changes within the tendofemoral complex and increased potential for the occurrence of chondormalacia, osteoarthritis, and osteochondritis when deep squats are performed [18; 22]. However, based upon the existing body of evidence, deep squatting performed with the correct technique has been shown to actually stimulate training-induced adaptations that can provide a protective effect for the knee joint [22; 23; 24]. Indeed, a review by [PERSON] et al. [22] presented compelling evidence that deep squatting does not place the knee, hip, or vertebral column at an increased risk for injury. In fact, the review suggested that deep squatting may actually create a protective adaptive scenario for the lower extremity. In support of [PERSON] et al., a more recent work by [PERSON] et al. [25] revealed that examined athletes who had a higher relative maximum parallel back squat strength displayed a substantially lower injury risk. In fact, based on these data, male athletes who squat >2.2 \(\times\) body mass and female athletes who squat >1.6 \(\times\) body mass had a significantly lower injury risk [25]. The squat is a closed kinetic chain, large muscle mass, multi-joint exercise that stresses the upper body and mid-section musculature, particularly the erect spinae (primarily isometric) and the leg and hip extensors (dynamically). From the performance perspective, evidence indicates that squatting movements, particularly deeper squats, have a substantial potential to increase performance capacity [23; 26; 27; 28]. This perspective is supported by improvements in traditional performance-based tests such as the vertical jump and sprinting assessments, as well as in sport performance. For example, [PERSON] et al. [29; 30] noted that alterations in absolute and relative 1 RM squat strength were statistically related to changes in tackling ability in rugby players.