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Back Squat Vs. Front Squat, a biomechanical comparison. How does the shift in centre of mass affect muscle activity and knee kinetics?

Back Squat Vs. Front Squat, a biomechanical comparison. How does the shift in centre of mass affect muscle activity and knee kinetics?

The squat is considered as a very effective exercise for increasing strength and stability of the muscles of the lower limbs (Gullett, Tillman, Gutierrez, & Chow, 2009). This is due to its activation of the large musculature of the hips and knee as well as its recruitment of the abdominals and spinal erectors and its potential to minimise injury (Gullett et al., 2009; Braidot, Brusa, Lestussi, & Parera, 2007; Escamilla, 2001). Many sports require high levels of strength and power, such as track and field and for athletes to generate force through their lower extremities into the playing surface (Escamilla, 2001; Waller & Townsend, 2007). Therefore when developing a strength program, an important area for the strength and conditioning (S&C) coach to consider is lower body strength (Bird & Casey, 2012). An S&C coach can use different forms of squatting such as the back squat (BSq), front squat (FSq), overhead squat and Bulgarian squat that have varying techniques (Bird & Casey, 2012). It is therefore suggested that it is important for an S&C coach to understand muscle activity and knee joint biomechanics while performing these variations as it would be useful for exercise prescription and rehabilitation (Gullett et al., 2009; Escamilla, 2001; Waller & Townsend, 2007). This essay will address the biomechanics of the BSq vs. FSq and provide S&C coaches with recommendations on what form to use with their athlete.
Barbell squats can be performed with the external load placed in a variety of positions (Schoenfeld, 2010). The BSq and FSq are characterised as sagittal plane movements involving the joints of the hip, knee and ankle (Sato & Heise, 2012). The BSq exercise consists of the low bar BSq, where the bar is placed slightly below the level of the acromion and high bar BSq, where the bar is placed slightly above the level of the acromion (Fry, Smith, & Schilling, 2003; Schoenfeld, 2010). In the FSq, the bar is held in front of the chest at the clavicle across the anterior deltoids (Pierce, 1997; Schoenfeld, 2010). The BSq and FSq share similar descending and ascending motions however during the FSq a more neutral spine alignment is required resulting in a more upright trunk position when compared to both low bar and high bar BSq (Bird & Casey, 2012; Fry & Kraemer, 1990; Wretenberg, Feng, & Arborelius, 1996).
The change in the position of the centre of mass of the bar between the BSq and FSq lifts, places more emphasis on the quadriceps and supporting muscles of the trunk during the FSq, while there is an increase in the degree of trunk flexion during the BSq resulting in more hip involvement (Cissik, 2000). This leads to activation of the hamstrings and glutes during the ascend to extend the hips back to the start position (Cissik, 2000; Stoppani & Merrit, 2011). Gullett et al. (2009) examined the differences in muscle activation using electromyography (EMG) during the BSq and FSq in 15 subjects squatting 70% of their one repetition maximum (1RM). Despite lifting more than 19kg during the BSq, this study found no significant difference in muscle activation of the hamstrings, quadriceps or erector spinae. The authors however did not compare low bar vs. high bar placement position during the BSq and also did not measure the activation of the gluteus maximus, which is usually associated with increased activation in the wide stance (WS) during the BSq (McCaw & Melrose, 1999). This is a stance athletes tend to take more than when performing the FSq. In another study Stuart, Meglan, Lutz, Growney & An (1996) compared knee extensor demands and low back injury risks of the FSq and BSq exercises and found that muscle activity was also equivalent during both lifts. They concluded that both exercises had similar low back injury risks for four of their subjects. The lower back injury risk was influenced more by trunk inclination leading to greater trunk extensor demands and lumbar shear forces rather than squat exercise type. It has been suggested by Russell & Phillips (1989) that trunk inclination during the squat contributes to trunk torque but these torques are not influenced by the type of barbell squat. However this was disputed by Fry & Kraemer (1990) who highlighted a number of issues with their research model. Fry & Kraemer (1990) argued that using 75% of the 1RM of the FSq for both measuring BSq and FSq is not the soundest approach, as the BSq weight is likely to be higher than the FSq. They go on to also disagree on the way the location of the shoulder joint centre was identified, which is important for correctly calculating correct trunk inclination. They state that Russell & Phillips (1989) calculation assumed that shoulder joint centre location is constant across both squat types when their anatomical location can actually vary by 7-15cm. The lack of EMG data and muscle activity levels to back their calculations brings uncertainty to their results. The angle of the hip for the high BSq is greater than the FSq and this suggests a greater compromise at the lower back due to the possible lumbar shear forces with considerably greater mean power absorbed by the hip in the high bar BSq (Braidot et al., 2007). Low bar BSq also generates greater hip extensor torque but less knee extensor torque compared with high BSq squats (Schoenfeld, 2010). Excessive trunk flexion contributes to an increase in hip torques (Fry et al., 2003) leading to an increase in stress on the lumbar spine and risk of injury (Braidot et al., 2007; McKean, Dunn, & J Burkett, 2010; Russell & Phillips, 1989). An advantage of having a more upright trunk when squatting therefore is a decrease in compressive force and spinal stress (Schoenfeld, 2010; Fry & Kraemer, 1990). Gullett et al. (2009) demonstrated that although more weight can be lifted during the BSq, the muscles tested were equally active during the FSq. Therefore as the FSq produces lower knee compression and lumbar stress in comparison with BSq, it can be concluded that the same workout can be achieved with the same load with less stress on the lumbar region making it a viable alternative for those suffering from back ailments (Fry et al., 2003).

According to Braidot et al. (2007) the adopted lumbar position during squatting will lead to variations in activation patterns of rectus abdominis and spine muscles. Gullett et al. (2009) found no significant difference in erector spinae and rectus femoris activity level between the BSq and FSq however there was significantly different muscle activity between the ascending and descending phases. In another study looking at trunk muscle activity during isometric trunk and dynamic strengthening exercises using EMG, Comfort, Pearson & Mather (2011) reported significantly different erector spinae (ES) activity (p, 0.01) during the FSq when compared to the BSq. The authors conclude that the FSq can be a valuable substitute to the BSq as higher muscle activity levels can be achieved using less weight when compared to the BSq. It is important to however note that the exercises were performed using a constant absolute load of 40kg which is unlikely to occur in athletic training were relative loads for each of the lifts will be used (Bird & Casey, 2012).

Another important aspect for an S&C to consider when comparing the two different squats is knee kinetics activity. Gullett et al. (2009) reported significantly higher compressive/tensile force at the knee (11.0 ± 2.3 N-kg-1 versus 9.3 ± 1.5 N-kg-1) and knee extensor moments during the BSq when compared to the FSq, however shear forces did not vary and were posteriorly directed. The increase in compressive forces and extensor moments observed during the BSq can be attributed to the increased load compared to the FSq (Bird & Casey, 2012). Russell & Phillips (1989) compared the maximum peak of the knee extensor moments between the BSq and FSq but found no significant differences exist. They conclude that maximum knee extensor moment comparison indicated similar knee extensor demands, so either squat exercise could be used to develop knee extensor strength. However it is important to note that the weight used by the authors was lower compared to the study by (Gullett et al., 2009). Stuart et al. (1996) investigated inter-segmental forces at the tibiofemoral joint and muscle activity during the low bar BSq, the FSq, and the lunge. They reported a posterior mean tibiofemoral shear force in both the BSq and FSq that increased with knee flexion during the descent phase of each exercise. They determined that the exercises do not produce excessive tibiofemoral shear or compressive force in anterior cruciate ligament (ACL)-intact subjects. Gullett et al. (2009) suggest that compressive forces are opposed within the knee by the meniscus and hyaline cartilage and shear stress is resisted in the knee joint by the anterior and posterior cruciate ligaments (PCL). Stuart et al. (1996) reported lower posterior shear forces compared to (Gullett et al., 2009), but they were unable to detect significant differences between BSq and FSq and make a distinction between the tibiofemoral joint compression forces that occurred in the two squat variations. Also the lower anterior/posterior shear forces measured can be attributed to the use of a lower mass during testing (22.7 kg) compared to the (61.8 kg) used by (Gullett et al., 2009). Compressive loading on the knee joint is an important variable when good joint health is a concern (Meyer & Haut, 2005). Chronic excessive loading on the knee joint, through heavy weight bearing exercise, could lead to deterioration or loss of the cartilage that acts as a protective cushion between bones (Gullett et al., 2009). Therefore by decreasing the compressive force encountered while performing squats, these risks and the pain associated with them may be reduced (Gullett et al., 2009). The low bar BSq position typically used by powerlifters generates higher hip extensor torque and less knee extensor torque due to greater forward inclination of the trunk compared with high bar BSq. This leads to less patellofemoral compression and ACL strain in the low bar BSq (Swinton, Lloyd, Keogh, Agouris, & Stewart, 2012). Patellofemoral compression values however do not exceed the strength threshold of the structure, therefore both bar positions can be used unless there is contradiction by an existing injury (Schoenfeld, 2010). The FSq may present a safer and possibly a more advantage option for athletes with pre-existing knee injuries than the BSq in terms of muscle recruitment and minimising compressive forces in the patellofemoral joint. The FSq allows a similar training stimulus to be achieved with less compressive forces on the knee which can be useful for athletes with osteoarthritic concerns (Bird & Casey, 2012)
It is important to note that many other variables such as movement speed (MS), width stance (WS), squat depth (SD), anterior displacement of the knee (ADK) and hand position (HP) can all affect the biomechanics of squatting (Fry & Kraemer, 1990). Wrist or shoulder pain is a common complaint when athletes are first introduced to the FSq because of hands position (Cissik, 2000). The S&C coach needs to determine if the cause of the pain is related to flexibility issues in the wrists or shoulders (Cissik, 2000). They can then work with the athlete to improve their flexibility in order to achieve proper exercise execution.
MS has been shown to increase both compression and shear forces with uncontrolled eccentric movement during the squat generating excessive high joint forces at the knee with the possibility of causing knee ligament damage (Donnelly, Berg, & Fiske, 2006). It is therefore important to balance between optimal transfer of performance and risk of injury and perform the squat decent in a controlled manner unless athletic goals require otherwise (Schoenfeld, 2010; Palmieri, 1987).
Hartmann et al. (2012) compared the effects of different squat depth variants on the development of 1RM and their transfer effects to jump performance. They reported that an increase in FSq and BSq depth when compared with quarter squats increases the transfer effect of dynamic maximal strength to dynamic speed strength capacity of hip and knee extensors. Quadriceps muscle activation is maximised by squatting to a parallel position, with no additional activity seen at higher flexion angles, whereas muscle activity of the gluteus maximus, which acts as a hip extensor, increases with SD (Caterisna et al 2002). Schoenfeld (2010) also states that an increase in SD increases hip extensor moments therefore athletes wishing to maximise strength of the hip musculature may benefit from doing deeper squats. It is however important to note that peak patellofemoral compressive forces occur at or near maximum knee flexion (Donnelly et al., 2006). Therefore athletes with patellofemoral disorders, existing injury or previous reconstruction of the PCL should avoid squatting at high flexion angles to minimise posterior shear (Donnelly et al., 2006; Dionisio, Marconi, dos Santos, & Almeida, 2011).

Fry et al. (2003) conducted a study to examine joint kinetics when forward displacement of the knees is restricted and unrestricted in order to scrutinise the recommendations that suggests keeping the shank as vertical as possible during the barbell squat, thus keeping the knees from moving past the toes. They found that squats that minimise ADK produced significantly greater torque at the hips and significantly less torque at the knees in the restricted squat when compared with the unrestricted squat. Forward motion of the knees beyond the toes is dependent on squatting depth and anterior trunk inclination (Yule, 2007). Anterior displacement of the system centre of mass (COM) occurs during the eccentric phase of the BSq and FSq exercise as the knee travels past the toes. In a low bar powerlifting stance BSq a vertical shin position is maintained resulting in posterior displacements of the system COM (Swinton et al., 2012). This torso position has been reported to increase forces and moments at the lumbar spine leading to increased risk of developing lower back injuries (Fry et al., 2003). This was supported by data from Swinton et al. (2012) who reported a significant effect (p, 0.05) on peak joint moments at the spine and ankle as a results of these differences in linear displacements. Therefore in order to minimise the risk of injury it is important to optimise the forces at all involved joints, by allowing the knee to travel past the toes (Fry et al., 2003).

Athletes when squatting use a variety of WS therefore it is important to understand the differences between them (Escamilla, Fleisig, Lowry, Barrentine, & Andrews, 2001a). Variation in WS can alter joint related forces with lower patellofemoral and tibiofemoral compression in a narrow stance (NS) and less forward knee lean in the wider stance reducing shear forces (Schoenfeld, 2010). Data from Escamilla, Fleisig, Zheng, et al. (2001b) shows greater tibiofemoral compressive forces in the wider stance whereas the NS squat produced greater gastrocnemius activity. When developing hip adductors and hip extensor strength a wider stance squat would be more beneficial whereas if the goal is gastrocnemius development then a NS squat would be more effective (Schoenfeld, 2010; Escamilla et al., 2001b). WS does not appear to affect quadriceps muscle activity (McCaw & Melrose, 1999; Escamilla et al., 2001b), however gluteus maximus muscular activity is significantly higher in the wide stance (McCaw & Melrose, 1999; Paoli, Marcolin, & Petrone, 2009). Therefore WS should be determined based on the athlete’s goal as well as their ability.

In order to determine which form of squatting to use with their athlete the S&C coach must take into account the biomechanical differences between the BSq and FSq as well as the variables mentioned above. It is clear that the FSq will result in a more upright trunk position than during both low bar and high bar BSq and may be advantageous for individuals with knee problems such as meniscus tears and general joint health when compared with BSq (Gullett et al., 2009). Using the BSq as major lower body mass builder allows for more weight to be lifted but this can results in higher stresses on the knee and back. However the shift in centre of mass does not alone influence the mechanics of the two lifts. A good S&C coach should be able to combine the different variables as well in order to create the ideal technical model of squatting for their athlete. For Example utilising a well executed deep FSq will still target the gluteus maximus perhaps not to the extent a wide stance low bar BSq will but there is less risk of lower back stress.

It is important for S&C to teach their athletes both varieties of the squatting exercise. Athletes can vary their use of both squats exercises to give variety to their training. Athletes who want to avoid lower knee compression and lumbar stress or who are worried about lower back injury issues can utilise the FSq exercise without compromising their lower body development. The FSq can also be used to safely build lower body strength while minimising compressive forces in the patellofemoral joint. When teaching the squat it also important for the S&C to not allow the athlete to rapidly eccentrically drop during the squatting movement as this may result in increased high joint forces at the knee leading to increased risk of injury. SD should be determined by the athletes goal and ability. If gluteus maximus development is priority then deeper squatting is required however for quadriceps development parallel squatting is sufficient. Athletes with existing knee injuries or previous reconstruction of the PCL should avoid deep squatting. It is important that ADK is not restricted during squatting movement and for the knee to travel past the toes to optimise the forces in the involved joints. Finally if lower patellofemoral and tibiofemoral compression is priority then a NS should be utilised however if gluteus maximus development is the target then a wider stance should be adopted. In conclusion, it is clear that a deeper understanding of muscle activity and knee joint biomechanics while performing squatting movements can allow the S&C to optimise results and reduce the risk of injury with their athletes.
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