Back vs front squats: the impact of the centre of gravity

The squat is generally used to improve strength and enhance sports performance, since squatting will strengthen the prime movers that are essential for explosive athletic movements such as running and jumping (1). Squats are also used to aid with rehabilitation (2,3) and can also be used as a screening tool to identify biomechanical deficits that can hinder movement patterns which could result in injury (4). From the various methods of squats available, the two most commonly used for athletic development are the front squat and high bar back squat. One distinct difference between these two squats is the barbell placement. During back squats, the athlete will place the barbell behind the head across the shoulders on the trapezius, slightly above the posterior aspect of the deltoids; during the front squat, the barbell will be positioned across the anterior deltoids, at the head of the clavicles with the elbows pointing forward with the humerus running parallel with the floor. Front squats do require more mobility at the ankle to allow the athlete to maintain a more vertical torso, and good mobility in the latissimus dorsi will allow athletes to keep the elbows up to support the barbell. Restrictions in areas such as the pecs, triceps, biceps, and thoracic region will limit the positions an athlete can achieve regarding positioning of the barbell. The front and back squats will have biomechanical differences that should be considered when prescribing training interventions. This discussion will consider these differences and how this can impact muscular recruitment patterns and biomechanics of the exercise, making critical judgements to the use of these for athletic development.

When considering barbell placement during the front squat, the athlete is required to maintain a vertical torso position to prevent the barbell from falling forward. This will alter the athletes centre of gravity, and in turn create a longer moment arm at the knee joint (Figure 1). This longer moment arm will therefore require greater execution of the knee extensors to overcome the force of the loaded barbell (5). Conversely, during the back squat, the moment arm of the hip joint is longer than the knee joint, suggesting the back squat requires greater hip involvement, placing the hip extensors under more stress to produce the required force to move the loaded bar (6). The difference between both squats will consequently have an impact on muscle recruitment.

Muscle activation

Both squats will largely activate the same lower body muscles with the greatest emphasis on the quadriceps, hips and glutes (2,7). Muscle activation levels are reported to be lower during the descend (7), with greater levels of muscle recruitment occurring at diverse joint angles (8). For example, when a parallel squat is achieved, the perpendicular distance between the line of action (Figure 1) at the hip joint and the line of movement will become its longest around 90º of knee flexion (9), resulting in maximum torque been required to ascend from the squat. Contrasting evidence has been published regarding the level of muscular recruitment during different squat depths, however, muscle recruitment levels will increase during deeper squats due to the greater levels of force to overcome during the ascend (8). In females, no significant differences were witnessed in muscle activation between the gluteus maximus, biceps femoris and vastus lateralis during front and back squats when performed to various depths (10). Conversely, a study performed on males has shown that back squats performed to 90º knee flexion angles had significantly greater activation levels in the gluteus maximus, biceps femoris and soleus muscles compared to 140º knee flexion angles (11). Caterisano et al (12) reported that the gluteus maximus achieved recruitment levels of 16.9%, 28% and 35.4% while performing partial, parallel and full depth squats, respectively, using loads equal to 100% and 125% bodyweight.

Vastus lateralis has been highlighted to have significantly greater levels of activation during front squats throughout the concentric phase (5,10), which can be explained by the longer moment arm at the knee during the front squat. Additionally, gluteus maximus, biceps femoris and semitendinosus will have significantly greater levels of activation during the concentric phase in both front and back squats (5); which is plausible as these muscles act as hip extensors, thus, greater forces are required to move the loaded barbell vertically.

It has been suggested that the hamstrings bicep femoris has minimal to moderate activity during back squats (12); although contrasting evidence has reported significant activation levels (5). Moreover, hypertrophy of the hamstrings appears to be non-existent following partial or full depth squats (14), which would suggest minimal activation. This may be explained by the muscles biarticular structure. The biceps femoris along with the rectus femoris muscles may act isometrically during squatting as they don’t change a great deal in length during the movement due to these muscles crossing two joints (6,15). Furthermore, the isometric contraction of the biceps femoris has been suggested to act as a stabilizer at the knee or as a transient to move energy across the hip and knee joints (6).

Trunk musculature will also play a part during squatting as the athlete attempts to stabilise. The erector spinae showed no differences in recruitment levels using either of the aforementioned squats (7,16). Contrastingly, in comparisons between the front and back squats, plank, superman and military press, the front squat had greater erector spinae activation during eccentric and concentric contractions compared to the back squat (17). The same study (17) reported no differences in rectus abdominis recruitment. Interestingly, the aforementioned study reported the plank to have the greatest muscular recruitment for the rectus abdominis muscles compared to the back squat. Empirical evidence suggests the front squat will recruit greater activation levels of the rectus abdominis due to barbell placement, however evidence suggests this may not be the case (16). Nesser and fleming (16) reported no differences in rectus abdominis activation levels between front and back squats. It must be noted, during back squats, unreliable electromyography measures have been reported for the rectus abdominis, which can be explained by the increased trunk inclination, resulting in increased skin folds and excessive motion of electromyographic artefacts (19). Clark and colleagues (20) reported that the trunk musculature activity will increase significantly with 10% load increments during eccentric contractions, with greater recruitment in the erector spinae and external oblique during concentric contractions. Research has shown that the level of recruitment for the erector spinae will increase during barbell squats due to the athlete being required to stabilise in all 3 planes of motion (18). Additionally, Nesser and Fleming (16) have observed greater external oblique activation during front squats as opposed to back squats, while the intermuscular coordination sequence during back squats will emphasise the hip and back extensors greatly (13).

Squat depth should be considered in sporting performance and rehabilitation. Squatting deep has been shown to improve jumping performance (13), whereas, quarter squats have been found to hinder isometric and explosive strength of the hip and knee extensors, thus not providing any significant increases of force during the acceleration process of reactive and concentric speed-strength (13). In addition, compression forces will surge as the depth of the squat increases due to greater level of muscle recruitment required (8), impacting athlete biomechanics.

“compression forces will surge as the depth of the squat increases due to greater level of muscle recruitment required”

Kinetic and kinematic differences

Joint biomechanics are also important to understand for programming guidelines. The centre of gravity shift between the two squats will effect hip joint kinematics significantly due to the forward lean of the torso which occurs during back squats (5); which is more prominent when using a low bar position (21). While knee joint kinematics are comparable between the two squatting movements (5), kinetic data of the knee joint has been suggested to differ significantly (7). Cotter et al (22) determined that the forces on the knee increased when performing squats using bodyweight to 50% of 1 rep max load, therefore, training strategies should be in place to allow untrained individuals to progressively increase their tolerance to the forces that are applied to the knee during squats (13,22). When the same relative load is used, the back squat has been shown to have increased knee joint compression forces in comparison to the front squat, 11 N.kg to 9.3 N.kg, respectively (7). Knee joint moments are also greater in the back squat compared to the front squat, 1 N.m.kg to 0.7 N.m.kg, respectively (7). This is important since forces around the knee joint have been shown to effect anatomical structures differently. Knee joint forces will be minimal when performing back squats with knee flexion angles up to 50º, suggesting this range would be ideal for knee rehabilitation interventions (23). In contrast, anterior cruciate ligament (ACL) forces are greatest whenever knee flexion angles are less than 60º due to the quadriceps producing an anterior directed force via the patella tendon on the proximal tibia (24). Research by Toutoungi et al has reported peak ACL forces to occur between 35-40º of knee flexion during the ascend, with no forces witnessed throughout the descending phase (25). The greatest force upon the ACL during squats has been reported to be 95N during the descent (25), and considering that the ACL can exhibit up to 2160N of force (26), the chances of increased stresses occurring to the ACL during squats is very unlikely.

Sheer force to the posterior cruciate ligament (PCL) will only occur when a knee angle greater than 60º is achieved (23). Maximum values of 2704N have been reported while squatting to a depth of 100º knee flexion in healthy individuals during the descending phase of the squat (25), and 2212N while squatting to a depth of 95º knee flexion (27). The PCL has been suggested to be capable of achieving maximum strength values of 4000N in young active individuals (28), emphasising the minimal risk of injury to the PCL during squats.

Furthermore, the stance width during the squat will impact kinematics, with an increase in hip flexion angles by up to 10º observed when using a narrow stance when knee flexion angles are greater than 45º (29). Forward translation of the knees has been reported to be greater during a narrow stance in comparison to a medium or wide stance, 21.7, 18 and 16cm, respectively (29). When kinematics are altered there is the chance this will also have impact on kinetics, therefore, stance width should be considered when designing training interventions.

Conclusions

Based on the information presented, the squat used will depend on a few different things. Muscle recruitment patterns and joint biomechanics will be influenced significantly as a result of the centre of gravity shift which occurs between front and back squats. The back squat will produce less stress on the knees because of the larger moment arm acting at the hip, while the isometric nature of the hamstring during back squats may help protect the ACL during rehab training. Since muscle activation has been reported to be low, squats will not be an ideal exercise to train the hamstrings. Using squatting exercises in isolation may create an imbalance between the hamstrings and quadriceps, this in turn may increase the risk of ACL injuries due to quadricep dominance, thus reducing eccentric strength capabilities of the athlete. On the other hand, the front squat will have a greater benefit for training the knee extensors and could even be used with individuals experiencing minor hip injuries. Although it is perfectly safe for healthy individuals to perform squats with very low risks associated regarding anatomical structures of the knee joint, progressions should be in place to allow individuals to gradually overload their body.

References

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