The squat and deadlift are exercises prescribed by strength and conditioning professionals for the purpose of strengthening the legs, hips, back and torso musculature. They are considered closed chain, compound lifts involving the integration of multiple joint systems and muscle groups. These exercises are therefore among the foundational movements that underpin highly sport specific motor patterns that contribute to a repertoire of well-developed athletic movement skills (Kitz, Cronin & Hume, 2009). It has also been shown that long-term lifting with squats and deadlifts not only promotes an increase in bone mineral density in young populations, but it may also help maintain this adaptation well into the later stages of life (Almstedt, et al, 2001, Walters et al, 2012).
When performed correctly, injuries related to these exercises are uncommon (Watkins, 1999), however poor technique or inappropriate prescription can lead to wide range of issues, especially in combination with heavy weights (Schoenfeld, 2010). Considering the complexity of these exercises and the variables related to performance, understanding the biomechanics is of great importance for achieving optimal muscular development as well as reducing training related injury. Therefore, the purpose of this report is to analyse and compare the squat and deadlift of two subjects and provide corrective advice where appropriate.
The Squat
The squat starts with the descent phase as the hips, knees and ankles all flex. A common cue is to descend until the thighs are parallel with the floor, and the hip joint is either parallel or below the knee joint (Schoenfeld, 2010). Ascent is performed primarily through triple extension of the hips knees and ankles, until the subject returns to the starting position.
Analysis of the squat can be achieved by sub categorising the movement into three comprehensive domains; upper body, lower body and movement mechanics. The upper body emphasises the stability and posture of the head, neck and torso, the lower body asses the joint positions of hips, knees and ankles, finally the movement mechanics assess the timing and co-ordination of the exercise (Meyer, 2014).
Upper Body
An incorrect head and neck position can result in improper spinal positioning and tracking throughout the movement (Donnelly, Berge & Frisk, 2006). It has been shown that both head and neck position can influence spinal kinetics and kinematics (Schoenfeld, 2010). For example, excessive cervical hyperextension can compensate for a lack of thoracic mobility, alternatively excessive cervical flexion may result in a tendency to extend the lumbar spine, which can increase lumbar compression forces (Meyer, 2014).
The head should maintain a neutral position (to slight extension) in relation to the spine, with the neck held in line to the plane of the torso. Although head position and gaze are related, it is important to recognise that they are independent of each other with the focal point or gaze instructed to be either straight ahead or slightly upward. When observing the bottom position of the squat both subject A and B display an adequate head position which is neutral to slight extension, although subject A displays a clear downward gaze (fig.1). A downward gaze during the ascent is not recommended as it has been hypothesised that movement tends to occur in the direction of the gaze (Meyer, 2014). Therefore, a slightly upward gaze may emphasise
the ascent to be lead with the head and chest rather than by the hips. Most faults relating to gaze can be fixed through verbal and visual cueing.
Squatting with a more vertical torso increases load onto the lower limbs resulting in reduced low back stress. By maintaining the trunk as upright as possible throughout the movement reduces shear forces associated with forward lean (McGill & Norman, 1985). A general guideline to ensure adequate trunk posture, is to keep the line of the trunk parallel to the line of the tibia. While subject A satisfies this criterion, subject B is unable to achieve this (fig 1.) resulting in an increased forward lean of the torso. This is likely due to the lack of dorsiflexion in the ankles, which shall be explored in the next section (lower body).
Both subjects display the ability to maintain a neutral spine throughout the movement. Proper squat technique requires a rigid spine that eliminates any planar motion (Schoenfeld, 2010). Squatting with a flexed lumbar spine decreases the moment arm for the erector spinae, reducing tolerance to compressive load resulting in the transference of the load from muscles to passive tissue (i.e. ligament) increasing the risk of injury (Noyes et al, 1984). Performing the squat with a stiffened torso and neutral lordotic curve is therefore essential for optimal performance.
Figure 1.
Lower Body
The squat exercise is often categorised into three groupings; partial squats (40° knee angle), half squats (70° - 90° knee angle) and deep squats (greater than 100° knee angle), however no standardised measures of quantification are universally recognised (Schoenfeld, 2010). Due to individual differences in joint mobility, joint stability and neuro-muscular control a blanket statement on appropriate squat depth cannot be made. Performing the full squat generally requires at least 15° - 20° of ankle dorsiflexion and 120° of hip flexion (Greene, 1994). It has been shown that individuals that lack sufficient ankle dorsiflexion may be at a greater risk of injury to the lower back, hips and knees during functional movements (Lun 2003, Powers 2003). Subject A clearly meets the criterion of the full squat with a hip angle in excess of 120°, a knee angle greater than 100° and over 20° of ankle dorsiflexion (fig 2.). In contrast subject B lacks the joint mobility to get into the full squat position highlighted by only 9° of ankle dorsiflexion at the bottom position which can be attributed in part to tightness in the soleus. Stretching the posterior ankle musculature can help correct this deficit and increase range of motion at the ankle. This will result in an increase in anterior tibial translation resulting in an increase in knee flexion and thus allowing the torso to become more vertical as the weight of the bar becomes more balanced over the centre of the foot. Hip flexion may be increased by performing the pole squat where the subject squats as deep as possible in front of a pole and uses the pole as assistance to pull the torso into the correct position and hold. Also, performing ball wall squats where the subject places a small ball between the lower back and a wall, and while pinning the ball to the wall slides down into a full squat.
Figure 2.
Movement Mechanics
The descent is initiated with the breaking of the hips or the ‘hip hinge’ which migrates the load to the posterior chain which is a safer strategy for knees and lumbar (Meyer, 2008). The descent should be controlled and a constant velocity and the ascent should follow the same vertical path in reverse. Subject A displays a slight horizontal anterior displacement of the bar on the ascent, although this may be due the perspective or parallax distortion of the camera, as it appears not to be exactly in line with the sagittal plane. Subject B displays a more vertical bar path but what is also observed is a bounce in the bottom position. This is indicated by the double spike in the bottom position of the bar path (fig. 3). Ariel (1974) showed that bouncing in the bottom positon of the squat can increase shear forces in the knee by 33%, and is therefore contraindicated. Excessive tibiofemoral shear forces can be injurious the cruciate ligaments (Ecamilla, 2001). This is likely caused by a high bar speed velocity and lack of control when approaching the bottom position during the ascent. A corrective solution for this could be using verbal cues to maintain a controlled velocity during the ascent, and by using isometric holds in the bottom position to develop strength and stability at this particular phase.
Figure 3.
The Deadlift
In contrast to the squat exercise, the deadlift begins with the ascent, with the hip, knee and ankles fully extending until the upright position is reached. Two common styles used are conventional and sumo. A comprehensive biomechanical analysis of both of these techniques was carried out by McGuigan and Wilson (2001) who revealed that the sumo variation offered certain mechanical advantages such as the more upright torso position, which results in less low back shear due to reduced spinal flexion. Alternatively, Escamilla (2000) found that the conventional style required an increased energy expenditure of approximately 25-40%, highlighting advantages of both styles. The choice of style is ultimately down to comfort, personal preference or the biomechanics of the individual.
Starting Position
The starting position places the bar over the mid foot of the lifter, while feet are approximately the same width as a flat footed vertical jump would require. The bar stays as close to the body as possible throughout the entire lift to minimise the moment arm on both knee and hip. The preferred back angle is approximately 45° but this is very much dependent on the anthropometrics of the lifter. For example, someone with long femurs and a short torso will be exhibit a torso more horizontal to the floor, reducing the back angle whereas someone with shorter femurs and a longer torso will be able to use a 45° back angle. Neither subject A or B can display a 45° back angle (fig. 4). Subject A displays a relatively neutral spine but subject B although showing a relatively neutral lumbar spine exhibits a degree of thoracic kyphosis. A corrective strategy that could be suggested to both lifters is to slightly raise the bar in order to reduce the amount of hip flexion required, while using a verbal cue to raise the chest. This would allow both lifters to perform the exercise with a degree of load, while maintaining a safe neutral spine.
Hales et al (2009) showed that squat and deadlift were markedly different in terms of kinematic analysis, concluding that the squat is a synergistic segmented movement, and deadlift a sequential or segmented movement. This is of particular interest when the squat and deadlift hip and knee angles of subject B are compared. Both lifts appear to exhibit similar hip and knee angles in the bottom position. The limiting factor again likely being lack of ankle dorsiflexion.
The shoulder blades should begin on top of the bar with the shoulders just in front. This represents the most effective technique. This is because the shoulder blades transfer force generated by the legs on the commencement of the lift, into the back and to the bar. As the bar is pulled in a vertical line over the mid foot gravity pulls in the opposite direction, therefore the shoulder blades must be above the bar to provide the most advantageous position to pull against the line of gravity. Subject A shows a good starting position with shoulder blades above the bar (fig.4) and shoulders just in front (represented by the red line). In contrast subject B starts with shoulders slightly behind the bar. The consequence of this is that the knees are pushed forward, which result in an ineffective bar path. This again could be corrected by raising the bar slightly off the ground using blocks. This will allow a more open hip angle and permit the shoulders to move in front of the bar.
Figure 4.
End Position
The lift ends at the top of the ascent, when hips knees and ankles are fully extended and spine remains in a neutral position. When comparing the bar paths of subject A and B in figure 5, both travel in vertical direction which is good. Subject A shows some posterior horizontal displacement of the bar during the descent, due to the bar rolling down the thighs. This is allows the bar to remain tight to the body and eliminates an increase in moment arm at the hip. Subject A shows a good lock out position with a vertical torso and neutral spine. In contrast subject B displays some lumbar hyperextension in the lock out which is contraindicated. This likely as a result of the less than adequate starting position of the shoulders and lack of pelvic control, potentially weakness in the gluteals. Verbal cueing to forcefully contract the gluteus maximus at and abdominals at the lock out position may help to pull the pelvis into a more neutral position. Corrective exercises such as the hip bridge may also help with this.
Figure 5.
Conclusion
Both the squat and deadlift are complex movements that require the integration of multiple joint systems and muscle groups. By breaking the movement down through a biomechanical analysis, the strength and conditioning professional can highlight strengths and weaknesses of the lift. This can assist in identifying corrective strategies to improve performance and aid in optimal training outcomes and reduced training related injury.
References
Ariel, B. G. Biomechanical analysis of the knee joint during deep knee bends with heavy loads. In: Biomechanics IV, R. Nelson and C. Morehouse (Eds.). Baltimore: University Park Press, 1974, pp. 44 –52.
Donnelly DV, Berg WP, and Fiske DM. (2006) The effect of the direction of gaze on the kinematics of the squat exercise. J Strength Cond Res 20: 145–150
Greene WB, Heckman JD. (1994) American Academy of Orthopedic Surgeons. The Clinical Measurement of Joint Motion. Chicago, IL.
Kritz, M., Cronin, J., & Hume, P. (2009). The bodyweight squat: A movement screen for the squat pattern. Strength and Conditioning Journal, 31, 76–85.
Lun, V. (2004). Relation Between Running Injury and Static Lower Limb Alignment In Recreational Runners. British Journal of Sports Medicine, 38(5), 576-580.
McGill SM and Norman RW. (1985) Dynamically and statically determined low back moments during lifting. J Biomech 18: 877–885
Myer GD, Chu DA, Brent JL, and Hewett TE. Trunk and hip control neuromuscular training for the prevention of knee joint injury. Clinical Sports Med 27: 425–448, 2008.
Noyes, FR, Butler, DL, Grood, ES, Zernicke, RF, and Hefzy, MS. (1984) Biomechanical analysis of human ligament grafts used in knee ligament repairs and reconstructions. J Bone Joint Surg 66A: 344– 352
Powers, C. (2003). The Influence of Altered Lower- Extremity Kinematics on Patellofemoral Joint Dysfunction: A Theoretical Perspective. Journal of Orthopaedic & Sports Physical Therapy, 33(11), 639-646.
Schoenfeld, B. (2010) Squatting kinematics and kinetics and their application to exercise performance. Journal of Strength and Conditioning Research 24(12)
Watkins, J. Structure and Function of the Musculoskeletal System. (1999) Champaign, IL: Human Kinetics Publishers,