Biomechanics of Hip Joint in a Squat Movement: Study of External Forces

Introduction

The purpose of this scientific report was to look at the movement of the hip joint in the eccentric phase of a squat and the external forces that act on it and to further investigate the role that biomechanics has had in the research and the development of a squat.

Key Terms

Biomechanics can be defined as the study of forces and how those forces act on living organisms mechanically (Hall, 2018). A system of interest (SoI) is derived from Newton’s second law of motion and can be defined as forces that are internal or external of the system (body) that is being studied (Hall, 2018).

System of Interest

The SoI that was being examined for this report was the hip joint. The internal forces of the hip joint are the musculoskeletal components that are used during a squat movement which includes the quadriceps, hamstrings, gluteus maximus, femur, tibia, pelvis and spine. The external forces acting on the hip joint are Newton’s law of gravitation, reaction forces and centre of gravity.

Main Body

The Hip Joint

The hip joint is a ball and socket joint. The hip joint consists of 3 planes of motion including the sagittal, frontal and transverse planes. The sagittal plane consists of the movement of flexion and extension, the frontal plane consists of abduction and adduction movement and the transverse plane consists of medial and lateral movement (Kritz, Cronin & Hume, 2009). Therefore, the hip joint has quite a large range of motion and movements that it is capable of completing due to the type of joint.

Newton’s Law of Gravitation

Newton’s law of gravitation states that all objects are attracted to another object and the greater the mass of either object, the greater the force of attraction between the two objects (Hall, 2018). Therefore, every object or being on Earth is being drawn towards the centre of the Earth due to the force of attraction between them and the force of acceleration occurs at a rate of 9.81 m/s at sea level. The external force of gravity is always acting or pushing down on an object or being. When a person begins the eccentric phase of a squat, the body goes into flexion and the relative angle of the hip joint begins to decrease. As displayed in figure 1, the force of gravity is pushing the body down at a rate of 9.81 m/s, however, the internal forces are all working together to push against gravity to keep the body upright and the hip joint in the correct position. This is also quite similar to reaction force.

Figure 1. The force of gravity acting on a body during a squat and the internal forces acting against gravity.

Reaction Force

Newton’s third law of motion states that for every action there is an equal and opposite action (Hall, 2018). Ground reaction force can be described as the amount of force a person puts into the ground, whether that is vertical or horizontal, the ground exerts an equal amount of force back towards the person. This is an equal and opposite action. As a person begins the eccentric phase of a squat, the amount of force that is exerted into the ground by the internal forces of the person, the ground exerts the same amount of force back to the person and this is known as vertical ground reaction force (Vahdat & Ghomsheh, 2018). As demonstrated in figure 2, the internal forces are applied forces (orange) and the external force is the reaction force (blue). For every applied force, there is an equal and opposite action produced by the reaction [image: ]force.

Figure 2. Ground reaction force in a squat.

Centre of Gravity

Centre of gravity can be defined as the point where a person’s mass is being naturally balanced (Hall, 2018). There are two set of criteria that is required for a body to reach natural balance and these are “1. All linear forces acting on the body must be balanced and 2. All the rotary forces (torques) must be balanced” (Hamilton, Weimar & Luttgens, 2011). This means that for the body to be balanced, all forces that act on the body must have an equal and opposite action and will therefore equal 0. The centre of gravity when a body is upright generally sits at hip level and approximately over the midfoot. This is where the body feels comfortable and naturally balanced. As demonstrated in figure 3, the centre of gravity does not overly change when a person is lowering into a squat position. The centre of gravity adjusts on the eccentric phase to keep the body balanced and resist external forces; however, the body still feels naturally balanced over the midfoot.

Figure 3. The centre of gravity standing and in squat position

Biomechanics of the Squat

At the start of a squat, a person begins in a standing or static position with their hips in a neutral position and their centre of gravity over their midfoot (Myer, Kushner, Brent, Schoenfeld, Hugentobler, Lloyd, Vermeil, Chu, Harbin & McGill, 2014). In the standing position, all of the angles between the joints are open and all internal forces are vertical to the force of gravity. When the eccentric phase begins, flexion occurs at the hip joint and knee joint which causes the hips to sit posteriorly and the relative angles between the hip joint and knee joint begin to decrease. The deeper the squat, the smaller the relative angle between the joints. As seen in table 1, the range of motion of the hip joint has a minimum angle at full extension of 0° and a maximum angle of flexion at 95.4° during a squat (Hemmerich, Brown, Smith, Marthandam & Wyss, 2006). The internal forces of the body, particularly the muscle groups of the gluteal and hamstrings, act as stabilisers for the hip joint and they assist in controlling the angle of the hip joint (Myer, et al. 2014). During the eccentric phase of a squat, the internal forces of the body resist the external forces. The internal forces hold the body and movement together, furthermore, the internal forces resist the external forces so the movement can be complete. As displayed in the squat position in figure 4, the internal force of the femur is almost horizontal to the force of gravity, and the spine and tibia sit on approximately a 45° angle from the hip joint and are quite similar in regard to the angle of the internal force.

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Table 1. Hip angle during flexion of a squat

Minimum Angle

(extension)

Maximum Angle

(flexion)

Angle (°)

95.4

Figure 4. Centre of gravity and decrease in relative angles during a squat.

Discussion

Biomechanics and Research

Biomechanics has assisted in the research of a squat as biomechanics gives a person a greater understanding of the internal and external forces that act on a body during a movement. A study was conducted that compared a traditional squat, a box squat and powerlifting squat. This study demonstrated several things such as the characteristics of different squat variations and what muscles are recruited in the different squat variations and goes quite into depth of the biomechanical differences between the squat variations. The study showed that a powerlifting squat had a more obvious upper torso lean and is more likely to recruit more muscles that surround the hips due to having a wider stance (Swinton, Lloyd, Keog, Agouris & Stewart, 2012). This type of research can be beneficial as it shows that biomechanics can assist a person in understanding what muscles are activated and when during a squat, it allows them to gain a greater understanding of the different types of squats, the actual squat movement itself and can assist in the improvements of technique in a squat.

Biomechanics Developing the Squat

Biomechanics can assist in the development of the squat as it breaks down the movement into internal and external forces and can assist in improving performance. This is important as it allows a person to gain a greater understanding of the movement as it is being broken down into segments. For example, a study was conducted between competitive high school students and novice powerlifters examining squats. The research displayed that novice powerlifters had a more controlled eccentric phase of the squat and had greater alignment in the trunk, hip and knee (Miletello, Beam & Cooper, 2009). This is important as it breaks down the technique into three phases. The first being the eccentric/flexion phase, the bottom of the squat/static phase and the concentric/extension phase. This allows for adjustments in the movement that can be beneficial for the athlete and/or coach to gain more knowledge, increase performance and safety including reducing the risk of injury. As a person has a greater biomechanical understanding of the movement, it allows for further development in technique improvement which can then lead to a greater performance.

Conclusion

The report has displayed the system of interest and important key terms. The report has discussed the internal and external forces that act on the body during a squat movement including Newton’s law of gravitation, reaction forces and the centre of gravity. The report further elaborated on the biomechanics of the squat with a focus on the hip joint. The report has also discussed the role that biomechanics has in the research of a squat as well as the role in the development of a squat.

References

1. Hall, S. J. (2018). Basic biomechanics (8th ed.). McGraw-Hill Education.
2. Hamilton N., Weimar W., Luttgens K. (2011). Kinesiology: Scientific Basis of Human Motion. McGraw-Hill Education – Europe.
3. Hemmerich A., Brown H., Smith S., Marthandom S. S. K., Wyss U. P. (2006). Hip, knee, and ankle kinematics of high range of motion activities of daily living. Journal of Orthopaedic Research, 24(4), 770-781. doi: https://doi.org/10.1002/jor.20114
4. Kritz M., Cronin J., Hume P. (2009). The bodyweight squat: a movement screen for the squat pattern. Strength and Conditioning Journal, 31(1), 76-85. doi: 10.1519/SSC.0b013e318195eb2f
5. Miletello W. M., Beam J. R., Cooper Z. C. (2009). A biomechanical analysis of the squat between competitive collegiate, competitive high school and novice powerlifters. Journal of Strength and Conditioning Research, 23(5), 1611-1617. doi: 10.1519/JSC.0b013e3181a3c6ef
6. Myer G. D., Kushner A. M., Brent J. L., Schoenfeld B. J., Hugentobler J., Lloyd R. S., Vermeil A., Chu D. A., Harbin J., McGill S. M. (2014). The back squat: a proposed assessment of functional deficits and technical factors that limit performance. Strength Cond J, 36(6), 4-27. doi: 10.1519/SSC.0000000000000103
7. Swinton, P. A., Lloyd R., Keogh J. W. L., Agouris I., & Stewart A. D. (2012). A biomechanical comparison of the traditional squat, powerlifting squat, and box squat. Journal of Strength and Conditioning Research, 26(7), 1805-1816. doi: 10.1519/JSC.0b013e3182577067
8. Vahdat I., Ghomsheh F. T. (2018). The effect of task execution variables on resultant verticle ground reaction force acting on foot sole during squat lifting. Journal of Bodywork and Movement Therapies, 22(3), 632-638. doi: https://doi.org/10.1016/j.jbmt.2017.10.010