What Is the Biomechanics of the Hip?
The hip joint is composed of a concave acetabulum and a convex femoral head. It belongs to a ball and socket structure and has inherent stability. The head of the hip joint and the acetabular cartilage surface are in contact with each other to transmit gravity, support the weight of the upper body and provide mobility of the lower limbs. Among the many movable joints, the hip joint is the most stable, and its structure can complete a wide range of actions required in daily life, such as walking, sitting and squatting. The disordered arrangement of ball and socket joints can lead to changes in the stress distribution of articular cartilage and bone, causing damage such as degenerative arthritis, and it is gradually aggravated by the huge forces on the joints.
- The hip joint is composed of a concave acetabulum and a convex femoral head. It belongs to a ball and socket structure and has inherent stability. The head of the hip joint and the acetabular cartilage surface are in contact with each other to transmit gravity, support the weight of the upper body and provide mobility of the lower limbs. Among the many movable joints, the hip joint is the most stable, and its structure can complete a wide range of actions required in daily life, such as walking, sitting and squatting. The disordered arrangement of ball and socket joints can lead to changes in the stress distribution of articular cartilage and bone, causing damage such as degenerative arthritis, and it is gradually aggravated by the huge forces on the joints.
Hip joint biomechanics
- The hip joint is the body's largest weight-bearing joint, and is mainly composed of the acetabular and femoral heads on the pelvis, and some soft tissues such as round ligaments and cartilage. The angle between the femoral neck and the femoral shaft is the neck shaft angle, which is about 110 ° to 141 ° for adults. This angle can increase the range of motion of the lower limbs and allow the force of the trunk: the amount to be transmitted to the wider base. Both hip varus (141 °) and hip varus (l10 °) caused by femoral shaft deflection will change the forces related to the hip joint. The angle between the long axis of the femoral neck and the transverse axis of the distal femoral condyle is the anteversion of the femoral neck, which is usually 12 ° -15 °, and the anteversion angle greater than 15 ° will cause some of the femoral head to lose acetabular coverage. The femoral moment is located inside and behind the femoral neck shaft junction. In the deep part of the trochanter, it is a bone plate composed of multiple dense bones, which is an extension of the posteromedial cortex of the femoral shaft. The femoral moment is the point of eccentric stress on the upper segment of the femur. It is the point of maximum compressive stress when standing upright, and it is also subject to bending moments and torques. Its presence increases the stress-bearing capacity of the neck-stem connection.
- Hu Gengdan, Wang Lejun, Niu Wenxin. Sports Biomechanics. Tongji University Press. December 2013, 1st edition.
- Figure 1 Human hip joint ( 1. Overall anatomy; 2. Coronal section)
- Under normal conditions, forces in all directions of the hip joint remain balanced. When symmetrically standing on both feet, the weight is evenly distributed to the lower limbs, and each hip bears 1/2 of the weight except the weight of the lower limbs. When the weight of one lower limb is loaded, the burden of the hip joint is the weight excluding the weight of one lower limb plus the abductor muscle strength. At this point, a fulcrum similar to the balance lever system is formed at the upper part of the weighted hip joint femoral head. In order to maintain body balance, the abductor muscles need to be tense and have a balancing effect. If the center of gravity is far away from the weight-bearing hip joint, the bearing capacity increases; if the center of gravity moves to the weight-bearing hip joint, the load bearing force decreases; if the center of gravity moves to the weight-bearing hip joint, the abductor bearing capacity is zero, and the hip only bears part of the weight The pressure.
Hip Biomechanics Hip Kinematics
- The hip joint is a ball bearing motion structure. The main movement can be decomposed into three mutually perpendicular planes: flexion and extension on the sagittal plane, adduction and abduction on the coronal plane, and internal and external rotation on the transverse plane. The range of these three plane movements is different. The maximum amplitude of the hip joint is in the sagittal plane. The range of forward flexion is 0 ° ~ 140 °, and the range of backward extension is 0 ° ~ 15 °. In the coronal plane, the abduction range is 0 ° -30 °, and the adduction range is 0 ° -25 °. In cross section, when the hip joint is flexed, the external rotation is 0 ° -90 °, and the internal rotation is 0 ° -70 °. When the hip joint is straightened, the rotation angle is smaller due to the restraint function of the soft tissue, and the internal and external rotation are 45 °. When moving up the stairs, the range of motion is large, and the range of flexion and extension is 67 °. Adduction, abduction and internal and external rotation are 28 and 26 °, respectively. When running, the range of flexion and extension on the sagittal plane will increase. The articular surface movement of the hip joint can be considered as the sliding of the femoral head in the acetabulum. Rotation of the ball and socket around the center of rotation of the femoral head in three planes causes sliding of the articular surface. If the femoral head is not compatible with the acetabulum, sliding will not be parallel to the surface or tangential to the surface, and the joint cartilage will be subjected to abnormal stress causing compression or separation.
Biomechanics of hip joints acting on hip joints and their biomechanical characteristics
- The hip joints are stressed differently in different positions, and they are simultaneously subjected to gravity and tensile forces of the abductors when standing. When standing and walking on one foot, because the center of gravity of the person is connected to the femoral heads on both sides, gravity produces a torque effect on the joints. At this time, the abductor muscles generate a reverse torque to maintain balance. Transverse circular and shear stresses are also accepted. When performing various actions, the hip muscles are often required to balance weight, which will cause a considerable amount of pressure on the hip joint. Because in this process, if the hip joint is used as the fulcrum, the arm from the fulcrum to the center of gravity of the body is much larger than the fulcrum to the hip muscles. The strength of the hip muscles is much greater than the weight of the human body, so the joint force will be greater than body weight.
- In addition to increasing stability, hip muscles can also regulate the stress state of the femur. When a normal person is standing, if the muscles (such as the gluteal muscles) are not tense, the femoral neck will receive a bending moment, which will cause tensile stress at the top and compressive stress at the bottom. Therefore, if the load is too large, tensile stress failure is likely to occur. The contraction effect produced by the muscles will offset the upper tensile stress and avoid femoral neck fractures.
- Hip joint movements are balanced and rhythmic during normal walking with minimal energy consumption. The two hips take turns bearing weights, and the center of gravity moves about 4.0 to 4.5 cm from side to side. During the gait cycle of the hip joint, there will be two stress peaks, which are when the heel is on the ground and the toe is off the ground. When walking slowly, the effect of inertial force can be ignored, which is the same as static. However, when the hip joint moves rapidly, the force will increase due to acceleration and deceleration. The total force is equal to the weight plus the inertial force, including ground recoil, gravity, acceleration, muscle strength, etc., and is generally considered to be 3.9 to 6.0 times the weight. While walking (speed 1.5
- The hip joint transmits gravity through the head and acetabular cartilage surfaces in contact with each other, and the load-bearing surface is the overlapping part of the upper hemisphere of the femoral head and the hemispherical acetabulum with the weight center as the pole. The elastic articular cartilage spreads the stress to the action points. The normal stress distribution of the femoral neck is that the resultant force passes below the center of the neck, with a higher compressive stress on the inside and a higher tensile stress on the outside. Classical beam test principle calculations, photoelasticity tests, finite element mechanical analysis combined with mathematical analysis, strain gauges, or measurements on bone surfaces coated with strain sensitive substances have all proven that the tensile stress at the upper femoral neck head-neck junction is the highest. When the hip joint deformed, the stress distribution changed: the medial pressure and lateral tension increased during hip varus; when the hip was valgus, the tensile stress gradually decreased with the increase of valgus and even disappeared. When the combined force passes through the center of the neck, the medial and lateral pressures are average. In order to analyze the stress of the hip joint, it is assumed that the entire body is concentrated at one point, which is called the center of gravity of the body. When standing still, the center of gravity and the common axis of both hips are in the same coronal plane and are located in front of the second sacrum. During normal walking, the hip joints take turns bearing weights, and the center of gravity moves back and forth. Therefore, the hip joint force will be different due to different exercise methods. Some experiments show that when the hip joint is subjected to 2,000 loads, cartilage will be severely vibrated and ulcers will be formed, causing irreversible deformation of cartilage and bone, causing extensive damage to bone. The hip joint activity of the elderly is about one million times a year. It is understandable that such a high load and high frequency will produce degenerative joint disease.
- Generally, the forces acting on the hip joint can be divided into tensile stress, compressive stress, bending stress, and shear stress. The effects of these forces are manifested through a combination of weight load and muscle contraction. To meet the needs of walking and labor, human hip joints have excellent mechanical properties and have the following biomechanical characteristics:
- (1) The upper end of the femur forms a multi-planar bending angle (neck-stem angle, anteversion angle), which has a multi-curved structure with the pelvis and lower limbs. Its bone trabecula has a multi-layer grid shape, the stress distribution is reasonable, the load bearing performance is the best, and its dead weight is light and heavy.
- (2) It has the characteristics of automatic feedback control to meet the needs of tensile stress and compressive stress. According to Wollf's law, the upper end of the femur has a unique fan-shaped pressure trabecular trabecular system and a bow-shaped transverse tension trabecular trabecular system; another trabecular system is formed at the rotor plane. The trabecular bone density can be increased or decreased by the automatic feedback system of the human body according to the magnitude of the force, so that the bone tissue can obtain the maximum effect with the smallest weight.
- (3) The biomechanical structure of the hip joint has variability. The quantity and quality of trabecular tissue structure are affected by many factors such as individual occupation, activity status, endocrine, material metabolism, nutrition, age, disease and so on.
- (4) The mechanical axis of the femoral shaft is from the center of rotation of the femoral head to the midpoint of the inner and outer condyles of the femur. The upper end of the femur is subjected to the largest shear stress. Therefore, the femoral neck is often fractured due to shear stress. Fracture from rotation stress. The hip biomechanical system is in dynamic equilibrium and can be adjusted at any time to keep the body's center of gravity stable. The distribution of bone trabeculae and the shape of the bone cross-section are adapted to the needs of external forces, especially to prevent the effects of bending stress to the greatest extent.