Hip flexion is the fundamental movement that brings the thigh closer to the torso. The hip joint, a ball-and-socket joint, allows for a wide range of motion necessary for human mobility. This action is central to nearly every activity requiring the leg to move forward, and its proper function is tied to overall lower body health.
Defining Hip Flexion
Hip flexion is an anatomical movement occurring in the sagittal plane, involving decreasing the angle between the femur (thigh bone) and the pelvis. When standing in the standard anatomical position, the hip joint is at zero degrees of flexion. The motion is visualized as the thigh moving forward and upward toward the trunk, such as when lifting the knee toward the chest. The typical range of motion for active hip flexion is approximately 90 to 125 degrees. This range is often greater when the knee is flexed because it slackens a major muscle.
Key Muscles Driving the Movement
Hip flexion is powered by a group of muscles collectively known as the hip flexors. The primary drivers are the two muscles forming the iliopsoas group: the psoas major and the iliacus. The psoas major originates on the lumbar spine, and the iliacus originates on the inside of the pelvis. These two muscles merge into a common tendon that attaches to the lesser trochanter on the upper part of the femur. Because they connect the torso to the thigh bone, their contraction pulls the femur upward. Other muscles act as secondary flexors. The rectus femoris assists in hip flexion while also extending the knee. The sartorius also helps with flexion, external rotation, and abduction of the hip.
Common Movements and Activities
Hip flexion is incorporated into countless daily and athletic movements. The swing phase of walking and running is the most frequent demonstration, where the leg lifts forward to clear the ground and prepare for the next step. Bringing the knee up toward the chest, such as during a high-knee march, is a pure, isolated example of hip flexion. The simple act of sitting down also involves this action, as the thigh moves toward the torso to lower the body. In sports, hip flexion is visible in activities like kicking a ball, where the forceful contraction drives the leg forward at high speed.
Causes of Restricted Hip Flexion
A limited range of hip flexion can significantly impede mobility. One widespread cause is muscular tightness, frequently resulting from prolonged periods of sitting. When the hip flexor muscles are held in a shortened position for hours each day, they can adaptively shorten and become less pliable over time, restricting the hip’s ability to fully move. Another factor is weakness in opposing muscle groups, such as the gluteal muscles and core, which forces the hip flexors to work harder to stabilize the pelvis. Beyond muscular causes, restricted motion can stem from structural issues within the joint itself. Conditions like hip osteoarthritis or a labral tear can physically limit the mechanical space for the joint to move, causing pain and a hard stop during flexion.