Flexion and extension are fundamental movements that occur in the sagittal plane (dividing the body into left and right halves), describing the decrease or increase in the angle between two body segments at a joint.
Flexion is a movement that decreases the angle between segments:
- Bending the elbow brings the hand toward the shoulder (decreasing the elbow angle)
- Bending the knee brings the heel toward the buttocks
- Bending forward at the waist (spinal flexion)
- Lifting the thigh forward (hip flexion)
Extension is a movement that increases the angle between segments:
- Straightening the elbow
- Straightening the knee
- Standing upright from a bent position (spinal extension)
- Moving the thigh backward (hip extension)
Hyperextension occurs when extension continues beyond the neutral anatomical position (angle greater than 180°), such as arching the back or extending the wrist backward.
These movements are essential in nearly all human activities:
- Walking/Running: Hip and knee flexion and extension drive locomotion
- Throwing: Shoulder and elbow extension generate throwing velocity
- Jumping: Hip, knee, and ankle extension produce upward force
- Lifting: Coordinated flexion and extension at multiple joints
In video analysis, tracking flexion and extension patterns helps:
- Assess range of motion limitations
- Evaluate movement technique and efficiency
- Identify asymmetries between limbs
- Monitor rehabilitation progress
- Optimize athletic performance
Understanding the timing, speed, and coordination of flexion/extension movements across multiple joints is key to analyzing complex movement patterns.