Flexion

Definition

Flexion is a joint movement that decreases the angle between two body segments, bringing them closer together. It is the opposite of extension. Common examples include bending the elbow (elbow flexion), bringing the knee toward the chest (hip flexion), squatting down (knee and hip flexion), and curling the trunk forward (spinal flexion).

Flexion occurs primarily in the sagittal plane and is produced by muscles positioned to pull body segments toward each other, such as the biceps (elbow flexion), hip flexors (hip flexion), and hamstrings (knee flexion).

Relevance to HYROX^®

Flexion patterns appear throughout a HYROX^® race. The descent phase of every wall ball begins with hip and knee flexion. Each lunge step requires controlled knee and hip flexion of the lead leg. The catch position on the rowing machine involves trunk, hip, and knee flexion.

Hip flexion is particularly important during running. The hip flexors drive the recovery phase of each running stride, bringing the thigh forward for the next ground contact. Over 8 km of cumulative running, hip flexor fatigue can reduce stride frequency and running economy.

Spinal flexion occurs during burpee broad jumps as athletes lower to the ground and during SkiErg pulls. Maintaining controlled spinal flexion (avoiding excessive rounding) is essential for protecting the lower back across hundreds of repetitions.

Key Details

• Primary flexors by joint: Biceps and brachialis (elbow), iliopsoas and rectus femoris (hip), hamstrings (knee), rectus abdominis (trunk)
• Plane of motion: Primarily sagittal plane
• Opposite movement: Extension
• Key HYROX^® applications: Squat descent in wall balls, lunge lowering phase, running stride recovery, rowing catch
• Risk area: Excessive lumbar flexion under load

Training Tips

Develop flexion strength through a full range of motion. For hip flexion, include exercises like hanging knee raises and marching drills with resistance bands. For knee flexion, Nordic hamstring curls and leg curls build the hamstrings in their flexion role.

Avoid excessive spinal flexion under load during HYROX^® training. Cue a braced, neutral spine during wall balls, lunges, and sled work. If your hip flexors fatigue during late running segments, add specific hip flexor strengthening (seated banded marches, 3 sets of 15 per side) to your program. Use ROXBASE to monitor if your later running splits slow disproportionately, which may indicate hip flexor endurance limitations.

Related Terms

Flexion is the opposite of extension and occurs in the sagittal plane. Ankle flexion is more specifically called dorsiflexion. Spinal flexion involves the lumbar spine and thoracic spine.

FAQ

What is the difference between flexion and extension?

Flexion decreases the angle at a joint (bending), while extension increases the angle (straightening). In a squat, the downward phase is flexion and the upward phase is extension. Both movements work as complementary pairs across all major joints.

Why do hip flexors matter for HYROX^® running?

The hip flexors drive the recovery phase of each running stride, pulling the thigh forward after push-off. Weak or fatigued hip flexors reduce stride frequency and increase ground contact time, which directly slows running pace across eight segments totaling 8 km.