Flexion
Flexion is a joint movement that generally decreases the angle between articulating body segments. Its direction and range depend on the joint and anatomical reference position.
Definition
Examples include bending the elbow or knee, moving the thigh anteriorly at the hip and flexing regions of the spine. Flexion commonly occurs in the sagittal plane and often contrasts with extension, but terminology is joint-specific: at the ankle, movement of the top of the foot toward the shin is dorsiflexion rather than simply “ankle flexion.” Brachialis and biceps contribute at the elbow, iliopsoas and rectus femoris at the hip, hamstrings at the knee, and several trunk muscles to spinal flexion.
OpenStax Anatomy & Physiology · body movements
HYROX® examples
Wall Ball descent, the lead leg during Sandbag Lunges and the Row catch can involve hip and knee flexion. Burpee Broad Jumps and SkiErg may include trunk or hip flexion depending on technique. These are coordinated whole-body tasks, and the rulebook specifies outcomes rather than one required joint pattern. It does not describe hundreds of spinal-flexion repetitions or define loaded lumbar flexion as a HYROX® injury mechanism.
HYROX® Singles Rulebook 2026/27 · OpenStax Anatomy & Physiology · body movements
Running claims
Hip flexors contribute to advancing the thigh during running, alongside momentum and coordinated pelvis, knee and ankle motion. The statement that weak or fatigued hip flexors necessarily reduce cadence, increase ground-contact time, worsen economy and directly slow all eight runs is not supported by the reviewed evidence. A 2024 meta-analysis found cadence had only a small association with economy, while contact time and hip-angle/range measures were generally not significantly associated. Late-split slowing cannot identify one muscle as the cause.
Van Hooren et al. · running biomechanics/economy meta-analysis
Training and spinal position
Hanging knee raises and resisted marches can train hip flexion; leg curls and Nordic exercises train knee flexors. Nordic training increases several knee-flexor strength measures, but evidence does not make it mandatory or define one universal dose. Use a controllable range, progressive resistance and task-appropriate volume. Lumbar flexion is a normal movement; a meta-analysis found low-quality evidence that greater flexion during lifting was not a low-back-pain risk factor or discriminator. Adjust posture and load for the task and symptoms rather than enforcing “neutral” in every movement.
Franke et al. · Nordic exercise strength meta-analysis · Saraceni et al. · lumbar flexion and low-back pain meta-analysis
Frequently Asked Questions
What is the difference between flexion and extension?
Flexion generally decreases and extension generally increases the angle between segments, but the exact description depends on the joint and anatomical position. They are not simple complementary pairs at every joint; the ankle, shoulder and spine use additional specific terms.
OpenStax Anatomy & Physiology · body movements
Why are hip flexors relevant to running?
They contribute to forward thigh motion during swing, but running performance emerges from whole-body coordination. Weakness or fatigue should be assessed directly rather than inferred from cadence, ground-contact time or late HYROX® splits.
Van Hooren et al. · running biomechanics/economy meta-analysis
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Use Repz to monitor if your later running splits slow disproportionately, which may indicate hip flexor endurance limitations.
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