What Truly Flexes the Knee During Walking?

When discussing the biomechanics of walking, a question often arises in training or clinical practice:
Which muscle flexes the knee during the swing phase?

The most common answer is: the hamstrings. And that’s understandable. These muscles, located at the back of the thigh, are well-known for their role as knee flexors. However, this answer, although technically correct, does not reflect the primary mechanism involved during normal-speed walking.

The True Dynamics of the Swing Phase

To understand what truly happens, one must observe the movement as a whole, and not focus solely on the knee.

The key muscle that initiates the swing phase is the iliopsoas, the primary hip flexor. By generating hip flexion, it pulls the thigh segment forward.
This initial movement causes, due to the inertia of the lower leg segment, knee flexion. In other words, the knee bends without a muscle directly acting to flex it at that precise moment.

This mechanism is essential for the fluidity and efficiency of the gait cycle. It perfectly illustrates the interdependence between the different segments of the lower limb.

The Role of the Hamstrings

The hamstrings, however, intervene at other times. Their role is then twofold:

• Braking knee extension just before heel strike,
• Stabilizing the joint at the beginning of the stance phase.

They therefore act more as brakes or stabilizers than as initiators of movement.

Why is this Distinction Important in Rehabilitation?

In gait rehabilitation, particularly in neurology or post-orthopedic surgery, it is crucial to target the right muscles at the right time.
Believing that the hamstrings are primarily responsible for knee flexion during oscillation, at normal walking speed, can lead to inappropriate or poorly prioritized exercise choices.

By understanding that it is hip flexion via the iliopsoas that initiates the movement, one can:

• More precisely target iliopsoas recruitment in leg raise or active walking exercises,
• Better adapt technical aids or compensation strategies for patients with hip flexion deficits,
• More clearly identify intersegmental coordination disorders in patients with neurological disorders.

Conclusion

Knee flexion during the swing phase of walking is primarily the result of hip flexion initiated by the iliopsoas.
The hamstrings, although knee flexors, are not the main actors in this specific functional context.

Understanding the biomechanical logic of walking allows one to refine therapeutic choices in rehabilitation, strengthen the correct muscle groups, and optimize the patient’s functional progression.