What muscle is primarily responsible for knee flexion, particularly in patients with cerebral palsy or other neurological conditions?

Muscles Responsible for Knee Flexion

The hamstring muscle group—consisting of the biceps femoris, semitendinosus, and semimembranosus—is primarily responsible for knee flexion, with the semitendinosus demonstrating the greatest activity during knee flexion exercises. 1, 2, 3

Primary Knee Flexor Muscles

The hamstring muscle complex, also known as the flexor femoris muscle complex (FFMC), comprises three distinct muscles that cross both the hip and knee joints 1:

• Biceps femoris (long head and short head): The long head is bi-articular, crossing both hip and knee joints, while the short head crosses only the knee joint 1, 4
• Semitendinosus: Demonstrates the highest electromyographic (EMG) activity during knee flexion exercises, with significantly greater activation than other hamstring muscles 2, 3
• Semimembranosus: Shows substantial EMG activity during knee flexion, though typically less than the semitendinosus 2, 4

Muscle Activation Patterns During Knee Flexion

Relative Muscle Contribution

The semitendinosus is the most active hamstring muscle during knee flexion exercises, followed by the semimembranosus, with the biceps femoris showing comparatively lower activation. 2, 3

• During knee flexion exercises with the hip flexed at 90°, the semitendinosus shows significantly greater muscle recruitment (measured by T2 MRI changes) than other hamstring muscles 3
• EMG studies demonstrate that the semimembranosus produces RMS values of 480.00 (±130.13) during knee flexion, compared to 303.50 (±63.31) for the semitendinosus and 188.54 (±46.10) for the biceps femoris long head 2

Effect of Joint Position on Muscle Activity

The hamstring muscles demonstrate variable activation depending on both hip and knee joint angles 4, 3:

• Hip position matters: Knee flexion exercises performed with the hip flexed at 90° produce significantly greater peak torque and total work than exercises with the hip fully extended 3
• Knee angle influences recruitment: EMG activity of the semitendinosus, semimembranosus, and biceps femoris short head increases significantly as the knee angle increases from 0° to 105° of flexion during isokinetic contraction 4
• The biceps femoris long head shows a different pattern, with greatest EMG activity at knee angles between 15-30° of flexion, then decreasing as flexion increases 4

Clinical Relevance for Neurological Conditions

Positioning Considerations

When assessing or treating patients with cerebral palsy or other neurological conditions affecting knee flexion 5:

• Avoid excessive hamstring stretch: Positions that stretch the hamstring muscle group beyond comfortable range may compromise sciatic nerve function, as the sciatic nerve crosses both hip and knee joints 5
• Limit hip flexion appropriately: Assess both hip extension and knee flexion when determining optimal positioning, since the sciatic nerve branches cross both joints 5
• The hamstring muscles function as both hip extensors and knee flexors, with their effectiveness as hip extensors depending on knee joint positioning 2

Muscle-Specific Injury Patterns

The biceps femoris is the most commonly injured muscle of the hamstring complex, despite showing lower activation during pure knee flexion exercises. 1, 3

• This paradox suggests that biceps femoris injuries may occur more frequently during activities involving combined hip extension and knee flexion, rather than isolated knee flexion 1, 3
• The semitendinosus, despite higher activation during knee flexion, may be relatively protected due to its more consistent recruitment pattern 3

Optimizing Hamstring Function

Exercise Selection Based on Target Muscle

To preferentially activate specific hamstring muscles 3, 6:

• For semitendinosus emphasis: Perform knee flexion exercises with the hip flexed at 90°, which produces the greatest semitendinosus activation 3
• For biceps femoris long head emphasis: Perform Nordic hamstring exercises at shallow knee flexion angles (less than 50°), which equalizes biceps femoris and semitendinosus activity 6
• For overall hamstring development: Hip extension exercises with the knee fully extended recruit all three hamstring muscles (biceps femoris long head, semitendinosus, and semimembranosus) more evenly 3

Common Pitfall

A critical error is assuming all hamstring muscles contribute equally to knee flexion—the semitendinosus is the dominant knee flexor, while the biceps femoris long head shows preferential activation during combined hip extension movements 2, 3. This distinction is essential when designing rehabilitation protocols for hamstring injuries or neurological conditions affecting knee flexion.