Muscle Activation vs Sensation During the Seated Leg Curl
MRI data shows that hip flexion angle shifts activation not just between hamstring muscles, but within specific longitudinal regions of the same muscle.
The hamstrings are not a single muscle, and they don’t activate uniformly during the leg curl. That much has been suspected for a while. What has been less clear is to what extent hip position changes which part of each hamstring muscle gets targeted. Also, whether what you feel during a set actually reflects what is happening in the muscle. A new study explored this, looking for insights on how leg curl variations could best be programmed.
Aims and Methods
The researchers wanted to know two things: first, whether changing the hip flexion angle (HFA) during the leg curl shifts activation not just between hamstring muscles, but within specific regions along the length of each muscle. Second, whether the subjective sensation of muscle contraction during the exercise actually lines up with what the MRI is showing.
Nine experienced male bodybuilders performed unilateral leg curl (LC) exercise under three different hip flexion angles: HFA 0° (hip near fully extended, like a prone leg curl), HFA 40° (intermediate), and HFA 80° (conventional seated leg curl). All sessions used the same load relative to each participant’s 1RM for that position.
To measure muscle activation, the researchers used MRI to calculate T2 relaxation times before and after each session. This method detects fluid shifts in activated muscle tissue and is considered a reliable marker of region-specific and longitudinally compartmentalised muscle activation. The hamstrings were divided into proximal, middle, and distal regions for the biceps femoris long head (BF-long), biceps femoris short head (BF-short), semitendinosus (ST), and semimembranosus (SM).
After each session, participants also rated their subjective contraction sensation using a numerical rating scale (NRS) across the same regions, which was then compared against the objective T2 data.
Results
The semitendinosus dominates the leg curl. Across every hip position tested, the ST showed greater activation than the other hamstring muscles. This held across the proximal, middle, and distal regions in most conditions, aligning with prior research showing that the ST contributes disproportionately to knee flexion torque.