Static stretching grew the hamstring aponeurosis but not the muscle
In short
Eleven untrained men trained one leg with eccentric knee-flexion exercise and the other with duration-matched static stretching for 10 weeks. MRI showed hamstring outer aponeurosis volume increased on both legs — roughly 19.0-31.8% for stretching and 15.8-22.8% for eccentric training — and those gains held through four weeks of detraining. Muscle hypertrophy appeared only on the eccentric leg, at roughly 22-42%, with no significant change in muscle volume from stretching.
Whether stretching builds muscle is an old argument. Splitting the two protocols across one person's left and right legs for 10 weeks gives a split answer: the connective tissue grew, the muscle did not.
How was it compared?
Eleven untrained men trained three times a week for 10 weeks. One leg performed eccentric knee flexion on an isokinetic dynamometer — 3 × 10 reps at 5°/s and 50% of maximal eccentric torque. The other leg performed three static stretch holds matched for time under tension. Comparing legs within the same person controls for genetics, diet, and sleep.
MRI quantified hamstring outer aponeurosis volume, inner aponeurosis volume and width, and muscle volume, then measured again after four weeks of detraining.
Both protocols grew the aponeurosis
Outer aponeurosis volume increased across all muscles under both protocols — roughly 19.0-31.8% for stretching against 15.8-22.8% for eccentric training, with stretching the larger of the two. Inner aponeurosis volume and width of biceps femoris long head and semimembranosus also rose about 7-23% under both.
The aponeurosis is the broad sheet of connective tissue through which muscle transmits force into tendon. Given how many hamstring injuries occur near the muscle-tendon junction, the fact that stretching alone thickened this structure is worth noting on its own.
Only the eccentric leg got bigger
Hypertrophy was eccentric-specific, at roughly 22-42% in the biarticular hamstrings. The stretching leg showed no significant change in muscle volume. And in almost every case, changes in muscle volume were not significantly associated with changes in aponeurosis dimensions — the two tissues responded independently.
Which one fades first when you stop?
Over four weeks of detraining, aponeurosis gains were preserved. Muscle volume stayed above baseline but was more sensitive to short-term detraining. Muscle reacts to time off first; connective tissue holds.
Eleven untrained men, single-joint work on an isokinetic dynamometer. A 22-42% muscle-volume increase reflects a large untrained-beginner effect, and there is no basis for assuming trained lifters get the same from Nordic curls or Romanian deadlifts.
Fitting it into a Big 3 program
The two jobs are distinct. Growing hamstrings requires eccentric loading, and stretching does not substitute for it. Looking only at connective tissue, stretching did its share. During a stretch where your deadlift is climbing fast, there is reason to keep both.
Read with eccentric overload adds sarcomeres, training without the eccentric, and hamstring stretching, before or after.
Frequently asked questions
Does static stretching build muscle?
Not in this trial. The leg that performed 10 weeks of duration-matched static stretching showed no significant change in muscle volume, while hypertrophy of roughly 22-42% appeared only on the eccentrically trained leg.
What did stretching change instead?
Connective tissue. Hamstring outer aponeurosis volume increased roughly 19.0-31.8%, and inner aponeurosis volume and width of biceps femoris long head and semimembranosus rose about 7-23% — larger increases than the eccentric leg's 15.8-22.8% outer aponeurosis change.
Are aponeurosis growth and muscle growth related?
In almost every case there was no significant association between changes in muscle volume and changes in aponeurosis dimensions, suggesting the two adaptations occur independently.
What fades first after four weeks off?
Muscle. Aponeurosis changes were preserved through four weeks of detraining, while muscle volume remained above baseline but proved more sensitive to short-term detraining.
Can these results be applied directly to trained lifters?
With caution. Participants were 11 untrained men and the exercise was single-joint eccentric knee flexion on an isokinetic dynamometer. The 22-42% muscle-volume increase reflects a substantial untrained-beginner effect.
Source: PubMed