Hamstrings hypertrophy is specific to the training exercise: Nordic hamstring versus hip extension

Key points

1. 01LSET grew total hamstring volume more than NHT (+18% vs +11%), with the biceps femoris long head growing 3.5 times more after LSET (+19%) than NHT (+5%)
2. 02NHT preferentially hypertrophied non-hip-extending knee flexors (biceps femoris short head, sartorius) at approximately twice the rate of LSET
3. 03LSET increased BFlh aponeurosis area by +9% versus only +3% after NHT; NHT did not differ from the control group for aponeurosis changes
4. 04Eccentric knee flexion strength improved with both LSET (+17%) and NHT (+11%), but only LSET differed significantly from the no-training control group
5. 05The pattern of hypertrophy matched the functional role of each muscle: muscles lengthened more during training grew more, regardless of which exercise was used

How it was conducted

Design

Parallel-group randomised controlled trial with a no-training control arm, 12 weeks

Participants

42 healthy young males (mean age 24-27 yr across groups), no lower-limb injury or systematic training in the previous 18 months

Groups

LSET: hip-flexed seated leg curl eccentric training (n=14); NHT: Nordic hamstring training (n=14); CON: habitual activity control (n=14)

Training dose

34 supervised sessions over 12 weeks; eccentric sets and reps equated between training groups

Primary outcomes

MRI-measured muscle volume of 7 individual knee flexors and BFlh aponeurosis area; maximum eccentric, isometric, and concentric knee flexion torque

Analysis

Repeated-measures ANCOVA with pre-training values as covariates; LSD post-hoc tests

What they found

• Total hamstring volume: LSET +18%, NHT +11%, CON no change; LSET > NHT > CON (all P < 0.001)
• BFlh volume: LSET +19%, NHT +5%; LSET greater than NHT and CON (both P < 0.001); NHT did not differ from CON (P = 0.053-0.949)
• SM volume: LSET +14%, NHT no significant change (P=0.423); LSET > NHT (P < 0.001)
• BFsh volume: NHT +22%, LSET +6%; NHT greater than LSET (P = 0.001-0.010)
• KF-and-HE group (BFlh+ST+SM): LSET 2.2-fold greater increase than NHT (P < 0.001)
• KF-not-HE group (BFsh+SAR+GRA+POP): NHT 1.9-fold greater increase than LSET (P <= 0.001)
• BFlh aponeurosis area: LSET +9%, NHT +3%, CON +2%; LSET greater than NHT (P=0.001) and CON (P < 0.001); NHT did not differ from CON (P=0.292)
• BFlh aponeurosis maximum width: LSET +8% (P < 0.001); NHT and CON showed no within-group change; LSET greater than NHT (P=0.031) and CON (P=0.038)
• Maximum eccentric torque: LSET +17%, NHT +11%, CON +4%; LSET greater than CON (P=0.009); LSET vs NHT not significant (P=0.237); NHT vs CON not significant (P=0.104)
• Maximum isometric torque: LSET +27%, NHT +25%, CON +14%; both training groups greater than CON (P=0.001-0.002); LSET vs NHT not significant (P=0.697)

Limitations

• The two regimes differed on multiple variables simultaneously (hip angle, bilateral vs unilateral eccentrics, loading mechanism, concentric component), so no single variable can be isolated as the cause of the differential hypertrophy
• Sample was restricted to healthy young males with no prior systematic training, limiting generalisability to females, older adults, or trained athletes
• Strength measurements did not extend to long muscle lengths (knee angles beyond 95 degrees) where LSET trains, leaving potential strength benefits at those angles unmeasured
• No prospective injury data were collected, so whether the greater BFlh and aponeurosis hypertrophy after LSET translates into fewer hamstring strain injuries remains unknown

Why it matters

For patients

Athletes or active people wanting to reduce hamstring strain injury risk may get greater protective muscle adaptations from hip-flexed eccentric leg curls than from Nordic curls alone.

For clinicians

For rehabilitation or injury prevention targeting the biceps femoris long head specifically, LSET (seated leg curl with accentuated hip flexion at 120 degrees) produces substantially more hypertrophy of that muscle and its aponeurosis than Nordic hamstring training over 12 weeks.

For readers

Exercise selection within the same joint action profoundly shapes which muscles grow, and training hamstrings at longer lengths (hip flexed) preferentially hypertrophies the injury-prone biceps femoris long head.