Muscles that stretch across two joints (like the hamstrings) don’t grow as well when one joint is bent — you need to position both joints right to fully stretch them and make them grow.
Scientific Claim
The hypertrophic response to training at longer muscle lengths may be influenced by muscle-specific biomechanics, such as whether a muscle is biarticular (crosses two joints) or monoarticular (crosses one joint), because biarticular muscles may not reach optimal length under certain joint configurations.
Original Statement
“The hamstrings are biarticular muscles... in both groups the hamstrings were trained at ML to LL, which may have resulted in optimal stimulation in both groups, contributing to similar hypertrophy... resistance exercises that require the muscle to produce force at longer lengths seem to optimize muscle growth.”
Evidence Quality Assessment
Claim Status
appropriately stated
Study Design Support
Design cannot support claim
Appropriate Language Strength
association
Can only show association/correlation
Assessment Explanation
The claim uses 'may be influenced' — a probabilistic verb — consistent with the narrative review design. The evidence from Nakao et al. [51] and Burke et al. [66] supports this as an observed association.
Gold Standard Evidence Needed
According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.
Randomized Controlled TrialLevel 1bWhether training biarticular muscles at optimal vs. suboptimal joint angles produces differential hypertrophy.
Whether training biarticular muscles at optimal vs. suboptimal joint angles produces differential hypertrophy.
What This Would Prove
Whether training biarticular muscles at optimal vs. suboptimal joint angles produces differential hypertrophy.
Ideal Study Design
A double-blind RCT of 60 untrained adults assigned to 12 weeks of hamstring training: Group A trains seated leg curl (knee flexion only); Group B trains Nordic hamstring curl (hip extended, knee flexed); Group C trains prone leg curl (hip flexed, knee flexed). Measure biceps femoris and semitendinosus volume via MRI.
Limitation: Cannot isolate neural adaptation from structural growth.
Animal Model StudyLevel 4Whether biarticular muscle fibers experience different mechanical tension than monoarticular fibers during lengthened contractions.
Whether biarticular muscle fibers experience different mechanical tension than monoarticular fibers during lengthened contractions.
What This Would Prove
Whether biarticular muscle fibers experience different mechanical tension than monoarticular fibers during lengthened contractions.
Ideal Study Design
A study in 24 rats with electrical stimulation of monoarticular (gastrocnemius) vs. biarticular (plantaris) muscles at long vs. short lengths, measuring sarcomere strain via high-speed imaging and Akt activation via Western blot.
Limitation: Cannot replicate human voluntary resistance training or load progression.
Cross-Sectional StudyLevel 3aWhether athletes who train biarticular muscles in lengthened positions have greater hypertrophy than those who don’t.
Whether athletes who train biarticular muscles in lengthened positions have greater hypertrophy than those who don’t.
What This Would Prove
Whether athletes who train biarticular muscles in lengthened positions have greater hypertrophy than those who don’t.
Ideal Study Design
A cross-sectional study of 80 elite athletes (powerlifters, sprinters, bodybuilders) measuring muscle thickness of biarticular (hamstrings, rectus femoris) and monoarticular (vastus lateralis, triceps) muscles via ultrasound, and analyzing their training logs for exercises that lengthen these muscles.
Limitation: Cannot determine if training caused hypertrophy or if hypertrophy influenced exercise choice.
Evidence from Studies
No evidence studies found yet.