Doing bicep curls only halfway up (keeping the arm stretched) might make the lower part of your bicep grow a tiny bit more than doing full curls, but the difference is very small.
Scientific Claim
In resistance-trained individuals, 8 weeks of partial range of motion training at long muscle lengths (0°–70°) may be associated with slightly greater hypertrophy in the distal elbow flexors (at 70% humeral length) compared to full range of motion training (0°–140°), with a trivial-to-small effect size (SMD = 0.10) and moderate Bayesian evidence (BF = 4.87), suggesting that sustained mechanical tension in stretched positions could enhance regional muscle growth.
Original Statement
“pROMinitial exhibited trivial to small superiority at 70% elbow flexor length (standardized mean difference [SMD] = 0.10 and Bayes factor = 4.87).”
Evidence Quality Assessment
Claim Status
appropriately stated
Study Design Support
Design supports claim
Appropriate Language Strength
probability
Can suggest probability/likelihood
Assessment Explanation
The study is an RCT with Bayesian analysis, supporting probabilistic claims. The language 'may be associated with slightly greater hypertrophy' correctly avoids definitive causation and reflects the small effect size and moderate evidence.
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.
Systematic Review & Meta-AnalysisLevel 1aWhether pROMinitial consistently produces greater distal elbow flexor hypertrophy than fROM across multiple RCTs in trained populations, with pooled effect sizes and subgroup analyses by training status and exercise type.
Whether pROMinitial consistently produces greater distal elbow flexor hypertrophy than fROM across multiple RCTs in trained populations, with pooled effect sizes and subgroup analyses by training status and exercise type.
What This Would Prove
Whether pROMinitial consistently produces greater distal elbow flexor hypertrophy than fROM across multiple RCTs in trained populations, with pooled effect sizes and subgroup analyses by training status and exercise type.
Ideal Study Design
A meta-analysis of all published RCTs (n≥10) comparing pROMinitial (0°–70°) vs. fROM (≥120°) in resistance-trained adults (≥2 years experience, age 18–40), using standardized ultrasound measurements of muscle thickness at 70% humeral length, with at least 6 weeks of training, and reporting SMDs with 95% CIs.
Limitation: Cannot establish causation or isolate mechanisms like sarcomerogenesis.
Randomized Controlled TrialLevel 1bIn EvidenceWhether pROMinitial causes greater distal elbow flexor hypertrophy than fROM in trained individuals under controlled conditions with blinding and larger sample size.
Whether pROMinitial causes greater distal elbow flexor hypertrophy than fROM in trained individuals under controlled conditions with blinding and larger sample size.
What This Would Prove
Whether pROMinitial causes greater distal elbow flexor hypertrophy than fROM in trained individuals under controlled conditions with blinding and larger sample size.
Ideal Study Design
A double-blind, randomized, crossover RCT with 50+ resistance-trained adults (age 20–35, ≥3 years experience), performing 12 weeks of unilateral preacher curls (pROMinitial vs. fROM), with pre/post ultrasound measurements of muscle thickness at 50% and 70% humeral length, DXA for lean mass, and blinded technicians, controlling for volume and intensity.
Limitation: Still cannot prove long-term or mechanistic effects like sarcomere addition.
Prospective Cohort StudyLevel 2bWhether individuals who habitually use pROMinitial in training show greater distal elbow flexor hypertrophy over time compared to those using fROM, in real-world settings.
Whether individuals who habitually use pROMinitial in training show greater distal elbow flexor hypertrophy over time compared to those using fROM, in real-world settings.
What This Would Prove
Whether individuals who habitually use pROMinitial in training show greater distal elbow flexor hypertrophy over time compared to those using fROM, in real-world settings.
Ideal Study Design
A 1-year prospective cohort of 200+ resistance-trained lifters self-selecting pROMinitial or fROM training, with quarterly ultrasound measurements of elbow flexor thickness at 70% humeral length, controlling for total volume, diet, and training history.
Limitation: Cannot control for confounding variables like nutrition or recovery habits.
Animal Model StudyLevel 4Whether prolonged stretch under tension during pROMinitial induces sarcomerogenesis or longitudinal hypertrophy in human-relevant muscle tissue.
Whether prolonged stretch under tension during pROMinitial induces sarcomerogenesis or longitudinal hypertrophy in human-relevant muscle tissue.
What This Would Prove
Whether prolonged stretch under tension during pROMinitial induces sarcomerogenesis or longitudinal hypertrophy in human-relevant muscle tissue.
Ideal Study Design
A controlled rat study with 40 animals, randomly assigned to 8 weeks of eccentric-only elbow flexor training under either full or partial (70°) ROM, with histological analysis of sarcomere number, fascicle length, and muscle fiber cross-sectional area in biceps brachii.
Limitation: Cannot directly translate to human muscle adaptation or training responses.
Case-Control StudyLevel 3Whether elite bodybuilders who use pROMinitial more frequently have greater distal elbow flexor hypertrophy than those who use fROM, after matching for training age and volume.
Whether elite bodybuilders who use pROMinitial more frequently have greater distal elbow flexor hypertrophy than those who use fROM, after matching for training age and volume.
What This Would Prove
Whether elite bodybuilders who use pROMinitial more frequently have greater distal elbow flexor hypertrophy than those who use fROM, after matching for training age and volume.
Ideal Study Design
A case-control study comparing 30 elite bodybuilders with high pROMinitial usage vs. 30 matched controls using fROM, measuring muscle thickness at 70% humeral length via ultrasound and comparing training logs, diet, and supplement use.
Limitation: Retrospective and subject to recall bias and selection bias.
Evidence from Studies
Supporting (1)
This study found that doing arm curls only halfway (0°–70°) led to slightly more muscle growth in the lower part of the biceps compared to doing full curls, even though the difference was tiny. This supports the idea that keeping muscles stretched during exercise might help them grow a bit more in specific spots.