No one has actually counted the tiny units inside muscle fibers to see if full-range lifting adds more of them—so we don’t know if that’s really what’s happening.
Scientific Claim
There is no direct evidence that longer-muscle-length resistance training increases serial sarcomere number in humans, as no study to date has measured sarcomere count using microendoscopy or histology.
Original Statement
“No studies to date have attempted to estimate serial sarcomere number changes from LML versus SML-RT... it remains unclear whether increases in fascicle length observed herein reflect an increase in the length of individual sarcomeres, an increase in the number of sarcomeres, or a combination thereof.”
Evidence Quality Assessment
Claim Status
appropriately stated
Study Design Support
Design supports claim
Appropriate Language Strength
association
Can only show association/correlation
Assessment Explanation
The claim accurately reflects the absence of direct evidence. The authors themselves admit this gap, and no study in the review attempted sarcomere quantification.
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 1bCausal effect of LML-RT on serial sarcomere number in human muscle.
Causal effect of LML-RT on serial sarcomere number in human muscle.
What This Would Prove
Causal effect of LML-RT on serial sarcomere number in human muscle.
Ideal Study Design
A double-blind RCT with 20 untrained adults, randomized to LML-RT or SML-RT for 12 weeks, with muscle biopsies taken pre- and post-intervention for microendoscopy and histological sarcomere counting in the vastus lateralis.
Limitation: Invasive biopsy limits feasibility, ethical approval, and sample size; cannot assess long-term effects.
Animal Model StudyLevel 4Direct causal link between muscle length during contraction and sarcomere addition in series.
Direct causal link between muscle length during contraction and sarcomere addition in series.
What This Would Prove
Direct causal link between muscle length during contraction and sarcomere addition in series.
Ideal Study Design
A controlled study in rats (n=60) undergoing 8 weeks of electrically stimulated eccentric contractions at long vs. short muscle lengths, with serial sarcomere counts via electron microscopy and in vivo microendoscopy.
Limitation: Cannot replicate human training protocols or neural adaptations.
Prospective Cohort StudyLevel 2aAssociation between long-term LML-RT exposure and sarcomere number in humans.
Association between long-term LML-RT exposure and sarcomere number in humans.
What This Would Prove
Association between long-term LML-RT exposure and sarcomere number in humans.
Ideal Study Design
A 3-year prospective cohort of 50 resistance-trained athletes with documented LML-RT history, undergoing muscle biopsy and microendoscopy to compare sarcomere number with matched SML-RT-trained controls.
Limitation: Ethical and practical barriers to repeated biopsies; selection bias likely.
Cross-Sectional StudyLevel 3Correlation between training history and sarcomere number in humans.
Correlation between training history and sarcomere number in humans.
What This Would Prove
Correlation between training history and sarcomere number in humans.
Ideal Study Design
A cross-sectional comparison of 40 resistance-trained individuals (20 with >5 years of LML-focused training vs. 20 with SML-focused training), with single muscle biopsy and microendoscopy for sarcomere counting.
Limitation: Cannot determine causality or timing of adaptation.
In Vitro Cell StudyLevel 5Molecular mechanism of sarcomere addition under mechanical strain at different lengths.
Molecular mechanism of sarcomere addition under mechanical strain at different lengths.
What This Would Prove
Molecular mechanism of sarcomere addition under mechanical strain at different lengths.
Ideal Study Design
Human myoblasts cultured under controlled mechanical stretch at long vs. short lengths, measuring gene expression of sarcomerogenesis markers (e.g., titin, nebulin) and sarcomere assembly via immunofluorescence.
Limitation: Cannot replicate whole-muscle biomechanics or systemic responses.
Evidence from Studies
Supporting (1)
Does longer-muscle length resistance training cause greater longitudinal growth in humans? A systematic review
The study looked at whether lifting weights with muscles stretched more might make muscle fibers grow longer, but it didn’t count the tiny parts inside muscles (sarcomeres) because no one has done that yet. So it agrees with the claim that we still don’t have direct proof.