Hybrid fiber alterations in exercising seniors suggest contribution to fast-to-slow muscle fiber shift.
Level 3 - non-randomized controlled study
Non-randomized interventional study with a matched comparison group
PubMed 30907516 · doi:10.1002/jcsm.12410
What was done
Twenty-two sedentary healthy elderly men and women completed a 16-week supervised endurance exercise intervention. Eighteen age- and gender-matched endurance-trained volunteers served as controls. Muscle fiber type distribution was determined by immunohistochemical analysis of vastus lateralis biopsies pre- and post-intervention, examining a total of 13,840 fibers.
What was found
At baseline, sedentary participants had significantly more Type IIa (P = 0.0301) and Type IIx fibers (P = 0.0328) than trained controls. Baseline hybrid fibers accounted for nearly 5% of total fibers in both groups, without significant differences between groups (I-IIa: P = 0.6719; IIa-IIx: P = 0.0998). The exercise intervention induced a shift toward increased Type I fibers and decreased Type II fibers, accompanied by a significant increase in Type I-IIa hybrid fibers (P = 0.0301).
Why it matters
This study provides evidence that hybrid fibers in older adults act as active intermediate states during fast-to-slow fiber transitions rather than fixed entities, supporting the reversibility of age-related muscle phenotype changes through endurance training.
Limits
The sample size was small (22 intervention participants, 18 controls) and the intervention was not randomized. The abstract does not report specific numerical values, percentages, or variance for the post-intervention changes, nor does it report functional strength or endurance outcomes.
Cited by
- supports Human muscle fibers possess the ability to transform between slow-twitch and fast-twitch phenotypes bidirectionally.