Author(s): VAN DE CASTEELE, F., VAN VOSSEL, K., HARDEEL, J., HORWATH, O., APRO, W., MOBERG, M., VAN THIENEN, R., LIEVENS, E., DERAVE, W., Institution: GHENT UNIVERSITY, Country: BELGIUM, Abstract-ID: 2120

INTRODUCTION:
Sports scientists typically measure an individual’s muscle fiber type composition (MFTC) by evaluating the myosin heavy chain isoform distribution in a single vastus lateralis biopsy. Given the substantial, non-systematic variation along the length of the muscle it is questionable if one biopsy gives an accurate representation of the whole muscle (1). Additionally, the vastus lateralis’ MFTC might not be representative for the other muscles, even though an across-muscle phenotype has been proposed (2). Therefore, we firstly investigated MFTC variation and how many fibers and biopsies one should analyze to determine MFTC within a certain margin of error and secondly, whether the vastus lateralis’ MFTC is representative for the gastrocnemius medialis.
METHODS:
Forty participants (20 women) were subjected to two biopsies in the vastus lateralis and two in the gastrocnemius medialis. For each biopsy, the outer needle was twisted 180° around its axis after the first cut to perform the second one and these were analyzed separately. Samples were cut into cross-sections, stained for myosin heavy chain type I, IIa and IIx and visualized. MFTC was determined as the percentage of type I fibers and absolute MFTC differences between cuts and biopsy sites were calculated. Pearson correlations examined the relationship between vastus lateralis and gastrocnemius medialis MFTC. Additionally, data from a recent study using near identical techniques, were reanalyzed (1). They took 10 vastus lateralis biopsies across two legs in 7 men. We determined the 95% limits of agreement for differences between the average MFTC of one to five biopsies and the average MFTC based on 10 biopsies, which was considered true MFTC. Within this analysis, we also tested the effect of the amount of analyzed fibers by randomly sampling subsets of 50 to 800 fibers.
RESULTS:
Absolute MFTC differences between first and second cut samples were smaller than between proximal and distal sites in vastus lateralis (4.5±4.0 vs. 6.9±6.0 percentage points, respectively; p=0.017), but not in gastrocnemius (4.4±4.1 vs. 5.5±4.2 percentage points, respectively; p=0.064). When increasing the amount of analyzed fibers in one biopsy from 100 to 200 fibers, the 95% limits of agreement for the difference with true MFTC narrowed from ±21 to ±20 percentage points. However, when analyzing 200 fibers from 3 biopsies, 95% of measured MFTCs lie within -10 and 10 percentage points of true MFTC. There is only a moderate positive correlation between MFTC in the vastus lateralis and the gastrocnemius (r²=0.22, p=0.006; at least 200 fibers in each of 3 to 4 cross-sections per muscle).
CONCLUSION:
To determine MFTC and stay within ±10 percentage points of true MFTC, at least 3 biopsies with at least 200 counted fibers need to be analyzed. Even when doing so, vastus lateralis MFTC could only explain 22% of variance in the gastrocnemius MFTC.

1 Horwath et al. (2021). J Appl Physiol. 131(1), 158-173
2 Vikne et al. (2012). Muscle Nerve. 45(4), 527-535