Development of a Non-invasive Methodology for the Assessment of Muscle Fibre Composition

Abstract

The percentage area of fast twitch fibres of a muscle is a major determinant of muscle mechanical power and, thus, an important biomarker for the evaluation of training processes. However, the invasive character of the assessment (muscle biopsy) limits the wide application of the biomarker in the training praxis. Our purpose was to develop a non-invasive method for the assessment of fast twitch fibre content in human soleus muscle. From a theoretical point of view, the maximum muscle mechanical power depends on the fibre composition, the muscle volume and muscle specific tension. Therefore, we hypothesised that the percentage area of type II fibres would show a correlation with the maximum muscle mechanical power normalised to muscle volume and specific muscle contractile strength (i.e., plantar flexion moment divided by muscle cross-sectional area). In 20 male adults, the percentage area of type II fibres, volume and maximum cross-sectional area of the soleus muscle as well as the maximum plantar flexion moment and the maximum mechanical power were measured using muscle biopsies, magnetic resonance imaging and dynamometry. The maximum mechanical power normalised to muscle volume and specific muscle contractile strength provided a significant relationship (r = 0.654, p = 0.002) with the percentage area of type II fibres. Although the proposed assessment parameter cannot fully replace histological measurements, the predictive power of 43% can provide a relevant contribution to performance diagnostics in the training praxis.

Keywords: diagnostic tool; fast twitch fibres; muscle biopsy; muscle power; soleus muscle.

Figure 1

Cryosection stained (anti-MHC II) images from two different participants (A and B) showing the slow type I (grey) and fast type II (red) fibres.

Figure 2

Representative plot of mechanical power (solid lines) and angular velocity of the ankle joint (dashed lines) as a function of the ankle joint angle during the maximum isokinetic contractions of one participant. The two vertical lines show the area where the mechanical power and the angular velocity of the ankle joint were determined.

Figure 3

(A) Relationship between percentage area of type II fibres and maximum mechanical power. (B) Relationship between percentage area of type II fibres and maximum mechanical power normalised to muscle volume and specific contractile strength.