Combining Non-Invasive Tests of Different Physiological Dimensions Leads to Better Estimation of Muscle Fiber Type Composition in Humans

Purpose: Muscle fiber type composition is a sports performance determinant, but its use in sports practice is hampered by the invasive muscle biopsy. We aimed to determine the best non-invasive method or combination of such methods to estimate muscle fiber type composition. We hypothesized that methods measuring different fiber type-specific characteristics, i.e., metabolism, power, contractility and endurance, would complement each other. Methods: Forty young, healthy and recreationally-to-competitively trained participants (20 men) underwent non-invasive tests and four muscle biopsies: two in the vastus lateralis and two in the gastrocnemius medialis. The average relative area occupied by type II fibers across the biopsies served as the criterion measure (multi-muscle FTarea%). On average, this was calculated based on the fiber type of ~5200 fibers and the cross-sectional area of ~1200 fibers per participant. Participants performed 30-m sprints, squat jumps, a Wingate with blood lactate measurements, isometric and isokinetic knee extensions, a maximal ramp incremental cycling exercise and underwent peripheral electrical stimulation of the quadriceps and a proton magnetic resonance spectroscopy scan to measure muscle carnosine. Results: Multi-muscle FTarea% ranged from 21.2 to 60.1%. Many parameters were significantly associated with multi-muscle FTarea%, but post-Wingate blood lactate concentration showed the strongest correlation (r=0.67; p<0.001). A multiple linear regression model with sex, post-Wingate blood lactate, peak knee extension torque at 300°.s -1 scaled to upper leg lean mass, average vastus lateralis and soleus carnosine and singlet contraction time as predictors could explain 68% of the multi-muscle FTarea% variance, confirming our hypothesis. Conclusions: The proposed model which combines non-invasively measured parameters of metabolism, power and contractility can provide a reasonably accurate estimate of vastus lateralis and gastrocnemius medialis fiber type composition.