Combining Noninvasive 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 noninvasive method or combination of such methods to estimate muscle fiber type composition. We hypothesized that methods measuring different fiber type-specific characteristics, that is, metabolism, power, contractility, and endurance, would complement each other. METHODS: Forty young, healthy, and recreationally-to-competitively trained participants (20 men) underwent noninvasive tests and four muscle biopsies: two in the vastus lateralis and two in the gastrocnemius medialis. The average relative area occupied by type 2 fibers across the biopsies served as the criterion measure (multimuscle 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: Multimuscle FTarea% ranged from 21.2% to 60.1%. Many parameters were significantly associated with multimuscle 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 multimuscle FTarea% variance, confirming our hypothesis. CONCLUSIONS: The proposed model, which combines noninvasively measured parameters of metabolism, power, and contractility, can provide a reasonably accurate estimate of vastus lateralis and gastrocnemius medialis fiber type composition.