Mixed
Concurrent strength and endurance training results in attenuated muscle hypertrophy compared to resistance training alone due to molecular competition between myofibrillar protein synthesis and mitochondrial biogenesis pathways.
If your goal is maximum muscle size, prioritize resistance training and minimize long-duration steady-state cardio, or separate them by at least 6 hours. If your goal is maximum endurance, prioritize running/cycling and limit heavy resistance training volume. Trying to do both at high intensities simultaneously will result in suboptimal gains for both due to conflicting molecular signals (mTOR vs AMPK).
An inverse relationship exists between striated muscle fiber size and its oxidative capacity. This relationship implies that muscle fibers, which are triggered to simultaneously increase their mass/strength (hypertrophy) and fatigue resistance (oxidative capacity), increase these properties (strength or fatigue resistance) to a lesser extent compared to fibers increasing either of these alone.
Why this rating
The paper is a comprehensive review synthesizing decades of experimental data, though it is not a primary clinical trial.
Source
The muscle fiber type–fiber size paradox: hypertrophy or oxidative metabolism?
T. van Wessel et al. · European Journal of Applied Physiology · 2010
DOI 10.1007/s00421-010-1545-0
More from this paper
- High oxidative muscle fibers remain small despite high protein synthesis capacity because increased mitochondrial biogenesis via AMPK attenuates myofibrillar protein synthesis and higher expression of E3-ligases increases protein degradation.Good
- The maximum rate of oxygen consumption (VO2max) per unit volume is inversely proportional to muscle fiber cross-sectional area (CSA) due to oxygen diffusion constraints, creating an evolutionary design constraint where larger fibers cannot support high oxidative capacity without developing anoxic cores.Good
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