STRENGTH TRAINING COULD HARM ENDURANCE TRAININGMacDougall, J. D., Sale, D. G., Moroz, J. R., Elder, G. C. B., Sutton, J. R., & Howard, H. (1979). Mitochondrial volume density in human skeletal muscle following heavy resistance training. Medicine and Science in Sports, 11, 164-166.
Muscular strength and endurance are poorly correlated: "The increase in contractile protein mass through hypertrophy, without a proportional increase in muscle oxidative capacity, might actually be viewed as detrimental to the performance of the endurance athlete and especially in body weight supported activities [e.g., swimming]." (p. 164)
Within the body, muscles have different fiber compositions. For example, the long head of the triceps is predominantly fast-twitch fibers whereas the vastus lateralis is considerably more heterogeneous with respect to fiber type distribution. The gluteals are primarily slow-twitch dominated.
Assuming that mitochondrial volume and density reflects the oxidative potential of muscle, it would seem that not only does heavy resistance training not enhance the endurance characteristics of skeletal muscle, it may even be detrimental to endurance performance by decreasing the oxidative potential per total muscle mass.
Implication. The use of weight training with endurance activities should be moderate and primarily of low load and higher repetitions so that hypertrophy does not occur.
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