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Jan. 20, 2000 (Atlanta) -- Optimal fitness seekers must decide if their #1 goal is strength or endurance. According to Michael Leveritt, PhD, from the Centre for Sport and Exercise Science at Waikato Polytechnic in Hamilton, New Zealand, if it's strength they're after, they should skip the running. Those who train for both types of fitness will probably be disappointed.

"Strength gains during combined strength and endurance training are not likely to be as great when compared with strength gains achieved through strength training alone," Leveritt tells WebMD.

For his report, which appears in the December issue of Sports Medicine, Leveritt studied previous research that looked at concurrent training. Although he says that the comparisons were often difficult because of varying study methods and fitness levels of participants, one overriding principle did emerge: Increases in strength will suffer if endurance exercise is attempted at the same time.

Not only are the physiological effects from strength or resistance and endurance training different, says Leveritt, but sometimes they may even be opposite. He cites numerous studies that show endurance training contributing to a loss in strength and decreased muscle-fiber size. "What might be happening," he tells WebMD, "is that when individuals perform a bout of strength training after they have done some endurance training -- even on the preceding day -- the quality of training and/or the physiological response to the strength training may be less than optimal." He notes, on the other hand, that he discovered instances in which resistance training can cause an increase in both.

According to Leveritt, running seems to have the greatest negative effect on strength when combined with training geared towards building strength. He found inconsistent results when cycling was studied. The two exercises that have the least negative effect on strength are endurance rowing and hand cranking.

The probable reasons given by Leveritt for combination training's negative effect on strength include:

  • contrasting effect on muscle fibers of the various exercises.
  • differences in hormonal needs for both types of exercise.
  • differences in demands placed on the neuromuscular system during endurance and strength training.
  • differences in level and type of fatigue. "There is evidence to suggest that residual fatigue from a bout of prior endurance exercise inhibits the quality of subsequent strength exercise," writes Leveritt.

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