Muscle filament spacing and short‐term heavy‐resistance exercise in humans.

1. Six weeks of a dynamic heavy‐resistance training of the quadriceps muscle in healthy young men resulted in a continuous increase in muscle strength, in an increase in muscle cross‐sectional area (significant only in the second half of the training period) and in an increase in radiological density of the muscle tissue of 3.1% (2P less than 0.001) in the first three weeks and 1.6% (2P less than 0.01) in the second three weeks. 2. The linear distance between myosin filaments (38.7 +/‐ 0.3 nm before, 38.7 +/‐ 0.4 nm after training; mean +/‐ S.E.M.) as well as the ratio of actin to myosin filaments (3.94 +/‐ 0.03 before, 3.86 +/‐ 0.06 after training) did not change with training. 3. These results refute the concept that the increases in muscle strength or radiological density during short‐term heavy‐resistance training are caused by changes in myofilament spacing.