Abstract Two independent pack-carrying studies were conducted in which the strength losses of involved muscles from nine 7.5-mile marches were determined. In the first series of marches, the rucksack had a clear advantage: not until the weight was 61 pounds were strength losses appreciable. Contrarily, in the second series, marching with the rucksack resulted in greater strength losses than with the combat pack. The difference is attributed to the superior physical condition of the subjects on the first rucksack march. The muscle groups showing greatest strength losses for the nine marches were: trunk extensors in eight marches; hip extensors in seven marches; and knee flexors in six marches. The neck extensors appeared in two of the three marches of the second series, but were not tested in series one.