Relationship of the Duration of Sustained Voluntary Isometric Contraction to Changes in Endurance and Strength

The research was undertaken to determine if an increase in the duration of a sustained voluntary isometric contraction is more closely related to changes in cardiovascular endurance or changes in strength. Subjects (N = 24) were tested before and after a training program to determine their endurance (oxygen consumption) and strength. The training program consisted of one daily sustained voluntary isometric contraction of the left forearm flexors at a prescribed percentage of the subject's maximum isometric strength. When the mean duration of contractions for each group showed a significant increase, training ceased. The groups which trained with contractions of 50, 75, and 100 percent of a maximum voluntary contraction showed significant increases in strength, but not in endurance. The group which trained with a 25 percent contraction increased significantly in endurance, but not in strength. The increased duration of contractions at percentages greater than or equal to 50 percent of a maximum voluntary contraction appeared to be due to increased strength. Hence, measuring cardiovascular endurance by the duration of a sustained voluntary isometric contraction at percentages greater than 25 percent seems unjustified.