Previous studies have compared muscle activity between different types of sit-ups and curl-ups. However, few have examined the exercises used by the armed forces or investigated the influence of exercise duration on muscle activation. The aim of this study was to compare abdominal and hip flexor muscle activity between the style of sit-up used by the British Army and 4 variations of a curl-up, at the start, middle, and end of a 2-minute exercise period. Surface electromyograms (EMGs) were recorded from the upper and lower rectus abdominis, external oblique, transversus abdominis and internal oblique, and the rectus femoris (RF) of 23 British Army personnel. Isometric maximal voluntary contractions were used to normalize integrated EMGs to allow them to be compared between exercises. Curl-ups with arms crossed and feet restrained produced the highest integrated EMG in all the abdominal muscles (p < 0.05). Feet-restrained sit-ups and curl-ups also resulted in significantly higher activity in the RF than in nonrestrained versions of the curl-up (p < 0.001). The significant increase observed in muscle activity between the start and the end of the exercises (p < 0.001) was deemed to be in response to a reduction in force producing capacity of existing motor units. The RF experienced the greatest increase during exercises that activated the muscle the most, that is, sit-ups and curl-ups with feet restrained (p < 0.001). Previous research has indicated that such exercises produce high shear and compressive forces in the lower back, which can be injurious. Thus, if an organization wishes to assess the endurance of abdominal muscles, rather than hip flexors, then curl-ups without restraint of the feet should be performed instead of exercises in which the feet are restrained.