To investigate the validity and reliability of the Biodex system 4 in the assessment of abdominal strength in patients with rectus diastasis, we studied 10 healthy volunteers and 10 patients with rectus diastasis of more than 3 cm. We assessed test-retest reliability at 30(o) and 60(o) of extension/flexion spinal angles, respectively, and isometric muscle strength with and without the abdominal girdle. Reliability was calculated using the interclass correlation coefficient (ICC). Validity was evaluated by correlation with the International Physical Activity Questionnaire (IPAQ) and a visual analogue scale (VAS) for self-assessment of abdominal muscular strength. The Kendall-Tau and Spearman tests were used. Reliability was excellent with ICC values ranging between 0.77 and 0.97. There was no significant difference in muscular strength for tests with or without a girdle except with 60(o) extension. The internal validity assessed as the correlation between VAS and isometric strength (p = 0.008) was excellent, whereas there was no correlation with IPAQ. The Biodex system 4 is a valuable and reliable instrument to measure abdominal wall strength in patients with rectus diastasis. The internal validity is excellent. The incongruence between abdominal muscle strength, as measured using the Biodex system 4, and IPAQ indicates that there is no relation between general physical activity (IPAQ) and muscular strength in patients with rectus diastasis.