Low back pain (LBP) patients may have reduced hamstring extensibility. The Instrumented Straight Leg Raise (ISLR) has been proposed as an objective extensibility measure; however, appropriate quantification methods, and test-retest reliability, are lacking. The lumbar flexion-relaxation response (FRR) is abnormal in LBP patients. The hamstring FRR has been identified in asymptomatic participants, but has not been explored in LBP participants. The strength of the lumbar and hip muscles are typically reduced in LBP participants, however the strength characteristics of the hamstring muscles is unknown. The ISLR appears reliable. The elastic moment correlated with flexibility, electromyographic activity, body-mass, age and gender. Male LBP participants had greater elastic moments than male controls, and female LBP participants had greater pelvic rotation than female controls. LBP participants presented with a reduced hamstring FRR, and higher hamstring eccentric/concentric ...