Selective segmental nerve blocks with local anesthetics are applied for diagnostic purposes in patients with chronic back pain to determine the segmental level of the pain. We performed this study to establish myotomal motor effects after L4 spinal nerve blocks by lidocaine and ropivacaine and to evaluate the relationship with pain. Therefore, 20 patients, of which 19 finished the complete protocol, with chronic lumbosacral radicular pain without neurological deficits underwent segmental nerve blocks at L4 with both lidocaine and ropivacaine. Pain intensity scores (verbal numeric rating scale; VNRS) and the maximum voluntary muscle force (MVMF; using a dynamometer expressed in newtons) of the tibialis anterior and quadriceps femoris muscles were measured on the painful side and on the control side. The median VNRS decrease was 4.0 (P < 0.00001; Wilcoxon's signed rank test), without significant differences between ropivacaine and lidocaine (Mann-Whitney U-test). A difference in effect on MVMF was found for affected versus control side (P = 0.016; Tukey test). Multiple regression revealed a significant negative correlation for change in VNRS score versus change in median MVMF (Spearman R = -0.48: P = 0.00001). This study demonstrates that in patients with unilateral chronic low back pain radiating to the leg, pain reduction induced by local anesthetic segmental nerve (L4) block is associated with increased quadriceps femoris and tibialis anterior MVMF, without differences for lidocaine and ropivacaine.