This will be a randomized, double-blind, double-dummy, placebo-controlled, parallel-group study in subjects 18-70 years of age with acute, painful, muscle spasm of the lower back. The study consists of a baseline screening (Study Day 1), during which subjects will be evaluated for inclusion/exclusion criteria, and a 7-day double-blind treatment period (Study Day 1 through Study Day 7). Subjects will be randomly assigned to be dosed twice daily with one of the following double-blind treatments: SR carisoprodol 500-mg tablets, SR carisoprodol 700-mg tablets, or placebo.