Efficacy of M640 in Acute Lumbosacral Musculoskeletal Conditions.

The goal of this clinical trial is to learn if a modification to metaxalone 640 mg can reduce low back and leg pain. The participants will be 18 to 80 years old, healthy with newly occurring back or leg pain. The main question aims to compare a group taking active treatment and a group taking a look-alike substance containing no active treatment. All participants will answer questions on Day 1, before treatment, and on 7-day after treatment, about: * Amount and quality of pain * Interference with physical activity * Interference with sleep