Assess the Effect of Metaxalone 640 mg (M640) Compared to Tizanidine 8 mg on Truck Driving Ability and Cognition

Every participant will receive active study drug and one (1 )comparator, in two (2) stages, one after the other. Each drug will be taken one (1) time after a high fat meal. Vital signs and discussion of medications, illness or injury are considered safety assessments and will be discussed at every visit. There will be ( four (4) visits.

Inclusion Criteria: * equal to or greater than 18 years old * equal to or less than 65 years old * weight at least 120 pounds * medically healthy * able to eat a high fat meal Exclusion Criteria: * medications known to affect sleep-wake cycle * current use of cimetidine * current use of certain anti-depressants * current use of certain antibiotics * positive urine drug test for mind altering medications