marathon: An organized 26.2-mile race for runners and walkers.

maximum heart rate: The maximum number of times your heart should beat in a minute without dangerously overexerting yourself.

meditation: A mental process involving focused attention, or calm awareness, which is also called mindfulness.

mountain bike: A fat-tire outdoors bicycle with upright handlebars that is built to withstand rough terrain.

multi-gym: A home gym contraption that looks like a bunch of health-club weight machines welded to each other.

muscular failure: In a strength training program, the point at which your last repetition with weights is so difficult that you cannot perform another repetition.