Smith machine

Machines guide the path of the weight, and on a leg press or Smith machine that path is a straight line.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 78.} The Smith machine is one of the most common rod machines, along with the leg press and hack squat.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 332.} These machines are usually plate loaded and move in a straight path only, which increases stability.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 332.} Most have safety catches that allow heavier loads without a spotter, though a spotter is still recommended.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 332.}

That stability changes the lift. In one comparison, back stabilizer activity was 30% lower in a Smith machine squat than in a free-weight squat.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 427.} A Smith squat is still a multi-joint lift, but it is less complex than a back squat, where the bar can drift forward and back.^{4Helms et al., The Muscle & Strength Pyramid: Training, 2nd ed., p. 151.} With the feet set out in front, Helms treats a Smith squat as a leg press variation.^{5Helms et al., The Muscle & Strength Pyramid: Training, 2nd ed., p. 250.}

In practice

• In a 12-week study of 49 untrained men, a varied routine of Smith squats, leg presses, lunges and deadlifts grew the quadriceps more evenly than Smith squats alone.^{6Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., pp. 110–111.} The Smith-squat-only group did not significantly grow the vastus medialis.^{6Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., pp. 110–111.}
• A Smith machine can make bench press throws and overhead press throws safer.^{7Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 474.}
• Stabilizing muscles generally work harder with free weights than with a Smith machine.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 427.}