The effect of handle diameter on manual effort in a simulated assembly task was investigated. The study group consisted of 16 right handed male volunteers, mean age 22.8 years. Right hand length, breadth, and inside grip diameter were measured. Maximum voluntary contraction (MVC) strength of the right hand and forearm were measured using standard isometric test techniques. The subjects performed a simulated industrial assembly task using three handles: a handle with diameter matched to the inside grip diameter of the subject (fit diameter), a handle diameter 1.0 centimeters (cm) smaller than the fit diameter, and a handle having a diameter 1.0cm larger than the fit diameter. Grip force and electromyographic activity of the forearm muscles were noted. Manual effort was evaluated by comparing the force exerted on the handle during the tasks to the maximum force generating capacity. The greatest grip forces were exerted when the smaller handle was used. Maximum grip strength increased 39% on the average for each 1.0cm decrease in handle diameter. Peak and average grip force exertion were not significantly affected by handle diameter, but did vary directly with resistance imposed by the handle and test apparatus. The electromyographic data indicated that muscle effort increased with increasing handle diameter. The authors conclude that small changes in handle diameter, on the order of +/-1.0cm, can have significant effects on manual effort. It may be beneficial to manufacture tools with different sized handles to allow users with larger and smaller grips than normal to select handles best suited for their hand size.