Evaluation of a new exercise machine designed to improve strength and speed of movement in the leg muscles. Investigators are trying to learn whether a new training method results in greater strength improvements as compared to a traditional method.

Inclusion Criteria:

• healthy
• able to exercise 3 times per week for 16 weeks at Mobility Research Center

Exclusion Criteria:

• under- or overweight
• chronic disease that limits ability to exercise
• hips and legs limited by arthritis or pain
• presently engaged in a strenuous exercise program
• planned absence greater than one week.