Age-related losses in lower extremity strength result in functional disabilities that diminish the quality of life for many older adults. Multiple factors, including type of muscle fiber, size of the muscle, length and speed of the muscle at contraction, age, and gender, affect the magnitude of strength generated. Assessment in clinical practice, in order to be cost and time effective, screens older adults for loss in strength. Further evaluation of strength loss requires the use of sophisticated procedures and equipment. Research into the causes of loss of strength and interventions to lessen or prevent loss of strength requires valid and reliable assessment tools. This article examines components of isokinetic muscle strength, the measurement of strength in clinical practice, methods to measure isokinetic strength, and validity and reliability of these measures.