Background and Purpose. The ability to perform 20 or more one-leg heel-rises is considered a "normal" grade for muscle strength (forcegenerating capacity of muscle) of the ankle plantar flexors, regardless of age and sex. Because muscle strength is closely related to age and sex, the "normal" test criterion was re-evaluated in different groups categorized by age and sex. Subjects and Methods. One hundred eighty sedentary volunteers (21-80 years of age) without lower-limb lesions performed as many repetitions of one-leg heel-rise as possible. Lunsford and Perry criteria were used to determine completion of the test. Results. The age and sex of the participants influenced the maximal repetitions of heel-rise, and the repetitions decreased with age and in female subjects. Discussion and Conclusion. The muscle strength of the ankle plantar flexors, as measured by manual muscle testing, varied with age and sex. Clinicians should consider the variances of age and sex when they perform manual muscle testing of the ankle plantar flexors. [Jan MH, Chai HM, Lin YF, et al. Effects of age and sex on the results of an ankle plantar-flexor manual muscle test. Phys Ther. 2005;85:1078 -1084 Key Words: Aging, Heel-rise, Manual muscle test, Plantar flexor. A nkle plantar-flexor strength (force-generating capacity of muscle) has an important role in standing balance, walking ability, and most activities of daily living. [1][2][3] During walking, the plantar flexors slow down the forward motion of the tibia over the foot during the stance phase and propel the body forward during push-off. People with weak plantar flexors usually present a crouch gait pattern during walking, difficulty in performing activities involving lower limbs or even inability to perform such activities, or inadequate standing balance. [2][3][4] For patients displaying these signs, examination of plantar-flexor strength is prudent. 2,4 Numerous methods have been developed to assess muscle strength, such as manual muscle testing (MMT), 5,6 tests using handheld dynamometers, 7,8 and isokinetic testing. 9 -11 Clinically, MMT is more frequently used than the other methods by virtue of its simplicity and convenience. No expensive instruments are required during testing, and it can be applied virtually instantaneously. 5,6 Although MMT might yield subjective data or can be onerous to perform, depending on the examiner's strength, in comparison with the other methods, it remains the most popular clinical method to identify muscle weakness. Mei-HwaAccording to the grading system of traditional MMT, muscle strength is judged by the muscle's ability to resist either gravity alone or gravity plus the examiner's strength. 5,6 The strength of the ankle plantar flexors graded in the prone position and against the examiner's strength is insufficient for the use required of the flexors in daily activities. Therefore, the challenge in designing MMT for the plantar flexors has been to find a technique that is able to exert a force to counter the force that supports bod...