This paper describes the application of the inverse dynamic method developed by Bosco and colleagues for measuring the mechanical properties of knee extensor muscles under ballistic conditions. Twenty elderly men performed a series of maximal ballistic leg extensions at different loads. Accurate measurements of friction and inertial effects during the movement were used to calculate the torque and power produced during extension. The error of the results was 6.2–45.3% when the friction and inertial effects were neglected. The torque-angular velocity relationships were linear (r = 0.92 to 0.99, p < .001). The peak measurements obtained were in agreement with published isokinetic values. This new ergometer allows assessment of movements similar to those performed in day-to-day activities and can be used. without training, by elderly or disabled subjects.