SECONDARY OUTCOMES Gait (stride length and walking velocity) was quantified with the use of a computerized 4.3-m (14 ft) walkway (GAITRite, CIR Systems). Participants were instructed to walk at their normal pace for four trials; the results were averaged to derive a score for each measure, with higher scores indicating better gait ability. Strength of bilateral knee extensors and flexors was measured at an angular velocity of 60 degrees per second with the use of an isokinetic dynamometer (Biodex System 3, Biodex Medical Systems). Summary peak torque values (in Newton meters [Nm]) of five cycles of maximal extension and flexion were calculated from the average of measurements of both limbs. The functional-reach test26 assessed the maximal distance a participant could reach forward beyond arm’s length while maintaining a fixed base of support in a standing position, with higher scores indicating better balance. The timed up-and-go test27 measured the time (in seconds) taken to rise from a chair, walk 3.1 m (10 ft), return, and sit down, with a shorter time indicating better mobility. Participants’ motor symptoms were assessed with the 14-item UPDRS III18; scores range from 0 to 56, with lower values indicating less motor disability. Assessors were trained by a board-certified neurologist according to the standard protocol.28 Interrater reliability was 0.96. Falls were monitored by means of daily “fall calendars” that were maintained by the study participants13 and collected monthly throughout the intervention or until a participant withdrew from the study.