 Maximal incremental shuttle walk. As originally described by Singh and colleagues, the incremental shuttle walk will be performed in an enclosed corridor on a flat 10-m-long course. The course will be identified by two cones, each positioned 0.5 m from either end to allow patients to walk in circle and thereby avoid the need for abrupt changes in direction. Patients will walk at a predetermined rhythm, as dictated by an audio signal played from a CD. Walking speed will initially be set at 0.50 msec and will be increased by 0.17 msec every minute until the patient reaches maximal capacity. Since the effects of encouragement on walking performance have been demonstrated, no encouragement will be given to patients throughout the test. The final measure will be distance walked, expressed in meters. 