The effect of a specially designed knee lock in a knee ankle orthosis was studied in 2 able-bodied volunteers and in 2 patients with spinal cord lesions. This lock allows the knee to bend during the swing phase but locks the knee securely during the stance phase. The device is intended to improve gait pattern and reduce oxygen consumption more than a standard knee lock which keeps the knee straight throughout the entire gait cycle. Energy savings with the knee locking mechanism were significant only at ambulation rates at or above 73 meters/min, a speed which could only be attained by able-bodied subjects. To achieve such high rates of ambulation a patient's hip flexors must be strong; yet a knee ankle orthosis is required only when the patient's knee extensors are weak. In patients with spinal cord injuries, this pattern of muscle strength is rarely encountered. The innervation of hip flexors or knee extensors overlaps, so voluntary control of these muscles will either be present or absent for both muscle groups. Therefore, most spinal cord injured persons who need a knee orthosis would not greatly benefit from this locking mechanism. There may be other conditions in which this device will prove valuable.