2. Both of these devices have a one-way valve that closes during inspiration so that the subject must breathe through a small diameter hole for the resistive trainer or against a spring loaded valve for the threshold trainer.
3. The one-way valve opens during expiration such that no load is imposed during the expiratory phase of respiration.
Figure 3: Threshold trainer (top) and Resistive trainer (bottom). Threshold and P-Flex trainers available from Respironics HealthScan Inc., 41 Canfield Rd., Cedar Grove, NJ7, 0009-1201. 1-800-962-1266.