This is the Treadmill Doctor Walking Belt for the Star Trac 4000 Treadmill. Model Number: 4000. Fits the following models: LifeSpan, TR3000-HRC (2008), Star Trac, 4221-SUSAPO, 5500 Pro S - 9-5532-SMEXP0, 5600 Pro - 9-5631-GUSAP0, 5600 Pro - 9-5631-GUSAP3, 5600 Pro - 9-5631-GUSAPX, 5600 Pro - 9-5631-SUSAP0, 5600 Pro - 9-5631-SUSAP3, 5600 Pro - 9-5632-GMEXP0, 5600 Pro - 9-5632-GOCEP0, 5600 Pro - 9-5632-GOULP0, 5600 Pro - 9-5632-SOCEP0, 5600 Pro - 9-5632-SPORP0, 5600 Pro Tread DC - 9-5633-SUSAP0 (PT), 6600 Pro Elite - 9-6631-SUSAP0 (PW), 9-4032-BUSAP0, 9-4512-BUSAP0 (TR4500, CONT HR, 220V, BLK, USA), 9-5531-GUSAP0 - 5500 Pro S, 9-6531-SUSAP0 - 6500 Pro IR, 9-7532-SOCEP0 - 7500 Pro S, 9-7532-SOULP0 - 7500 Pro S, 9-7532-SUKIP0 - 7500 Pro S, 9-7532-SUSAP0 - 7500 Pro S, 9-7533-SUSAP0 - 7500 Pro S, Pro Tread DC 5500 - 9-5531-SUSAP0 (PS), TR1800 - 9-1821-BUSAP0 - Black (Lxxxxx), TR3900 Series, TR4000 Pre-Soft, TR4000 PreSoftmill - 9-4011-BUSAP0 - Grey (4xxxxx), TR4000 PreSoftmill - 9-4031-GUSAP0 - Grey (4xxxxx), TR4000 Softmill, TR4200 - 9-4221-SJPNP0, TR4500 - 9-4532-BUSAP0 (Dxxxxx), TR4500 2.5hp - 9-4531-BUSAP0, TR4500 Series.