A treadmill includes a base assembly, a collapsible treadmill deck and a locking device that includes a telescopic tube and a latch. The telescopic tube supports the treadmill deck in a folded position and includes an inside tube and an outside tube. The outside tube has a pawl hole and a bottom end pivotally mounted to the base assembly. The inside tube is telescopically mounted in the outside tube and has an outside end pivotally mounted on the treadmill deck and multiple positioning holes aligned with the pawl hole. The latch includes a stationary bracket fastened on the outside tube and a pivot pawl pivotally mounted on the stationary bracket. The pivot pawl has an inside end extended into the pawl hole to engage one of positioning holes to interlock the inside tube with the outside tube to firmly hold the folded treadmill deck in position.

A locking device to lock a collapsible treadmill deck in a folded position includes a telescopic tube and a lock. The telescopic tube includes an outside tube and an inside tube. The outside tube has a though hole. The inside tube is telescopically mounted in the outside tube and has multiple positioning holes corresponding to the through hole. The lock is attached to the outside tube, aligned with the through hole and includes a mounting bracket, an adjusting knob and a detent assembly. The adjusting knob is rotatably and retractably mounted on the mounting bracket. The detent assembly includes a rotating shaft with a locking pin movably mounted in the mounting bracket and connected to the adjusting knob. Therefore, the locking pin will be pulled into one of the positioning holes to engage the inside tube by pushing the adjusting knob toward the mounting bracket.