It depends on what kind of tendonitis you have. Rest does help, but the tendonitis can be related to mobility issues. For example, I have been dealing with costochondritis which is an inflammation of the cartilage that connects the ribs to the breastbone. Most doctors will prescribe an anti-inflammatory and be done with it, but that's not actually treating the root cause which is related to the mobility of the ribs and thoracic spine. So basically go easy on it, but also figure out if there's a root cause for it that you can work on such as mobility.