In the fitness industry pop pins, which go by several names including, pop pin assemblies, retractable spring plungers and retractable spring loaded plungers are widely used on residential and commercial strength equipment for various purposes. Some common functions are to set and lock in seat height, range of motion (ROM) settings, back pad adjustments and roller pad comfort settings.

The pop pin assemblies consist of a plunger, a compression spring, a threaded locking cap and a knob. Although pop pin assemblies appear to be a minor part of a piece of single station strength unit or a home gym, they really fill important roles. If they are not functioning at all, a seat may have to be used at the lowest setting, making a position like a pec deck unusable for all but the tallest users or if the pop pin is used to set a range of motion on a position like a leg extension, it could force the user to have an incomplete exercise or to have to over extend just to start the exercise.

More serious failures are when they slip out of the setting during use, while under a load. This type of failure could easily cause a serious injury to the user.

Several thing can go wrong with these retractable spring loaded plungers. A common problem is that the pull knobs, which could be round knobs, T-knobs or draw knob style, can come off and get lost. Although this problem is easily resolved by replacing the missing knob, in the meantime, the ROM adjustment or seat adjustment can't be used. Another common failure is that the internal compression springs break or lose their strength. This is a serious failure because the tip of the plunger may not stay engaged throughout the complete exercise routine. Once again, this type of failure can cause serious injury to the user. Occasionally, the plunger it self breaks, but more often, the functional tip of the plunger will wear and when that happens, the pop pin won't stay in place when under a load.

Frequently, what is reported as a failure of a pop pin is actually wear of the receptacle holes on the range of motion cam or the seat support or some similar device, that the tip of the pop pin plunger locks into. With wear, the holes in the range of motion cam or seat support tube, get elongated or oversized, which causes the pop pin assembly to slip out of place at times, depending upon which settings were chosen.

Routine preventive maintenance of strength equipment should include an examination and lubrication of the pop pin assemblies to ensure that the are safe and that they continue to function as designed.