Various devices, systems, methods, and apparatuses utilize mechanical assemblies to prevent, detect, and/or mitigate objects from entering or moving close to or under a treadmill or exercise machine. In some embodiments, a guard, or guard assembly, is attached or otherwise fixed to a rear area of a treadmill. The guard assembly may include an end cap and pivoting guard configured to be retrofit to existing treadmills, such as to a rear end or area of a deck or frame of a treadmill.