GRAVITY! As long as we have that we can use our body to strengthen & build muscle! Seriously though…I’d have to say pull-up bar…there is much to be accomplished with a pull-up bar:
Hip Flexors, ABS, Back, Biceps, Lats, Rhomboids, Traps, Pecs, Triceps, Legs.
Even better would b a “power station” – a multi-exercise piece of exercise equipment that features a rectangle-shaped frame with both dip and pull-up bars.