A selectively adjustable, portable exercise system with first and second resistance mechanisms, each with a rotatable spool, a cord wrapped around the spool, a handle secured to the cord, and a resistance stack formed by plural resistance packs, which can be modular. The spool and the resistance pack have engagement patterns. Each resistance pack presents a resistance to rotation, such as by a coil spring. Each of a plurality of keys has a rod portion with an engagement pattern with a spool engagement pattern to lock the spool relative to the key and a resistance pack engagement pattern for locking one or more resistance packs relative to the key and the spool. The resistance pack engagement patterns of at least some of the keys vary to lock different combinations of resistance packs relative to the key and the spool. A belt, base platforms, or both can form a retaining structure.