This invention describes a novel gravity-independent exercise unit designed for use in microgravity, or on the ground, as a means by which to counter muscle atrophy and bone degradation due to disuse or underuse. Modular resistive packs comprising constant torque springs provide constant force opposing the withdrawal of an exercise cable from the device. In addition to uses within the space program, the compact resistive packs of the CFREU allow the unit to be small enough for easy use as a home gym for personal use, or as a supplement for rehabilitation programs. Resistive packs may be changed conveniently out of the CFREU according to the desired exercise regimen. Thus, the resistive packs replace the need for expensive, heavy, and bulky traditional weight plates. The CFREU may be employed by hospitals, rehabilitation and physical therapy clinics, and other related professional businesses.