A training harness (11) for exercising the inspiratory muscles of an athlete (10) is adapted to embrace the thorax (12) of the athlete (10) during the performance of aerobic exercise. The harness (11) has a back plate (13), a chest plate (14) and linkages (15-24) interconnecting said plates (13,14) by extending around the thorax (12) of the athlete (10). The linkages (15-24) define a pre-determined circumference of the harness (11), which may be increased when the athlete's thorax (12) expands upon inspiration—however such an increase in the circumference of the harness (11) requires the athlete (10) to work against an applied resistive load.