We have recently characterised, using time-resolved USAXS and SAXS data at ID02 (Brunello et al., 2020, PNAS 117:8177-8186), the kinetics of the structural changes in the cardiac thick filament during the unitary mechanical response elicited by electrical stimulation of an intact cardiac trabecula at constant length. In particular we were able to determine the structure, dynamics and function of the myosin motor domains in different regions of the thick filament during contraction. Now we propose to determine the role of thick filament stress in controlling the rate of relaxation in cardiac trabeculae, using mechanical protocols that mimic the physiological working cycle of the heartbeat.