Calcium kinetics (Ca absorption, bone deposition, bone resorption, and calcium balance) during and after bed rest with (EX) and without exercise [CON(EX)] and with (AG) and without artificial gravity [CON(AG)]. Asterisk indicates significant difference from pre-bed rest in all four groups. **P<0.05, significantly different from pre-bed rest in the AG group; #P<0.05, significantly different from pre-bed rest in the CON(AG), AG, and EX groups; ##P<0.05, significantly different from pre-bed rest in the CON(EX) and AG groups