Spending a long time in weightlessness is unhealthy. One of the

Question

most important negative effects experienced by astronauts is a loss of
muscle and bone mass. Since an ordinary scale won’t work for an astronaut
in orbit, what is a possible way of monitoring this change in mass?
(Measuring the astronaut’s waist or biceps with a measuring tape is not
good enough, because it doesn’t tell anything about bone mass, or about
the replacement of muscle with fat.)

Answer 1

A centrifuge with a strain/stress gauge could be used, or the frequency of vibration on a spring could be measured.

The article at the link shown below describes the method actually used by NASA:

http://www-istp.gsfc.nasa.gov/stargaze/Smass.htm