Current NASA standards for preflight physical training are:
- To be selected it is not necessary to be a high performer in sports activities (some-times it can be even a “handicap” i.e. marathon runner / cardio-vascular status).
- During their whole career they have to maintain a good physical shape (safe sports practice 2 – 3 times per week, = with minimum injury’ risk)
- On NASA side, the sports practices are left on the initiative of each astronaut “self practice”, in Star City the sports’ practices are scheduled and managed in the training time-tables.
Preflight training includes strength training (using free weight exercises like the squat and deadlift). There doesn't seem to be a fixed regimen, just a set of tests for strength and agility the astronauts have to pass before flight.
During missions in the ISS, training is more structured:
astronauts are scheduled training sessions for the International Space Station (ISS) treadmill (TVIS) and cycle ergometer (CEVIS), as well as the Advanced Resistive Exercise Device (ARED). In-flight programs are designed to maintain or even improve the astronauts’ pre-flight levels of fitness, bone health, muscle strength, power and aerobic capacity. In-flight countermeasure sessions are scheduled in 2.5 h blocks, six days a week, which includes 1.5 h for resistive training and 1 h for aerobic exercise.
Loss of muscle is not the biggest concern.
After flight recovery within 1 - 6 weeks depending of the flight duration and after flight physical training intensity.
E.g. bone loss is a bigger problem (takes longer to recover from). Strength training helps increase bone density.
In conclusion, astronauts do a fair amount of strength training, just not to the level reached by bodybuilders and other strength athletes. At competition level, those athletes train several hours a day, every day. That would reduce the amount of time available for other aspects of training.
There's also the question, how much training is necessary? Competition athletes are several times stronger than average, but if there's only 10% muscle loss then you only need to have 110% of average strength at the start of a mission to come out okay.