Prolonged weightlessness causes osteoporosis, which can be reduced by exercise and medicine. Research onboard the International Space Station has led to even better and more effective training programs being drawn up, and new machines being made specifically for astronauts. Conjointly, there have been major leaps forward in medications capable of partially preventing declining calcium levels.
A recent study about 14 ISS astronauts, who were 4-6 month in space, showed a maximum bone loss of 1.5% / month in the most vulnerable (from a bone loss point of view) region - the hip. Therefore, once landed on Mars after 7 months of flight, the bone loss would be in the worst case scenario 10.5%.
When they arrive on a planet with 62% less gravity, they would have 100% more bone density compared to humans under earth gravity.