びよう is rather feminine word used in the context involving such as cosmetics or plastic surgery.

筋トレ (きんとれ / abbr.) or 筋肉トレーニング (きんにくトレーニング) works fine. The literal translation of the word is “muscle training”. You can use it when to describe the training to shape your figure, both in order to lose your weight or gain your weight in a healthy way.