As a (amateur) bodybuilder, I feel like giving you some advice. I have been practicing bodybuilding since I was 16 years old.
I disagree with Simon on the issue of looseness.
I would say to focus attention on three fundamental elements useful for a pianist of any level.
1) Stretching (not the Anderson method) but above all the promising PNF (Proprioceptive Neuromuscular Facilitation)
You can find the main characteristics in the excellent work of Rolf Wirhead, ANATOMY OF MOVEMENT AND ATHLETIC ABILITY, with wonderful pencil drawings by this physiologist.
2) Posture and GRIP. Carlo Lonati (teacher of Vidusso and Pollini) recommended pull-ups on a bar (with full extension for a few seconds, both in supinated and pronated grip) in addition to clearly swimming. Extra-rotations and intra-rotations are FUN-DA-MEN-TAL both for general shoulder stability and to improve the fatigue resistance of the pianist's prolonged posture.
I also recommend the Arlaud movement with a maximum of 2 kg per hand, to ensure formidable lumbar and posterior deltoid functionality!
The levator scapulae and the supraspinatus must be treated with extreme seriousness (the entire rotator cuff is fundamental for the pianist/vibraphonist/guitarist/violinist and whoever else).
Other muscles that can create posture problems are the lower back, the abdominals (which should always be kept under training), and the cervical muscles. I recommend that what was said in point 1 applies here as well!
The grip is extremely important! It is precisely that which can ruin our piano skills. I absolutely advise against proceeding with push-ups using the palms of the hands.
One should plan to purchase a pair of handles (perhaps even the rotating ones, which are more physiological). Alternatively, use fists with appropriate cushions underneath to avoid bruising.
I recommend using good bodybuilding gloves. NEVER use a false grip or one without the thumb in opposition.
3) Rely on fundamental exercises with free weights, and rarely with machines. In fact, in the long run, machines can cause repetitive stress trauma much more frequently. Furthermore, if we want to create muscle hypertrophy, I remind you that bodyweight/free weight exercises are much more effective if associated with correct execution, good nutrition, a minimum of 8 hours of sleep, and correct recovery times (to be evaluated SUBJECTIVELY).
One last thing, I advise against using the flat bench, which can lead to non-physiological shoulder movements. Personally, I train at home and have not used a bench for at least 5 years, without having problems with results in terms of hypertrophy and elasticity.
I am a firm supporter of old-school Natural Bodybuilding (J. Vigna is the authority here). Valid advice can also be found in Tozzi's "Science of Natural Bodybuilding."
Before a concert, I recommend performing general stretching sessions with arm rotations (perhaps some light swimming motions). For me, it is a panacea.