Most of what is involved in piano tuning, voicing, repair and regulation does not require much strength at all. Indeed, there are many women technicians and many elderly who do this work. Many who had more strength in their youth but suffered the impairment of rotators cuff injuries in both shoulders and tendonitis continue to do this work as a full time occupation. There are occasionally moving and lifting tasks but those who do not have the strength to do those find ways around it such as a little assistance from someone else.
The first thing that comes to mind is that the skills directly involved with tuning are apt to be contrary to those involved with highly skilled playing of the piano but there are ways to learn to do each without one impairing the other. To play the piano well, one must have nimble fingers, hands and wrists. To tune the piano well, one must operate the keys very firmly and manipulate the tuning hammer with very exacting movements.
After tuning, the thing to do would be to relax, take stretching breaks and do some warm up finger exercises such as one may do in any case before attempting to launch into any advanced repertoire. One may have tensed the muscles in the forearms, wrists and hands while tuning in a way that one never would in playing. It would be important to shake off that kind of tension and prepare for a different kind of use of these muscles before playing.
In learning tuning, most novices will immediately think in terms of "turning" the tuning pins much the same as one would think about turning the pegs of a guitar or the key of a harpsichord. Of course, the principle is the same but the piano is different in the sheer mass of the steel wires and their tension that is involved. The tuning pin is driven into a massive wood product made denser under pressure. What it takes to move a tuning pin, even without a piano wire under tension attached is many times greater than a harpsichord tuning pin or guitar peg.
The piano string is a steel wire under the average tension of a fully grown man. Imagine suspending yourself from the ground with one of these wires as your only support and what it would take to make the tiniest increase or decrease in tension from that to make the difference between an in tune or out of tune string and multiply that an average of 230 times just to finely tune a piano, let alone what it takes to get the piano close enough to accept a fine tuning!
While there are many technicians who use and even advocate a "slow pull" technique for tuning a piano, that technique requires much more use of arm, wrist, hand and finger muscles than an impact type technique. The slow pull (and push) type technique can be tiring and exhaustive to the muscles and tendons and can also cause such conditions as tendonitis.
Those who advocate the slow pull technique say that they need to "feel" the tuning pin move. First of all, one feels the pin move when using the impact technique, so there is no advantage in the slow pull technique in that regard. Most importantly, however, because the tuning pin is so tightly gripped by the pinblock, slow pulling and pushing upon it will twist and bend it. This, then requires many more compensating movements along with more muscle and tendon strain to tune each pin.
It is a far better idea, therefore to begin to learn piano tuning by using an impact type technique or even an impact type tuning hammer. The hand and wrist remain relaxed. The forearm also uses only the strength required to lift itself. The total energy and strain expended are far less after a pitch adjustment and fine tuning. Total time spent tuning a piano is also generally at least a half hour less.