Can you improve arm strength?

Technically speaking, you don't lose fast-twitch fibers per se. What happens as we age is that we tend to lose lean muscle mass (natural decline in testosterone and GH), which in turn, reduces the number of fast-twitch fibers available for recruitment.

Inversely, you will see an increase in slow-twitch fibers.

If an aging athlete wants to preserve fast-twitch fibers, they'll need to ensure that they're maintaining or even adding lean muscle mass and incorporating strength training on a regular basis.

In a sport subject to a strict anti-doping policy, it's tough to pack on lean muscle mass in your later years. It can be done naturally, but it's much more difficult vs. an enhanced athlete.

I am not sure, but TRT probably isn't an option for 40 plus men who have declining test levels in the NFL (insert Brady joke here).