Keyboard typing for long periods of time is one way to build up strain gradually. Secondly, when we do any kind of activity without balancing it with rest, stretching, and therapeutic exercise to refresh the muscles, they eventually build up to that critical stress we discussed earlier. After a long day of typing, if you notice soreness in your shoulders, arms, or hands, you are experiencing this problem. Of course, we have deadlines, competition, and the need to perform well at our jobs to avoid being replaced, so we override these signals. If you continually stress your arms and hands, they will eventually break down.
Some bodies are more resilient than others.
Younger bodies generally recover faster than older bodies. But eventually, something gives. What happens at this point is extremely interesting. Many times, you don't even register that your arm functions are breaking down, because after the Motor Control Center sees that you are not going to stop to rest and recuperate your strained muscles, it becomes a trooper and figures out a way to keep going as best as it can. As fatigued muscles weaken and switch off, the MCC looks around for other muscles that it may recruit and substitute.