What's astounding about it is the all-at-once nature of the change. Usually its one set of muscles one week, and a different set the next week, so it all seems gradual. In this morning's strength training class I discovered that 15 pound weights are no longer challenging enough for the overhead press, and I had to switch to 17.5 for the second set. Then on the bicep curls I got through all the repetitions on both sets with the 17.5 pound weights, which means I'll have to scavenge for a pair of 20s next time. Then it was big news on my weak muscles. My triceps which had just graduated to 12.5 pound weights are now ready for 15s, and my anterior deltoids are finally ready to move from 8 to 10 pound weights.
This is the thing I love about strength training, the feeling that you get when you can suddenly do things that seem impossible or at least implausible before.