If individual muscles are take off the bone before going into rigor, or after going into rigor but before leaving it, they can contract or stay contracted. This makes them tougher, and aging won't help. A similar thing happens when meat freezes before or during rigor.
Based on my own experience, I think there's something to this. Two years ago I shot a 2 1/2 year-old fat mulie buck right before dartk. I dressed it, sliced out the backstraps within the hour, and hung the hams and shoulders bone-in overnight. I boned the rest out the next morning and hauled it all out. At home I tried the backstrap first and it was tough. So I was kind of dreading chewing on the rest, but the rest of it (besides the backstrap) was tender, just like you'd expect. Same thing with the cow elk I shot last year--shot it right before dark, skinned it, cut out the backstraps, and hung the rest bone-in overnight. Tough backstrap and tender chops from the rest.
