Quote: Schwingding wrote: Any other things to suspect?
This sounds like something that would be near impossible to diagnose over a forum.
Anyhow, I'll throw out some food for thought: New shorts with seams in a new place? Saddle worn out? Gain/lose significant weight? Change your shoes/cleats/pedals? Change your bottom bracket spindle length? Change the crank length or Q factor? Since you have a tri background, any running injuries? Long term wear from running? Onset of arthritis? Damage to sciatic nerve? Other musculoskeletal injuries that may have altered your biomechanics? Tri folks tend to mash rather than spin. This is usually harder on the knees, but hey, hips are in there working too. Combination of any of the above?
I understand you think your muscles are giving out first, but I would believe those strengthen over time and use (unless injured). So I'm sorta working on the premise that the problem is elsewhere and manifesting itself as muscle fatigue.
Start keeping a diary of your modifications and take very accurate measurments of the new positions. Make changes systematically and remember that you may not notice anything for quite a while.