The majority of the heat that we generate is provided by muscular activity and is the by product of work (force times distance). The more work we do the hotter we get. The fitter we are the more work and heat we can generate. During strenuous activity the heat output from our muscles can be 40 times that from all other tissue (please see this link from Cornell University). At maximum heart rate it may be 16 times the heat output at rest!
Why is this relevant to mountain sports? Well, just because we can power up that hill, at maximum respiratory rate, lungs bursting, doesn't mean that we should. If you want to be comfortable throughout a long day in the hills it may be prudent to leave the fastest ascents to the fell runners and adopt an alpine plod instead. This will result in greater endurance and a more stable temperature, requiring less clothing adjustment through the day, saving on the faff factor that slows many ascents.
(Thanks to Lonsdale Fell Runners for the picture)