Precisely, Gal.
In a race the stress is all centred on the hips, knees, ankles and lower back - all the weight goes through these joints.
I agree with Skyliner too that other forms of riding put stress on - and I also agree that sitting trot for 20 minutes is no walk in the park!!!
I'm surprised that you give no credence to race-riding (or other forms of riding for that matter) Mike when it comes to placing stress on joints - it is one of the most notorious for making cripples out of people! So many people who ride a lot for long periods of time end up having hip/knee replacements.
Ballet, also, is another sport which is notorious for producing cripples in a similar manner - although lithe & light in movement, whilst performing a jeté, a pas de chat or other jump, for example, the weight is thrown into the air and on landing concentrates almost entirely on the knees & ankles which places immense amounts of stress on them.