I'm about to have evlt for venous insuffic. Why is exercise post proc forbidden? Is it the impact, the muscle tension, or the raised heart rate?

It is not forbidden. Exercise, within reason, is OK & actually encouraged after EVLT as it can help reduce the incidence of blood clots & help with recovery. Usually it is recommended to avoid excessive exercise such as heavy weight lifting or extreme exercise such as prolonged running, as there is some theoretic risk of these activities increasing the possibility that the treated vein does not respond appropriately.
Can exercise. I have been performing endogenous laser vein ablation for over 15 years. With the newer wavelengths and better techniques, it is not necessary to stop exercise post closure. I encourage my patients to get back to routine activities as soon as possible.I do recommend no pounding exercises such as jogging or squats for only 3 days post treatment but i don't feel that this makes any difference either.