It is better to say: Both dukha vedana (eg., an injury or cancer) and sukha vedana (eg. a good massage) arise due to kamma vipaka and are exhibited in the physical body. Vipaka means is a result, so one may not be able to completely stop a dukha vedana due to an injury, for example, but can take steps to make it better (but that also involves some more dukkha as we see below; one has to do things to make the situation better; so that involves sankhara dukkha).
-By the way, both sukha and dukkha are not cetasika (mental factors), consistent with the fact that they don’t arise in the mind (even though they are of course eventually felt by the mind).
– But the problem is, normal humans GENERATE additional mental suffering (mainly by worrying about the physical suffering by generating vaci sankhara).
You said: ‘When sentient beings strive to avoid dukha vedana and prolong sukha vedana as much as possible, they engage in sankhara dukkha (the suffering of burdensome activities)…”
– That is right.
You asked: “One question is: Do activities to maintain our health and daily lives count as sankhara dukkha? For example, is exercise to maintain our health a form of sankhara dukkha? What about brushing the teeth, showering and eating?”.
– Yes. We have to do things (sankhara) to “maintain a healthy body”, and that is “extra work”, that is necessary. But it is important to note that these are not necessarily ABHISANKHARA that will lead to future vipaka. These are really more dukha associated with moving the body, etc (For example, in the case of an injury, we have to go to a doctor or go and get whatever is need to apply to the wound, etc).
You said: “I understand that in order to have cooked food on our dinner tables, it is necessary to go grocery shopping, pay for the purchases, wash and cook the food, and clean up after the meals, etc. But what about the mere act of eating? There is surely effort involved, even if it is very minor”. Yes. All these involve an effort (sankhara), even though some of it is “masked” by our anticipated “pleasures”. Nevertheless, all that involve more work.
You asked: “This is for me to understand whether the Buddha Himself was subjected to sankhara dukkha. For example, when He suffered from bodily aches and pain due to previous vipaka, might he have continually changed His position or posture to lessen the pain? Would that be considered a form of sankhara dukkha?
The Buddha needed to eat to continue to live. When going round on his alms round to obtain alms food, would that be considered a form of sankhara dukkha?”
– Yes. The only suffering that stops for a Buddha or an Arahant DURING the life is “samphassa ja vedana“; see, “Vēdanā (Feelings) Arise in Two Ways“.
– Anyone with a physical body will be subjected to all three types of dukkha (dukkha dukkha, sankhara dukkha, viparinama dukkha). They continue until Parinibbana. After that, all suffering stops.