Combine the last two exercises and give them in alternation. First,
lift the body, then rest, then lift both feet, then the body, and
so on.
This alternate movement will bring great relief. The muscles are
more or less opposed; at any rate, the activity concerned in each
exercise will receive a rest during the other action.
This, of course, uses rhythm as an aid. True, natural rhythm is always
helpful and should be introduced whenever possible.