low-impact – adj. designed to provide exercise without being over-strenuous

vertical – adj. positioned up and down rather than from side to side : going straight up

horizontal – adj. positioned from side to side rather than up and down : parallel to the ground

spare – v. to prevent (someone or something) from experiencing or being affected by something unpleasant, harmful, etc.

ache – v. to produce a dull continuous pain : to hurt in a way that is constant but not severe

relieve – v. to reduce or remove (something, such as pain or an unpleasant feeling)

stroke – n. one of a series of repeated movements of your arms in swimming or rowing that you make to move yourself or the boat through the water

terrestrial – adj. relating to or occurring on the earth

treading water – expression keeping the body nearly upright in the water and the head above water by a treading motion of the feet usually aided by the hands