Exercise for the achilles tendon is most commonly done via eccentric exercise which involves contracting a muscle as it lengthens. The opposite is termed a concentric contraction where the muscle shortens as it contracts. The force generated during an eccentric exercise is higher than a concentric contraction. The subsequent loading of a tendon occurring with an eccentric contraction is the basis for the Alfredson protocol of Achilles tendon rehabilitation.
An example of an eccentric contraction is for the calf muscle and achilles tendon complex. If you stand with both forefeet on a step and push up onto your toes, a concentric contraction results where the muscles shorten as they contract. If you then lift one foot off the ground and lower yourself down on the other foot, an eccentric contraction occurs with the muscle contracting but lengthening at the same time. Higher force is generated in the Achilles tendon during this movement which is the basis for the Alfredson protocol. Repeated loading of the tendon via this exercise results in the tendon being able to tolerate higher loads. Not all achilles tendon problems are suited to this approach. A good place to start is an individual assessment by a NU Moves physiotherapist.