Tendon

Overview

Tendon is a strong structure of our body, but once injured, it recovers very slowly （often takes 8 weeks or even more).

It takes a long time to recover, e.g., 8 weeks or more – more patience is needed.
There is very few blood vessel, so it often does not get much nutrition – extra care is often needed to speed up recovery, e.g., massage, heat, cold, compression.
There is very little nerve, which makes it hard to evaluate if the exercise/impact is too much on the spot – often, it relies on the sensation the next morning to tell if the previous day’s exercise is too much.
It is strongly related to calf muscle groups, or even the entire lower body system. Imbalance elsewhere often causes over compensation – we will need to treat it as a whole system, not just isolated tendon.

The best exercise if isometric (still-motion) exercise, which is mild while stimulate recovery. It is often undervalued, because its impact is relatively small on muscle. However, long and steady isometric exercise is important for tendon health. Modern recommendation suggests very heavy weight - the heaviest weight that you can last for 45 secs, but take it slowly to build up the weight.
Heavy Slow Resistance exercise is a slow transition from isometric to typical weighted exercise. It takes 3-5 seconds to finish one movement. In addition to tendon, it also put more stress on muscles. The weight is set to be possible with 10-15 repetitions.
The next level is plyometrics, where it adapts the tendon to take more quick stress - absorb and release energy quickly, but without really facing the full load in running.
Flexibility/stretching the corresponding muscle is often helpful to reduce stress on the tendon. Tendon issue is often due too much impact, which ideally could be shared/dampened by muscle. Stronger/more flexible muscle is definitely helpful.

Here has more discussion on Achilles Tendon Injuries.