Isometrics are contractions without visible movement: pushing against an immovable bar, holding a wall sit or a calf raise at mid-range. They are simple, need little equipment and are popular in tendon rehabilitation.
Heavy isometric holds of around 30-45 seconds are often used for tendons such as the patellar and Achilles tendon
Some studies reported short-term pain relief after isometrics in tendinopathy, but later research has been mixed, so it is not a guaranteed effect
Strength gains from isometrics are greatest near the joint angle trained
They can also be used as a low-fatigue strength stimulus during heavy endurance blocks
A common template: 5 holds of 30-45 seconds at a hard but tolerable effort, 1-2 minutes rest between. Examples: Spanish squat or wall sit for the quads and patellar tendon, a bent-knee calf raise hold for the soleus and Achilles.