The next phase in treating pain associated with the patellar tendon is the isotonic phase. Progression to this phase can be done once isotonic exercises can be performed with less than 3/10 pain.
Isotonic exercises are defined as exercises that are performed under a constant load. This means the same amount of weight is applied while the muscle is changing in length. Examples of these exercises include weight machines (leg press, knee extension, etc) and body weighted exercises (lunges, squats, etc.)
Research suggests staying within 10-60 degrees of knee flexion during isotonic exercises . This is because full knee extension and deep knee flexion (90* or greater) can induce pain in the patellar tendon. It also suggests starting isotonic exercises with both legs before progressing to single leg exercises.
It is still important to self assess knee irritability during this phase, especially before progressing intensity. If pain lasts longer than 24 hours, weight should not be progressed.
Tips for Isotonic Strengthening: 3-4 sets of 15 reps
- 15 reps should be challenging in order to produce tissue changes in the tendon
- Progress the load until you’re performing 3-4 sets of 6 reps (optimally your 6 rep max)
- Perform these exercises every other day, allowing a day of rest
- On “rest” days, continue with exercises from isometric phase to manage soreness
Below are two videos demonstrating isotonic exercises: