In the first part of the the study two new outcome measurements (force reproducibility and tendon stiffness) will be tested for reproducibility. Therefore 30 healthy people will be assessed.

In a second part of the study 60 patients will be randomly allocated to two groups. Group A (n=30) will perform a traditional training program and group B (n=30) will perform an eccentric training program.

Before the onset of the training programs, pain, function, maximal force, range of motion, tendon stiffness and force reproducibility will be assessed. Both training programs will be accomplished at home. The first four weeks there will be an appointment with the therapist once a week to explain, correct and when necessary, aggravate the exercises. The next eight weeks these appointments will be diminished to once every two weeks.

After the 12 weeks of training the patients will be reassessed for all the parameters.

For detection of long term effects of the eccentric training program, the patients will be assessed a third time at three months after the end of the training program.