Final answer:
The knee flexion angle should be measured at the start of the exercise program to establish a baseline, during the program if the client experiences pain, and at the end to evaluate progress.
Step-by-step explanation:
The knee flexion angle necessary to stand from a chair should ideally be measured at the start of the exercise program. This initial assessment provides a baseline for the individual's current functional capacity. Subsequent measurements can be taken after several weeks of training to track progress, or when the client experiences pain to adjust the exercise program accordingly. Lastly, it's also beneficial to measure the knee flexion angle at the end of the exercise program to evaluate the overall effectiveness of the training and determine the next steps.