It's still early days and it often takes longer to achieve knee extension (straightening) than flexion (bending).
Here are a few exercises you could try - just choose one or two that you like and start off by doing them for a very short time. Stop before they hurt.
Extension: exercises for improvement
Here are a few exercises you could try - just choose one or two that you like and start off by doing them for a very short time. Stop before they hurt.
Extension: exercises for improvement
New Zealand

