I Have Anterior Knee Pain That I Can't Relate To

I have anterior knee pain that I can't relate to anything specific. I do have a weaker leg on that side with nerve damage following a slipping disc 15 years ago . I have wasted muscle particularly on that calf . What's the best approach for accurate diagnosis . ? I keep fit generally , don't run. Walk regularly ( it often hurts at the beginning and more after but seems to ease part way through ? I cycle regularly and reasonable distances . It's always been a bit of a dodgy knee but am really worried that walking may cause more damage and am training for an endurance walk event in July .