Unable to straighten leg

Hi,  Over a week ago i did a bike ride, then i did a run which i ran faster than my usual and then the following day because i felt ok i went to a spinning class and then some weight machines for my quads and inner and outer thighs (don't know the names of the resistance machines) - again all ok until later that night my knee cap felt sore.  Not sore to the touch but whilst trying to straighten my leg from a sitting position or when bearing weight on it - especially when bending the leg whilst standing on it and then the tenderness is felt as the leg straightens. 

 So first of all, i'm not sure if it was the resistance machines or the increase in training or all of these contributing factors.  

 Basically i'm miserable as i can't run - i've been swimming atleast.

Do you think this is patella tendinitis/runners knee?

Some time ago I was told about two common causes of knee (patellar) tracking problems other than running:

1: Using leg (quad extension) machines with inadequate support under the knee joint.  The leg going beyond 90 deg. before contraction.  Too much weight.  Pointing your toes forward on contraction.  All things I was told not to do when I had runner's knee.  I was also told that using a loose weight attached to the ankle and short range leg extensions (with good under knee support) was best for correcting patellar mal-tracking.

2.  If you pedal a bike the saddle should be adjusted so that the leg is fully extended on the down pedal.  Otherwise this tends to neglect the inner thigh muscles (vastus medialis) which is important to correct patella tracking. 

See a sports-oriented physio, ask about the points I've suggested before trying anything, after all, sports physiotherapy evolves over the years and what worked for me some time back may not be applicable to you as an individual.  Sounds like you're just doing too much anyway.  You're a runner first, I take it and not training to take on David Haye

Hi,

 After 2 weeks off running, just swimming and a couple of spin classes my knee is improving but it still feels weak whilst walking and still can't straighten leg when trying to extend in sitting position.  So i went off to physio today.  After various exercises he deduced that i've strained  a medial quad tendon which is causing the issue.  He put ultrasound on the knee area and gave me some strenghtening exercises to do to strenghten glutes and quads.  Said to try a gentle jog and if it hurts to stop.

 He doesn't think it's patella tendinitis.