Anterior cruciate ligament (ACL) deficiency:
There is usually a plateau or double peak that occurs in the midportion of knee extension. See below.
This is because the ACL controls a portion of the glide and rotation during mid range knee extension.
Think of it like this:
The quadriceps pull which shears the tibia forwards.
The ACL is unable to control the movement and by reflex turns off the quadriceps temporarily.
This gives the dip which is corrected for by further quadriceps activity.
With ACL deficiency there are often three rapid spikes in the early part of the hamstring curve (see below) I hypothesise that this is due to reflex inhibition.
Think of it like this:
The knee is fully extended and the hamstrings pull to bend it.
This trigers an eccentric reflex from the quadriceps which causeses an anterior tibial shear.
This creates a reflex inhibition from the ACL as it is unable to fully control the movement.
This turns off the hamstrings giving a peak and dip in the graph.
The cycle is repeated.
A double hump (M shape) will often be seen (see below) in people with meniscal dysfunction.
This is probably due to impingement during normal joint motion and reflex inhibition.
Think of it like this:
The joint surfaces rotate around within the menisci.
As the knee is extended / flexed a point is reached where the damaged meniscus is under alot of pressure from the joint surfaces.
The meniscus reacts with pain inhibition which temprarily turns off the muscles.
This gives the dip (and is often only seen ever third repetition).
Generally the peak torque will be lower.
It must be stressed that normal isokinetic test results do not exclude meniscal pathology.
Please see patello-femoral section.
Opposite curve deformations are often seen in the hamstrings and quadriceps curve data. See below.
This is probably a reflection of the pain experienced during the movement and is present in both curves at the same point in ROM because the articular surfaces are involved.
The torque curve deviation will be almost repeated throughout the repetitions.
