ACL

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.

Ohkoshi et al. (1998) found after tendon harvest at all isokinetic speeds more than 80% of cases showed a change in curve shape with no peak in the later half, and a shifted early peak. This reflected a change in the peak torque angle during flexion of the donor knee.

Normal Hamstring Curve: 

 Early Peak in Hamstring Curve:

Ikeda H, Kurosawa, and Kim (2002) looked at strength and torque curves in ACL deficient knees these are the curves they described.

Erratic First Curve.

Concave Mid Section (eccentric).

Ayalon, Barak, and Rubinstein (2002) did a qualitative analysis of the isokinetic moment curve of the knee extensors following ACL reconstruction. They found that after three weeks of isokinetic exercise the torque curve of the quadriceps concentrically improved to such a degree that it was noticeable to the human eye. They concluded that analysis of the torque curve was a valuable tool following ACL reconstruction. The original torque curves often looked like the one below.

Erratic Last 30 Degrees.