The Forgotten Muscles of the Stifle
Why stifle stability is more than just the cruciate ligament
When we talk about stifle stability in dogs, the conversation usually centers on the cranial cruciate ligament.
But the stifle is not stabilized by ligaments alone.
Several muscles around the joint play important roles in controlling rotation, supporting weight bearing, and coordinating limb movement.
In our recent Animal Rehab Solutions/ANZCVS Sports Medicine & Rehab Chapter Journal Club, we discussed some of these often-overlooked contributors to stifle function:
• Popliteus
• Long digital extensor (LDE)
• Superficial digital flexor (SDF)
• Gastrocnemius
Understanding how these muscles function can change how we think about stifle instability, and rehabilitation strategies.
Popliteus: the rotational stabilizer
The popliteus is a small muscle located caudal to the stifle joint.
Its primary function is internal rotation of the tibia and dynamic stabilization of the stifle during movement.
This muscle becomes particularly relevant when considering rotational instability of the stifle, which is often overlooked compared with cranial–caudal instability.
In dogs with cruciate disease, the popliteus may act as a secondary stabilizer, helping control rotational forces during stance and gait.
However, pain and inflammation within the stifle can lead to arthrogenic muscle inhibition, reducing activation of this important dynamic stabilizer.
Long Digital Extensor: more than a digit extensor
The long digital extensor (LDE) is often thought of simply as a muscle responsible for digit extension.
However, its anatomical relationship with the stifle suggests a more complex role.
The LDE tendon starts from the extensor fossa of the femur and passes through the extensor groove in the proximal tibia. This positioning means that the LDE may contribute to dynamic stabilization of the stifle.
While the cranial cruciate ligament remains the primary restraint against cranial tibial translation, surrounding structures such as the LDE tendon within the extensor groove may provide additional mechanical support during movement.
Gastrocnemius: propulsion with biomechanical consequences
The gastrocnemius is one of the most powerful muscles of the pelvic limb.
It extends the hock and contributes significantly to propulsion during gait.
However, in a cranial cruciate ligament–deficient stifle, its biomechanics become more complex.
When the gastrocnemius contracts, it generates forces that contribute to cranial tibial thrust, pushing the tibia cranially relative to the femur during weight bearing.
In a stable joint, this force is resisted by the cranial cruciate ligament.
But when the ligament is compromised, this force can contribute to joint instability.
Pain, inflammation, and instability within the stifle may then lead to arthrogenic muscle inhibition, altering activation of the surrounding musculature.
As a result, propulsion, limb loading, and gait mechanics can all be affected.
Superficial Digital Flexor: part of the caudal support system
The superficial digital flexor works together with the gastrocnemius as part of the caudal musculature of the crus.
These muscles contribute to supporting of the limb during stance, propulsion during gait and flexing the digits.
When stifle disease is present, changes in loading patterns can significantly alter how these muscles function.
When the stifle hurts, the entire crus is affected
One key concept from our journal club discussion was how closely connected the stifle is to the muscles of the crus.
The popliteus, LDE, superficial digital flexor, and gastrocnemius all function within a kinetic chain that extends from the stifle to the hock and ultimately to the digits.
When the stifle becomes painful or unstable, the effects are not limited to the joint itself.
Joint inflammation can lead to arthrogenic muscle inhibition, reducing activation of surrounding musculature.
As a result, the entire crus muscle group may change how it functions during gait.
This can lead to:
• altered propulsion
• changes in limb advancement
• altered paw loading patterns
• compensatory digital mechanics
In other words, a painful stifle can influence movement all the way down to the digits.
Rotational stability: more dynamic than we think
Another interesting point from the literature is how much rotational laxity actually exists even in normal dogs.
One study (Faulkner et al. 2026) reported that internal rotational laxity of the stifle can reach around 20° in clinically normal dogs who is predisposed to CCLD/MPL, and this laxity is significantly greater than what has been described in Greyhounds.
This suggests that rotational stability of the stifle is not controlled solely by passive structures such as ligaments. Instead, it likely relies on dynamic muscular control around the joint as well.
When these dynamic stabilizers are inhibited or dysfunctional — for example due to pain, inflammation, or cruciate disease — rotational instability may become more pronounced.
This may also help explain phenomena such as the pivot shift, where combined rotational and translational instability occurs within the stifle.
Why this matters clinically
We think of stifle disease primarily as a ligament problem.
But the stifle is supported by a complex system of dynamic stabilizers and kinetic chains.
Understanding these relationships helps explain:
• rotational instability of the stifle
• compensatory gait patterns
• variability in recovery after surgery
• why rehabilitation is important for restoring coordinated limb function
Sometimes the key to improving mobility is not just correcting mechanical instability — but restoring coordinated muscle function across the entire limb.
Rehabilitation perspective
When the stifle becomes painful or inflamed, the first step is addressing pain and joint inflammation. Once these are controlled, much of the arthrogenic muscle inhibition affecting the surrounding musculature can begin to resolve.
At that point, rehabilitation becomes about providing the right stimulus at the right time.
This includes gradually restoring neuromuscular activation and coordinated function across the entire limb.
From a rehabilitation perspective, it is important not to focus on the stifle alone.The crus musculature — including the popliteus, long digital extensor, superficial digital flexor, and gastrocnemius — should all be considered, as well as how the limb ultimately loads through the paw and digits.
Final thought
The cranial cruciate ligament may be the most famous structure of the stifle.
But all the muscles like the popliteus, long digital extensor, etc remind us that joint stability is a team effort. And when the stifle hurts, the consequences can travel all the way down the digits.
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