It’s often said that you should not allow your knee to go past your toes when performing lower body exercises. Indeed, for many years I was the personal trainer telling my clients exactly this.
The ‘knee over toe’ conundrum has been around for a long time. The general recommendations for split squat technique focus on the relative position of the leading knee and ankle, especially at the bottom of the movement. In fact, in a recent NSCA journal article on split squat exercise technique, the author stated that:
[the] knee moving forward of the foot at the bottom of the movement should be avoided as this compromises the knee and makes the exercise problematic.
There was no qualifier for this statement, so I presume this means it will be problematic for everyone. The concern focuses on the forces acting upon the knee, specifically the ACL.
Bilateral Squat Research
Prior studies on the bilateral squat have shown that restricting the motion of the knees over the toes can indeed reduce the amount of forces on the knee. However, if you decrease the moment arm 1 at one joint, you invariably increase the moment arm at another – increasing the relative amount of force to other joints such as the hips and lumbar spine.
But What About The Trailing Knee?
If, during a bilateral squat, restricting the knee moving forward increased forces at the hip and lumbar spine – what would happen during a split squat? How would the forces re-distribute?
When I was studying for my MSc in Biomechanics, I had access to a lab that had force platforms, a VICON infrared camera system and more. I wanted to collect some data on what actually happens when you select a ‘traditional split squat’ (not allowing the knee to move force) vs a ‘full active-range split squat,’ which does allow the knee to move forward. But first, a review…
The ACL and SHEAR!
Quick review 1: the Anterior Cruciate Ligament (ACL) resists anterior translation of the tibia 2. It works in conjunction with other active tissues that perform the same job such as the hamstrings and the muscles that attach into the IT Band (Glute Max, TFL, Glute Minimus, etc.) that connects into Gerdy’s Tubercle.
Quick review 2: shear is a dirty word in fitness, but shear is simply a name of a type of force. Shear is a force, not a motion. Therefore, it would be an anterior shear force that would cause anterior translation of the tibia. Possibly creating a need for the ACL to resist that motion.
Quick review 3: during lower body exercises, it is the knee extensors (vastus lateralis, medialis, etc.) that create an anterior shear force in response to an external load. In fact, if we didn’t have shear, we wouldn’t have joint motion.
It’s the quads that did it!
If we are talking exercise, which we are, and not an injurious situation, which we are not - the majority of forces acting upon the ACL come via the knee extensors, which attach into the tibial tuberosity, via the patella ligament.
The knee extensors generate tension based upon external resistance demands. This external demand is called torque.
If the knee flexors do not generate enough force to counteract the force of the extensors, and maintain joint integrity, this could put undue stress onto the ACL. However, this isn't necessarily a bad thing. Your ACL may well tolerate those forces where someone else's may not.
The Study (a little experiment)
I took one person (me!) and performed two split squats (SS1 & SS2). SS1 was the traditional version; I prevented my knee from travelling forward. For SS2, I tested my exercise specific active range of motion and performed a full range of motion without going into passive ranges (and allowed my leading knee to travel forward.)
I performed 10 repetitions and had a colleague ensure that my technique was as planned. I paused at the lowest position to minimise the effect of inertia on my results. I wanted to collect the data at the amortisation phase 3 as most authors suggest this is the critical point.
I collected quite a bit of data from this experiment. However, as the focus of this article is on the forces acting upon the ACL, which come via the knee extensors (in turn responding to an external resistance demand), we’ll focus on the external resistance demand and assess torque present at the knee joint. Torque is measured in Newton-metres 4.
- During the traditional split squat (SS1), the angle of my ankle joint reached 7.6º from neutral, however, during SS2, it reached 39º of dorsiflexion
- For the SS1, the amount of torque present in the leading knee was -0.8 N.m, meaning there was an extensor torque rather than flexor – creating a bigger requirement for my knee flexors!
- During the SS2, the amount of torque was 42.6 N.m, so we can see that by allowing the leading knee to travel forward there was a large increase in torque at the knee (and this time, it was a flexor torque creating a demand for the knee extensors)
The Trailing Leg
This was interesting: during SS1 the torque was 86.9 N.m. This is a bigger force than either position on the leading leg. By allowing the leading knee to travel forward in SS2, the forces decreased on the trailing knee to 63.7 N.m.
In an effort to protect the ACL:
- Preventing the knee from moving forward over the foot during the split squat is often suggested as a safe way to protect against anterior tibial translation.
- We know that anterior translation is caused by the knee extensors in response to the external resistance demands.
- We’ve seen, in this little experiment, that limiting ankle range of motion in the split squat will assist in reducing the forces acting upon the ACL of the leading knee. However, this variation of the split squat will be little use if the ACL that requires protection is in the trailing leg.
In reality, it doesn’t come down to a relationship between the ankle and the knee, or the toes and the knees. It comes down to the relationship of each axis/joint to the line(s) of force acting upon the body.
What is THE Goal of the exercise?
What if your goal is to increase the strength of your knee extensors? The cue above from the NSCA and many other sources is based on an idea that we want to minimise forces – effectively minimising the external resistance demands, therefore minimising the work that the knee extensors are required to perform.
But what if our aim was to increase the strength and size of the knee extensors? Minimising work would not be the most appropriate scenario for this goal.
Of course, looking at the forces acting upon the trailing leg was certainly not an original idea of mine. The first time I thought about this was in an RTS class when Tom Purvis threw a comment out:
"... we are so busy worrying about the leading knee being in front of the foot, that we fail to see that the trailing knee is FAR in front of the foot."
It’s nice to get some numbers.
This is a N=1 study. It’s based on my short legs and my specific exercise technique. But it paints a useful picture. But what could change?
The way I perform the split squat involves a ground reaction force split between legs of approximately 1.6:1. (I had about 60% more force going through the leading leg than the trailing leg). Cueing a similar or different distribution should be part of your cueing expertise as a personal trainer. What do you want to affect?
I wanted to record the data in a static position. Inertia would have created increases in demand at various joints and decreased at others. Similarly, walking lunges, anterior lunges, etc. will all offer different external resistance demands due in large part to the inertial properties of the mass - you (plus whatever extra load).
What I haven't discussed here is the specific joint forces: shear, compression, etc. Or other issues that we need to be aware of: patella compression syndrome. I'll look at that in another article.
The moment arm (MA) is the shortest distance between an axis and a line of force. The longer the moment arm, the greater the effect the force has to rotate. In this case, decreasing a moment arm makes the muscles of the knee work less.↩
The ACL attaches onto the posterior surface of the femur and the anterior surface of the tibia, so is best placed to prevent the tibia being pulled forward↩
The amortisation phase is the transition between the eccentric and concentric portion of a movement, popularised in gait and plyometric work↩
or Moments of Force (N.m)↩