[Agility Lab] Cutting Technical Models: It’s Not That Complicated

I am going to be talking about these points in the context of the ‘Cuts and Curves’ location of the COD landscape which I have previously outlined. High speed, relatively low space constraint changes of direction where performance is mainly defined by the maintenance of velocity.

The foundation is that everything starts with sprinting. It is an innate form of human locomotion. All that happens when you start thinking about cutting and curves is you have to consider the impact the extra constraints have on the movement. We will concentrate on the angle which the athlete needs to cut to and the time available to do it. This means that we will consider braking force, frontal plane force application and therefore injury risk. As these pesky ACL tears don’t seem to be going away anytime soon.

So what do we take forward from sprinting? Well sprinters don’t only run-in straight lines, they around a track with bends. So naturally my mind thinks “What can we learn about cutting and curve performance from a sprinter running a bend?”

When a sprinter runs a bend, they deviate from the linear speed technical model as little as possible. There needs to be a small amount of lateral ground reaction force relative to the COM, but that is about it. Or at least that is as far as we need to go for me to make my point.

I am not going to go into sprinting in lots of detail and will come back to it later in the article. But a few principles to take forward.

  1. Optimum projection comes from the sagittal plane
  2. Efficient projection needs the direction of GRF to pass through the COM
  3. Safety comes from maintaining frontal plane alignment

Understanding Injury Considerations

From an injury perspective, the main thing to be aware of and avoid is a Knee Abduction Moment (KAM). Meaning forces which act to move the foot laterally and the knee medially. Foot out, knee in (in the frontal plane). This is our biggest ACL risk factor. So in our technical models, we should simply be asking, how do we make the KAM or the forces driving the foot out and the knee in lower? Exactly how to analyse and calculate this is for another day.

Applying Lateral Ground Reaction Force

In order to apply lateral ground reaction force, the base of support (BOS) needs to be placed outside of the COM. Simply push to the right to move the COM left. A sprinter does this safest and most optimally to maintain speed (which in most cuts is the most important thing). All they do is tilt their entire body to one side. BOS moves outside the COM a little, but the pelvis tilts and the shoulders tilt so that they remain relatively perpendicular to the direction of force. This reduced frontal plane forces and also helps the GRF pass through the centre of the COM.

In a perfect case scenario to optimise speed and minimise injury risk. This is what we want athletes to do. But of course they can’t as the constraints are different.

The Impact of Less Time

Less time to identify the need to change direction means that the larger and greater momentum carrying parts of our body can’t be manipulated as effectively as we need. We can in a split-second move our limbs as out muscular structure is designed to do that and they aren’t very heavy. But we can’t do that with our trunk as effectively. And if we can’t adjust our trunk position in a short amount of time, we can’t create the full body tilt position which we know to be the safest and the fastest. So what happens?

Our manipulation becomes distal to proximal.

Very little time is enough to move our leg outside out COM and create the desired GRF to complete the task. We re-direct the GRF from hip abduction. This ticks the performance box but doesn’t tick the injury box as hip abduction is an ACL risk factor for two reasons. It moves the BOS location outside the COM (without the trunk tilt to maintain COM and GRF alignment) which increases our KAM. And the hip abduction also leads to a muscular hip ADDuction moment, further increasing the knee abduction moment due to it wanting to move the knee in, while the foot is planted ‘out’. Not ideal.

If there is a little more time, or a little more skills. The athlete can move the next proximal structure which is the pelvis. So the athlete gets the BOS to the same location outside of the COM, but has achieved this by the pelvis tilting (improved maintenance of the GRF being perpendicular to the pelvis line which means less frontal plane loading). This is an emerging area of the research and something which I think is going to be quite impactful. Only a handful of studies have reported hip abduction amount but controlled for pelvic tilt/positioning as far as I am aware. What the pelvic tilt does is reduce the hip adduction moment and therefore the KAM. Once you watch enough performance with a focus on this you see it in a lot of elite performers. Even in high chaos situations you see the trunk remain relatively static but the pelvis rock left and right with the location of the lower limbs.

Large Angle Changes More Likely Need Deceleration

If the athlete needs to decelerate this means the BOS moving in-front of the COM to apply a braking force. This immediately increases injury risk as we increase the anterior draw of the tibia at the knee and therefore ACL loading. This links to our high-risk kinematics via an increased chance of heel strike. But it also impacts our trunk. When we apply braking force, we are able to apply a much greater level of impulse that in propulsion. We have a huge spike in braking impulse which needs to be transferred from its application location on the ground through the COM to be effective. Therefore, across multiple joints including the hips and pelvis. Naturally, this creates further muscular demand with the need to control the trunk. But the sudden impulse can be so high the GRF will be directed straight through the pelvis (effectively transferred across the joints of the lower body) but poorly transferred across the pelvis to the trunk as the musculature just can’t keep up. This means the pelvis slows more than the trunk, resulting in increased and potentially excessive trunk/hip flexion. Once again, a chance of increased injury risk (impact on moment arms and if multi-directional, potentially KAM). So a need to decelerate isn’t ideal. But if the situation demands it, you are a bit stuck. We can’t dodge physics.

If both the need to decelerate and minimal time availability are combined, we have a really high risk situation. Made worse if an athlete has got themselves to a high velocity without the relevant physical capacity to deal with it. One of the many reasons why speed control (cognitively and physically) and perception skills to give yourself more time are so important. Yet rarely trained.

Here is a summary:

I am conscious that I started this post with ‘it’s not that complicated’ then rambled on about biomechanics for 1000 words. But these are my take aways.

  1. General Principles:
    • We want minimal frontal plane forces; these are minimised by ‘perpendicular’ relationships between joints. i.e pelvis tilts to keep a ‘perpendicular’ relationship with the leg/direction of GRF.
    • Excess speed in situations where is doesn’t suit is a major problem
    • Optimum projection comes from the sagittal plane
    • Efficient projection needs the direction of GRF to pass through the COM
  2. Stuff you want:
    • A straight line between BOS location and the head
    • If it can’t be straight to the head it should be straight from BOS to pelvis, with the pelvis tilted towards the BOS direction, then curved through the spine to the head.
    • If it can’t be that then the athlete better be resilient to deal with the forces coming their way.
  3. Stuff you don’t:
    • Multiple ‘kinks’ and angles in the frontal plane. Commonly from the foot to the hip (a ‘kink’ at the knee and more KAM).
    • The femur abducted and the pelvis still level with the ground