Reflexes and Sporting Movements
If you reach out to touch a hot stove by mistake, you will involuntarily pull back the hand very quickly without conscious intention or perception of danger. This is an example of the withdrawal reflex, which works like this: When your hand detects danger, sensory signals go to the spinal cord, which directly executes the motor commands to retract the hand. This reflex arc takes about 40-50 milliseconds, saving the additional 200-500 milliseconds that would be required if the signal traveled all the way to the brain and back.
A similar withdrawal reflex protects our feet, but with an important difference. If you step down onto something dangerous, and then immediately withdraw the foot away from the danger, you're going to fall. The solution is to find support from the opposite leg by quickly extending it. This pattern - flexion of one leg with extension of the other - is known as the crossed extensor reflex.
Here's how the reflex works to protect you from what some claim is one of life’s great pains: stepping on a Lego. Within milliseconds of making contact, your affected foot rapidly retracts to your center by “triple” flexion at the hip, knee, and ankle. Almost immediately thereafter, your opposite leg performs a triple extension to push into the floor, support your body weight, and maintain balance. This happens far faster than you can think about it.
You could try this for yourself, although it's kind of hard to do it on purpose. Step your right foot down onto an imaginary sharp object and then try to immediately withdraw the foot. You'll feel the left leg extend to the ground for support.
The stumble reflex has a similar pattern, also involving flexion of one leg and extension of the other, but it happens in a different part of the gait cycle. Imagine walking and then your right foot trips on a rock. You will fall on your face if you don't take quick protective action. The stumble reflex quickly lifts the foot up over the obstacle, which also serves to drive the foot forward in a lunging action to prevent the fall. At the same time, the left leg is powerfully extending to keep your center of mass high.
Why Study Reflexes?
Reflexes are interesting because they reveal something about how the body coordinates its movements on a fundamental level, and balances considerations of protection with performance.
For example, consider the pattern of movement involved in the hand-on-the-hot-stove example: the danger was encountered because the hand extended away from the body, and the hand was protected by retracting it to the body. This pattern – extension away from the center of the body for exploration, and then flexion for protection – is a fundamental part of the movement behavior of even the simplest organisms. There is something primally protective about flexion (think fetal position) and something inherently bold about extension. Movement that is safe and functional is always striking a balance.
Another key fact about protective reflexes is that they form the fundamental building blocks for basic movements. The crossed extensor reflex is the pattern for walking, running and sprinting. With each step you take, you're flexing one leg while extending the other.
The stumble reflex is involved in sprint acceleration. When a sprinter launches from the blocks, they are almost falling onto their face. This activates the stumble reflex, creating the powerful knee drive and back foot push that is necessary to catch the fall.
The “Lego-movement” of quickly withdrawing a foot from the ground appears constantly in sports that require “quick feet”, such as basketball, soccer or tennis. In these dynamic environments, athletes are constantly perceiving that one of their feet needs to be repositioned. In essence, the foot has landed or is about to land in the “wrong” place. This activates the flexors of that foot to quickly remove it from the ground or redirect it to a better place. The extensors of the opposite leg must activate quickly provide the support to allow this to happen.
When athletes attempt to develop quick feet through agility ladder drills, they're basically stepping on and off Legos in choreographed patterns as fast as possible.
Practical Applications
So what can we do with this information? Here are some practical points, mostly related to training for sport.
Flexion is underrated
Triple extension tends to get all the credit for athletic movements, because it does the work of driving the body where it needs to go. And therefore athletes in the gym tend to spend a lot of time training the posterior chain and knee extensors.
Flexion patterns tend to get neglected, and perhaps this is a mistake because the speed of flexion might act as a governor on the speed of extension, given that these are paired movements, and that protection is a high priority. Indeed, in the protective reflexes described above, flexion occurs slightly prior to extension.
In sprint technique, a common error is prolonged contact time of the foot, along with prolonged extension of the support leg behind the runner, and slow movement into lifting of the foot and knee. Research shows that improving "front side mechanics" — by shortening ground contact time and lifting the knee earlier — can enhance speed and reduce the risk of hamstring injury. The biomechanical details are fairly complex, but there is a relatively simple takeaway: quick flexion matters. Here are some technical resources making this point: a study, an article, and a video.
Use Reciprocal Facilitation
Since flexion of one leg facilitates extension of the other, you can enhance performance of either movement by cueing its opposite.
In a step-up exercise, the quality of the push by the supporting leg (using triple-extension) can be improved by focusing on lifting the opposite knee (triple flexion).
The reverse is also true - a marching or running drill seeking good knee lift could be improved by focusing on the quality of the supporting leg pushing into the ground.
Protective cues
For agility drills requiring quick foot movement, visualization cues like "imagine stepping on a Lego" might access reflexive pathways that produce faster responses.
For sprinting, some coaches cue runners to imagine running over hot coals to promote shorter ground contact times and better front side mechanics.
Allowing the body to fall forward a little bit is a good way to activate the stumble reflex. There are lots of variations on how you can break the fall: by pressing the hand or the stumbling foot into a wall (easy); accelerating into a sprint (harder); or quickly decelerating by reaching the stumbling foot into a lunge (hardest!).
Thank about bigger patterns
Other fundamental movement patterns are built around these reflexes as well, such as the contralateral movement of the arms during running, and the “hip lock” pattern of muscle activation described by Frans Bosch. The hip lock involves muscles on all sides of the pelvis working to pull it up on the side of the flexing leg (QL and abs) and down on the side of the extending leg (glute med and hamstrings.) You can find many videos demonstrating varieties of hip lock exercises here.
Try to think of how you can use hip lock style movement patterns in supine or prone. There are many! If you'd like me to film a video of some, let me know in the comments.