Sherrington’s Law of Irradiation: Why Tension Is Important

“Your muscles are already capable of lifting a car, they just don’t know it yet.”—Pavel Tsatsouline

Your muscular system is a lot stronger than you might realize. You are already capable of lifting incredibly heavy objects. However, your body has limiting factors built in, so you don’t accidentally do things like tear your tendons from your bones.

These limitations are good, but also tend to be overprotective. There are a few skills of strength we can use that help us (safely) reduce these limitations—and therefore increase our performance.

Get Tight

If you pay attention to StrongFirst content or attend an event, you might notice a consistent message of “get tight.” Why is that referenced so often? Why is tightness important to pushing a heavy kettlebell overhead, pulling a heavy object off the ground, or doing a one-handed push-up?

Sherrington’s Law of Irradiation: Why Tension Is ImportantThe first reason might seem obvious. When we press or pull an object from the ground, the force of the press needs to get transferred into the ground. If we are loose like a noodle, our press must overcome the wobble. If we are tight, the force gets directly transferred into the ground. If we were building a multi-story building, we would not start the first floor with thin wood as it would have too much flexion. Rather, we start it with concrete or steel beams.

The second purpose of tension is to inhibit safety signals that protect our muscle and tendons from hurting themselves. These safety mechanisms protect us from danger (see here and here for great videos that illustrate these mechanisms), and these systems respond quickly and automatically. One example is when you go to the doctor and he or she taps your knee. When a medical professional taps on a ligament in your knee, your knee quickly responds with a kick (called the stretch reflex). The signal does not go to your brain and then you consciously decide to kick. Rather, a signal from the tendon stretching went into your spinal cord and the muscle contracted.

Another safety mechanism, the inverse stretch reflex, leads our muscles to relax when the tendon is pulled too much. If we were able to inhibit this safety signal, we could create a stronger muscular contraction. Tension inhibits the circuitry that tells the muscle to relax.

Sherrington’s Law of Irradiation: Why Tension Is Important

Charles Sherrington, a famous physiologist, created a model for neuromuscular facilitation and inhibition. Sherrington’s Law of Irradiation describes when muscles recruit nearby muscles.1 Pavel called this law “muscle cheering,” as nearby tight muscles cheer the working muscles to work harder.2

Here is how Pavel described it:

“Make a tight fist. Where do you feel the tension? Your forearm and biceps, right? Even tighter! White knuckles! Do you feel your shoulder and even chest flexing too? In reality, when the demand for force increases, other muscles jump in on the action. Like a stone dropped in the water sends ripples across the surface, tension spreads—irradiates—from the muscle directly responsible for the job at hand towards others. The bigger the stone, the taller are the waves and the further they spread! … It states that a muscle working hard recruits the neighborhood muscles, and if they are already a part of the action, it amplifies their strength! Not by cheating, as some complement their barbell curls with a back swing, but by ‘cheering’. The neural impulses emitted by the contracting muscle reach other muscles and ‘turn them on’ as electric current starts a motor.”

An Experiment

This effect has been tested thousands of times by many individuals and occurs in every StrongFirst event. But recently, I decided to test it in what is perhaps an unexpected place—in a class I am teaching on statistics. The experiment served a dual purpose of creating data for the students to analyze, as well as teaching them stronger handshakes when they go for important job interviews.

The students are part of a doctoral level program and probably would not consider themselves elite athletes. Students were on average in their mid-twenties. Students were given a hand dynamometer and asked to squeeze it as hard as possible with both their strong hand and their stronger hand.

The Intervention

We then practiced an intervention where they first learn to squeeze their opposite hand as hard as possible while gripping. We then added tightening up the glutes and thigh muscles. We finally added in tensing the ab muscles. After the students learned how to tense these opposing muscle groups, we asked them to retry the hand dynamometer.

Below are the results. Most students showed an improvement on grip-strength, no matter which hand they used. On average, there was significant improvement.

Grip Strength: Strong Arm
Grip Strength: Strong Arm
Grip Strength: Stronger Arm
Grip Strength: Stronger Arm


This two-minute intervention led a group of untrained students to have a marked improvement in hand strength. Think about what could occur with repeated training of this skill of tension. It might be a way for you to kettlebell press half your bodyweight, deadlift 2.5 times your bodyweight, or do a one-arm push-up.

In the StrongFirst Kettlebell Course and Certification, students learn to hold a partially emptied water bottle in their free hand. Right before they press, they are instructed to squeeze the bottle as hard as possible. It serves the purpose of generating strength in the pressing arm, both cheering muscles on the opposite side and inhibiting safety signals from the tendons. If you have not tried this before, give it a shot and see whether you can press the weight easier.

So the next time you read something from StrongFirst or attend a Course or Certification and notice that we always say, “Get tight!”—you know the research-backed reasons why.

1. Austregesilo, A. 1928. “Synreflexia (Association of Reflexes).” The Journal of Nervous and Mental Disease 68 (1): 1–10.
2. Tsatsouline, Pavel. 2000. Power to the People!: Russian Strength Training Secrets for Every American. Dragon Door Publications.
Craig Marker
Ph.D., Team Leader, SFG II, SFL, SFB
Craig Marker, Ph.D., Team Leader, CSCS, is a fitness enthusiast who has spent his life trying to help people improve their lives. As a professor, he works with students on how best to understand research and place it into context. He has published over fifty articles, chapters, and textbooks on psychology and research methods.

As a researcher, he understands the cutting edge of strength, sports performance, body composition, and nutrition. As a psychologist, he has focused on research and treatment of anxiety disorders, which positions him to understand motivation and the fear of making life changes.

Craig’s upcoming book, the AntiFragile Self, takes on the topic of building a stronger person in the mental and physical domains.

As a certified StrongFirst Kettlebell Instructor, Craig views kettlebells as one tool in the trade of forging a better person. He uses the Functional Movement Screen and multiple corrective movements to make sure his students are performing at their best for the rest of their lives.

Visit his intentional community in Atlanta: Armour Building.

3 thoughts on “Sherrington’s Law of Irradiation: Why Tension Is Important

  • Craig,
    Whilst it is obvious that the people in the experiment that improved did so significantly, I notice there are a few lines sloping in the opposite direction? Any possible explanation as to why this might have occurred? I would have thought fatigue, but the significant improvement of the other subjects suggests sufficient rest time was given.

  • Nice article but diagrams do not have proper explanations (1st diagram has many unexplained terms and typos) or legends (2nd and 3rd diagrams show innervation on x-axis). I know strong and stronger refer to non-dominant and dominant hands but still kind of confusing. Thanks.

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