Core training has been all the hype the last several years - from an aesthetic point of view (achieving 6 pack abs) to it's potential role in reducing lower back pain and related symptoms. This article won't be touching on any of those points. I'm speaking more from a performance-enhancement perspective, specifically, the role of core stiffness and it's relation to performance outcomes.

What is the "core"?

The musculature and structures between your shoulder girdle and your hip; the superficial and deep abdominal muscles.

What is the function of the "core" and what is "core stiffness"?

Ask a performance coach and they'll give you one answer. Ask a physical therapist and they might give you a different one. From a physical therapy or rehabilitation setting, core stiffness may refer to increased muscle tone that leads to lower back pain and movement restrictions. In the performance sports setting, core stiffness may refer to the core's ability to maintain stability in response to the influence of external forces, which may vary in magnitude, direction and speed. For the purpose of this write up, I'll be talking about the latter.

To my knowledge, the function of the core is to transmit force, prevent force/power leaks and maintain the position of certain joints and body segments. In the words of professional lower back and spine expert Stuart McGill, he describes the function of the core to "stiffen the torso to prevent motion, [and] enhance motion of the distal limb segments".

In the majority of sporting performances, the ability to accelerate distal limbs is the key to high power output: think about throwing a baseball, kicking a soccer ball, jumping, pushing, punching. Stiffening the torso to prevent excessive motion and enhances force production and power output because it acts as a medium for force transmission - ensuring all the force you generated is directed towards your intended direction. A floppy and weak midsection results in a weak kick, a weak throw, a weak jump and a weak landing.

an overview, with examples.

The idea of core stiffness allowing for better force production/transfer can be applied to many different scenarios. I'll give 3 examples from 3 different sporting types, a mixed sport (Mixed martial arts/combat sports), an endurance-dominant sport (Cycling) and a strength-dominant sport (Powerlifting).

In each example, the RED highlighted area represents the "core": the superficial abdominal muscles (rectus abdominis), the deeper muscles like the transverse abdominis as well as surround muscles like the obliques (for rotation), latissimus dorsi and rotator cuff muscles.

The GREEN highlighted areas represent the prime movers that are responsible for generating force through concentric muscle action, and/or the distal limb segments that are used to transmit force into the intended direction or to move a certain object.

Example #1 - The Punch and Kick in Martial Arts

The straight right/left punch is one of the most basic, yet intricate techniques in martial arts. Contrary to what many think, the prime movers of the straight punch are actually the back leg, the muscles surrounding the hip and the shoulder girdle, NOT the arms. Power is generated by pushing off the back foot, contracting the glute, and directing that force into the distal limb segment (which is the fist in this scenario).

In order to effectively transmit that power from foot to fist, the core must rigid. The obliques must create enough deceleration force to prevent your torso from over-rotating (shifting you off balance), and your shoulder girdle must be sturdy enough so loosen up on impact. Additional, these types of movements (punches, kicks, hitting/impact-movements) exhibit a "double peak" in regards to muscle activity (measured through Electromyography (EMG)). In other words, whenever performing a powerful movement, the muscles will go through 3 phases: contract/stiffen, relax, and contract/stiffen once again upon impact, showing a "double" peak in activation (will be pictured below).

Stuart McGill et al. performed a study in 2010 on elite mixed martial artists (Georges St. Pierre, David Loiseau to name a few) that introduced me to this concept so I'm sure he can explain it more clearly:

“Many of the
strikes intend to create fast motion and finish with a very large
striking force, demonstrating a ‘‘double peak’’ of muscle activity.
An initial peak was timed with the initiation of motion
presumably to enhance stiffness and stability through the body
before motion. This appeared to create an inertial mass in the
large ‘‘core’’ for limb muscles to ‘‘pry’’ against to initiate limb
motion. Then, some muscles underwent a relaxation phase as
speed of limb motion increased. A second peak was observed
upon contact with the opponent (heavy bag). It was postulated
that this would increase stiffness through the body linkage,
resulting in a higher effective mass behind the strike and likely
a higher strike force.”

— McGill et al. (2010)

To further illustrate the "double peak" in activation, and to tie it back to the discussion earlier about prime movers and the core, here's a figure taken from McGill et al's 2010 study (green and red highlights made by myself).

This figure illustrates the muscle activation of the left back muscles during a left leg kick to a heavy bag.

#1 in green represents the initial activation of the muscle (phase 1 - contraction), where the foot pushes off the floor and the hip flexors and obliques contract in order for the leg to be lifted into the air. The prime movers are the hip, quadriceps and left back.

#2 in red represents the relaxation phase (phase 2) where the leg is in mid air and force is being transmitted through the core, to hit the intended target (shin/foot hitting the heavy bag).

#3 in green represents the second activation of the muscle (phase 3 - contraction again), where the muscles stiffen up once again on impact.

An athlete with a weak core, or an inability to stiffen up the core will experience power leaks in #2, which subsequently results in a lower striking force upon impact in #3.

Optimal force and power production happens when an athlete has:

Good body positioning
The ability to show double activation during strikes/impact movements
The ability to efficiently cycle through contract-relax-contract muscle action if repeated punches/kicks/impacts are performed
Strong and resilient core musculature.

Example #2 - cycling

Operating at up to 300RPM (revolutions per minute) and putting out up to 2,200 watts, track cyclists are known for their lower limb strength and power, as well as their ability to maintain extremely high power outputs for extended periods of time.

Watch the video below, notice the rigid and stable core - supported by the shoulders and arms, and how that translates to incredibly fast distal limb segment movement (in this case, the feet).

Track bike roller rev out.

Yes, the thigh musculature are still the primary movers, but the core/torso also contribute greatly, acting like a shock absorber to negate most of the horizontal and vertical bodyweight shifting that happens during high RPMs. A strong core ensures as much of the force generated by the prime movers can be directed towards spinning the pedal.

Picture taken from TeamNater (Youtube), editted by myself.

The principles of contract-relax-contract discussed above also apply to this example. The quadricep has to relax while the hamstring is active during the (pedal) upstroke, and vice versa during the downstroke. Pair it with the fact that this is all happening at 150, 200, 250+ RPM and you'll realize the amount of motor control needed to be an elite cyclist.

Example #3 - Powerlifting

Our last example is a strength sport - powerlifting. Powerlifting is a maximal strength sport, meaning the velocity of distal limbs and the velocity of the bar is relatively low compared to other sports such as Olympic Lifting or various throwing sports. As a result, core stiffness plays a slightly different role - a slower isometric role. Although there is no contract-relax-contract cycle in powerlifting, the core still acts as a stopper for potential force leakages. The main role of the having a rigid torso/core is to support the lumbar and thoracic spine under heavy load.

In a sport where limb length and leverages are highly influential on performance outcomes, a lifter must maintain the position of their lumbar and thoracic spine so that their chest does not collapse under the weight, and their lower back doesn't go into excessive flexion. Losing position means the lifter will not be able to lift the weight up, regardless of how strong their prime movers are.

Below is an example of one of the lifters on my Powerlifting team, Amos So, bracing for a squat at a local powerlifting meet.

Context Specific

Throughout the article, I've used terms like "stable", "rigid", "strong" and "resilient" to describe the properties of a well-functioning core. Different coaches and trainers will use different terms to describe the core, so terminology is sometimes confusing.

However, what matters is how you apply these principles to your own training, or the training of your clients and athletes. Improving performance measures via increased core stiffness will depend on several things:

The nature of the sport and the energy systems involved (How long is the core braced for? Does the athlete need more strength? Or more endurance?)
The particular movement you're trying to improve by increasing core stiffness
The properties of the movement itself and the speed of core stiffening
The magnitude of core stiffening (stiffer doesn't always mean better, remember: fast and powerful athletes also need to learn how to relax at the right times - like the example of the punches and kicks we used earlier).

Much like the development of max strength for athletic performance, there are also most likely diminishing returns, so don't fall into the trap of "more core training = higher performance".