Hidden Potential
Key concepts of muscle physiology to unlock the body’s true power
Muscle physiology provides the foundation through which we can both understand and improve human movement. A basic knowledge of this science is all a coach and athlete need to maximize the body’s hidden potential. In this article we break down a few key principles including key types of muscle contractions as well as the stretch reflex.
Muscle Contractions
There are several types of muscle contractions in the human body, and to cover them all would take a fair bit of time, so for ease of understanding and simplicity, we will be covering the two that are primarily responsible for movement. They include concentric and eccentric muscle contractions.
In a concentric muscle contraction, a muscle or group of muscles shortens to produce force and subsequent movement at a joint.
The example most commonly used to help picture this movement is that of the bicep muscle during the upward phase of the dumbbell curl. You can physically see the muscle shortening, providing the necessary force to lift the weight upwards. This type of muscle contraction is the most widely known and easiest to understand.
The second, lesser known, and arguably most important type is the eccentric muscle contraction.
In an eccentric muscle contraction a muscle builds force or energy when it lengthens or stretches, similar to how a rubber band builds elastic/potential energy when stretched.
Looking at the same example of the dumbbell curl, this would be the lowering phase of the exercise where you can physically see the muscle lengthening to control the weight downwards. The bicep is still contracting or providing force, but it’s doing so while it lengthens.
And though this concept may be easy to grasp in the context of weight lifting, when applied to actual movements in sport it becomes a bit harder to visualize. To aid in our understanding, we will apply these concepts to the counter movement jump, the overhead throw, and the kick.
During the squat or counter movement jump, as the body compresses downward or flexes, the muscles and tendons that cross the ankle, knee and hip, those responsible for lifting the body, are actually eccentrically contracting and building elastic energy. We refer to this as a pre-stretch or pre-load. This ‘priming’ phase allows us to produce stronger concentric muscle contractions, and subsequently more upward force.
An easy way to test this is to jump as high as you can, first from a standing position where you utilize the preload or counter movement jump, and then second starting from a squat position with no preload. You can probably guess which will yield more vertical height (answer: counter-movement).
In the volleyball spike the simultaneous forward rotation of the torso and backward rotation of the arm, pre-stretches the muscles on the front of the shoulder. Once these muscles reach a critical length, they rebound (think a rubber band snapping back to its resting length) and rapidly accelerate the arm forward.
In a technically efficient soccer kick, the player will first rapidly extend the leg backwards, which pre-stretches or eccentrically loads the muscles on the front of the hip, which are responsible for the rapid acceleration of the leg. Once they reach a critical length, similar to that of the internal rotators during the overhead throw, they recoil or concentrically contract maximally to drive the leg towards the soccer ball.
These principles not only play a critical role in the rapid acceleration of our limbs, but also in their deceleration, which is the phase where most injuries actually occur. For instance, after ball release during the pitch and javelin throw, or ball contact during the spike, the muscles on the back of the shoulder, the external rotators, contract eccentrically to slow the rapidly moving arm down (picture the bicep lengthening during the downward phase of the curl).
Because of the incredibly high forces placed on these muscles, they are much more susceptible to injury, which is why it’s imperative that athletes learn proper deceleration mechanics and maintain a high level of strength and flexibility in these areas.
The Stretch Reflex
The other crucial principle in muscle physiology that must be addressed and understood is the Stretch Reflex.
In a stretch reflex, a muscle or group of muscles involuntarily (key word) contracts in response to a stretching sensation.
The key component responsible for the activation of this reflex is called the muscle spindle. A muscle spindle is a stretch receptor built into our skeletal muscles to help regulate their length and to prevent them from overstretching or tearing. If a muscle stretches either too far or too forcefully, the muscle spindle activates. A signal is then sent to the spinal cord, which then sends a signal right back to the muscle, known as a spinal reflex, causing the muscle to rapidly contract or shorten (stretch reflex). The faster and more forcefully a muscle stretches, the stronger the stretch reflex will be. Reflexes also produce stronger concentric contractions, primarily because they are protective in nature.
The stretch reflex is active in all of the movements previously discussed above. During the counter movement jump, as the ankles, knees, and hips bend forcefully, elastic energy is stored, the muscle spindles become active, and the stretch reflex is initiated. Both activities assist the muscles in generating more force and increased jump height. The rapid external rotation of the shoulder in the overhead throw, as well as the rapid extension of the leg backwards during the kick, pre-stretch both the internal rotators of the shoulder and the hip flexors of the leg. Muscle spindle activity is triggered and an activation of the stretch reflex occurs. To reiterate, this is an involuntary and automatic response of the body (reflex), which can often be very counterintuitive for some athletes.
To summarize, our muscles can produce force in different ways. The lesser known eccentric muscle contraction and the stretch reflex work in conjunction with each other, exploiting our muscles ability to generate elastic energy and a powerful reflex. Both produce stronger, and more forceful concentric muscle contractions and increased energy output. Harnessing these key concepts can be the difference between average and high level athletic performance as well as injury prevention and longevity.
