The desire to both understand and incorporate functional movement into our assessment and training of the body continues to grow within the fitness industry. Unlike genres of training that have a specific set of movements or assessments, functional movement training allows us to tailor fitness programming to each client’s unique movement needs and abilities. To do this, we have to understand the functional movement mechanisms that allow us to move in an effective and energy-efficient way. In this sense, functional movement is the pinnacle of personal training because it forces the trainer to go outside a rigid set of movements and assessments or screens to accommodate the vast number of functional variations that are possible.
This article deals specifically with muscle function during functional movement, which is very different from what you see during the fixed or relatively static movement that is typical in more traditional gym training. Much of the early research into muscles and their function was done in a very static environment, sometimes even using cadavers. As we all know, dead bodies don’t move! This old-school scientific approach hindered researchers’ ability to understand and therefore teach the body’s function in a moving context until the late 20th century, when the work of Gary Gray and Dr. David Tiberio began injecting a more dynamic element into functional movement.
Load to Explode
The traditional view is that when in active motion, muscles create a concentric (shortening) force moment that moves a bone around a static axis of rotation; this rotational force around an axis is known as torque. During function-related movement, muscles work very differently than conventionally understood. In fact, what we see is an initial lengthening or eccentric contraction from our muscles in the opposite direction to the subsequent concentric contraction. Gary Gray, a pioneer of understanding and teaching functional movement, dubbed this “load to explode” (Gray, 2004).
The initial “load” (muscular lengthening) contributes to or even defines the “explode” (shortening force). We see this in all functional movement – especially sport – from swinging a golf club to throwing a football to kicking a soccer ball to jumping. If we want to throw the ball to the left, we first rotate to the right. If we want to jump up, we first go down. This is a primary component of the universal human function of gait. Even during a simple activity such as standing up from the seated position, we first lean forward, tilting our pelvis to the anterior and stretching the hip extensors during the resulting hip flexion to initiate hip extension.
So, if this is such an integral part of functional human movement, why is it not well-documented or a widely used principle when performing our assessment and training of the body?
The eccentric loading of our muscles is not a sudden developmental phenomenon in human function. In fact, it has been well-documented previously in many different guises. Back in 1918, physiologist Ernest Starling stated, "The mechanical energy set free in the passage from the resting to the active state is a function of the length of the fiber." Starling’s law of the heart says that the larger the diastolic phase of filling with blood and stretching of the heart, the larger systolic contraction and pumping of blood. This highlights the universal applicability of the concept of “load to explode,” from smooth cardiac muscle to the striated muscles of the skeleton.
The stretch-shorten cycle (SSC) is an example of the “load to explode” concept at work. The SSC describes the increased muscular tension created by tensile force on the muscle that can be used to increase concentric force production and is seen as the basis for plyometric training (Siff, 2003). Plyometric training is an explosive type of training comprised of aggressive jumping and hopping that aims to increase the tensile force on the muscle. It uses the myotatic reflex as a basis for increasing force production.
The myotatic reflex is another well-documented reflex arc. The myotatic or “stretch" reflex shortens muscle fibers after the muscles intrafusal fiber length has been rapidly increased or “stretched.” This process occurs through the gamma motor neurons. The explosive shortening through the alpha motor neurons of the larger extrafusal or muscle fibers helps regulate fiber length. The common denominator in all these training modalities is a muscular lengthening before shortening or “load” before “explode.”
We seem to have failed to make the connection between these types of training and the same mechanisms used in the basic premise of human movement. The muscular process of “load to explode” is constantly occurring and not just in an extreme context as the word “explode” would imply.
So, why do we load to explode? The answer lies in the body’s use of subconscious muscular information and stored mechanical energy.
Subconscious Muscular Information
Muscular activation is often seen as a conscious neurological innervation of motor units. If we want to perform a bicep curl, we consciously and concentrically activate the bicep to create elbow flexion. This is the basic concept of traditional gym training.
During function-related movement, however, our muscles are not activated consciously. The body has developed subconscious information mechanisms that activate muscles to create the force to move. As we walk, we do not create hip flexion followed by extension through conscious activation. All that laborious, time- and energy-consuming conscious activation would get in the way of crossing the road or talking on the phone! As previously noted, if we want to jump up, we must first go down. This is the opposite of our conscious intent, so there must be another way to create the intended movement.
Instead of conscious concentric contraction, our muscles respond to subconscious mechanical eccentric tension created by movement patterning. Our clever bodies have motor patterns ingrained that take movement out of our consciousness to find the most efficient ways to move.
The majority of the proprioceptive sensory organs in the body – from the fascia to the joints – are mechanoreceptors and therefore mechanical. They simply respond to the mechanical tension created by eccentric movement by sending afferent (towards) information to the central nervous system (CNS) for the appropriate muscular response, which generally initiates concentric movement in the opposite direction. And all this has happened while you were thinking about what to have for dinner!
Let’s use gait or walking as an example. As we step forward, ground reaction forces, gravity and momentum force our bones to create flexion at the ankle, knee and hip joints. This also helps decelerate the movement of our center of mass. The extensors or plantar flexors of the ankle feel the tensile stretch created by dorsiflexion and then create plantar flexion. This extends the knee and allows the hips to go through effective extension as the opposite leg goes through swing phase. This tensile stretch then creates load before the explode of the hip into flexion to swing again and repeat the cycle (Farley, 1998; Neumann, 2010).
Inside the muscle the muscles, spindles or intrafusal fibers along with the Golgi tendon organs respond to the increased length and tension of these movements. They send the information created through mechanical tension back to the CNS (gamma motor neuron) that then innervates the appropriate muscles fibers to create the opposite concentric shortening movement (Matthews, 1964; Orlikowski, 1973).
During a throw, the relative joint motions of abduction and external rotation at the shoulder along with scapulae retraction will eccentrically lengthen the lats, pecs, abdominals and some rotator cuff muscles. In response they create concentric shoulder adduction and internal rotation along with scapulae protraction to propel the ball.
This same opposite direction external motion and internal feedback loop of information occurs for the vast majority of functional movement. Any function that does not do this would be an exception rather than the rule.
Stored Mechanical Energy: A Reusable Energy Source
The harnessing of mechanical energy is another reason for a “load to explode.” We can store elastic energy through the passive components of the muscle complex, such as the tendons, as well as the active muscle fibers themselves (Cavagna, 1977). This happens as the passive and active components of the muscle go through the tensile stretch associated with the loading portion of a functional movement (Bosco, 1982).
We can then use the stored energy to contribute to the explosive and more energy expensive concentric muscular shortening. This means that we don’t have to use as much metabolic energy stored within the body. As any endurance athlete knows, if someone offers you some energy for free, take it!
For a more in-depth explanation of elastic energy, read Elastic Energy Storage, Part 1 and Elastic Energy Storage, Part 2 on PTontheNet.
Putting Theory into Practice
Research clearly supports the validity of the “load to explode” concept. Functional movement was the evolutionary result of the body’s requirement for subconscious and energy-efficient movement.
The question is, then, why is this not a widely used principle of movement and muscle function for both our assessment procedures and program design when training our clients? If we want to increase our personal training clients’ ability to move effectively and efficiently, we should incorporate more function-related movement principles such as “load to explode” into assessments and program designs.
One simple way to achieve this is by getting our clients to shift away from only moving vertically (up and down), which is prevalent in gym workouts. By moving our center of mass forwards and backwards and side to side, we have to decelerate movement, which in turn creates muscle lengthening or loading.
One great example of this is a multi-directional lunge. A good lunge should shift our center of mass over onto the leg we are stepping with rather than trying to remain to upright. A clear indicator of this is how much the front knee bends; too little of a shift towards the toes, and we will remain more vertical and transfer less of our mass to load the muscles for a subsequent explode back to the start position.
The co-ordinates (in degrees) in the diagram to the right will move the body through 3 planes of motion for effective multi-dimensional loading and unloading/exploding. This pattern can be easily used in a relatively confined space such as a gym.
Although movement assessment of our clients is beyond the scope of this article, we need to think about whether we are assessing our clients statically. Static postural assessments will not tell us anything about our clients' ability to load muscles dynamically, so a more dynamic approach to assessment would allow us to better evaluate our clients' functional ability or state.
Gait assessment is a useful tool in this respect. It is a universal human function that incorporates a load to explode cycle, transferring our mass, and is useful in showing asymmetrical patterns to the left and right half of our bodies rather than the front and back, as tends to be the focus in most gym-based postural assessment.
References
Bosco, C. et al. (1982). Store and recoil of elastic energy in slow and fast types of human skeletal muscles. Acta Physiol Scand, 116: 343-349.
Cavagna, G. (1977). Storage and utilization of elastic energy in skeletal muscle. Exercise & Sports Sciences Rev 4: 89-129.
Farley, C. & Ferris, D. (1998). Biomechanics of walking and running: centre of mass movements to muscle action. Exercise & Sport Sciences Review, 21 (1): 253-286.
Gray, G. (2004). Functional Video Digest Series. Gray Institute.
Matthews, P.B.C. (1964). Muscle spindles and motor control. Physiology Review 44: 219-288.
Neumann, D. (2010). Kinesiology of the Musculoskeletal System. Elsevier.
Orlikowski, Z. & Moncha, S.L. (1973). Muscle spindle. In Bourne GH: The Structure and Function of Muscle, Part 2. New York Academic Press.
Siff, M. (2003). Supertraining. Supertraining Institute. Denver.
Starling, E.H. (1918). The Linacre Lecture on the Law of the Heart. London, UK: Longmans, Green & Co.