Do you know what the nervous system pays attention to? It pays attention to things that are interesting. Therefore, from a programming perspective, activities that stimulate, challenge and engage both the body and the mind can be particularly beneficial. One approach to creating novelty in your programming is by incorporating reactive training and its secondary benefits, which are referred to as reactive variability. 

What Is Reactive Training?

Reactive (adjective): Readily responsive to a stimulus

Reactive training challenges the body to successfully respond to imposed stimuli with minimal or no conscious thought, with the fastest response time for the purpose of:

  • Maintaining postural equilibrium
  • Improving timing, coordination and force production of muscular contractions
  • Providing optimal joint stability

The execution of all forms of reactive training is based on the function of the sensory motor system. First and foremost, this system determines the current state of the body at the time of receiving the stimulus. This includes joint angles, muscle lengths and tension, acceleration/deceleration of the body, available ground reaction forces, surface friction, visual input and vestibular states. 

The sensory motor system uses all of this information to elicit an appropriate response. In other words, it works to:

  • Maintain or regain your center of gravity over your base of support and achieve postural equilibrium for control or preparation for the next movement or stopping a movement
  • Determine the rate at which the needed muscles fire in harmony with the other involved muscle groups, and all with the necessary force for a smooth and coordinated movement
  • Simultaneously ensure the deeper, more intrinsic muscles provide the necessary joint stability for the required force production and safety 

These motor responses are continuously received by the sensory system; the motor system uses these ongoing updates to modify or produce motion and/or stability. The feedback loop is continuous, operates largely through spinal reflexes and is outside of conscious thought. This is good news, because conscious thought is slower than the body’s spinal reflexes and would interfere with the motor system optimizing the responses necessary for fast, smooth, coordinated movements. 

The speed of spinal reflexes and their associated muscle contractions are what protect the joints from harm by stabilizing them or resisting external force. Multiple studies have determined that contractions slower than 32−110 milliseconds respond too late to protect a joint from damage against external forces (DeMers, Hicks and Delp, 2017; Wagner et al., 2012).

Conscious vs. Unconscious Reactive Training

As a health and exercise professional, you may be familiar with forms of reactive training that utilize various lighting systems that flash on and off and require a touch or wave over to record your response. Others involve responding to visual or verbal cues from a coach for directional changes during running or footwork drills. Drills or activities that require a rapid response to verbal, visual or tactile cues involve conscious processing of the information prior to the reaction. Conscious reactive training is slower because the information must travel to higher levels in the brain where a decision for an output must be made and then executed. Although slower, it is essential in many day-to-day activities as well as sport. 

A baseball pitcher who has a line-drive hit directly at him has to get his glove on the ball to protect his face just before it hits him. To do this, he must first see the ball, determine its trajectory and even approximate the speed at which the ball is traveling to move the glove from where it would normally land during his follow through. This is an example of conscious reactive training, which is much the same as reacting to cues from a lighting system (as described above). 

In this example, unconscious reaction occurs at all the muscles that stabilize the joints, from the hand, through the body and all the way to the feet, for the purpose of stiffening the wrist and arm at impact. As the ball contacts the pocket of the glove, and vibration and force are sensed via the proprioceptive pathways in the hand and arm, there is a reflexive increase in stiffness of the hand, elbow and shoulder joints to keep the glove on and fixed with impact. The information to stabilize with impact does not have to go to the brain to be processed before a response occurs. It need only travel to the spinal cord and back, skipping the higher-level decision-making process. 

In this case, both the conscious reaction and the unconscious reaction were critical for the pitcher to execute the defensive move with his glove. 

Now consider the exact same scenario, but the person on the pitcher's mound is a 65-year-old sedentary client with mild cognitive decline. Her ability to successfully perform the safety measure both consciously (cognitive processing) and unconsciously (motor system’s execution) in time to avoid injury is likely compromised. Reactive success is based on both the speed of interpreting the stimulus and the speed of executing the response. 

Use of Unstable Surfaces

The use of unstable surfaces in fitness is an excellent example of reactive training. Although many health and exercise professionals utilize liable surfaces (e.g., BOSU, Core-Tex, Airex pads) in their programming for balance or core work, they are, in fact, utilizing the broader principles of reactive training. 

"The use of unstable surfaces in fitness is an excellent example of reactive training."

By their nature, unstable surfaces create a variable environment. The tools themselves aren’t unstable—they simply reflect the perturbations of the user. For example, the movement of the Core-Tex can be described as a continuous “pitch and catch” between the user and Core-Tex. 

The straight-arm plank is an exercise that demonstrates the functional value of this description. When an exerciser engages a Core-Tex by placing his or her hands on the platform (the “pitch”), the Core-Tex responds (the “catch”) by moving and reflecting where the majority of the exerciser’s force or weight has been applied. Because of this responsiveness, the user must unconsciously react. At a minimum, reaction(s) that occur are the necessary stability requirements (from toes to wrists) and possibly with a counter movement (another “pitch) where the user brings Core-Tex back to center or purposefully in another direction (see video, below). The Core-Tex reacts again to the purposeful counter movement or overcorrection by the user and the interaction continues. Therefore, each repetition is different than the previous one. As such, the motor system learns to create “solutions” for a much broader set of scenarios and the proprioceptive system is challenged to potentially improve its sensitivity and conversion of the mechanical signals from the receptors at the periphery.  


For example, if the environment continually is changing during this pitch and catch, a host of things throughout the body must change as well, including joint angles, the mass and momentum of the body, and the muscle synergies necessary to maintain postural control of the body as a whole and stability at the joints. This reactive variability also works in the body’s favor by avoiding loading the same structures (joints and tissues) over and over; instead, the stress is dissipated over a wider area. This response reactively trains the proprioceptive system to be more discriminating in its calculations of the input and trains the neuromuscular system to be more efficient in its responses.

By contrast, consider a static straight-arm plank performed on the floor. Here, the singular problem can easily be solved with one fixed strategy and the challenge is limited to strength and/or endurance based on the hold time. This is not to say a static plank is not of value, but it offers fewer functional benefits than a plank performed on an unstable surface. 

What Is Reactive Variability?

The Greek philosopher Heraclitus wrote “no man ever steps in the same river twice.” He was referring to the ever-changing world around us and the variability that is constant in nature. This same variability exists in the way we physiologically, biomechanically, neurologically, cognitively and emotionally interact with the world around us, all day, every day. 

Activity of all types and levels of difficulty have at least one thing in common: variability. Variability has been shown to be extremely beneficial to biological systems. From heart rate to nutrition, variability gives the body opportunity to prepare, to progress and to thrive.

Variability moves along a continuum from subtle to extreme. With movement, it encompasses the physiological, neurological, myofascial and skeletal systems. Something as familiar as walking has variability of these systems between every step. Sports such as basketball or martial arts, for example, feature a high degree of variability demands. 

Reactive variability involves applying the principles of reactive training to continuously unpredictable environments that produce ongoing novel inputs and outputs. These environments may include the surface we stand on or the tools we use. Programming for clients also includes both intra-exercise and inter-exercise variability. Intra-exercise variability is the variability occurring within multiple reps of the same exercise. Inter-exercise variability is the programming of exercises and environments that minimizes the repetition of similar movement patterns within or between sessions. 


With intra-exercise variability, even if the final outcome looks identical after 10 repetitions, the path the body took to get there would be different. This has been demonstrated in multiple studies with high-level performers in a variety of sporting activities, including pistol shooting and free throws (Nakano, Fukashiro and Yoshioka, 2018; Scholz, Schöner and Latash, 2000). These experts are capable of following many different paths to the same outcome. Novices will also show a great deal of variability but will be unable to consistently achieve the desired outcome. For the novice, this is not inherently bad; rather, it can be viewed as an opportunity for the motor system to discover multiple solutions to the same or similar movement scenarios. 

For this reason, the human body and the motor system that drives output must have what some have referred to as motor dexterity or motor creativity. In this context, “dexterity” or “creativity” is not related so much to the movement or posture, but how the motor system adapts to the environment or circumstances. 


For example, consider a drill where the client runs directly at you. When he or she is about 10 feet from you, you point either left or right. The client must cut off the outside foot, turn and run perpendicular to his or her original path. Now consider the differences if this drill is done on:

  • A basketball court
  • Rubber gym flooring
  • Gravel
  • Wet grass
  • Deep sand
  • Ice

Now imagine you were watching a video of a client performing this drill. However, you can only see the client from the ankles up and you are unable to see the surface on which he or she is running. Unbeknownst to you, while running toward you the client was on gravel, but when cutting left he or she was on ice and when cutting right he or she was on rubber gym flooring. Would there be a difference in the movement strategy the client used for each surface? Absolutely—and he or she would require the necessary movement dexterity while transitioning from one to the other. 

Programming for reactive variability for your clients does not require such dramatic scenarios. Begin by incorporating tools and cues that promote reactive responses in a variable environment into your clients’ training programs.


In this ever-changing world, the ability to react quickly and decisively is essential. Consider this quote from an article on complex living systems, published in the Journal of the Royal Society Interface: 

“…a reactive system does not seek equilibrium, has no set point and no state of rest. A reactive system holds itself together as a system just by reacting. A reactive system succeeds not by reaching homeostasis; a brain in homeostasis is clinically dead. A reactive system succeeds by being both robust and resilient. The reactive system responds to simultaneous perturbations and continues to survive; thanks to its reactive dynamics” (Cohen and Harel, 2007). 

Reactive variability supplies the exposure necessary to create an adaptation to, or a preparation for, the constant change encountered every day in life and sport. Acute injury rarely occurs in the movement zone that is trained week in and week out. Rather, injuries typically occur in the zone to which the individual has not been exposed. Repetitive injuries, on the other hand, are the exact opposite of variability. They occur due to lack of exposure to variety in the movement zone. 

Reactive training and reactive variability are elements of your programming that have evidence-based benefits. They also provide an opportunity to engage the movement interests of our clients as well as interact with them more from a coaching perspective. As we view health and fitness through a much broader lens, we realize that motivation, compliance and traditional metrics (strength, cardiovascular capacity, body composition, etc.) can be achieved in many ways. Reactive training and the associated benefits are an aspect of health and wellness that should not be overlooked and can easily be implanted as part of any fitness program. 


Beard, D.J. et al. (1993). Proprioception after rupture of the anterior cruciate ligament: An indication of the need for surgery? Journal of Bone and Joint Surgery Bone Joint Surgery, 75, 2, 311−315. 

Cohen, I.R. and Harel, D. (2007). Explaining a complex living system: Dynamics, multi-scaling and emergence. Journal of the Royal Society Interface, 4, 13, 175–182

DeMers, M.S., Hicks, J.L. and Delp, S.L. (2017). Preparatory co-activation of the ankle muscles may prevent ankle inversion injuries. Journal of Biomechanics, 52, 17−23.

Nakano, N. Fukashiro, S. and Yoshioka, S. (2018). Variability of release parameters in basketball free throwInternational Society of Biomechanics in Sport, 36, 1.

Scholz, J.P., Schöner, G. and Latash, M.L. (2000). Identifying the control structure of multijoint coordination during pistol shooting. Experimental Brain Research, 135, 3, 382–404.

Wagner, H. et al. (2012). Movement variability and skill level of various throwing techniques. Human Movement Science, 31, 1, 78−90

Wojtys, E.M. and Huston, L.J. (1994). Neuromuscular performance in normal and anterior cruciate ligament-deficient lower extremities. American Journal of Sports Medicine, 22, 1, 89−104.