Introduction 

Managing daily Neuromuscular fatigue in strength and conditioning can be paramount to the success of any  program targeted at maximizing athletic performance and overall adaptation. As a human performance professional, there are a great deal of variables to consider and account for when  aiming to optimize an athlete’s overall performance. Those variables tend to provide a delicate  and ever-changing balance between the athlete participating in the sport, sport specific skill  practice, strength and conditioning work, all in addition to daily life stressors. Not only this, but  each athlete has a unique response to every stressor, thus making fatigue monitoring a relative necessity. With that, there are many methods that can be utilized to assess and monitor an  athlete’s readiness and fatigue. While there are a vast number of ways to monitor athlete fatigue,  a few that are very common within sports science are, but not limited to: neuromuscular fatigue  monitoring (NMFM) thought force plate assessments, central nervous system readiness via direct  current potential (DC Potential), as well as monitoring and tracking heart rate variability (HRV),  resting heart rate (RHR), and recovery or readiness scores from various pieces of wearable  technology (Casey et al., 2017; Coyne et al., 2020; Heishman et al., 2018, 2020; Merrigan et al.,  2020). While each of these techniques have their respective place in sports science and athlete  monitoring, the primary focus going forward will be on neuromuscular fatigue, what it means,  how it can inform decision making, as well as how it can be applied by sports scientists and  strength and conditioning professionals alike.  

Background 

Neuromuscular System 

For a brief and extremely oversimplified review, while staying relevant to the topic at hand, the  neuromuscular system is an intricate system within the human body, in which provides the  connection of the brain and spinal cord (central nervous system, CNS) to both muscles, as well  as the nerves that innervate them (peripheral nervous system, PNS) (Faude and Donath, 2019).  Within the human performance realm, the neuromuscular system can be seen having an  overabundance of functions, such as driving and controlling overall movement, coordinating  central motor drive, reflex activity, excitation-contraction coupling within skeletal muscle, as  well as providing both the coordination and integration of sensory feedback in the generation of  movement (Faude & Donath, 2019). More specifically, a highly functioning neuromuscular  system can facilitate adequate or enhanced levels of force production within a respective task,  coordinate movements to protect bodily structures, as well as allow for the maintenance of  stability by way of allowing for the human organism to maintain a stable posture in the presence  of potential change (Faude & Donath, 2019). For the sake of brevity, understand that there are considerable layers of complexity and intricacy of the neuromuscular system, the likes of which  this paper will not cover in its entirety. 

Readiness and Fatigue 

Continuing to delve deeper, understanding the meaning of both readiness and fatigue are vital to  the application of NMFM. In this context, fatigue can be loosely defined as the reduction of an individual’s ability to produce force, and or perform work, thus resulting in a decrease in 

performance (Heishman et al., 2018). To further expand the meaning of fatigue within human performance, low-frequency fatigue activities take center stage. When an athlete partakes in activities that involve high-intensity movements, repetitive eccentric movements of moderate-to high force, or motions that involve a repeated stretch-shortening cycle, there has been a reported potential decline in performance over time if fatigue is not identified, quantified, and addressed  correctly (Heishman et al., 2018). If programmed incorrectly, these movements can accumulate  throughout a day, week, or month, and the athlete will begin to show clear signs of fatigue, as  higher intensity movements will drive high amounts of fatigue. The importance of this, as it  relates to readiness and fatigue, is that within this context it provides a basic framework to  understanding fatigue, and ultimately will provide insight as to how to establish athlete readiness within human performance.  

As with every strength and conditioning program, the main goal is to provide a stimulus, then  allow for adequate recovery, all in efforts to elicit adaptation, and a subsequent increase in  performance (Gabbet, 2015; Heishman et al., 2018; McFarlane, 1985; Mooney et al., 2013).  With this as the main goal, it is important to ensure that the athlete maintains the highest  respective level of readiness to continue to participate in the daily training (Heishman et al.,  2018; Watkins et al., 2017). Readiness has enumerable ways of being calculated, however,  remaining with the topic of NMFM and readiness, one of the most common methods is through  the counter movement jump (CMJ), which has been found to be both a valid and a reliable measure of neuromuscular fatigue, and overall athlete readiness (Alba-Jimenez et al., 2022;  Claudino et al., 2016; Heishman et al., 2018, 2020; Merrigan et al., 2020; Watkins et al., 2017).  In application with NMFM, through the CMJ, the human performance professional can gain  insight into a quantitative analysis of both the readiness and recovery of the athlete, in addition to  showcasing potential quantitative adaptations to the program (Heishman et al., 2018; Merrigan et  al., 2020; Wu et al., 2019). Moreover, predictive models can be established with the CMJ, which  can ultimately offer prospective insight into central and metabolic fatigue (Wu et al., 2019). 

Monitoring 

Neuromuscular Fatigue Applied 

With the background and reasoning established to NMFM, administering the CMJ with various  pieces of technology is a clear next step. A frequently utilized piece of sports science technology  are force platforms, otherwise known as force plates / force decks (Alba-Jimenez et al., 2022;  Heishman et al., 2020; Merrigan et al., 2020). Although they may not always be feasible, force  plates can offer an extensive insight into the force and time characteristics of the athlete’s movement. For some, other pieces of technology like jump-apps, contact mats, and so forth, may present to be more feasible methods of tracking this over time. However, this paper will  now aim to provide the human performance professional with a few examples of what to look for  from their respective force platform software, in addition to what can be done with the given  information.  

As with most things within sports science, there are enumerable methods to come to the same  relative conclusions, and broadly speaking, no one method is particularly more correct than the 

other, when used correctly. Despite this, there are a few methods that are valid and reliable, and  utilized by much of the field of human performance professionals who are tracking readiness,  NMF, and performance overtime. Within the CMJ, depending on the application, there are an  abundance of tracking methods. Commonly, when looking into human performance, the  following metrics are often analyzed over time for NMFM and readiness: Jump Height, Reactive  Strength Index Modified, Flight Time:Contraction Time, Concentric Rate of Force Development,  Eccentric Rate of Force Development, Eccentric Duration, Force at Zero Velocity, Peak Landing  Force, Peak Landing Force Asymmetry, as well as various other relevant metrics (Alba-Jimenez et al., 2022; Heishman et al., 2018, 2020; Merrigan et al., 2020; Watkins et al., 2017). While  there are many ways to track CMJ metrics and fatigue, finding one that works best for one’s  respective population is key to establishing standardization and norms to compare against. Once  the metric(s) in which to track have been determined, then deciphering what they mean, and  what to do with them is of clear importance. Following a consistent test protocol, such as  providing the CMJ test prior to activity, and requiring the athlete to do 3-5 jumps within the test,  can provide the human performance professional an overabundance of foundational data of which will assist in making an informed decision (Heishman et al., 2020; Wu et al., 2019). 

For a practical example, suppose a facility or team comes to agreement that they will monitor  neuromuscular fatigue through the CMJ, with 3 jumps per test, with hands on hips, and they  determine that they want to examine metrics from the peak jump within each testing bout. So,  over the long term, the team determines that they would like to track Jump Height (impulse  momentum, in.), Reactive Strength Index Modified (jump height/time to takeoff), and Eccentric  Duration (ms). From these metrics, Jump Height provides a peak height of the jump, showing  the outcome of the athlete’s ability to absorb, redirect, and generate and exert maximal force and  power output in a jumping pattern. Moreover, Reactive Strength Index Modified (RSI-Mod.)  showcases a ratio between the athlete’s jump height, and their time to take off (Bishop et al.,  2022). In other words, RSI-Mod. would show how high the athlete got in the air, compared to  how quickly they were able to leave the ground from the point where they went into the braking  phase and propulsive phase, from their unweighting phase of the CMJ, which in translation  showcases their elastic and explosive capabilities to both quickly absorb and develop force.  Eccentric Duration is a measure of the amount of time in which the athlete applies the  metaphorical “brakes”, or how quickly they can slow themselves down, ultimately to redirect the  force concentrically within the propulsive phase to jump vertically. In conjunction with one  another, these respective portions of analysis in that of, Jump Height, Reactive Strength Index  Modified, and Eccentric Duration can provide plenty of information for the performance  professional to make data-informed decisions with.  

With these metrics in place, the team can track them over time, and in this case, compare the  athletes’ outcomes against their previous test trials. From there, the team can establish a criteria in which to make decisions. For instance, if an athlete’s eccentric duration is >20% of what their  normal eccentric duration is, or over what their previous session was, which would be a clear  indicator of neuromuscular fatigue, what decisions should the human performance team make  based upon that? Context is key within these situations, as some human performance teams may  choose to cut training that day, whereas others may choose to cut volume or intensity by a

predetermined percentage. In the end, establishing and following a standardized protocol with  sound reasoning based on the presented data is a potential way to safeguard the athlete from  potential injury, or at a minimum, ensure that they are in the best possible position to execute the  plan of the day within training (Alba-Jimenez et al., 2022; Merrigan et al., 2020; Wu et al.,  2019).  

Conclusion 

The neuromuscular system is an intricate and integral part of human movement, the likes of  which can be examined within readiness and neuromuscular fatigue measures, such as the  Counter Movement Jump. Upon completion of the Counter Movement Jump, with the assistance  of proper technology, human performance teams can track and measure interindividual  neuromuscular fatigue and readiness for daily tasks or activities. Establishing valid and reliable  metrics that can be examined over time, along with instituting what the team is to do with the  data once it is gathered, is paramount to the athlete’s success when measuring fatigue and  readiness. In the end, neuromuscular fatigue monitoring and readiness tracking, through  movements like the Counter Movement Jump, can provide human performance teams with  plenty of information to make data driven decisions on go- or no-go criteria, should they so  choose, for their athlete to not only adapt at the most effective rate, but also potentially contribute to injury prevention.

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