The physical and tactical training of the military is designed to maximize operational performance capacity. The purpose of training regimens prior to mission engagement is to adequately prepare military personnel for the physical and psychological demands of their mission [1
]. To be effective, military personnel must possess enough muscular strength, power, and endurance capabilities to complete endurance marches across rugged terrain, swiftly maneuver under fire, and engage in close quarters combat, all of which may be sustained for long durations while carrying external loads [1
] (Figure 1
). Although tactical training strategies are designed to produce positive longitudinal adaptations, overtraining or chronic fatigue-driven decrements in operational performance can occur [4
]. Deeper insights can be achieved by objectively measuring neuromuscular performance (NP) capabilities, which encompasses monitoring: (1) physical readiness, (2) neuromuscular fatigue (NMF), (3) injury risks, and (4) adaptations to training cycles.
Objective monitoring of NMF, which may be defined as decrements in force generating capabilities, is a common practice in human performance sciences [5
]. In accordance with the long lasting and widely accepted Selye’s General Adaptation Syndrome (GAS), if progressive overloading is not properly managed, military personnel can expect to reach the physiological state of overtraining, as indicated by accumulated NMF [6
]. However, GAS suggests a minimum amount of training-induced NMF, sometimes referred to as functional overreaching, is necessary to initiate adaptations to prepare physiological systems for future high-level stressors. Yet, since there is an exceptionally high demand for physical readiness in tactical environments, military training frequently comprises bouts of high intensities and/or volumes of training in maladaptive conditions, which are often accompanied by high incidences of musculoskeletal injuries (MSKI) [8
]. The overaccumulations of NMF, which are present in military factions all over the globe, are inadvertently costing inordinate amounts of funding and resources, annually [10
]. Excessive NMF is a major contributor to overuse injuries that are rampant in military personnel [11
], such as muscle strains, joint sprains, and stress fractures [11
]. Consequently, mission success is directly impacted by the deleterious side effect from over accumulated NMF, such as decreased strength and power production, decreased cardiorespiratory endurance, and increased risk for MSKI [4
]. The MSKIs subsequently result in restricted operational capacities and increased compensatory workforce strain, provided other military personnel may then be expected to account for the absence (due to injury) of others [8
]. Consequently, it is imperative military groups strongly consider strategies for (1) objectively quantifying the acute effects of training loads and (2) track longitudinal responses to training stimuli to ensure positive neuromuscular performance (NP) adaptations and mitigation of injury risk [8
Fortunately, decades of performance science research supports a variety of effective strategies for reliably and objectively assessing neuromuscular performance under virtually any circumstance (e.g., regardless of budgets, personnel resources, or time availability). One of the most common, and arguably the easiest, strategies to monitor NP in tactical populations is countermovement jump (CMJ) testing [3
]. The variations of CMJ testing (e.g., tape measure, inertial measurement units, linear transducers, optical measurement systems, jump mats, and force plates) retain degrees of reliability for jump height estimates while allowing practical options for numerous occasions (e.g., widely affordable options based on budget), which explains the popularity in utilizing this testing strategy. The CMJ, a measure of lower body power output, is positively associated with occupational performances in tactical settings [25
]. Further, the CMJ has been used for neuromuscular performance testing in sport and tactical environments to identify performance adaptations [23
], resiliency to fatigue [35
], and risks for injury [37
]. Traditional jump testing is most simply conducted using a tape measure, vertically affixed to a wall. However, this method restricts practitioners to a single outcome measure, vertical jump height. Although this metric is generally useful, it fails to provide more valuable information on movement strategies (i.e., how the individual arrived at this outcome) and relevant weaknesses. For example, military personnel may exhibit the same jump heights before and after bouts of rigorous training, but the neuromuscular strategies used could be drastically different (e.g., countermovement depth, impulse). Thus, monitoring force-time metrics during testing may be more conducive for detecting NMF than the physical outcome (e.g., jump height) [38
Recently, due to increased availability of both hardware and software, more sophisticated procedures including force plates have been adopted by practitioners (i.e., administrators and testers, tactical strength and conditioning facilitators). The application of NP monitoring via force plate testing presents practitioners a multitude of software and testing application options. Most solution sets provide data outputs composed of hundreds of different testing metrics, leaving it up to the practitioner to choose which metrics to monitor. This blessing and curse often leave practitioners quickly inundated with mass quantities of testing data that is poorly understood and often improperly actioned compared to common singular outputs (i.e., jump height). Therefore, it is particularly important that practitioners planning on using force plates take great care in educating themselves on the best practices of force plate testing prior to their use as monitoring tools. Under the right circumstances, force plates may be considered the most reliable method of measuring vertical jump height [39
]. Additionally, due to the high sampling frequencies typically housed within force plate testing hardware, force-time data metrics are more sensitive (than jump height) to changes in the neuromuscular status of an individual, making them more effective NP monitoring strategies [40
]. In short, reliable and quantifiable NP monitoring can be obtained using force plate testing when utilizing best practices.
Force plates provide the ability to test a large variety of neuromuscular assessments for physical capacities of strength and power. For example, isometric strength testing (e.g., isometric mid-thigh pull, squat, or bench press) may be a more appropriate means to analyze maximal force production (maximal strength) than one repetition maximum testing, as they are relatively simple to administer, time efficient, have a lower potential for injury risks, and possess high degrees of reliability under the correct testing conditions [42
]. Thus, force plates provide the ability to monitor neuromuscular function more frequently and allow for testing of upper and lower body power and strength, all key components of performance monitoring in tactical settings. Therefore, this review has two purposes (1) describe how the use of force plate testing may improve physical training in the military, (2) explain the importance of using the correct applications, assessments, and metrics of interest for particular applications within tactical training of the military.
2. Defining Ground Reaction Forces
Biomechanics refers to the study of mechanics utilized during and resulting from body movements and is made up of kinematic and kinetic characteristics. The kinematics represent movement through positional and time data (spatiotemporal features), which in turn provides description of the movement’s position and velocity in respect to time. Kinetics refers to the interactions of forces and torques created in the body and those that may act on the body during movement (i.e., contact forces). Establishing an understanding of the forces during movement provides knowledge on how an individual achieves a desired outcome (jump height) and the effect of the forces (absorption in lower extremity joints during landing, Figure 1
) on the body. Thus, by assessing applied and resultant forces during movement, analyses from force plate testing can help to explain why and how the body moves in the way that it does. Here, the end goal is to optimize movement strategies, which can translate to enhanced operational readiness via improved physical performance capacity, and reduced injury risks.
Motion results through some combination of muscle forces and external resistance forces (although it is possible that no motion occurs, and the action is isometric). It is important to differentiate force from the term torque, as torque is referring to the application of forces resulting in a rotation of a body segment. However, the focus of this review will be on the resultant vertical ground reaction forces (vGRF; i.e., forces acting on the body from the ground), which are measured in Newtons (N). Force induced movement may primarily be explained by Newton’s three Laws of Motion [45
]. The first, Law of Inertia, states that an object in motion will stay in motion unless acted upon by a force. Thus, if someone is moving quickly in a sprint or falling from an obstacle, they must produce enough muscular force to overcome the inertia and controllably cease movement, making maximal strength and power valuable physical traits for military personnel. The second, Law of Acceleration, explains that the likelihood of an object to change speed or direction (acceleration) is a function of the object’s mass and force applied by it (Force = mass × acceleration). This law of motion is key for explaining the importance of force producing capabilities of tactical populations. If an individual improves leg strength and maintains body mass, they will improve their ability to accelerate and decelerate their body using their legs, which, for example, may be quantifiably observed by improved drop jump performances. The third, Law of Reaction, states that for every action there is an equal and opposite reaction, which explains the interaction between the muscle forces in the body and opposing forces of the environment. During a CMJ, the forces created by the legs against the ground will push back on the body and result in the body moving in the direction of the forces, in this case upwards. By combining these laws, it is understood that the amount of resultant forces dictates the movement characteristics that represent neuromuscular performance capabilities (i.e., greater forces created by an individual elicits a higher CMJ).
Although the amount of forces during the vertical jump may dictate the maximal height reached, there are many techniques to generate these forces that may result in the same jump height. Here, we consider the impulse (explained in detail later), which may be a result of a high amount of force over a short duration or a lower amount of force in a longer duration. Of course, vertical jump height is a valuable performance metric, but it may not be as sensitive of a metric for assessing changes over time compared to the force characteristics produced during the movement (known as the force-time curve) [35
]. It is also possible to quantify the motion, known as momentum, of an individual as the product of velocity and body mass. In short, a greater impulse will yield increased momentum and if mass remains the same, velocity, and subsequently jump height, would be increased (referred to as the impulse-momentum theorem).
In order to alter the impulse during movement, one may increase their ability to produce force through training both physiologically and technically. Although anatomical structures, such as varying limb length ratios, may influence the mechanics of movement, they cannot be completely altered. However, utilizing the most efficient and safest mechanics of movement may help to improve performance and reduce the risk of injury [30
]. In ergonomics, forces may be assessed to improve footwear designs or equipment load dispersion to improve the impact on forces and limb imbalances on movement quality, subsequently decreasing injury risks [46
]. Since military boots may alter movement kinetics [46
] it is necessary to standardize, as much as possible, the equipment used during testing. Using partial or full-kit military equipment during testing has altered movement mechanics and increased ground reaction forces and will more closely resemble that of real-world scenarios [47
]. However, these controlled environments may still have limitations to the transferability to typical military movements, as fatigue, vision impairment, and distractions from secondary cognitive tasks may have further effects on landing forces [48
]. Yet, as mentioned throughout this review, these force plate assessments provide valuable insight into muscular power and strength capabilities, which are closely associated to many occupational tasks within the military. Notably, force plate data are in reference to the body as a whole (i.e., center of mass) but can be broken down into right and left body segments via bipedal force plates. Thus, the use of bipedal force plates allows the ability to assess limb asymmetries during movements and provide another biomechanical metric to determine likelihood for injury and profiling of an individual. In summary, analyzing the forces of gross movement are a critical component to program effectiveness for neuromuscular performance monitoring.
3. Force Plate Functions
Before selecting the test of interest, it is important to understand the basic foundational principles of force plates to improve the quality of data received. Extensive developments in force plates contributed to more sophisticated builds that include either strain gauges or piezo-electric materials, which allow higher sampling frequencies, less deformation, and more reliable data. A detailed explanation of these devices may be found elsewhere [51
], but in short, piezo-electric sensors are more sensitive to detecting small rapid forces, while strain gauges are more conducive for measuring large forces particularly during longitudinal monitoring (due to the lower likelihood of signal drift). Specifically, force plates measure six variables representing reaction forces placed on the body, which equal the action forces applied by the body (i.e., Newton’s third law of motion). The ground reaction forces examined in the aforementioned tests are a cumulation of the reaction forces (Fx
, Figure 1
and Figure 2
), which are components of the four loading cells in each corner of the force plate. It is important to be aware of the direction of each axis (Fx
) as this is subject to change across force plate manufacturers. For example, the vertical ground reaction force and focus of this review may be identified in the Y or Z axis depending on the defining source (i.e., force plate manufacturers). Additionally, it is important to consider the force plate is utilizing action-reaction principles to display force-time curves. Thus, a downward force acting on the platform is recorded as a positive force output and vice versa, making it critical to verify the direction of the output.
When considering force plate testing in NP monitoring, it is essential to identify the equipment specifications needed to ensure accurate readings. For example, the strain gauge and piezo-electric sensors may better serve different purposes (athlete fatigue monitoring and profile testing versus finger dexterity). In addition, there are certain specifications force plate signaling conditions should meet, which are explained elsewhere in more detail [52
]. Ultimately, modern devices should be suitable for the purposes mentioned in this text, but practitioners should be aware of differences between devices. One concern is comparing results between different devices due to the influence of varying specifications. Additionally, to ensure reliable data, device installation should conform to manufacturer guidelines (i.e., not to place the platform on suspended floors) and practitioners should be aware potential problems may exist if these guidelines are not met. Location and use of the platforms may result in force artifacts during signal acquisition, which require post-process filtering to ensure accurate readings. For example, results will likely differ if testing takes place on a grass field compared to a level concrete floor. Although, the chief concern is ensuring identical data collection procedures across comparisons (or testing timepoints); it is also important to meet minimum specifications considering sampling rates below 300 Hz may underestimate jump height [39
Signals from the force plate voltage sensors are recorded through analogue to digital conversion hardware linked to the collection device (e.g., computer), using a 12 or 16-bit device. Typically, the 12-bit is adequate for up to 10,000 N, within limits of most sports force measurements, while a 16-bit converter may provide excessive sensitivity. Typically, this is not a concern for most force plate practitioners, as signal processing is completed by the manufacturer’s proprietary software. Of course, there is the ability to custom write software with appropriate skills and packages, such as MatLab (MathWorks, Natick, MA, USA) or Labview (National Instruments, Austin, TX, USA). However, it is important to understand manufacturer processing methods to ensure they meet testing requirements. For example, manufacturers or custom software will utilize different filtering and processing techniques, which may under- or over-estimate the force-time curve results. It should also be noted, compared to various cutoff frequencies of filtering processes, raw vertical force data seem to produce the least amount of error [39
]. Residual analysis via visual identification of a best fit line (that does not attenuate peak forces of various cut-off frequencies) is recommended for determining low pass filtering cut-off frequencies in custom software.
The force plate calibration process, which is often overlooked, must also be confirmed. Although relatively easy to use, force plates can be complex and require caution and attention to detail to ensure accurate data [54
]. Before the platform is shipped it is typically calibrated by the manufacturer, but this does not account for individual variations of signal noise due to software processing, installation, and local environment, as well as any changes to these variables over time. Calibration processes for vertical forces are relatively simple and normally consists of placing a dead known weight on the force plate, which has been utilized by many investigators [54
]. However, calibration of the horizontal forces is much more difficult to accomplish and, without verified experience, should be left to the manufacturer to reduce the risk of errors, as the magnitude and placement of the load used are critical [54
]. None-the-less, calibration of the vertical force axis is easily conducted and should be incorporated into testing protocol schedules.
5. The Force-Time Curve Considerations
Selecting the right variables form the force-time curve is of upmost importance. Typically, the forces, as either mean, net, or peak forces, and duration of movement phases are of interest. Additionally, some variables may be derived directly from this force-time curve, such as impulse and RFD. The impulse of a movement refers to the area under the curve and is now typically calculated with integration techniques using the trapezium rule [39
]. This variable is most often useful when a movement is broken down into phases, such as the braking and propulsive phases of a CMJ or landing phase of a drop jump. As previously mentioned, this may help explain different techniques for obtaining a similar outcome of maximal jump height, for example. The RFD is typically of interest to exercise biomechanists as many sport tasks occur in very small timeframes which require forces to be developed rapidly. Conversely, the rate of loading during the drop jump landing may be of interest and pertains to an individual’s ability to absorb forces during rapid movements, such as landing from a vehicle or obstacle. In either case, there are numerous ways to calculate RFD. One method is calculating the average slope between two arbitrary time points, be it a percentage of peak force (PF) or time bands from initiation of movement (i.e., 0–200 milliseconds). Another method is differentiating the force-time curve and computing an instantaneous value with the highest rate of force development between two time points along the force-time curve. Some have found RFD to be more sensitive to changes in NP than measures such as PF during an IMTP [69
]; however, variables are typically more trustworthy and sensitive and to changes over time or comparisons between individuals when the metric is reliable.
5.1. Selecting the Right Variables
The number of variables that may be analyzed from a force-time curve is quite spectacular and, as a result, many manufacturers will provide virtually all of them, which most times is more than necessary. Unfortunately, this can easily lead to confusion for both the practitioner and tactical personnel and lead to poor understanding and compliance. Fortunately, there are a few key strategies one may use to help guide them to selecting the best variables of interest for a given test. In general, the variables selected should be easy to digest and independently informative. In other words, the variables have face validity (are a depiction of what is measured by the eye) and each tell a different story with minimum overlap among them. A good example of this is peak velocity (which typically occurs near take off) and jump height during a CMJ. Typically, jump height is calculated using take off velocity, which means the variables are highly correlated, and therefore tell the same story. However, it is easier for someone to comprehend what jump height means compared to peak vertical velocity. There is an added bonus to analyzing jump height, as the individual being tested will aim to achieve the highest jump possible and ensures that all the data obtained is from maximal effort attempts. This is a critical component to testing since a lack of effort on a given test will significantly disrupt efforts to monitor NMF and NP adaptations. This is a point of emphasis for both the practitioner administering the test and the tactical personnel performing the test: maximal effort must be ensured by all concerning parties prior to testing to ensure the data is in fact reliable and valid. The maximal height achieved during a jump is also a metric that may be easily calculated from other tools such as jump mats, vertecs, or a simple measuring tape on the wall, which is important if force plates are not available during certain times. However, as mentioned previously, these tools will not give insight into how that jump height was achieved.
Additionally, it is important to remove unreliable variables from the analysis that are subject to error. This is an easy way to reduce the numerous variables from your force plate data outputs while also ensuring the metrics being monitored are sensitive to change (fatigue or adaptation), which will make monitoring NP much more effective. The next factor to consider are the variables that directly influence the CMJ performance and neuromuscular function, which as previously mentioned will be the forces applied to the ground and all of its components. In other words, consider the peak or mean forces, the net forces, or the impulse during the entire movement or select phases of the movement. Further, the strategy one uses to achieve these forces may be useful to track. For example, the time taken to deliver the net mean forces during the braking phase of a CMJ provides insight on how long it took to descend before initiating the ascent and take off to perform the jump (longer eccentric duration that may be suggestive of NMF). From the force-time curve, impulse is calculated and is the driving factor of the movement. However, the same impulse may be achieved by having more mean force spanning less time (ideal; more explosive) or less mean force across more time. Therefore, net force, duration, and impulse are typically the most influential factors to jump performance and its associations with NP.
5.2. Reliable Metrics and Reliability Testing
No matter the force plate test, the key to obtaining reliable data is to have each test be performed under the same conditions across time points. Any lack of consistency in testing procedures increases the likelihood of Type I (false positive) and Type II (false negative) errors, which further lends itself to poor neuromuscular performance monitoring because of increased difficulty interpreting intraindividual changes over time or interindividual comparisons. To determine reliability, often, the intraclass correlation coefficient (ICC) is used as the degree of consistency and agreement between two sets of data; however, it does not account for systematic errors that may result in correlated data sets to be non-repeatable. Thus, it is important that the coefficient of variation (CV) is also reported to provide the typical measurement error [71
]. For force-time characteristics there is no minimal acceptable thresholds for these values, but acceptable reliability is typically considered as an ICC ≥ 0.80 and a CV ≤ 10% [72
]. More robust procedures would also include the confidence intervals (95% is most common) around the reliability measures to indicate the range of reliability found during testing, since the limits above and below the average value may not be acceptable.
For RFD, time bands of 0–50, 0–150, and 0–250 milliseconds are more reliable than the peak RFD or average RFD. The time bands are calculated as the change in forces divided by the time (50–250 milliseconds), while the peak RFD is the highest rate of force development occurring at a sampling window. Average RFD is calculated as the peak force divided by the time to achieve peak force. The issues lie in the fact that the time bands are so small for the highest RFD that it is likely to vary quite a bit and/or be attributed to artifact. However, if the slope of the force-time curve is steeper in the beginning of the movement, more total net forces are achieved in that duration and thus, the total impulse would also be increased. Furthermore, impulse is a more reliable measure and assessing various time bands from 0–100, 0–200, and 0–300 milliseconds are recommended over peak or average RFD over the entire IMTP effort. There have been other calculations for isometric testing, such as index of explosiveness, reactivity coefficient, S-gradient, A gradient, but have been found to be particularly unreliable [42
]. Consequently, for isometric strength assessments it is recommended to analyze the peak force of the entire movement and impulse at epochs from 0–100 to 0–300 milliseconds, as these are the most reliable measures and most likely to accurately detect change in strength metrics over time.
5.3. Conducting Multiple Versus Single Trials
Further, it is recommended that multiple trials of each test be recorded if time permits. Although a singular trial may be sufficient for monitoring NMF if multiple trials are not feasible [23
], an individual’s performance may increase from trial 1 to 2, as the musculature potentiates from the previous trial. Yet, the maximal effort of an individual is often difficult to determine and if maximal effort is not always provided the recording and analysis of the maximal effort strength and power tests over time may not be accurate. For this reason, others have found that the average values are more appropriate for monitoring NMF and performance adaptations [73
], while others have found similar results between the highest and average jump height performance [74
]. From a statistical standpoint, the average of scores is also more likely to be the truer performance result, and when monitoring NP it is essential to obtain “true” scores to accurately detect change [36
]. Thus, multiple trials allow for in house testing of reliability checks, as well as average trial results which typically are more sensitive to changes in neuromuscular performances [36
]. Lastly, if a single trial is used, it is strongly encouraged that standardized and supervised warm-up procedures are used including multiple jump trials prior to collecting data on a single trial to improve the test’s validity.
In the military, it is necessary to ensure progressive overloading is properly managed, which we now know is possible through controlled, objective monitoring of training loads and physical capacities, to improve performances and reduce the risk of injury. Due to the information provided in this review, it is recommended that force plate testing become a routine part of standard operating procedures in the military for NP monitoring and additionally for both acute and chronic NMF. Practitioners will then be able to engage in evidence-based practices to more effectively prescribe training loads over time so that physical characteristics such as explosiveness, maximal strength, and endurance are at the very least maintained, if not ideally enhanced, over the course of military personnel’s tenure. However, the complexity of force plate testing comes with required necessary precautions to ensure the monitoring procedures in place are reliable and faithful enough to make sound decisions. In conclusion, even though force plate testing may be a beneficial objective tool for monitoring, there is still a subjective nature to the quality of data that relies on solid preparation of the testing protocols, coaching and cueing to confirm maximal efforts, and selection of reliable and practical metrics that can be easily portrayed.