Neuromuscular testing and profiling in predicting readiness to return to sport following ACL reconstruction
It is prevalent in team sports such as football, which requires rapid deceleration and change of direction, as well as individual sports where the knee joint goes from almost full extension to full flexion at high speeds and within the space of milliseconds, such as alpine skiing, where racers compete in an unpredictable environment with repeated bidirectional turning composed of forceful concentric but predominantly eccentric movements that elicit near maximal levels of lower body muscle activation (Jordan et al., 2015). Even though ACL reconstruction surgery now has a much higher success rate than previously, the success of the return to sport (RTS) process depends highly on individualization of the rehabilitation programme and regular monitoring of lower limb neuromuscular function. Functional symmetry in lower limb strength and power seems to be an important key performance indicator (KPI) for RTS, as reinjury rates seem to be significantly reduced after 9 months post-surgery and after the injured limb has achieved more than 90% quadriceps strength relative to the uninjured limb, respectively (Grindem et al., 2016).
With the increasing accessibility of dual force plate systems, it is now possible to monitor several parameters of neuromuscular readiness separately for the injured and uninjured limb, thus providing objective data on functional lower limb asymmetry, which can aid practitioners in making decisions on when to progress an injured athlete to a new stage of rehabilitation and ultimately when to return to competition in their respective sport.
VERTICAL JUMP ASSESSMENT AS A GUIDE TO NEUROMUSCULAR STATUS
Assessing vertical jump performance and strategy is considered a standardized and repeatable method for evaluating neuromuscular readiness and monitoring strength and power abilities in high-performance athletes (Coutts et al., 2007; Gathercole et al., 2015a; Gathercole et al., 2015b). Vertical jump testing may be carried out on a weekly basis and considered alongside measures of training load and athlete wellness to evaluate athlete readiness (Coutts et al., 2007). There have been recent studies involving world-class athletes where dual force plate systems have been used to assess lower limb kinetic (force) asymmetries in athletes returning from injury, (Jordan et al., 2015; Jordan et al., 2017), and to detect injury susceptibility in non-injured athletes (Jordan et al., 2015). Despite practitioners applying numerous testing methods in elite performance environments, dual force plate systems have been established as a gold standard in neuromuscular testing, particularly as they allow the practitioner to identify the force signature separately for both limbs, which provides valuable insight on asymmetry as regards production of force and impulse during the concentric and eccentric phases of a jump, respectively. A review by McMaster et al. (2014) in Sports Medicine provides a nice overview of various testing methods, and their reliability/validity.
How an athlete applies force may be the single most important variable to evaluate in the context of assessing the strength and power abilities. In fact, barbell and system mass velocities assessed with accelerometry or linear position transducers are a result of how the athlete applied force. This is given to us by Newton’s second law of motion (F=ma) and the impulse-momentum relationship (Impulse = Mass * the change in Velocity). Until recently, force assessments were uncommon in the daily training environments of athletes. Force plates and force sensors were typically found only in laboratories at universities or colleges. However, this technology is now becoming increasingly more available to coaches, as well as software that allows for easier interpretation of data. As the velocity and acceleration of an object are governed by force application and the fundamental laws of physics, force assessments are perceived as a gold-standard measurement among biomechanists and scientists. There also may be invaluable insights obtained for evaluating not only how much but also how an athlete applies force (as outlined in the graphics below, adapted from Jordan et al., 2015). This concept distinguishes the performance outcomes from the movement or force application strategy, which may be important for injury prevention, monitoring athletes after injury and in sports were producing as much force as possible is not the main driver of performance, including team sports such as football. Jordan quotes the period of time when an ACL injury occurs in skiing to be as short as 60 ms, which means that the rate of force development during an athlete’s jump strategy could be much more important than performance outcomes such as peak force and jump height, respectively.
Therefore, when testing, profiling and monitoring athletes that are returning to sport following an ACL injury, the time to peak force, jump phase duration, and concentric to eccentric impulse ratio during a countermovement jump could all be used as markers that guide the decision to advance athletes in their rehabilitation programme and ultimately clear them to return to their respective sport.
Force-based assessments can include dynamic or ballistic movements and isometric actions. While force application in these movements is easily quantified with the help of tools such as dual sensor force plates, more specific measures of force application such as rate of force development (RFD) may exhibit high variation, thus compromising the ability to detect meaningful changes in performance. Once regarded as a common means of testing, isometric testing often receives scrutiny due to a suspected lack of specificity to dynamic movements found in sport. However, high correlations between performance in ballistic movements and isometric RFD have been found (Haff et al., 2005; McGuigan et al., 2010). Nevertheless, isometric testing enables the coach to assess the strength curve or the torque produced throughout the range of motion of a joint (Kulig et al., 1984) and also provides a more reliable measure of RFD (Maffiuletti et al., 2016). Isometric RFD may also serve as a valuable neuromuscular monitoring metric, as it appears to be highly sensitive to fatigue and muscle damage (Jenkins et al., 2014; Penailillo et al., 2015). Moreover, isometric testing allows for objective testing to be conducted on athletes returning to the sport in their early stages of rehabilitation (Bousquet et al., 2018). Dual sensor force plates allow the practitioner to determine the rate of force development and impulse for both limbs individually, which provides an objective parameter in assessing whether the injured limb is within the required strength and force development levels of the uninjured limb. Whilst RTS protocols are normally designed to return athletes to sport within 9 to 12 months post-surgery, research has indicated that significant lingering asymmetries in the rate of force development can persist in the injured limb for as long as 24 months post-surgery (Jordan et al., 2015). It seems that achieving a goal of at least 15% (or less) of functional lower limb asymmetry significantly reduces the risk of re-injury of the ACL and this could be a benchmark the practitioner should chase when assessing neuromuscular abilities in tasks such as jumping (Myer et al., 2006).
ESTABLISHING A LOWER LIMB ASYMMETRY INDEX
In a study of elite skiers, Matt Jordan found significant lower limb asymmetry between ski racers with and without ACL reconstruction, leading him to term the phrase kinetic impulse asymmetry index. The asymmetry index is essentially a ratio between the impulse of the uninjured limb in relation to the uninjured limb, represented by the equation below (Jordan et al., 2015).
The latest software solutions developed by dual sensor force plate manufacturers now allow these types of asymmetries to be fed back to the athletes in real time, thus providing valuable information to the athlete and the practitioner, who is able to modify the training programme depending on the required strength or RFD curve with respect to the athlete’s sport. The figures below show the testing dashboard with real-time data on a series of countermovement jumps tested on ForceDecks Lite series.
The decision of when to allow an athlete to return to train and/or compete in their sport is a complex one and should involve quantitative as well as qualitative movement analysis of sport-specific tasks. Such a decision is best made jointly by the athlete, the strength coach and the therapist in order to avoid emotionally charged decisions and avoid individual bias as much as possible.
Neuromuscular testing and profiling can start early in the post-operative phase with isometric testing of RFD and progress to more ballistic movements later in the rehabilitation process. A non-exhaustive list of strength and power diagnostics is provided in the table below, with considerations for RTS load planning (Taberner, Allen & Cohen, 2019).
The importance of regularly monitoring these qualities over the long term to establish chronic tissue adaptations is fundamental in order to allow the performance team to make the most informed decision possible when allowing athletes to return to compete in their respective sport. While neuromuscular testing and profiling of athletes with ACL injuries is an important part of the RTS process, it is important not to lose sight of the chaotic nature of most sports, with unpredictive tasks imposed on athletes at high speeds (such as in team sports), coupled with unpredictable environmental conditions at high speeds (such as alpine ski racing, for example).
Therefore, a rehabilitation process moving from high control to ultimately high chaos has been suggested as an optimal model to ensure athletes return to their sport with a robust neuromuscular and coordination system, which is ready to respond to a stimulus at high speed (Taberner, Allen & Cohen, 2019).
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