Rugby League is a heavy collision sport involving frequent bouts of high-intensity exercise (sprinting, tackling, and running) separated by bouts of low-intensity exercise (walking and standing) (10,16,24). Recent studies (16,24,27) have added to our understanding of player movement characteristics during Rugby League match play. Advances in match analysis technologies such as portable global positioning system (GPS) and integrated accelerometry have enabled investigators to accurately quantify activity profiles and the impact associated with collisions in contact sports. Further, the accurate determination of impact forces experienced by players during repeated collisions with opponents during match play provides new insight into the physical demands of competition.
The movement patterns of elite Rugby League players during competitive match play using GPS technology and integrated accelerometry have been described (21,24), and the impact forces experienced by players during high-velocity collisions that are synonymous with elite Rugby League match play have been presented (22). To augment our understanding of the demands of Rugby League match play, compromised neuromuscular function has been established for up to 48 hours postmatch (5,20,23), whereas significant correlations have been reported between high-intensity collisions and elevated markers of skeletal muscle damage after elite Rugby League competition (22). Despite previous analysis of elite Rugby League match play (16,20–24,27), uncertainty remains regarding the influence of repeated blunt force trauma and impact on acute neuromuscular fatigue postmatch and the resolution of neuromuscular performance during the short-term recovery period before subsequent competition.
Participation in contact sport such as Rugby League that involves high-intensity, intermittent exercise and blunt force trauma is a complex phenomenon and has been associated with the manifestation of significant neuromuscular fatigue (23). Neuromuscular fatigue has been described in humans as any exercise-induced reduction in the maximal voluntary force or power produced by a muscle or muscle group (1,11) and is determined by the type of muscle contraction, the intensity of exercise, and the duration of the exercise (8,17). Traditionally, neuromuscular fatigue has been examined using isolated forms of isometric, concentric, or eccentric movements (11). Recent evidence however suggests that the incorporation of movements involving the stretch-shortening cycle (SSC) (13,17,25) provides a more specific examination of neuromuscular fatigue.
Movements involving the SSC incorporate metabolic, mechanical, and neural elements of fatigue together with impairment of the stretch-reflex activation (25). Typically, the SSC involves a preactivated muscle that is first stretched (eccentric action) and then shortened (concentric action) and is common to activities that involve different phases of running, jumping, or hopping (17). Recovery after impaired SSC function occurs in 2 phases: (a) identified by a significant initial decrement in SSC function immediately postexercise and (b) a phase of transient recovery then followed by a subsequent decrement in performance, resulting in a peak reduction in the SSC function 48–72 hours postexercise (14,17).
The countermovement jump (CMJ) is commonly used to assess the SSC and sport performance and to determine the effect of competitive contact team sport match play on neuromuscular fatigue (2,5,12,20,23). Those data that are available however are conflicting (2,5,12,20,23). Hoffman et al. (12) reported various changes in peak power (PP), peak force (PF), and peak rate of force development (PRFD) throughout the course of American Football match play with no significant decrement in PP, PF, or PRFD postmatch. An analysis of Australian Rules Football by Cormack et al. (2) also found no significant change in the average force and average power immediately after competition. Research studies (5,20,23) examining the neuromuscular response to elite Rugby League match play however have reported consistent patterns of reduced force and power measures during the CMJ and increased neuromuscular fatigue for a period of 48 hours postmatch that return to prematch levels within 96 hours postmatch.
Rugby League match play presents a unique model to study neuromuscular fatigue generated by high-intensity competition, combining movement patterns and sprinting profiles similar to Rugby Union, running volumes similar to soccer and Australian Rules Football and blunt force trauma that is characteristic of American Football match play. The Australian National Rugby League (NRL) season comprises a 24-match schedule conducted over a 26-week period from March to September. In preparation for the upcoming season, teams will complete several months of training and then participate in several trial matches in the month before the first match of the NRL season. Players therefore may participate in up to 46 weeks of high-volume and high-intensity training throughout the off-season, preseason, and in-season competition periods each year. Anecdotally, a common feature of team performance during the course of the NRL season is a midseason slump in performance that may be associated with overwork and or underrecovery of players because of training and match play demands that may include representative commitments. The extended exposure of players to the demands of training and match play may lead to altered neuromuscular function and may contribute to the development of underrecovery, overreaching, or overtraining syndrome in team sport athletes (6,20). Uncertainty remains however regarding the manifestation and resolution of neuromuscular in response to elite level contact sport, and the influence of blunt force trauma and impact on neuromuscular fatigue after Rugby League match play is unknown.
A greater understanding of the neuromuscular responses to repeated collisions during elite Rugby League match play may provide scope for improved individualized postmatch strategies, reduce the risk of residual and cumulative fatigue, and potentially decrease the incidence of musculoskeletal injury. The present research systematically examined the time course of neurosmuscular responses of players pre, during, and after elite Rugby League match play. Despite the existence of an elite NRL competition and the professional status of players participating in that competition, there remains a paucity of research regarding the requirements of actual match play and subsequent recovery periods in preparation for subsequent performance. The aim of this study therefore was to examine the acute and short-term neuromuscular responses to the intensity, number and distribution of impact associated with collisions during competitive Rugby League match play. We hypothesize that blunt force trauma associated with repeated high-intensity impacts that are characteristic of Rugby League match play will result in a substantial reduction in neuromuscular performance postmatch. Further, an investigation of portable GPS, integrated accelerometry, and markers of neuromuscular fatigue provides a more detailed and specific analysis of the influence of repeated blunt force trauma and collisions during elite Rugby League match play than was achieved previously.
Experimental Approach to the Problem
A single group repeated-measures pre-post match design was used in this study. To examine the impact characteristics of the independent variable of player collisions during elite Rugby League match play, portable GPS and accelerometer data were collected from the players during 8 matches of an 80-minute duration between 2 NRL teams. All the participants played a minimum of 30 minutes in each of the two 40-minute halves of each match. The players were separated into forwards and backs for comparison. This study examined SSC performance to determine neuromuscular fatigue after elite Rugby League match play. Measurements of the dependent variables of PRFD, PP, and PF during a CMJ were performed on a portable force plate prematch and postmatch play. An understanding of the influence of blunt force trauma and impact on neuromuscular markers of fatigue after elite Rugby League match play is important to determine postmatch recovery strategies, monitor residual and or cumulative fatigue throughout the competitive season, and effectively manage player preparation for subsequent matches.
Twenty-two elite male Rugby League players, age 24.2 ± 7.3 years, height 188 ± 20.1 cm, and mass 94.6 ± 26.8 kg; mean ± SD, representing an NRL team volunteered to participate in the study. Because of the minimum time on field match play requirements, data were analyzed from 15 players (Forwards n = 8; Backs n = 7). Before the commencement of the study, the players attended a presentation outlining the purpose, benefits, and procedures associated with the study. All the players were made aware of their ability to withdraw from testing at any time. Written informed consent was obtained from all the players who participated in the study. The study was approved by the Bond University Human Research Ethics Committee (BUHREC) and the NRL club from which players volunteered.
The CMJ data were collected 24 hours prematch; 30 minutes prematch; within 30 minutes postmatch; and at 24, 48, 72, 96, and 120 hours postmatch. The daily training and CMJ data collection schedule is outlined in Table 1. The subjects were asked to refrain from strenuous exercise during the 24 hours before baseline data collection 24 hours prematch. The CMJ data were collected daily between 1530 and 1630 hours with the exception of the 30 minutes postmatch data that were collected between 1700 and 2200 hours because of the scheduled time of match play. The players performed the CMJ within 30 minutes of match completion and before participation in postmatch recovery activities. Data were examined for each subject at each CMJ data collection time point. Throughout the postmatch data collection period, the subjects participated in all of the team's scheduled postmatch recovery and daily training sessions (Table 1). An example of a training week during the in-season competition phase of the NRL is outlined in Table 2.
This study used commercially available 5-Hz GPS receivers (SPI-Pro, GPSports, Canberra, ACT, Australia) (Figure 1), which operated in the nondifferential mode and provided data in real time. The GPS is a satellite-based navigation system that enables real-time data collection during training and competition (4,7,19). Information with respect to the intensity, number, and distribution of gravitational forces (G) experienced by players during collision is recorded simultaneously via satellite communication with a portable GPS receiver and integrated accelerometry worn by a player. The GPS typically uses a network of 24 satellites in orbit around the Earth. Each satellite is equipped with an atomic clock that emits, at the speed of light, the exact time and position of the satellite. The GPS receiver compares the time emitted by each satellite signal with the lag time, measured by each receiver, translated into distance by trigonometry. By calculating the distance to at least 4 satellites, the exact position and altitude of the receiver on the Earth's surface can be determined (29). Speed of displacement is determined via the Doppler shift method, by measuring the rate of change of the satellites' signal frequency attributable to movement of the receiver (29).
The SPI-Pro GPS units used in this study contain a triaxis (x, y, z axes) integrated accelerometer, which measures accelerations in gravitational force (G-force) on 3 planes, namely, forwards-backwards, up-down, and tilt left-right. The integrated accelerometer within the GPS unit measures accelerations and decelerations (meters per second squared) for each plane, with known gravity of 9.8 m·s−2 equal to 1 G. The integrated accelerometer measures the rate of acceleration and deceleration on each plane and divides the value by 9.8 m·s−2 to determine the combined G-force as the sum of the G-force measured on each directional axis. The GPS model used in this study (76 g; 48 mm × 20 mm × 87 mm) was worn in a purpose-designed vest (GPSports) to ensure that the range of movement of the upper limbs was not restricted. The GPS unit was worn in a padded mini backpack contained in the vest and positioned in the center area of the upper back slightly superior to the shoulder blades at the level of approximately the first thoracic vertebrae (Figure 2).
The participants had previously worn the GPS units with integrated accelerometers in all outdoor training sessions that included Rugby League specific running, skill-related, and match-simulated contact activities during a 12-week preseason training period before the commencement of the regular season of weekly competition. The participants had also worn the units in 2 preseason practice matches conducted 14 and 21 days before the first match examined in this study. The players continued to wear the GPS units with integrated accelerometers during all in-season outdoor training sessions for the duration of the 26-week regular season phase of the NRL. No participants complained of discomfort or impediment to their normal range of movement or performance from wearing the GPS equipment during training or competitive match play. Data provided from the GPS integrated triaxial accelerometer for examination in this study included impact (G Force) data (intensity, number, and distribution) and total distance traveled characteristics. Raw accelerometer data were available in real-time via Wireless Fidelity (WiFi) communication and were displayed using commercially available software (Team AMS, GPSports). The accuracy and reliability of the SPI-Pro have been reported previously (15,24,26) and has been assessed by our laboratory over distances from 5 to 8,000 m on a synthetic 400-m athletics track with <3% variation in the total distance and the reliability of speed assessed with electronic light gates (Smartspeed, Fusion Sports, Brisbane, Australia) from walking speed (6.0 km·h−1) to maximum sprint speed (>22.0 km·−1) with variation <5.5%. Our results are similar to the results of others (26).
Impact Classification System
Player exposure to impact was determined via accelerometer data provided in ‘G’ force. A zone classification system forms the basis of the analysis performed by the Team AMS (GPSports) software, allowing 6 ranges impact (zones 1–6) to be preset and used for subsequent analysis. Zone 1 indicates the lowest impact or lowest velocity of collision with each zone progressively categorizing impact force and movement intensity to zone 6 indicting the highest impact and intensity of movement. The impact classification system used in this study was based on methods presented previously in Rugby League (24) and Rugby Union (7) and manufacturer guidelines (GPSports). Each impact was coded to 1 of 6 zones based on acceleration G-force characteristics recorded via portable accelerometry. Impact zone characteristics used in this study are listed in Table 3.
Tackle Count and Hit-Up Number Data
The average number of tackles and the number of ball carries (hit-ups) completed by forwards and backs were determined via postmatch analysis of video recordings of each match (Table 4) performed by 3 NRL coaches, each with >15-year experience in elite coaching. For the purposes of this study, a tackle was defined as an event involving physical contact that halted the progress of an opponent in possession of the ball. A ball carry (hit-up) was defined as a player being tackled in possession of the ball during match play.
The Countermovement Jump
Before performing the unloaded CMJ test, the subjects completed a warm-up session consisting of 10 minutes of self-paced stationary cycling followed by 5 minutes of prescribed dynamic stretching. Once positioned on the force plate, the subjects performed 1 submaximal practice jump. Each subject then performed 3 CMJs, with 3 minutes of rest between each CMJ. The CMJ was commenced in the standing position; the subject then dropped into the squat position and immediately jumped vertically incorporating arm swing to jump as high as possible. The depth of knee flexion and the amount of arm movement used during the CMJ was individually determined by each subject. Take-off from 2 feet was strictly monitored with no preliminary steps or shuffling permitted during the eccentric or transition phases of the CMJ technique. The best result from the 3 CMJs was used for analysis.
The CMJ was performed on a commercially available force plate (ONSPOT 200-1, Innervations, Muncine, IN, USA), which sampled at a rate of 1,000 Hz, and the analog signal was converted to a digital signal using a PowerLab 30 series data acquisition system (ADInstruments, Sydney, Australia). The vertical force-time data were filtered using a fourth-order Butterworth low-pass filter with a cutoff frequency of 17 Hz.
Calculation of Force Variables
The force-time data from the CMJ included PRFD, PP, and PF. A CMJ was deemed to have started when the vertical force exceeded 10 N greater than the mass of the subject. The PRFD was calculated from the maximum force that occurred over the first derivative of the force-time curve. Peak force was calculated as the maximum force achieved over the force-time curve during the CMJ. The vertical velocity was calculated from the integration of the force-time trace and was used to calculate PP. The vertical force was multiplied by the velocity throughout the propulsive phase of the CMJ to determine PP.
The data for each of the dependent variables are represented as mean (±SEM) using standard statistical methodology. Changes in force-power characteristics were analyzed using a one-way repeated-measures analysis of variance. Significant differences were located by a Bonferroni post hoc test. Differences in tackles, hit-ups, and impact zones between backs and forwards were determined using Student's unpaired t-test. The criterion level for statistical significance was set at p ≤ 0.05. The correlations between changes in PF-power characteristics, total tackles, number of hit-ups, and impact zone entries were analyzed using the Pearson Product-Moment Correlation Coefficient. The data are expressed as mean ± SD. All statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS for Windows, version 14.0; SPSS, Inc., Chicago, IL, USA).
Changes in the force-power characteristics after elite Rugby League match play are shown in Table 5. The PRFD was significantly (p = 0.026) lower 30 minutes postmatch and 24 hours postmatch (p = 0.042) compared with 30 minutes prematch. The PRFD values returned to 24 hours prematch values 48 hours postmatch. The PP was significantly lower 30 minutes postmatch (p = 0.005) and 24 hours postmatch (p = 0.034) when compared with 30 minutes prematch. The PP had returned to 24 hours prematch and 30 minutes prematch levels after 48 hours postmatch. The PF was significantly (p = 0.031) lower 30 minutes postmatch but had returned to prematch levels after 24 hours.
There was no significant difference in the total distance traveled during match play between backs (7,886 ± 1,695 m) and forwards (7,462 ± 1,566 m) (Table 6). The number (mean ± SD) of tackles and ball carries for forwards and backs during match play are listed in Table 4. Although there were no significant differences in the number of hit-ups between forwards and backs, forwards completed significantly (p < 0.05) more tackles during match play compared with backs (Table 4). Impact zone entries during match play are listed in Table 4. Significant (p < 0.05) correlations were found between the total number of impacts and PRFD and PP 30 minutes postmatch. Impact zones 4–6 were significantly (p < 0.05) correlated to PRFD and PP 30 minutes postmatch with impact zones 5 and 6 significantly (p < 0.05) correlated to PRFD and PP 24 hours postmatch (Table 7). There were no significant correlations between tackles and hit-ups and force and power characteristics. There was no significant difference in the number of recorded impacts in each zone between forwards and backs. The grouping of match impacts within zones 4–6 (heavy + very heavy + severe) revealed that the players experience high-level impacts approximately every 50 seconds during match play.
Rugby League match play is characterized by repeated eccentric muscle contractions, intermittent high-intensity exercise and frequent blunt force trauma. This study examined the neuromuscular responses to the incidence and intensity of impact Rugby League match play collisions to determine if the CMJ could be used to monitor neuromuscular fatigue and recovery after competition. The main findings of this study were as follows: (a) PRFD and PP measured during a CMJ were decreased for up to 48 hours postmatch with PF decreased 30 minutes postmatch; (b) significant correlations were found between the total number of impacts and PRFD and PP 30 minutes postmatch; (c) heavy, very heavy and severe impacts were significantly correlated to PRFD and PP 30 minutes postmatch; (d) very heavy and severe impacts were significantly correlated to PRFD and PP 24 hours postmatch.
Previous work examining force-power variables including PRFD, PF, and PP subsequent to contact team sport participation during Australian Rules Football (2), and College American Football (12) found no significant difference between prematch and postmatch force and power measures and suggested that contact team sport athletes may be able to maintain PRFD, PF, and PP after match play. The results of these studies (2,12) when compared with the results of this study and others (5,20,23) that have examined the neuromuscular response to Rugby League match play are surprising. The competitive requirements of Australian Rules Football and American Football involve repeated blunt trauma that could be expected to have an effect on the SSC and a reduction in CMJ performance as found in this study.
Peak rate of force development, a measure of explosive muscle strength, is an important measure of performance in Rugby League. In this study, PRFD significantly decreased 30 minutes postmatch and 24 hours postmatch in comparison to 24 hours prematch. The postmatch PRFD remained below prematch values for up to 48 hours postmatch, and may reflect the influence of impaired excitation-contraction coupling reported with low-frequency fatigue (LFF) (18) on decreased PRFD, PP, and PF 24 hours after Rugby League match play.
Peak force decreased significantly 30 minutes postmatch, returning to prematch values within 24 hours postmatch. Peak power in this study was significantly lower 30 minutes postmatch and 24 hours postmatch compared with 30 minutes prematch values. It would appear that both the velocity of the CMJ, evidenced by the reduction in PRFD, and the force as evidenced by the reduction in PF 30 minutes postmatch, may have contributed to the decrease in PP. The decrease in PP remained until 48 hours postmatch suggesting that the velocity component of PP was more sensitive to fatigue than the force component and is consistent with recent theories that suggest power measures to be useful to monitor LFF (9).
Not all studies (2,12) have reported a decrease in PP and PF after competitive contact sport match play. The cause of the decrease in PF observed 30 minutes postmatch in this study and as suggested by others (23,28) may be because of a combination of central fatigue in the form of reduced central drive and peripheral fatigue in the form of an impairment in action potential propagation over the sarcolemma (high-frequency fatigue) or impairment of the excitation-contraction coupling mechanism (LFF). The difference between PP and PF found 30 minutes postmatch in this study and other studies (2,12) is most likely because of the variation in the physiological demands of match play in sports such as Australian Rules Football, American Football, and Rugby League and the characteristics of repeated blunt force trauma during collisions experienced by players from different contact sports. Our results indicate that PF (also referred to as maximal strength) recovers more quickly than PP or PRFD after Rugby League match play.
The reduction in PRFD and PP 30 minutes postmatch in this study is in contrast to the results of studies examining the neuromuscular response of players to Australian Rules Football (2) and College American Football (12) match play. Few data exist however regarding the frequency and intensity of match play collisions and impact during Australian Rules Football and College American Football competition. Differences in PRFD and PP found in this study in comparison to those of other studies (2,12) may be attributed to running profiles, sprinting profiles, tackling and wrestling, and heavy blunt force trauma demands placed on elite Rugby League players but not on Australian Rules Football or College American Football players during match play. Although College American Football players are likely to experience heavy contact during match play, fewer total blunt force trauma episodes combined with reduced running volumes and extended rest periods between competitive efforts may contribute to the maintenance of PRFD and PP after College American Football match play.
The results of this study are consistent with the findings of others (5,20,23) that have examined neuromuscular performance and fatigue after professional Rugby League match play. McLellan et al. (23) found significantly reduced PRFD, PP, and PF after elite Rugby League match play that retuned to prematch levels within 48 hours postmatch, whereas McLean et al. (20) reported significantly reduced power and flight time measures during the CMJ for a minimum of 48 hours after Rugby League match play. Similarly, Coutts et al. (5) described a significant decrease in force and power measures during the CMJ 24 hours after professional Rugby League match play that remained reduced for at least 48 hours postmatch. Present and previous (5,20,23) results indicate that neuromuscular fatigue after Rugby League match play can be monitored via CMJ variables that include measures of force and power. The specific influence of repeated blunt force trauma during impact experienced by Rugby League players during match play collisions however remains unknown.
Body impacts experienced by players during high-intensity collisions between opposing players are associated with gravitational impact forces in zones 4–6.
This study found a significant correlation between the number of zone 4–6 impacts and PRFD and PP 30 minutes postmatch. Zone 5 and 6 impacts were significantly correlated with PRFD and PP 24 hours postmatch. Our results indicate that exposure to high-impact collisions >7.1 G caused significant decrement in neuromuscular performance and increased neuromuscular fatigue 30 minutes postmatch. Collisions that involved impacts >8.1 G resulted in prolonged neuromuscular fatigue for at least 24 hours postmatch. Regardless of the nature of impact forces experienced by players during match play however, a return to prematch PRFD, PP, and PF was observed within 48 hours postmatch.
This study found no significant difference between the total number of impacts or the number of entries in each impact zone between forwards and backs during offensive and defensive match play. The total number of impacts for forwards and backs was significantly related to reductions in PRFD and PP 30 minutes postmatch is consistent with the findings of others (5,20,23) who have examined neuromuscular fatigue after Rugby League match play. Variation in mean total tackles and hit-ups in comparison to the total number of zones 4–6 entries and total impacts during match play was found in this study. The disparity in the total number of tackles and hit-ups compared with the number of impact zone entries is likely to be because of impact experienced by players during missed tackles, incomplete tackles, line breaks during hit-ups, and second effort during tackles and or hit-ups that were not included in this study. Our findings with respect to the total number of impacts, zones 4–6 entries and postmatch PRFD and PP indicated that neuromuscular fatigue after Rugby League match play is dependent upon the number and intensity of collisions experienced by players.
To our knowledge, no studies have examined neuromuscular fatigue in response to impacts experienced by players during elite Rugby League match play. The relationship between neuromuscular fatigue and the frequency and intensity of impacts experienced by players during Rugby League match play using integrated accelerometry is likely to reflect greater LFF associated with heavy blunt force trauma during repeated high-intensity collisions. On the basis of the present and previous (2,3,5,20,23), we suggest that PRFD and PP during the CMJ can be used to monitor acute recovery from elite Rugby League match play.
This study provides an insight into the relationship between the prematch and short-term postmatch neuromuscular responses to the intensity, number, and distribution of impacts associated with collisions during elite Rugby League match play in the NRL that have not been reported previously. The findings of this study suggest that repeated high-intensity collisions during elite Rugby League match play are associated with significant neuromuscular fatigue 30 minutes postmatch that remain evident for up to 48 hours postmatch. Players can also expect to sustain blunt force trauma and impact >7.1G because of high-intensity collision approximately every 50 seconds during match play. The number of heavy to severe impacts experienced by players during match play is correlated with significantly reduced PRFD and PP 24 hours postmatch.
A substantial acute decrease in neuromuscular performance in response to match play was identified followed by a return to homeostasis within 48 hours, substantiating the value of the CMJ as a viable analysis method to monitor acute and chronic LFF and supports the implementation of at least 2 days of modified activity postmatch to facilitate recovery and optimize training in preparation for subsequent competition in the NRL. When integrated with GPS and accelerometer technologies, neuromuscular measures can assist coaches to determine the demands of competition and establish a comprehensive profile of individual responses and adaptations to elite Rugby League match play.
The authors wish to thank the players and staff of the Gold Coast Titans Rugby League Football Club, Australia, for participation and facilitation of this study. No grant aid was received in conjunction with this study, and no conflicts of interest are declared.
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