The Countermovement Jump
Before performing the unloaded CMJ test, 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, subjects performed 1 submaximal practice jump. Each subject then performed 3 CMJ, 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 ft 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 2000-1) 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. The PF 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.
Plasma Creatine Kinase Sampling and Analysis
Plasma [CK] was determined from 30-μL capillarized whole-blood samples collected via fingertip puncture made using a spring-loaded single-use disposable lancet. Blood samples were collected from subjects simultaneously at the time of saliva sample collection. Whole-blood samples were centrifuged (Heraeus, Function Line) at 3,000 rpm for 10 minutes; separated plasma was stored at a temperature of −30°C until analysis. Plasma samples were analyzed using a Reflotron spectrophotometer (Abbott Architect) via an optimized UV test.
Salivary Cortisol Sampling and Analysis
Unstimulated saliva was collected via passive drool into a plastic tube for analysis of Cortisol. The [sCort] are independent of salivary flow rate (31), and a significant correlation has been reported between saliva and serum unbound cortisol concentration at rest (r = 0.93) and during exercise (r = 0.90) (28). All subjects were requested to avoid the ingestion of food and fluids other than water during the 60 minutes before providing each saliva sample and refrain from brushing their teeth 2 hours before each saliva sample-collection session. Subjects were instructed to wait for a period of 10 minutes after their last consumption of water before commencing the saliva sample-collection process. Saliva samples were stored at −80°C until analysis.
[sCort] was analyzed in duplicate via a commercially available enzyme-linked immunosorbent assay (Salimetrics LLC, State College, PA, USA) using a microplate reader (SpectraMax 190, Molecular Devices, Fullerton, CA, USA). Standard curves were constructed as per the manufacturer's instructions, and commercially available standards and quality control samples were used for the assays (Salimetrics LLC). Cortisol sensitivity was 0.007 ng·mL−1 with an average intraassay CV of 2.6%. All samples were analyzed in the same series to avoid interassay variability.
Endocrine and biochemical variables analyzed prematch and postmatch included [sCort] and [CK]. Before statistical analysis, log transformation was applied to the endocrine and biochemical data to normalize the distribution and reduce nonuniformity bias. All data are expressed as mean ± SD. Changes in force-power characteristics, biochemical concentrations and endocrine concentrations were analyzed using a 1-way repeated measures analysis of variance. Significant differences were located by a Bonferroni post hoc test. The criterion level for statistical significance was set at p ≤ 0.05. The correlation between peak changes in force-power characteristics, biochemical and endocrine characteristics was analyzed using the Pearson Product-Moment Correlation Coefficient. The mean coefficient of variation (CV) for CK assays was 6.1%. All statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS for Windows, version 14.0; SPSS, Inc., Chicago, IL).
Changes in the force-power characteristics after elite Rugby League match play are shown in Table 3. The PRFD was significantly lower 30 minutes postmatch (p = 0.026) and 24 hours postmatch (p = 0.042) compared with 30 minutes prematch. Peak rate of force development values returned to 24 hours prematch values 48 hours postmatch. However, at 72 and 96 hours postmatch, the PRFD values were significantly higher than both prematch values (p = 0.012, p = 0.044, respectively). Peak power was significantly lower 30 minutes postmatch (p = 0.005) and 24 hours postmatch (p = 0.034) when compared with 30 minutes prematch. Peak power had returned to 24 hours prematch and 30 minutes prematch levels after 48 hours postmatch. Peak force was significantly lower 30 minutes postmatch (p = 0.031) but had returned to 30 minutes prematch levels after 24 hours.
There was a significant (p = 0.003) increase in plasma [CK] 30 minutes postmatch with a further significant (p = 0.002) increase in [CK], peaking at 24 hours postmatch (Table 4). Significant increases in plasma [CK] were also found 48 hours postmatch (p < 0.006), 72 hours postmatch (p = 0.004), 96 hours postmatch (p = 0.013), and 120 hours postmatch (p = 0.043) compared with 30 minutes prematch. There was a significant correlation between plasma [CK] and PRFD 30 minutes postmatch (p = 0.044, r = −0.64) and 24 hours postmatch (p = 0.033, r = −0.58) compared to 30 minutes prematch values.
The [sCort] was significantly (p = 0.043) higher 30 minutes prematch compared with 24 hours prematch. Significant increases in [sCort] were also found 30 minutes postmatch (p < 0.001) and 24 hours postmatch (p < 0.000) when compared to 24 hours prematch (Table 4). The [sCort] was significantly (p = 0.042) lower 96 hours postmatch compared with 24 hours prematch. There was a significant correlation between the percent change in [sCort] and PF 30 minutes postmatch (p = 0.048, r = −0.58) compared with 30 minutes prematch values.
The present study examined the neuromuscular, biochemical and endocrine responses to Rugby League match play to determine if the CMJ, plasma [CK] or [sCort] response could be used to monitor neuromuscular fatigue after competition. The main findings of the present study were (a) PRFD and PP measured during a CMJ were decreased for up to 48 hours postmatch with PF decreased 30 minutes postmatch. Peak rate of force development also increased above prematch values 72 and 96 hours postmatch; (b) plasma [CK] and [sCort] increased postmatch with plasma [CK] remaining elevated for up to 120 hours postmatch; (c) significant correlations between the change in plasma [CK] and PRFD and the change in [sCort] and PF were found postmatch.
Previous work examining force-power variables including PRFD, PF, and PP subsequent to team sport participation during Australian Rules Football (7), American Football (15), and Soccer (36) found no significant difference between pre and postmatch force and power measures and suggested that team sport athletes may be able to maintain PRFD, PF, and PP after match play. The results of these studies (7,15,36) when compared with the results of the present study are surprising. The competitive requirements of Australian Rules Football, American Football and Soccer that involve periods of sprinting, jumping, rapid changes of direction, and blunt trauma could be expected to have an effect on the SSC and a reduction in CMJ performance as found in the present study.
Peak rate of force development, a measure of explosive muscle strength, is an important measure of performance in Rugby League. In the present study, PRFD significantly decreased 30 minutes postmatch and 24 hours postmatch in comparison to 24 hours prematch. Peak rate of force development remained below prematch values for 48 hours postmatch, and may reflect the influence of impaired excitation-contraction coupling reported with low-frequency fatigue (LFF) (22) on decreased PRFD, PP, and PF 24 hours after Rugby League match play.
The reduction in PRFD 30 minutes postmatch (35%) in the present study agrees with some (37) but not others (15,36). The differences in PRFD, PP, and PF between the present study and other studies (15,36) may be because of the running volumes, sprinting profiles, tackling and wrestling, and heavy blunt force trauma demands placed on Rugby League players but not upon Soccer or American Football players during match play. Although American Football players are likely to experience heavy contact during match play, episodes of contact involving blunt force trauma are dispersed through protective padding so as to reduce the effect of such trauma upon skeletal muscle tissue. 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, PP, and PF after American Football match play.
We observed a significant increase in PRFD at 72 hours (18%) and 96 hours (17%) postmatch that preceded a return to prematch values 120 hours postmatch. Our results regarding PRFD at 72 and 96 hours postmatch appear to be in contrast to the reported bimodal trend of neuromuscular fatigue involving SSC exercise (20,26). An immediate decrease in neuromuscular performance after exhaustive SSC exercise has been attributed primarily to metabolic disturbances, (e.g., metabolite accumulation, depletion of energy substrates and phosphate, and decrease in mitochondrial respiratory control) (22), whereas the secondary decrease in neuromuscular performance may coincide with the inflammatory processes associated with muscle damage during exhaustive SSC exercise (10). Studies that reported a bimodal trend of neuromuscular fatigue and recovery (20,27) used exhaustive eccentric exercise and not team sport activity as used in the present study. Therefore, the pattern of PRFD recovery observed at 72 and 96 hours postmatch may reflect the specific sprinting, SSC activity, blunt force trauma, and high-intensity intermittent nature of Rugby League match play and the influence of postmatch recovery methods.
The cause of the significant increase in PRFD after 72 and 96 hours postmatch in the present study is unclear. The CMJ is an effort-dependent SSC activity that may have been performed submaximally by players 30 minutes post and 24 hours postmatch. Submaximal CMJ effort 30 minutes post and 24 hours postmatch may be one of the consequences of competitive Rugby League match play, influenced by skeletal muscle soreness and SSC fatigue resulting from high-intensity intermittent exercise and repeated blunt force trauma. During competitive Rugby League match play however, the extent of SSC fatigue may not be exhaustive. Incomplete exhaustion of SSC performance during Rugby League match play may result in a pattern of postmatch PRFD recovery that is not consistent with the bimodal concept of fatigue-induced SSC performance decreases and the subsequent short-term recovery process.
During the short-term postmatch recovery period, players completed a structured recovery program of cold water immersion therapy (12°C), low-intensity deep water running and low-intensity mobility-resistance exercise (<40% 1RM). Strength training that included complex training methods incorporating high-intensity loads (>85% 1RM) coupled with explosive SSC exercise for the upper and lower limbs was conducted 48 hours postmatch. Rugby League specific speed and agility exercise consisting of high-intensity, short-duration sprint and SSC exercise was conducted 72 hours postmatch.
Our results suggest that the explosive strength, speed, and SSC exercise conducted during the structured recovery period (48 and 72 hours postmatch) may have resulted in a compensatory effect observed in PRFD at 72 hours post and 96 hours postmatch. The impaired excitation-contraction coupling reported with LFF (34) in the presence of exercise-induced muscle damage (20,22) may have been attenuated by the structured postmatch recovery strategies. These structured recovery strategies, if responsible for the increase in PRFD 72 and 96 hours postmatch reinforce the need for recovery strategies that enhance the return to prematch function so that the optimal training stimulus can be provided to players in preparation for subsequent match play.
Peak power in the present 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. Peak force decreased significantly 30 minutes postmatch, returning to prematch values within 24 hours postmatch. Not all studies (7,15,16) have reported a decrease in PP and PF after competitive match play or sporting activity. The cause of the decrease in PF observed 30 minutes postmatch as suggested by others (1,34) 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 impaired excitation-contraction coupling (LFF). The difference between PP and PF found 30 minutes postmatch in the present study and other studies (7,15,16) is most likely because of physical demands placed on players from different sports and the incorporation of a structured recovery program during the short-term recovery period. Our results indicate that PF (also referred to as maximal strength) recovers more quickly than PP or PRFD after rugby league match play.
Other studies (6,32) have found that PF recovers more rapidly than PP and PRFD. Byrne and Eston (6) reported the recovery of PP after exhausting squat exercise was 2 days longer than the recovery of isometric strength and suggested that PP, unlike strength that recovers more rapidly, may be affected by delayed onset muscle soreness (39) and the inflammatory responses to exercise-induced muscle damage. Although both PF and PP are important qualities in elite Rugby League players (3), PP and PRFD measured during a CMJ may be more useful than PF in monitoring neuromuscular fatigue post-Rugby League match play. The nature of Rugby League match play includes sprinting, jumping, high-speed directional changes, and a rapid summation of forces characteristic of PP and PRFD. Significant reductions in PP and PRFD found beyond 24 hours postmatch in the present study may reflect the continued influence of LFF and delayed recovery of the neuromuscular system.
In the present study, plasma [CK] and [sCort] were used as indirect markers of muscle damage and physiological stress after Rugby League match play. Although eccentric muscular work has traditionally been seen as the major contributor to increases in plasma [CK] after exercise (5), recent evidence suggests that significant increases in plasma [CK] may occur as a result of physical impact and blunt force trauma (15,32,34). The present study found that participation in Rugby League match play, which is characterized by physical impact and blunt force trauma, significantly increased plasma [CK] 30 minutes postmatch, with a peak plasma [CK] occurring 24 hours postmatch. Plasma [CK] remained significantly elevated above 24 hours prematch concentrations for up to 120 hours postmatch. The present study adds new knowledge in the time course of [CK] after Rugby League match play.
Although plasma [CK] as a marker of exercise-induced muscle damage has been challenged (40), plasma [CK] continues to be used as an indirect marker of skeletal-muscle damage (5,20). Our results using plasma [CK] suggests that the exercise-induced muscle damage after Rugby League match play does occur in significant amounts and remains elevated for at least 120 hours postmatch. Peak plasma [CK] (941 ± 392 U·L−1) found in the present study is similar to plasma [CK] reported in other sports that involve high-impact collisions between players (33,35). Although Takarada (35) recorded similar peak plasma [CK] 24 hours postcollege Rugby Union match play, the return to prematch [CK] was shorter (48 hours) postmatch than in the present study. The prolonged elevation of plasma [CK] found from 48 hours postmatch until 120 hours postmatch in the present study may have also been influenced by recovery strategies during the first 24 hours postmatch and the subsequent training sessions undertaken by the players after 48 hours postmatch.
The effect of elevated [CK] upon athletic performance is unclear. We observed a significant correlation between the increase in plasma [CK] and decreased PRFD 30 minutes postmatch (p = 0.044, r = −0.65) and 24 hours postmatch (p = 0.033, r = −0.58). Our results suggest that the decrease in PRFD in the present study at 30 minutes postmatch and 24 hours postmatch is causally related to the increase in plasma [CK], consistent with the findings of others (2,27). Andersson et al. (2) reported that CMJ height was reduced in the presence of a significant rise in plasma [CK] after elite female soccer match play. Nicol et al. (27) also reported an association between an increase in plasma [CK] and decreased drop jump performance during the first 2 days after exhaustive SSC exercise.
The associated increase in [CK] with decreased PRFD suggests that a CMJ may be used as an indirect estimate of the exercise-induced muscle damage from Rugby League match play. Support for a CMJ as an indirect estimate of PRFD is based on the reported relationship between [CK] and decreased SSC performance (20). The use of the CMJ as a functional indicator of PRFD, PP, PF, and exercise-induced muscle damage may therefore provide an appropriate method of functional impairment analysis associated with skeletal-muscle damage and recovery times after Rugby League match play. The association between increased [CK] and decrements in PRFD may be modified by structured recovery programs during the short-term postmatch recovery period.
In the present study, there was a significant increase in [sCort] 30 minutes prematch, 30 minutes postmatch, and 24 hours postmatch compared with 24 hours prematch. Cortisol is used as an indicator of “physiological stress” imposed during strenuous physical activity (23) and “psychological stress” because of the competitive environment of sport (25). The increase in prematch [sCort] found in the present study is consistent with the results of others (12) and is likely to be associated with prematch “psychological stress” because of anticipation and anxiety (29).
In the present study, the peak [sCort] was found 30 minutes postmatch, and by 48 hours postmatch [sCort] had returned to 24 hours prematch levels. Similar postmatch cortisol levels have been reported after games of Australian Rules Football (7), American Football (15), and Rugby Union (8). Elevations in [sCort] have been reported to be dependent on the duration and intensity of exercise (13). The increase in postmatch [sCort] found in the present study may be a reflection of exercise duration, intensity, and combative nature of Rugby League match play and psychological influences of anxiety and perceived stress of competition (29).
The present study found a significant correlation between change in [sCort] and the decrease in PF 30 minutes postmatch (p = 0.048, r = −0.58). No other study has examined the relation between [sCort] and PRFD, PP, and PF after Rugby League match play. Although it is not clear what the relationship between decreased PF and increased [sCort] may be, our results indicate that those players with the largest decrement in PF also produced the highest [sCort] 30 minutes postmatch. Decreased PF may reflect neuromuscular fatigue via a Rugby League match play-induced decline in central drive and failure of the excitation contraction mechanism (1) in players who experienced greater psychological stress or completed more high-intensity activity for longer duration, resulting in higher postmatch [sCort].
In summary, the findings of the present study indicate that the PRFD measured during a CMJ may be used as a mechanism to determine the neuromuscular fatigue associated with competitive Rugby League match play. Elevated plasma [CK] for up to 120 hours postmatch suggests significant damage to muscle tissue as a result of the blunt force trauma associated with high-speed collisions among Rugby League players. These neuromuscular and biochemical markers show promise as predictors of fatigue, recovery, and readiness for subsequent training after a Rugby League match play.
The ability to quantify decrements in key performance indicators such as lower body force and power characteristics and establish the neuromuscular status of players postmatch via an easy to administer functional test enables coaches to make informed decisions regarding recovery protocols and subsequent training programs and schedules. Our findings indicate skeletal-muscle damage occurs as a result of the rigors of elite Rugby League match play, and is reflected by peak [CK] measured 24 hours postmatch. Elevated [CK] persisted in comparison to prematch levels despite 120 hours of modified activity postmatch suggesting a prolonged recovery phase of at least 5 days is required to achieve a full recovery of muscle damage after match play. The [sCort] profile depicted in the present study identified a substantial acute [sCort] increase in response to Rugby League match play followed by a return to homeostasis within 48 hours. A minimum period of 48 hours of modified activity postmatch is therefore recommended to enable [sCort] to return to prematch rested levels.
Peak rate of force development, PP, and PF returned to prematch levels within 48 hours postmatch, indicating an absence of neuromuscular fatigue and a preparedness of players to undertake strength training despite a prolonged presence of muscle damage as indicated by elevated [CK]. The present study found that a return to modified strength training activities 48 hours postmatch may result in a compensatory increase in PRFD and supports the early implementation of strength training methods to facilitate the short-term postmatch recovery period. By comparing neuromuscular, biochemical and endocrine responses to match play, coaches are able to establish a comprehensive profile of individual responses and adaptation to elite Rugby League match play.
The author wishes 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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Keywords:Copyright © 2011 by the National Strength & Conditioning Association.
countermovement jump; creatine kinase; cortisol; contact sport