The peak torque (PT) of isometric twitch in skeletal muscles is transiently increased after a brief maximum voluntary contraction (MVC) (16,32). This enhancement, referred as postactivation potentiation (PAP), has been shown in a variety of human muscles, including knee extensor (KE) (10,15,25,26,29) or plantar-flexor muscles (14,23,24,38). PAP is maximal immediately after the conditioning brief (∼10 s) MVC (10,15) and declines exponentially over the time but is still evident for 5-10 min (15,29,38). In addition to an increase in isometric twitch PT, PAP is often associated with a shortening of twitch contraction (CT) and half-relaxation (HRT) times (14-16,38) and increased twitch maximal rates of torque development (RTD) and relaxation (RR) (3,16,24,25). The most accepted mechanism of PAP is considered to be the phosphorylation of myosin regulatory light chains via myosin light chain kinase during the conditioning contraction, which renders actin-myosin more sensitive to Ca2+ released from sarcoplasmic reticulum in a subsequent twitch and thereby enhances the force of the twitch contraction (13,36,37,40).
The most important muscle characteristic affecting PAP magnitude is fiber type (13,15,35). PAP is greater in muscles with the shortest twitch contraction (CT) and half-relaxation (HRT) times and highest proportion of type II muscle fibers (11,15,36-38). The greater PAP in type II fibers seems to be related to their greater capacity of myosin regulatory light chain phosphorylation in response to activation (13,40). On the basis of these results, it would be expected that subjects with the greatest performance in maximal intensity activities that depend on type II muscle fibers (i.e., maximal sprinting, (9)) would also show the greatest PAP in the involved muscles. However, there are no studies in which the magnitude of PAP has been related with the performance in activities requiring power and speed.
The capacity for PAP in skeletal muscles has been used for evaluation of specificity adaptation of the human neuromuscular system to different types of systematic training (14,23-25). Cross-sectional experiments have been conducted using sedentary subjects, elite endurance-trained and power-trained athletes (14,23-25). However, no studies have assessed the magnitude of PAP in athletes who combine power and endurance training simultaneously as in soccer (34).
Therefore, the purpose of the present study was to examine the relationship between twitch PAP in KE muscles and sprinting and vertical jumping performance in professional male soccer players. Similar to previous studies (17,34), vertical jump height and 15-m sprint time were taken as indicators of performance in activities requiring power and speed during a soccer game.
Experimental Approach to the Problem
To determine the relationship between twitch PAP in KE muscles and functional performance in a group of professional soccer players, various isometric and dynamic measures of the lower extremities were assessed. These included supramaximal twitch contractile properties of knee extensor muscles at rest and in potentiated state, vertical jumping height in squat (SJ) and counter-movement (CMJ) jumps and 15-m sprint time. Correlation analyses were performed to examine the relationship between each of these measures. The results were then analyzed to determine the degree of relation between each of the studied measures. The experimental phase took place during the end part of the preseason period. Players attended three testing sessions and each session was separated by 7 days. The first two sessions were designated as familiarization sessions. The subjects were instructed to avoid any strenuous physical activity during the duration of the experiment and to maintain their dietary habits for the whole duration of the study.
Fourteen professional male soccer players with mean (SD) age, height and body mass 20.0 (3.6) years, 177.9 (6.9) cm and 70.5 (5.7) kg, respectively, participated in this study. Their training experience was 12-15 years. All soccer players studied were full time professionals who trained 2-4 h per day. Subjects were experienced in all testing procedures as a result of their performance in previous studies (more than 4 times in the last 2 years). They were informed of the purpose, experimental risks and procedures of the study and their written informed consent was obtained. Three subjects were under age of 18 and a parental informed consent was required. The study carried the approval of the University of Tartu Ethics Commitee for Studies Involving the use of Human Subjects and was performed in accordance with the Helsinki Declaration.
Measurement of Twitch Contractile Properties of the Knee Extensor Muscles
During measurement the subjects sat in a custom-made dynamometric chair with the knee and hip angles equal to 90° and 110°, respectively. The body position of the subjects was secured by three Velcro belts placed over the chest, hip and thigh. The unilateral knee extension torque was recorded by a chair-fixed standard strain-gauge transducer (DST 1778, Russia) connected with the plate by a rigid bar. The strain-gauge transducer pad was placed approximately 3 cm above the apex of the lateral malleolus on the anterior aspect of the leg. Signals from the strain gauge transducer were linear from 0 to 2500 N. The force signals were sampled at the frequency of 1 kHz and stored on a hard disk of a computer using software WSportLab (Urania, Estonia). Skin temperature of the tested muscle group was continuously controlled and maintained at 35°C with an infrared lamp. The reproducibility of the torque measurements was calculated with repeated static load on the plate. The relative error between trials and the relative difference were less than 0.5%. High reliability of isometric MVC torque measurements using the above mentioned chair-fixed dynamometer (test-retest reliability with 5-day-interval between measurements was r = 0.96) was demonstrated in the previous study (28).
To assess the contractile properties of the KE muscles, electrically evoked isometric twitches were elicited by percutaneous electrical nerve stimulation. Two surface (2-mm thick) self-adhesive stimulating electrodes (Medicompex SA, Switzerland) were used. The cathode (5 × 5 cm) was placed on the skin over the femoral nerve in the inguinal crease and the anode (5 × 10 cm) was placed over the mid-portion of the thigh. Prior to attaching the stimulating electrodes, electrode gel was applied to the contact surface, and the underlying skin was cleaned with isopropyl alcohol. The electrical stimuli were rectangular pulses of 1-ms duration applied at supramaximal intensity (130-150 V) delivered from an isolated voltage stimulator (Medicor MG-440, Hungary). To determine the supramaximal stimulation intensity, the voltage of the rectangular electrical pulse was progressively increased to obtain a plateau in the twitch torque, i.e. when twitch torque failed to increase despite additional increases in stimulation intensity. A stimulation intensity of 20-30% greater than that needed for maximal twitch response was used for further twitch measurements. The following characteristics of isometric twitch contraction were calculated: PT - the highest value of isometric torque production, CT - the time to twitch maximal force, HRT - the time of half of decline in twitch maximal force, maximal rate of torque development (RTD) - the first derivate of the development of torque (dF/dt) and maximal rate of relaxation (RR) as the first derivate of decline of torque (−dF/dt).
Vertical Jumping and Sprinting Performance Testing
Vertical jump height was determined using a force platform with specifically designed software (Bioware, Kistler, Switzerland). Jump height was determined as the centre of mass displacement calculated from force development and measured body mass. Two types of vertical jumps were performed: squat (SJ) and counter-movement (CMJ) jumps. SJ was started from a static semi-squatting position with a knee angle of 90 deg of knee flexion, followed by subsequent action, during which the leg and hip extensor muscles contracted concentrically. In CMJ, each subject stood erect on the force platform and performed a preparatory movement down to approximately 90 deg of the knee flexion, stretching the leg extensor muscles (eccentric contraction), followed by an explosive maximal extension in the opposite direction (concentric contraction).
Fifteen meter sprint time was measured using photocells (Brower Timing, Fairlee, Vermont, USA) at the start and finish lines. The players performed 20 min of individual warm up including several accelerations. They then carried out four trials separated by a 3 min rest interval, and the best trial was used for the subsequent statistical analysis. The players decided themselves when to start each test from static position 30 cm behind the photocell, with the time being recorded from when the subjects intercepted the photocell beam.
The experimental phase took place during the end part of the preseason period. Each subject was tested separately, instructed, and verbally encouraged to give maximal effort in all tests. Players attended three testing sessions at approximately the same time of the day. The first two sessions were designated as familiarization sessions. During both sessions subjects were familiarized with the experimental setup and protocol, and isometric MVC measurement and electrical stimulation procedures were demonstrated. Each session was separated by 7 days. The third session was designated as testing session. Subjects were asked to abstain from intensive exercise and from drinking caffeine-containing beverages 24 h prior to the testing session.
During the testing session, on reporting to the laboratory, the subject sat resting for ∼30 min before the experiment for minimizing any potentiation effect from walking to the laboratory. Two twitches of KE muscles with 10 s intervals between stimulations were elicited in resting condition. Five minutes after the pre-MVC testing of twitch contractile characteristics, a conditioning MVC of the KE muscles was applied. The subject was asked to exert maximum voluntary isometric knee extension against the pad of the strain-gauge transducer as forcefully as possible during 10 s. Strong verbal encouragement and visual online feedback were used to motivate the subject. After the end of conditioning MVC, the subject remained seated without moving his legs. The post-MVC maximal isometric twitch was evoked at 2 s after the conditioning MVC. After measurement of twitch contractile properties of KE muscles, each subject performed vertical jumping and 15-m sprint tests. A period of 5 min was allowed between tests. The measurement of twitch PAP and vertical jumping test were performed in laboratory with the temperature of 21°C. Sprinting test was performed on indoor track and field court with an air temperature ranged from 23°C to 26°C. Data obtained from the two familiarization sessions allowed us to calculate the reproducibility of the measurements of 1) isometric MVC torque of KE muscles, 2) supramaximal twitch characteristics at rest and in potentiated state, and 3) vertical jump height and sprint time. Test-retest reliabilities for the experimental tests demonstrated intraclass correlations ranging from 0.87 to 0.95.
Values are expressed as means and standard deviations (SD). A one-factor ANOVA was used to test whether the conditioning MVC changed the twitch contractile properties. This was done both for the measures expressed in the units of measurement and as relative (percentage) changes from of the pre-MVC value. Relationships between variables were described using Pearson product-moment correlation coefficients. A level of p ≤ 0.05 was considered significant.
Twitch Characteristics in Unpotentiated Condition and MVC Torque
Table 1 shows the electrically evoked isometric twitch characteristics in unpotentiated (initial) condition before the conditioning contraction and isometric MVC torque of the KE muscles. During the conditioning 10-s MVC, isometric torque decreased (p < 0.05) by 21.2 (5.4) % for measured group.
Post-MVC Twitch Characteristics
Immediately after the conditioning MVC, twitch PT, RTD and RR potentiated (p < 0.05) by 45.6, 103.9 and 139.9 % from initial values, respectively. No significant changes in time-course characteristics in isometric twitch (CT and HRT) were found after the conditioning MVC as compared to initial level. A significant (p < 0.05) positive correlation were found between PAP of twitch PT and pre-MVC twitch RTD and RR (r = 0.56 and r = 0.57, respectively).
Vertical Jumping and Sprinting Performance
The mean (SD) value of jump height was 37.3 (5.6) and 32.3 (4.9) for CMJ and SJ, respectively, and the mean value of 15-m sprint time was 2.28 (0.16) s. A significant (p < 0.05) negative correlation was found between 15 m sprint time and jump height in CMJ (r = −0.63) and SJ (r = −0.57). No significant correlation was found between isometric MVC torque of KE muscles, and 15-m sprint time and jump height in CMJ and SJ.
Correlations Between Twitch PAP and Vertical Jumping and Sprinting Performance
PAP of twitch PT correlated significantly (p < 0.05) positively with jump height in CMJ (r = 0.61; Fig 1A) and SJ (r = 0.64; Fig 1B) and negatively with 15 m sprint time (r = −0.59; Fig 1C). Twitch RTD potentiation correlated positively with vertical jumping (r = 0.54 and r = 0.58 for CMJ and SJ, respectively, p < 0.05) and sprint time (r = −0.59, p < 0.05). However, the relative value of RR, CT and HRT in potentiated state was not correlated with both vertical jumping and sprinting tests. No significant correlations were found between the resting twitch RTD, RR, CT and HRT and jump height and 15-m sprinting time. A significant (p < 0.05) positive correlation was found between PAP of twitch PT and isometric MVC torque of KE muscles (r = 0.54).
The main finding of the present study was the significant relationship observed between PAP of twitch PT and RTD in KE muscles and the performance in vertical jumping and 15-m sprinting. Moreover, after systematic soccer training the potentiation observed was similar to that previously published in power trained athletes and higher than in endurance-trained and non-trained subjects.
In agreement with previous studies performed on KE muscles (10,15,26,29), the current study demonstrated a significant potentiation of twitch PT, RTD and RR after brief conditioning MVC. Immediately post-MVC, twitch PT was potentiated by 45.6 % from initial levels. This is similar (26) to or lower (10,15) than these potentiation levels of twitch PT which have been reported previously in moderately physically active subjects. Our previous studies showed that PAP of twitch PT was 51% and 30% in KE muscles in elite power-trained and endurance-trained women, respectively (25), 44% and 21% in plantar-flexor muscles power-trained and untrained women, respectively (23), and 54% and 28% in plantar-flexor muscles measured in male power and endurance trained athletes, respectively (24). Taking in to account the differences in measured muscles, gender of subjects and duration (5 vs. 10 s) of the conditioning isometric MVC applied, it could be suggested on the basis of the results observed that PAP value in professional soccer players tend to be similar to that observed in male power-trained athletes (sprinters and jumpers). In this regard, soccer game is composed of high-intensity actions interspersed by periods of medium- or low-intensity activity (34). During a 90-min soccer match, a sprint bout occur every ∼90 s, each lasting an average of 2-4 s (34). Systematic soccer training may induce neuro-muscular adaptations tending to increase PAP, such as a greater hypertrophy of type II muscle fibers (21) and/or an increased ability to activate high-threshold fast motor units during conditioning MVC (14). However, further studies comparing PAP in elite male power-trained, endurance-trained and soccer players, who train power and endurance simultaneously, are required.
We found a significant positive correlation between potentiation of twitch PT after the conditioning MVC and pre-MVC twitch RTD and RR. Twitch RTD and RR are indicators of muscle contraction and relaxation speed, respectively. Twitch RTD depends largely on rate of formation of cross-bridges during contraction (20) while twitch RR is related with speed of muscle relaxation (18). This is in agreement with previous studies indicating greater PAP of twitch PT in muscles consisting of a high percentage of fast-twitch fibers (13-15,36-38).
Numerous studies (17,34) have measured vertical jump height as indicator of muscle power of the lower limbs in soccer players. The results obtained in the present study were similar (2,7) to or lower (39) than those published previously in articles on professional soccer players.
Differences in soccer players´ training history (17,39) (i.e., with or without systematic strength training), competitive level (34) and in procedures used to measure vertical jumping performance may explain these discrepancies.
To our knowledge, only two studies (4,12) have measured 15-m sprint time in elite soccer players. Both studies reported similar values (2.30 and 2.35 s, respectively) to those obtained in the present study (2.28 s). In several studies 10-m or 30-m distances have been selected for sprint testing (34). In the present study, 15-m distance was selected because during a soccer game each sprint bout last on an average for 2-3 seconds (17,34).
In the present study, a significant negative correlation was found between vertical jump height in CMJ and SJ and 15-m sprint time. This is in accordance with previous studies in senior (39) and young (5,12) elite soccer players. For example, Wisloff et al., (39) observed in Norwegian senior elite players a significant correlation between vertical jump height and 10-m and 30-m sprint time. These results confirm the relation between vertical jump height and short-duration sprint time and agree with those biomechanical analyses of sprinting that have shown that short-distance sprint is highly dependent on the subject´s ability to generate powerful extensions of the KE, hip extensor and plantar-flexors muscles (22,35). In general, force produced at high velocities (i.e., vertical jumping) has been reported to be significantly related to sprinting performance (31).
In spite of the low overall inter-subject variability (coefficient of variation was between 5 and 17% for all the variables measured), the main result of this study was the significant correlation between PAP of twitch PT and RTD in KE muscles and the performance in vertical jumping and 15-m sprinting. Taking in account the limitations of a correlational design and the moderate “r” values obtained (accounting for less than 50% of the variance); this result confirmed our hypothesis and agrees with the published literature. One the one hand, it is well-known that high-threshold fast motor units of KE muscles are recruited during maximal intensity actions as vertical jumping or short-duration sprinting (6,9,19). On the other hand, PAP is greater for high-threshold compared with the low-threshold motor units (11). Small mammalian (1,11,13,36,37) and human (15,36) muscles with shorter twitch contraction times and a higher percentage of type II fibers exhibit greater twitch PAP. Hamada et al. (15), examined the correlation between fiber-type distribution and PAP in human KE muscles and showed that individuals with higher percentage of type II fibers demonstrated greater PAP. The greater PAP in type II muscle fibers is explained by a greater phosphorylation of myosin regulatory light chains in response to a conditioning activity (13,36), the likely mechanism of PAP (40). In consequence, the percentage of type II fibers in KE muscles may explain the relationship found between the magnitudes of PAP, the performance in vertical jumping and 15-m sprinting.
The present results are in agreement with those studies that considered PAP as a mechanism that can influence human neuromuscular performance (16,32). In mammalian models, PAP has been shown to enhance PT and RTD during isometric contractions (3,36,37) as well as peak power output during concentric contractions (1,13). In humans, there are reports of improved explosive muscle performance (i.e., vertical jumping) after the carrying out of conditioning contractions (for review see 16,32). Studies performed in our laboratory have shown as volitional (i.e., two sets of four reps at 80% of 1RM in parallel squat) (31) or electrical induced (7 s high frequency tetanus induced by percutaneous electrical stimulation on knee extensors) (30) conditioning contractions improve acute vertical jumping and isokinetic knee extension performance, respectively. Similarly, French et al., (8) demonstrated acute increases ranging 5-10% in drop jump and isokinetic knee extension after three sets of three MVICs interspersed by 3 min rest periods. The performance enhancement observed in these studies has been partly explained by PAP, although its presence was not tested. However, a recent study (3) performed on human adductor pollicis muscle has reported a significant enhancement of RTD (range: 9-24%) in ballistic voluntary contractions (10, 20, 50 and 75% of MVC) associated with PAP. All these findings argue that PAP, through subsequent increased RTD and acceleration, would increase peak velocity and power attained during ballistic voluntary contractions (3,13,30). Nevertheless, it is important to point out that a number of studies have not found any enhancement in acute explosive performance following a conditioning activity (16,32). Although the comparison between all these studies is difficult because of differences in methodology and design (i.e., intensity, duration and mode of the conditioning activity); the main discrepancy between them is probably related to the conditioning activity's dual effects of inducing both PAP and fatigue (10,27,32). It is well known that fatigue and potentiation are two opposed forms of force regulation during repeated activation that coexists in human skeletal muscles (27). Establishing a strategy for obtain the greatest benefit from PAP it's a complex process affected by a number of variables such as 1) the mode and intensity of the conditioning activity selected, affecting the relative amounts of PAP and fatigue produced (3,10,27); 2) the length of the rest interval between the end of the conditioning activity and the start of the performance to be improved, affecting the net effect of PAP decay and recovery from fatigue (10,32) and 3) the great inter-subject variability observed in response to the conditioning activity (14-16,23-25). Thus, further research attempts using strategies to produce PAP and its subsequent effects on human muscles are still needed to understand the role of PAP in muscle performance.
In conclusion, twitch PAP in KE muscles was significantly correlated with performance in vertical jumping and sprinting in male professional soccer players, whereas the magnitude of PAP in soccer players was similar to that observed previously in power-trained athletes (sprinters and jumpers). These results may be explained by the greater twitch PAP value in muscles with a higher percentage of type II fibers and suggest that (i) systematic soccer training may induce neuro-muscular adaptations tending to increase muscle PAP capacity and (ii) PAP is a mechanism related with the performance in activities requiring power and speed.
Although the correlations can only give insights of associations between variables and not into cause and effect, the results of the present study show that soccer players with a greater performance in 15-m sprinting and vertical jumping with and without countermovement tend to exhibit a greater postactivation potentiation in their knee extensor muscles. Moreover, it was observed that after systematic soccer training the potentiation measured was similar to that previously published in power trained athletes and higher than in endurance-trained and non-trained subjects. These results observed in highly-trained soccer players permit us suggest that PAP is a mechanism related with the performance in activities requiring power and speed. This suggestion is in agreement with previous studies that have shown an ergogenic acute effect during muscle potentiated state on power/speed activities (16). However, it is also important to note that there are many experiments in which this effect was not observed (16,32). Differences in the sample selected, the protocol designed to induce PAP and the activity used to measure PAP may principally explain these conflicting results. Our results permit us recommend to the coaches that PAP induced by means of a suitable conditioning activity (in mode, duration and intensity) and with an optimal recovery time between efforts individually selected for each athlete (allowing the muscle recover from fatigue and still be potentiated) is a mechanism that can be utilized to improve the performance in activities requiring power and speed. Specifically, PAP mechanism needs to be taken into account by the coaches during the design of (i) training sessions in which the athletes execute activities at maximal or near maximal intensity in which their fast-twitch MUs are generally recruited and, (ii) warm-ups before a competition in speed/strength sports with the aim of start the sport practice at the highest level of muscle potentiation. In addition, always that possible, we recommend including supramaximal twitch measurement at rest and during muscle potentiated state as part of the battery of tests used to control neuromuscular adaptations after a period of training.
The authors disclose professional relationships with companies or manufacturers who will benefit from the results of this study. The results of this study do not constitute endorsement of the product by the authors of the NSCA.
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