LA FATICA NEUROMUSCOLARE NEL CALCIO: ASPETTI CENTRALI E PERIFERICI

Soccer is a team sport that requires prolonged high-intensity intermittent exercise. Physical aspects of match performance have changed a lot in the last forty years; for example total distance has increased from 7000-8000 m in the seventies to 10000-11000 m reported in recent investigations. Nowadays, during the match, players change activity on average every 5 seconds and perform approximately 1300 actions, with 200 of these being completed at high-intensity. In addition, the game involves other intense actions such as decelerations, kicking, dribbling, and tackling. All of these efforts exacerbate the physical strain imposed on the players and contribute to making football highly physiologically demanding. Furthermore, soccer players that compete at high level perform up to 70 matches during a single season. During the second half of a match, the total distance and high-intensity running decline markedly, with the amount of high-intensity running 20% to 40% lower in the last 15 min of the game compared with the initial 15-min period. A greater decrement in running is observed when more activity is performed in the first half. Furthermore, in the 5-min period following the most demanding 5-min of the game, the distance covered at high intensity is reduced by 6% to 12% compared with the game average. Collectively, these results indicate that players experience fatigue toward the end of a match and temporarily during a game. Accordingly, both single and repeated-sprint test performances are impaired after a high-intensity period during as well as at the end of the game. Fatigue may also have a negative impact on passing precision, with the less fit players showing a more pronounced deterioration in technical performance. Furthermore, maximal strength and jumping ability are also both reduced immediately after a match. The time required for a full recovery of these qualities in male players may be very long (> 72 h) (permanent fatigue). However, there is limited information regarding recovery time in high level athletes. In literature, fatigue can be defined as an acute impairment of performance that includes both an increase in the perceived of the effort necessary to exert a desired force or power and/or reduction in the ability to exert maximal force or power. It has been demonstrated that the neuromuscular mechanisms that contribute to performance impairment differ according to the contraction mode, exercise intensity and the duration. Fatigue can be classified as central when the origin is proximal and/or peripheral when the origin is distal to the neuromuscular junction. However, to date, no studies have quantified the relative contribution of central and peripheral factors in determining fatigue in soccer. Peripheral skeletal muscle function can be determined using electrical stimulations and classified as high-frequency and low-frequency (or long-lasting fatigue). Long-lasting fatigue can be evaluated using tetanic nerve stimulations at different frequencies (low- to high-frequency force ratio). Nonetheless, tetanic nerve stimulations are very painful, and their usability is limited. Paired stimulations have been proposed as a surrogate of the stimulation trains, and recent research demonstrates the validity of this method to quantify muscle fatigue induced by eccentric exercise. In chapter 2, a study was conducted to determine the extent to which neuromuscular fatigue occurs in high-level professional soccer players. In addition to this, we aimed to examine recovery after a match and to examine its relationship with central and peripheral fatigue indicators. Maximal voluntary contraction (MVC), sprint and passing abilities, muscle soreness, maximal voluntary activation, EMG activity, and evoked quadriceps contractile properties (using different electrical stimulations) were determined before, immediately after, and at 24 and 48 h after the match in 20 male professional players. Immediately after a game, fatigue was evident from the reduction in MVC and sprint performance (-11%, P < 0.001 and -3%, P < 0.001, respectively) as well as increased muscle soreness (P < 0.001) compared with baseline. However, short-passing ability was seen to be preserved. This was observed alongside maximal voluntary activation and EMG activity being reduced by 8% (P < 0.001) and 12% (P = 0.001), respectively. M-wave characteristics were unchanged, whereas quadriceps mechanical responses to single and paired stimulations at 10 Hz resulted significantly reduced (peak torque = -8%, P < 0.001 and -9%, P < 0.001). Despite the observation of a partial recovery 24 h after the game, all variables returned to baseline values 48 h after the match. In high-level professional players, the fatigue induced by a soccer match occurs to a lesser extent than in lower level athletes and appears to be recovered in a faster fashion. Match-related fatigue is determined by a combination of central and peripheral factors. A relationship exists between central fatigue indicators, MVC and sprint performance decrements, whereas muscle soreness appears to be linked to peripheral fatigue indicators, in particular to mechanical responses at low-frequency stimulations. In chapter 3, a study was conducted to determine the possible relationship between match related fatigue and goal scoring. Some preliminary observations indicate that goal scoring increases over time during the game; however, these preliminary results were obtained analyzing minor soccer leagues and/or using a limited amount of matches. The aim of this study was to examine the goal scoring patterns in four major European leagues (English Premier League, French Football Ligue 1, Italian Serie A and Spanish Football Liga) across three consecutive seasons (2008–09, 2009–10 and 2010–11). Chi square analysis revealed a significantly (P < 0.001) higher goal scoring frequency in the second half (55.1 %) compared to the first half (44.9 %). With 15-min interval analysis highlighting a significant (P < 0.001) upward trend in the number of goals scored as time progressed (13.7, 15.1, 16.2, 17.7, 17.2 and 20.2 %, for 0–15, 15–30, 30–45, 45–60, 60–75 and 75–90 intervals respectively). The main finding of the present study is that, in the four major European soccer leagues investigated, the amount of goals scored is greater in the second half, with the scoring rate being highest in the last 15-min of a match. Fatigue occurring during the game may play a role; however, fatigue alone cannot explain the differences. The state of the games result and the reduced time available for scoring are other potential contributing factors to the increased goal scoring patterns observed in the latter stages of the game. Repeated-sprint ability (RSA) is defined by the ability to perform several sprints interspersed with brief recovery periods. RSA is an essential component of many team sports like soccer and is deemed to be an important determinant of physical performance. Several studies have been carried out to establish the physiological determinants of RSA and to investigate the fatigue mechanisms related to this particular form of exercise in order to improve performance. However, no study to date has investigated the high- and low-frequency fatigue induced in more than one muscle group involved during fatiguing RSA exercise. In chapter 4, a study was presented aimed to investigate peripheral fatigue in Knee Extensor (KE) and Plantar Flexor (PF) muscles RSA cycling and running tests. Both RSA tests consisted of 5 x 6 s sprints and peripheral fatigue was quantified using diverse electrical stimulations (from a single stimulus to paired stimulations at 100 Hz). The main findings were that RSA cycling induced higher decrements in peak torque (PT), maximal rate of torque development and relaxation of KE compared with running (PT decrements at different stimulation frequencies: -39% to -53% cycling vs -16% to -39% running, P < 0.049). The PT ratios of KE measured using several low- and high-frequency stimulations did not highlight differences in low-frequency fatigue induced by the two exercise modes. On the contrary, no major differences were noted in PT decrements of PF (P > 0.231). However, the higher decrements of some PT ratios of the PF (10/100 Hz, 20/50 Hz and 20/100 Hz) confirmed the presence of low-frequency fatigue following RSA cycling. Subjects also reported significantly higher values for RPE leg following RSA cycling (8.2 vs 7.3 respectively, P = 0.018) despite no differences in blood lactate, hydrogen ions and bicarbonates concentrations (P > 0.467). The higher level of peripheral fatigue induced by RSA cycling may be partially related to longer fractional duration of muscle contraction phases which can limit local blood flow. The discrepancies in neuromuscular fatigue between KE and PF can be partially explained by differences in muscle fibre composition and/or muscle contributions during RSA tests. A number of studies have determined the reproducibility of peripheral muscle contractile properties at rest reporting good values of coefficient of variation. Only one study has assessed the reliability of the quadriceps contractile properties in a fatigued state. This was achieved using a sustained (2 minutes) maximal isometric contraction. However, this form of exercise is far from specific for the majority of sporting performances. In order to use electrical stimulations as a tool to determine the extent of peripheral muscle fatigue experienced by athletes it is necessary to carry out methodological studies that establish the reliability of peripheral muscle fatigue measures caused by a specific exercise. For these reasons, in chapter 5 a reliability study was conducted to determine the reproducibility of peripheral fatigue induced in knee extensors by high-intensity intermittent running test (HIT). The reliability of the assessment of peripheral fatigue was established in ten amateur soccer players (age: 18 ± 1, height: 178 ± 5 cm, weight: 68 ± 8 kg). Transcutaneous electrical stimulations before and after HIT determined knee extensors muscle contractile properties on three separate occasions (T1, T2 and T3), each separated by 7 days. No significant differences were noted for any of the parameters measured (P = 0.101). The ICC values for peak torque (PT) varied from moderate to high, with the exception of PT at 10 Hz calculated comparing T2 vs T1 (range PRE = 0.78 - 0.92; POST = 0.76 - 0.97). The ICC derived from PT percentage decrements data were all low, with the exception of PT decrements at 1 Hz calculated comparing T3 vs T2 (ICC = 0.85, moderate). The TE for all contractile parameters obtained using 1 Hz and 100 Hz electrical stimulations were below 10%, including some that demonstrated a TE lower than 5%. Muscle contractile properties determined using 10 Hz stimulations showed a higher level of TE (range: 3.2 - 15.1%). Similar results were obtained for maximal rate of torque development and torque relaxation. From the results of this study it can be concluded that muscle contractile properties express a good level of reliability in baseline and post-exercise measures following familiarization. In chapter 6, seasonal variations in peripheral neuromuscular fatigue induced by HIT were established in two groups of professional soccer players: eleven adult players (age: 23 ± 3, height: 181 ± 4 cm, weight: 78 ± 6 kg) and twelve young players (age: 17 ± 1, height: 181 ± 4 cm, weight: 71 ± 5 kg). Transcutaneous electrical stimulations before (PRE) and after (POST) HIT determined knee extensors muscle contractile properties on four separate occasions during a season (before the preparation period Test 1, after the preparation period Test 2, in the mid of competitive period Test 3 and towards the end of the competitive period Test 4). Muscle contractile properties at rest were partially reduced during the competitive period both in adult and young professional players (reduction of PRE maximal rate of torque development at 1 Hz (P < 0.024) and the same parameter at 10 Hz (P < 0.022)). Analysis of POST parameters, confirmed the reduction of the high-frequency fatigue levels during the competitive period for adult players (higher values of POST peak torque at 100 Hz (P = 0.006) and POST maximal rate of torque relaxation at 100 Hz, P = 0.009). Furthermore, in young players, the reduction of both high-frequency fatigue and low-frequency fatigue were evident (POST maximal rate of torque development at 100 Hz (P = 0.035), POST maximal rate of torque relaxation at 100 Hz (P = 0.009), POST peak torque at 10 Hz (P = 0.038) and POST 10/100 Hz ratio (P = 0.012). Considering percentage decrements, several parameters confirm the presence of lower level high-frequency and low-frequency peripheral fatigue both in adult and young players during the competitive period. These results suggest that it is possible to use the assessment of muscle contractile properties to monitor soccer players physical condition. In conclusion, fatigue in soccer is determined by a combination of central and peripheral factors. Central fatigue is evident immediately after and in the hours of recovery following a match (permanent fatigue). While peripheral fatigue is more evident considering some aspects of soccer performance characterized by short high-intensity periods (temporary fatigue). Peripheral fatigue include both high-frequency and low-frequency fatigue with the latter being more evident in young players. Training interventions and strategies applied to improve performance levels should take these aspects into consideration.