Neuromuscular abilities play a determinant role in contemporary soccer. Professional players are required to perform progressively greater sprinting distances per match (Barnes et al., 2014), with straight sprints known to be the most frequent action to create opportunities for goal scoring (Faude et al., 2012). Hence, it is a paradox that typical soccer preparation negatively affects the players' neuromuscular development due to the interference phenomenon between endurance (i.e., technical-tactical sessions) and strength-power training (Faude et al., 2013b; Loturco et al., 2015c; Noon et al., 2015). This phenomenon represents a challenge to the coaching staff that needs to conciliate adequate ball-related training volume--aerobic loading--and preservation of the neuromuscular capacities that support the increasingly high soccer physical match demands (Barnes et al., 2014; Folgado et al., 2014).
Of note, the combination of reduced technical-tactical training volume and adequate neuromuscular stimuli (e.g., plyometrics and jump squat exercises) can produce positive effects on strength-power abilities (Loturco et al., 2015a; 2015d; Ramirez-Campillo et al., 2015). As an example, it was reported that either increasing or decreasing the bar velocity during jump squat training were capable of improving the jumping ability and sprinting speed in soccer players undertaking less than 300 min of accumulated technical-tactical training per week (Loturco et al., 2015a), with the strategy of increasing the bar velocity producing superior adaptations in sprinting speed. Accordingly, "unloaded" plyometric training using vertical and horizontal jumps has been shown to be effective in improving sprinting and jumping abilities during soccer preseasons performed with similarly low technical-tactical training exposure. Furthermore, combining strength and power training during the competitive season was able to improve isometric peak strength and rate of force development, without altering sprinting speed (Faude et al., 2013a). On the other hand, the absence of specific strength-power interventions can impair sprinting speed and isokinetic strength throughout the competitive season (Eniseler et al., 2012; Magal et al., 2009). In summary, the concurrent training effects and/or the detraining of power-speed abilities will result in loss of "sport form" in soccer players, with negative effects on match running locomotor performance (Silva et al., 2013).
In spite of the widely described seasonal changes in the neuromuscular abilities in soccer players, less is known about the mechanical responses of the trained muscles during the course of different training periods and their respective consequences on speed and power performance. In this regard, tensiomyography (TMG) has been used as a simple and non-invasive tool to assess the mechanical properties of skeletal muscles. Several contractile parameters can be derived from the application of this technique, such as the contraction time (Tc), delay time (Td) and maximal radial displacement (Dm). Contraction time (which is obtained by determining the time lapse from 10% to 90% of Dm) has been related to the speed of force generation, while Td is related to the mus cle fiber conduction velocity and Dm to the muscle belly stiffness (Simunic et al., 2011; Tous-Fajardo et al., 2010). These indices are highly reproducible (ICC = 0.92 to 0.97; CV = 2.7 to 4.7%) and correlated with physiological and performance characteristics (Tous-Fajardo et al., 2010). For example, Tc presented a correlation of 0.88 with the proportion of myosin heavy chain I of the vastus lateralis muscle (Simunic et al., 2011), whereas Td of the same muscle in sprinters and endurance runners presented a correlation of -0.72 with countermovement jump height (Loturco et al., 2014). Interestingly, a recent study (Garcia-Garcia et al., 2016) involving soccer players showed that, in response to 10 weeks of training, knee extensor muscles presented a 17.7% to 22.7% decrease in Tc, an 8.7% to 9.9% decrease in Td, and a 12.2% to 14.2% decrease in Dm, while there was an 11.9% increase in Td and a 24.5% increase in Dm in the knee flexors. Unfortunately, the investigated players did not execute any performance test. Consequently, it was not possible to link the changes in each of the TMG parameters, measured in the different muscle groups, to specific functional performance changes. In addition, the differences in the magnitude of the percentages and the dissimilarity in the direction of changes observed in each one of the examined contractile parameters have hampered a more integrated and functional view of their prospective neuromuscular adaptations.
Hence, although the TMG-derived measures acceptably represent some "isolated" muscle contractile properties, it is very difficult for sports coaches to correctly interpret and practically implement the outcomes in the athletes' training routines. One possible way to make the TMG outcomes simpler to use and interpret is by combining them into a single index that integrates several of the meaningful and reliable mechanical outcomes. In this regard, by arranging the TMG variables, the velocity of contraction (Vc) can be calculated as follows: Vc = Dm/Td+Tc. This index seems to be a practical and useful way to assess the mechanical functionality of the muscles. Some authors have already suggested some indices related to the muscle contraction velocity to properly assess muscle functionality in top-level athletes (Rodriguez-Matoso et al., 2012); however, its sensitivity to training effects (especially in soccer where TMG has been proposed to be useful (Barcelona, 2015)) and possible relation with changes in soccer players' neuromuscular performance need to be addressed.
Therefore, the aim of this study was twofold: 1) to verify whether professional soccer players express changes in speed-power related abilities and muscle mechanical properties during the an 8-week training period and; 2) to examine the similarities between the direction of these mechanical and performance changes. It was hypothesized that professional soccer players would experience adverse changes in speed-power related abilities [due to concurrent training practices commonly found in soccer (Loturco et al., 2015c; Silva et al., 2013)] and these changes would be accompanied by impairments in muscle mechanical properties (i.e., TMG-derived Vc).
Twenty-two male Brazilian elite soccer players (age: 23.8 [+ or -] 4.2 years, height: 1.77 [+ or -] 0.07 m, body mass: 76.2 [+ or -] 8.0 kg) from the same professional soccer team took part in this study. The investigated team had recently finished participating in the most competitive Brazilian regional championship (Sao Paulo State 1st division) and, at the time of this investigation, was in preparation for the inferior division of the Brazilian National championship. After being informed of the experimental risks, the soccer players signed a written informed consent to participate in this study. The research was approved by the local Ethics Committee.
This is a repeated measures design study assessing elite soccer players' jumping and sprinting abilities, along with muscle contraction properties provided by TMG, prior to and after an 8-week training period. This period was implemented in the transition between the State and the National championships. One-week off was provided to allow players time to recover from participation in the State championship. The pre-training tests were performed 24-h prior to the first training session, while the post-training tests were executed 48-h after the final training session. The first competitive match took place 2 weeks following the post-tests. Both pre-tests and posttests were performed in the following order: 1) TMG measurement; 2) squat jump (SJ), countermovement jump (CMJ), and drop jump (DJ); 3) jump squat (JS); 4) 20-m linear sprint and 5) change of direction (COD) sprint. The TMG indices were assessed with the players not performing any warm-up, as it can bias the muscular contraction mechanical parameters. Prior to testing sessions, a general and specific warm-up routine was performed, involving light running (5-min at a self-selected pace followed by 3-min lower limb active stretching) and submaximal attempts at each testing exercise (e.g., submaximal vertical jumps or sprints). All tests took place in the same sports laboratory and commenced at the same time of day ([approximately equal to] 9 a.m.) on both occasions. All athletes had been previously familiarized with the tests due to their regular training and testing routines. During the 8 weeks of training, the athletes completed, on average, 8 training sessions per week, which consisted of [approximately equal to] 6 technical and tactical (game-based training) and [approximately equal to] 2 strength-power (e.g., unloaded vertical and horizontal jumps, and JS) training sessions. Table 1 shows a typical weekly training routine during the respective training period.
The Dm, Tc, Td and Vc were recorded from both the rectus (RF) and biceps femoris (BF) muscles from the dominant leg (Rey et al., 2012), using a Tensiomyography device (TMG Measurement System, TMG-BMC Ltd., Ljubljana, Slovenia) (Figure 1). The Dm corresponds to the radial movement of the muscle belly expressed in millimeters...