La velocidad de ejecución como variable para el control y la dosificación del entrenamiento y como factor determinante de las adaptaciones producidas por el entrenamiento de fuerza

Resumen

Movement Velocity as a Critical Variable for Prescribing and Monitoring Resistance Exercise, and as a Determinant Factor of the Resistance Training Induced-Adaptations This thesis encompassed three consecutive studies that built upon each other’s findings and were aimed at investigating the role played by movement velocity as a variable for prescribing and monitoring resistance exercises, and as a critical variable for determining the adaptations to resistance training (RT). In the first study (Study I), we analyzed (1) the pattern of repetition velocity decline during a single set to muscle failure in the bench press (BP) and full squat (SQ) exercises against different submaximal loads; (2) the reliability of the percentage of performed repetitions with respect to the maximum number that can be completed for different magnitudes of velocity loss within a set to failure; and (3) the estimated degree of fatigue and its recovery after a single set to muscle failure through isometric and dynamic activations. In the second study (Study II), we examined the acute mechanical and physiological responses to 16 resistance exercises protocols performed with different level of effort in both the SQ and BP exercises. Finally, in the third study (Study III), we compared the effects of RT programs with different magnitudes of repetition velocity loss allowed in each set (10%, 30% and 45% in the SQ exercises, and 15%, 40% and 55% in the BP exercise) and different relative intensities (55-70% 1RM and 70-85% 1RM) on functional, neural and hormonal adaptations. Resistance training is recognized as an effective method for improving athletic performance because it typically results in increases in muscle strength and hypertrophy, power output, speed, and local muscular endurance (Kraemer & Ratamess, 2004; Ratamess et al., 2009). However, one of the main problems faced by coaches, strength and conditioning professionals and researchers is the issue of how to objectively quantify and monitor the actual training load undertaken by athletes in order to maximize performance (González-Badillo & Sánchez-Medina, 2010). Although several acute variables have been described for the design of RT programs (Bird, Tarpenning, & Marino, 2005; González-Badillo & Ribas, 2002; Kraemer & Ratamess, 2004; Ratamess, et al., 2009), it appears that exercise intensity and volume are the two most critical factors in determining the type and extent of the resulting neuromuscular adaptations (Bird, et al., 2005; Fry, 2004; González-Badillo, Marques, & Sánchez-Medina, 2011; González-Badillo & Ribas, 2002; Tan, 1999). Exercise intensity during RT has been traditionally identified with relative load (percentage of one-repetition maximum, %1RM) or with the maximal load that can be lifted a given number of repetitions in each set (e. g., 5RM, 10RM, 15 RM) (Fry, 2004; González-Badillo, et al., 2011; González-Badillo & Ribas, 2002; González-Badillo & Sánchez-Medina, 2010; Tan, 1999). However, these methods appear to have some potential disadvantages (González-Badillo, et al., 2011; González-Badillo & Sánchez-Medina, 2010). As an alternative, recent research has examined the possibility of using movement velocity as an indicator of relative load during resistance exercise (González-Badillo, et al., 2011; González-Badillo & Sánchez-Medina, 2010; Sánchez-Medina, González-Badillo, Perez, & Pallares, 2014; Sánchez-Medina, Pallarés, Pérez, Morán-Navarro, & González-Badillo, 2017; Sánchez-Moreno, Rodríguez-Rosell, Pareja-Blanco, Mora-Custodio, & González-Badillo, 2017). Close relationships between movement velocity and %1RM have been found for exercises such as the bench press (BP), prone bench pull, squat and pull-up (González-Badillo, et al., 2011; González-Badillo & Sánchez-Medina, 2010; Sánchez-Medina, et al., 2014; Sánchez-Medina, et al., 2017; Sánchez-Moreno, et al., 2017), which makes it possible to determine with considerable precision the %1RM that is being used as soon as the first repetition of a set is performed with maximal voluntary velocity (González-Badillo & Sánchez-Medina, 2010). Such findings open up the possibility of monitoring, in real time, the actual load (%1RM) being used by measuring repetition velocity during RT, thus allowing to determine whether the proposed load (kg) truly represents the %1RM that was intended for each session. On the other hand, training intensity per se is not sufficient to define the training stimulus and should be associated with other acute training variables, especially with the training volume (Bird, et al., 2005; Fry, 2004; González-Badillo & Ribas, 2002). The training volume is generally determined from the total number of sets and repetitions performed during a training session (Bird, et al., 2005; Hass, Feigenbaum, & Franklin, 2001; Kraemer & Ratamess, 2004). Thus, when training volume is prescribed, the vast majority of studies use a specific number of repetitions to complete in each exercise set for all participants. However, the maximal number of repetitions that can be completed against a given relative load has been found to present a large variability between individuals (Douris et al., 2006; Sakamoto & Sinclair, 2006; Shimano et al., 2006; Terzis, Spengos, Manta, Sarris, & Georgiadis, 2008). Therefore, if during a training session all participants perform the same number of repetitions per set against the same relative load (e. g., 70 % 1RM), it is possible that they are exerting a different level of effort (i. e., the number of repetitions left in reserve in each set may vary considerably between individuals). These considerations suggest it is necessary to find better ways to objectively monitor training volume during RT. Accordingly, rather than performing a fixed, predetermined, number of repetitions, it seems more appropriate to stop or terminate each training set as soon as a certain level of neuromuscular fatigue is detected (which, in turn, will depend on the specific goal being pursued) (González-Badillo, et al., 2011; Pareja-Blanco, Rodríguez-Rosell, Sánchez-Medina, Sanchis-Moysi, et al., 2016; Sánchez-Medina & González-Badillo, 2011). During RT in isoinertial conditions, and assuming every repetition is performed with maximal voluntary effort, an unintentional decrease in force, velocity and hence power output is observed as fatigue develops and the number of repetitions approaches failure (Izquierdo, González-Badillo, et al., 2006; Pareja-Blanco, Rodríguez-Rosell, Sánchez-Medina, Sanchis-Moysi, et al., 2016; Sánchez-Medina & González-Badillo, 2011). Recent research has shown that monitoring repetition velocity is an objective, practical and non-invasive indicator of the acute metabolic stress, hormonal response and mechanical fatigue induced by RT (González-Badillo et al., 2016; Pareja-Blanco, Rodríguez-Rosell, Sánchez-Medina, Ribas-Serna, et al., 2016; Sánchez-Medina & González-Badillo, 2011). Repetition velocity has shown a very similar pattern of decrease during a single set to failure for loads ranging from 60 % to 75 % (Izquierdo, González-Badillo, et al., 2006). However, to our knowledge, the question of how many repetitions remain undone (left in reserve) in an exercise set when a given magnitude of velocity loss is reached (e. g., 20, 30 or 40 % reduction in repetition velocity) has not yet been investigated. Therefore, in the context of a velocity-based resistance training approach, in the Study I of the present Thesis we analyzed (1) the pattern of repetition velocity decline during a single set to muscle failure in the bench press (BP) and full squat (SQ) exercises against different submaximal loads; (2) the reliability of the percentage of performed repetitions with respect to the maximum number that can be completed for different magnitudes of velocity loss within a set to failure; and (3) the estimated degree of fatigue and its recovery after a single set to muscle failure through isometric and dynamic activations. To this end, three separate partial studies were undertaken: Study I.1, Study I.2 and Study I.3. Knowledge of the mechanical and physiological aspects underlying RT is essential to improve our understanding of the stimuli affecting the neuromuscular adaptations (Crewther, Cronin, & Keogh, 2005; Crewther, Keogh, Cronin, & Cook, 2006; Sánchez-Medina & González-Badillo, 2011). The combination of the different acute resistance training variables result in very different training protocols and, consequently, different stimuli for neuromuscular adaptations (Kraemer & Ratamess, 2004; Spiering, et al., 2008; Toigo & Boutellier, 2006). Therefore, it seems important to carry out investigations that allow us to know how to modulate the neuromuscular, metabolic and endocrine response modifying the main variables that define the training load (volume and intensity) Different studies have analyzed the mechanical and physiology response after: (1) different resistance training protocols to muscle failures using different load magnitudes (e. g., 10RM vs. 5RM) (Kraemer, Dziados, et al., 1993; Kraemer, Fleck, et al., 1993; Kraemer et al., 1990; Leite et al., 2011; Smilios, Pilianidis, Karamouzis, & Tokmakidis, 2003), (2) resistance exercise protocols performing maximum (to failure) vs. half-maximum number of repetitions per set against the same load (e.g., 5 of 10 possible repetitions with a given load compared with performing all repetitions (10RM) (González-Badillo, et al., 2016; Gorostiaga, Navarro-Amezqueta, Calbet, et al., 2012; Gorostiaga et al., 2014; Gorostiaga, Navarro-Amezqueta, González-Izal, et al., 2012; Pareja-Blanco, Rodríguez-Rosell, Sánchez-Medina, Ribas-Serna, et al., 2016); and (3) resistance training protocols performing maximum and forced repetition (Ahtiainen & Hakkinen, 2009; Ahtiainen, Pakarinen, Kraemer, & Hakkinen, 2003, 2004; Gentil et al., 2006). In addition, a recent study (Sánchez-Medina & González-Badillo, 2011) analyzed the acute mechanical and metabolic response to 15 resistance exercise protocols differing in the number of repetitions performed in each set with respect to the maximum predicted number against a given load. The results of this study (Sánchez-Medina & González-Badillo, 2011) allowed us to know the validity and practical relevance of the concept level of effort (relationship between the degree of requeriment demanded by performing a determined physical effort and the actual possibilities of a subject at a given moment, i.e., the relationship between what was done and what could be done) as a variable for monitoring and dosing the resistance training. However, as a result of studies published in relation to the use of movement velocity for monitoring resistance exercise (González-Badillo, et al., 2014; González-Badillo & Sánchez-Medina, 2010; Izquierdo, et al., 2006; Pareja-Blanco, et al., 2014; Pareja-Blanco, Rodríguez-Rosell, Sánchez-Medina, Sanchis-Moysi, et al., 2016; Sánchez-Medina & González-Badillo, 2011), rather than a determined, fixe, load (kg) and number of repetitions, the level of effort during resistance training should be prescribed using two variables: (1) first repetition’s mean velocity, which is intrinsically related to loading intensity (González-Badillo & Sánchez-Medina, 2010), and (2) the magnitude of velocity loss attained in each exercise set because it is closely linked to the actual level of effort being incurred (Sánchez-Medina & González-Badillo, 2011). Therefore, the acute mechanical, metabolic and neural response to 16 resistance training protocols defined by the best mean propulsive velocity (MPV) (usually the first repetitions) over the set and the percentage of MPV loss (%VL) within the set in both SQ and BP exercises were analyzed in Study II of this Thesis. Although several studies (Ahtiainen, Pakarinen, Alen, Kraemer, & Hakkinen, 2003; Bird, et al., 2005; Drinkwater et al., 2005; Kraemer & Ratamess, 2004; Rooney, Herbert, & Balnave, 1994; Willardson, 2007; Willardson, Norton, & Wilson, 2010) have suggested that performing repetitions to failure may be necessary to maximize muscle strength and hypertrophy, other studies seem to indicate that similar, and even greater, strength gains and improvements in athletic performance can be obtained without reaching muscle failure (Folland, Irish, Roberts, Tarr, & Jones, 2002; Izquierdo-Gabarren et al., 2010; Izquierdo, Ibanez, et al., 2006; Pareja-Blanco, Rodríguez-Rosell, Sánchez-Medina, Sanchis-Moysi, et al., 2016; Pareja-Blanco, Sánchez-Medina, Suarez-Arrones, & González-Badillo, 2016; Sampson & Groeller, 2016). The rationale for performing resistance exercises to failure is to maximize motor unit recruitment (Davies, Orr, Halaki, & Hackett, 2016; Willardson, 2007), although it has also been hypothesized that failure compared with non-failure training could lead to greater elevation of anabolic hormone levels (Davies, et al., 2016; Drinkwater, et al., 2005; Schoenfeld, 2010), which may contribute to resistance training-induced changes in muscular strength (Ahtiainen, et al., 2003; Davies, et al., 2016; Ronnestad, Nygaard, & Raastad, 2011). However, recent evidence have shown that motor unit recruitment can be maximized without the need to perform resistance exercise to failure (Desmedt & Godaux, 1977; Gorostiaga et al., 2012; Sundstrup et al., 2012), and elevation of anabolic hormone levels is not required for significant increases in muscular strength (West et al., 2010; West et al., 2009; Wilkinson, Tarnopolsky, Grant, Correia, & Phillips, 2006). Therefore, the controversial results found in the literature clearly emphasize the need to conduct further research on this topic. As mentioned above, movement velocity is a critical variable for accurately estimate training intensity and volume during resistance training. Thus, movement velocity should be used as a reference variable for monitoring and dosing the resistance exercise. In relation to this novel velocity-based resistance training approach, a recent study (Pareja-Blanco, Rodríguez-Rosell, Sánchez-Medina, Sanchis-Moysi, et al., 2016) compared the effects of two RT programs in SQ exercise only differing in the level of effort quantify by repetition velocity loss allowed in each set: 20% (VL20%) vs. 40% (VL40%) on muscle structural and functional adaptations. Relative magnitude of training loads (Determined through the first repetition’s mean velocity), number of sets (three), and inter-set recoveries (4 min) were kept identical for both groups in each training session. The results of this study (Pareja-Blanco, Rodríguez-Rosell, Sánchez-Medina, Sanchis-Moysi, et al., 2016) showed that VL40% resulted in greater muscle hypertrophy and reduction of myosin heavy chain IIX percentage than VL20%. Although VL20% resulted in lower hypertrophic response, this group resulted in similar squat strength gains than VL40% and greater improvements in CMJ and sprint performance. Thus, it seems that performing resistance training only using SQ exercise at 70-85% 1RM and low degree of fatigue (20% velocity loss) in each training set induce greater improvements on neuromuscular performance than performing the same training program with maximum or near maximum degree of fatigue (40% velocity loss) in each training set. However, considering that, against load magnitudes of 70-85% 1RM in SQ exercises, (1) a 20% velocity loss involves performing approximately 50% of maximum possible repetitions, and (2) a 40% velocity loss means performing approximately 70-80% of maximum possible repetitions, there exists a wide range of percentage of performed repetitions remains unanalyzed. Thus, based on studies published to date, we can not assert that velocity losses in the series above 20% tend to produce lower gains on strength and athletic performance. In addition, the level of effort depends on velocity loss in the set (volume), but also of the first repetition’s mean velocity (relative intensity), and that such level of effort is different depending on the exercise used (SQ or BP exercise) (Sánchez-Medina & González-Badillo, 2011). Therefore, the Study III was aimed to analyze the effects of different MPV losses in the set and load magnitudes in SQ and BP exercises on the changes in neuromuscular performance and resting hormonal concentration. To this end, two separate partial studies were undertaken: Study III.1 and Study III.2.   STUDY I.1 Title: Analysis of movement velocity decline during a single set to muscle failure against 8 submaximal load magnitudes (50, 55, 60, 65, 70, 75, 80 and 85% 1RM) in the BP exercise. Purpose: The main aim of this study was to analyze the pattern of repetition velocity decline during a single set to failure against different submaximal loads (50 - 85 % 1RM) in the bench press exercise. A secondary aim was to analyzed the relationship between the maximal number of completed repetitions against a given load magnitude and different mechanical and anthropometric variables. Methods: A group of 22 young healthy men (mean ± SD: age 24.6 ± 3.6 years; height 1.76 ± 0.06 m; body mass 75.8 ± 7.2 kg) volunteered to participate in this study. Participants performed 8 testing sessions in random order, every 6 - 7 days. During each session, participants performed a test of maximum number of repetitions to failure (MNR test) against the corresponding load: 50, 55, 60, 65, 70, 75, 80, and 85% 1RM. Relative loads were determined from the load-velocity relationship for the BP. Results: A very close relationship was found between the relative loss of velocity in a set and the percentage of performed repetitions for all 8 relative load used. This relationship was very similar for all loads, but particularly for 50 - 70 % 1RM, even though the maximal number of repetitions completed at each load was significantly different. Equations to predict the percentage of performed repetitions from relative velocity loss are provided. The number of repetitions performed against each load showed no relationship to either anthropometric (body mass, height, arm length) or mechanical variables (MPV of the fastest repetition in the set, MPV of the last repetition in the set, loss of MPV over the set, estimated 1RM and relative strength ratio). Conclusions and practical applications: By monitoring repetition velocity and using these equations provided, one can estimate, with considerable precision, how many repetitions are left in reserve in a bench press exercise set. Therefore, our results highlight the practical importance of using the loss of repetition velocity for monitoring the level of effort and the training volume during resistance exercise.   STUDY I.2 Title: Estimation of neuromuscular fatigue during isometric and dynamic activations after a single set to muscle failure at 60% 1RM. Reliability of percentage of repetitions performed against different velocity losses in the set. Purpose: This study aimed: (1) to analyze the reliability of the percentages of performed repetitions with respect to the maximum possible number that can be completed in a set to failure at 60% 1RM when a given magnitude of MPV loss (15 - 75 %) is reached; and (2) to compare the degree of estimated fatigue and its recovery during isometric and dynamic activations after a single set to muscle failure at 60% 1RM in the BP exercise. Methods: Twenty-eight young healthy men (mean ± SD: age 23.5 ± 2.9 years; height 1.77 ± 0.07 m; body mass 75.5 ± 8.1 kg) volunteered to participate in this study. After initial evaluations, participants performed two testing sessions separated by 5 - 7 days. During each session, participants carried out a single set to muscle failure (MNR test) at 60% 1RM in the BP exercise. As estimation of the degree of fatigue induced during the set to failure, and its later recovery, it was analyzed the change in different mechanical variables [MPV, peak velocity (PV), peak force (PF), peak power (PP) and rate of force development (RFD)] measured during isometric and dynamic activations immediately after the effort (Post 1) and at 3 (Post 2), 5 (Post 3), 10 (Post 4), 15 (Post 5), 20 (Post 6) min post-effort. During the first testing session, fatigue was estimated by dynamic activations, whereas in the second testing session fatigue was estimated by isometric activations. Results: No significant differences were found for any mechanical variable between the MNR test 1 and 2. Paired t-tests revealed no significant differences between trials for any percentage of repetitions completed at each magnitude of MPV loss. The percentages of repetitions completed had very high absolute reliability (CV: 2.1 - 6.6 %), with lower CV values as the loss of MPV over the set increased. All kinetic and kinematic variables assessed during dynamic and isometric activations showed a significant decrease immediately after the MNR test, and none of them recovered the initial values following 20 min rest time, except the PF during isometric activations. In additions, the degree of loss and recovery of PF and RFD variables was similar for isometric and dynamic activations in all time intervals assessed. Conclusions and practical applications: Considering de strong correlation between the percentage of MPV loss and the percentages of performed repetitions with respect to the maximum possible number that can be completed as well as the high absolute reliability of the percentage of performed repetitions when a given magnitude of MPV loss is reached, it is recommended to use the loss of repetition velocity for monitoring the training volume during resistance exercise. In addition, the degree of fatigue and recovery following resistance exercises should be assessed by dynamic activations and preferably using movement velocity (MPV and PV) and RFD variables.   STUDY I.3 Title: Analysis and comparison of movement velocity decline during and after a single set to muscle failure against 4 submaximal load magnitudes (50, 60, 70 and 80% 1RM) in the BP and SQ exercises. Purpose: The present study was designed to analyze and compare: (1) the magnitude of velocity loss during and after a single set to muscle failure against 4 different relative intensities; and (2) the percentage of velocity loss after each set to muscle failure depending on the maximal number of repetitions completed in both the BP and SQ exercises. Methods: Twenty young healthy men (mean ± SD: age 25.0 ± 3.5 years; height 1.77 ± 0.06 m; body mass 76.0 ± 7.2 kg) volunteered to participate in this study. Participants performed 8 testing sessions in random order, every 6–7 days. During each session, participants performed a MNR test (4 in the BP exercise and 4 in the SQ exercise) against 4 submaximal load magnitudes: 50, 60, 70 and 80% 1RM. Relative loads were determined from the load-velocity relationship for the each exercise. The degree of fatigue after each MNR test was estimated by percentage of velocity loss attained with the individual load that elicited a ~1 m•s-1 (L1m•s-1) before the effort. Results: A very close relationship was found between the relative loss of velocity in a set and the percentage of performed repetitions for all 4 relative loads used in the BP and SQ exercises. Comparisons between both exercises showed that for the same percentage of MPV loss over the set, the percentage of performed repetitions with respect to the maximum possible number that can be completed was greater in the SQ than BP exercise for all four loads used. The percentage of loss of MPV with the L1 m•s-1 load was significantly greater for the BP than the SQ exercise in all four loads used. No significant differences in the average loss of MPV with L1 m•s-1 were found between groups for any of load magnitudes used in the BP or SQ exercise. Conclusions and practical applications: Coaches and strength and conditioning professionals should consider using the magnitude of velocity loss attained in each exercise set for monitoring training volume during RT. In addition, our results have also shown that there are some differences in term of percentage of repetitions completed depending on percentage of velocity loss over the set distinctive of each exercise and load magnitude used that should be taken into account when prescribing training volume through velocity loss.   STUDY II Title: Acute effect of different resistance training protocols determined by the first repetition’s mean velocity and maximum percent velocity loss to be allowed in each set on mechanical, metabolic and neuromuscular response. Purpose: This study aimed to analyze the acute mechanical, metabolic and neural response to resistance exercise protocols (REP) differing in the first repetition’s mean velocity (MPVbest) and percentage of velocity loss in the set (VL). Methods: Twenty-one young healthy men (11 in the SQ exercise [mean ± SD: age 23.5 ± 2.9 years; height 1.77 ± 0.07 m; body mass 75.5 ± 8.1 kg] and 10 in the BP exercise [mean ± SD: age 23.5 ± 2.9 years; height 1.77 ± 0.07 m; body mass 75.5 ± 8.1 kg]) volunteered to participate in this study. During a period of approximately 10 weeks, each participant performed 17 testing sessions in the following order: 1) an initial test with increasing loads for the individual determination of 1RM strength and full load-velocity relationship in the SQ or BP exercise, 2) 16 REP determined by the best MPV (usually the first repetitions) over the set (MPVbest) and the percentage of MPV loss (%VL) within the set (S). Thus, REP in the SQ exercise were configured as follows (S x MPVBEST [%VL]): 3 x 1.13[10%], 3 x 1.13[20%], 3 x 1.13[30%], 3 x 1.13[45%], 3 x 0.98[10%], 3 x 0.98[20%], 3 x 0.98[30%], 3 x 0.98[45%], 3 x 0.82[10%], 3 x 0.82[20%], 3 x 0.82[30%], 3 x 0.82[45%], 3 x 0.68[10%], 3 x 0.68[20%], 3 x 0.68[30%], 3 x 0.68[45%], whereas REP in the BP exercise were configured as follows: 3 x 0.93[10%], 3 x 0.93[20%], 3 x 0.93[30%], 3 x 0.93[45%], 3 x 0.79[10%], 3 x 0.79[20%], 3 x 0.79[30%], 3 x 0.79[45%], 3 x 0.63[10%], 3 x 0.63[20%], 3 x 0.63[30%], 3 x 0.63[45%], 3 x 0.47[10%], 3 x 0.47[20%], 3 x 0.47[30%], 3 x 0.47[45%]. These 16 REP sessions were randomized for each participant. In order to analyze the acute mechanical, metabolic and neural response to each REP, participants undertaken a battery of measurements at before and immediately after each effort: blood lactate concentration, and the individual load that elicited a ~1.00 m•s-1 (± 0.03 m•s-1) MPV (L1m•s-1), and vertical countermovement jump (CMJ) height, surface EMG signals and sprint time in 20 m (T20) (only in SQ group). In addition, as an estimation of the level of effort induced by each REP, it was calculated an Effort Index as follow: MPVbest x average %VL in the training session. All testing sessions were conducted on separate days, with at least 72 h of recovery time. Results: In both exercises, the MPV loss with L1m•s-1 and post-effort blood lactate concentration were progressively greater as the percentage of MPV over the set increased at each relative intensity used. For the same percentage of MPV loss over the set, the MPV loss against L1m•s-1 and post-effort blood lactate concentration were progressively greater as the relative intensity decreased. The changes for CMJ and T20 after the different REP were similar to those shown by the MPV loss with L1m•s-1. The MPV loss against L1m•s-1 was significantly greater for BP compared with SQ and strongly correlated to peak post-exercise lactate (r = 0.95 - 0.96) for both the SQ and BP. The Effort Index showed a strong relationship with (1) the MPV loss with L1m•s-1 (r = 0.92 - 0.98), (2) peak post-exercise lactate concentration (r = 0.91 - 0.95), (3) CMJ height loss (r = 0.93), (4) changes in T20 (r = 0.77), and (5) EMG signal (r = 0.54 - 0.80). Conclusions and practical applications: The high correlations found between the Effort Index and mechanical (velocity against L1m•s-1, CMJ height and T20 losses), metabolic (lactate) and neural measures of fatigue support the validity of using the Effort Index to objectively estimate neuromuscular fatigue during resistance training.   STUDY III.1 Title: Comparison of two percentages of MPV loss in the set (10% and 30% in the SQ exercise, and 15% and 40% in the BP exercise) against the same relative intensities (70 - 85% 1RM) on neuromuscular performance and chronic hormonal changes. Purpose: This study aimed to compare the effects of two RT programs only differing in the level of effort achieved in each set, objectively quantified by repetition velocity loss allowed in each set: low-velocity loss group (VLlow; 10% and 15% for SQ and BP exercise, respectively) and medium-velocity loss group (VLmedium; 30% and 40% for SQ and BP exercise, respectively) on neuromuscular adaptations and resting hormonal concentration. Methods: Twenty-five young males were randomly assigned to a VLlow (n = 12) or VLmedium (n = 13) training group. Participants followed an 8-week velocity-based RT program using the SQ and BP exercises with relative loads of 70-85% 1RM and executing every repetition at maximal intended velocity. Movement velocity in all repetitions was registered by a linear velocity transducer. The 1RM strength, full load-velocity profile in SQ and BP exercise, CMJ height, T20 and resting hormonal concentration were determined pre- and post-training. In addition, EMG signal of vastus lateralis (VL) and rectus femoralis (RF), and pectoralis major (PM) and triceps brachii (TB) was recorded during the isoinertial loading test in SQ and BP exercises, respectively. Results: For the SQ exercise, both experimental groups showed significant improvements in all variables analyzed, except in T20 for VLmedium. The percentage of change and intra-group effect size was greater for VLlow compared to VLmedium in all variables assessed during SQ exercise. In addition, VLlow showed an increment in the neural activity after resistance training period, whereas EMG variables in VLmedium remain unchanged. For the BP exercise, both experimental groups also showed significant improvements in all strength and muscle endurance variables. However, unlike the SQ exercise, the VLmedium resulted in greater percentage of change in all variables assessed than VLlow. Moreover, increments in EMG amplitude and power frequency of PM and TB were found in both training groups. On the other hand, VLmedium showed greater muscle damage and worse testosterone-cortisol balance than VLlow. Conclusions and practical applications: Training with a low compared to medium percentage of MPV loss in the set during SQ exercise produces greater gains in muscle strength, EMG activity and high-speed actions such as jump and sprint, as well as lower hormonal stress. However, in order to obtain greater beneficial effects in BP exercise, training sessions should be conducted allowing a greater percentage of velocity loss in the set.   STUDY III.2 Title: Comparison of three percentages of MPV loss in the set (10, 30, and 45% in the SQ exercise, and 15, 40 and 55% in the BP exercise) against the same relative intensities (55 - 70% 1RM) on neuromuscular performance and chronic hormonal changes. Purpose: This study aimed to compare the effects of three RT programs only differing in the level of effort achieved in each set, objectively quantified by repetition velocity loss allowed in each set: low-velocity loss group (VLlow; 10% and 15% for SQ and BP exercise, respectively), medium-velocity loss group (VLmedium; 30% and 40% for SQ and BP exercise, respectively) and high-velocity loss group (VLhigh; 45% and 55% for SQ and BP exercise, respectively) on neuromuscular adaptations and resting hormonal concentration. Methods: Thirty-four young males were randomly assigned to a VLlow (n = 11), VLmedium (n = 11) or VLhigh (n = 13) training group. Participants followed an 8-week velocity-based RT program using the SQ and BP exercises with relative loads of 55 - 70% 1RM and executing every repetition at maximal intended velocity. Movement velocity in all repetitions was registered by a linear velocity transducer. The 1RM strength, full load-velocity profile in SQ and BP exercise, CMJ height, T20 and resting hormonal concentration were determined pre- and post-training. In addition, EMG signal of VL and RF, and PM and TB was recorded during the isoinertial loading test in SQ and BP exercises, respectively. Results: For the SQ exercise, the training group reaching the lower velocity loss in the set (VLlow) obtained greater improvements in jump and sprint performance, and similar or even greater gains in strength and muscle endurance, depending on the variable analyzed, than the VLmedium and VLhigh. In addition, the VLmedium showed greater sprint improvements and lower increments in jump performance than VLhigh. The improvements in VLlow were accompanied by an increment in electrical muscle activity (EMG) for RF and VL muscles, whereas EMG variables in VLmedium and VLhigh remain unchanged. For the BP exercise, all three experimental groups showed significant improvements in all strength and muscle endurance variables. However, the VLmedium resulted in greater percentage of change in all variables assessed than VLlow and VLhigh, whereas the changes in VLhigh were slightly greater than in VLlow. No changes were observed in EMG variables for any muscle assessed during the BP exercise. On the other hand, VLhigh showed greater muscle damage and worse testosterone-cortisol balance than VLlow and VLmedium. Conclusions and practical applications: Resistance training involving low compared to medium or high percentage of MPV loss in the set during SQ exercise produces greater gains in muscle strength, EMG activity and high-speed actions such as jump and sprint, as well as lower muscle damage and catabolic status. However, in order to obtain greater beneficial effects in BP exercise, training sessions should be conducted allowing a greater percentage of velocity loss in the set.