The Effects of the “Sling Shot” Device on Bench Press Performance, Mechanical Properties of Muscle, and Movement Kinematics : The Journal of Strength & Conditioning Research

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The Effects of the “Sling Shot” Device on Bench Press Performance, Mechanical Properties of Muscle, and Movement Kinematics

Wojdała, Grzegorz; Krzysztofik, Michał

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Journal of Strength and Conditioning Research 37(4):p 780-786, April 2023. | DOI: 10.1519/JSC.0000000000004349
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Upper-limb strength improvement is a substantial aspect of recreation and competitive sports training, whereas the bench press (BP) is often used for training, testing, or research purposes (30). The BP may be considered as a leading exercise used for developing strength, power, and hypertrophy of the upper body, particularly the pectoralis major, anterior deltoid, and triceps brachii muscles (10,30). However, it should be noted that the technique of the BP should reflect the specific requirements of a sports discipline (10). Furthermore, the course of the exercise itself can be configured by many factors, including exercise intensity and volume, movement tempo and velocity, time under tension, or range of motion (21,25). Moreover, athletes are diversifying their training routine more frequently to implement an additional stimulus to break through plateaus, avoid monotony, consolidate different training goals, or reduce the duration of training sessions (20). An accessory that is increasingly used in sports training, which significantly affects the kinematics of the BP movement along with the intensity and volume, is the sling shot (SS) supportive device. The SS is made of 2 resilient sleeves wrapping around the elbows connected with extensible material. The stretching of the material that makes up the SS provides additional elastic energy that supports the athlete in the eccentric phase of the movement and allows for a rebound effect in the concentric phase of the lift (36,37). Research has shown that SS affects BP performance variables, increasing results of the one repetition maximum test (1RM), the generated power and bar velocity, and the number of repetitions performed until muscular failure. The use of the SS also changes the surface electromyography readings (sEMG), thus changing the activity pattern of the prime movers depending on the applied external load (6,26,27,36–38).

Despite partial evidence of the effectiveness of the SS, previous studies lacked a unified approach to testing methodology, with the main objection being that the 1RM test was not measured separately for SS-assisted BP (6,26,36). According to available knowledge, only 2 studies have included a separate 1RM measurement for both (RAW) and SS-assisted BP conditions (9,38). The study by Gavanda et al. (9) showed no significant differences in the increases in maximal strength and muscle mass within the chest and arms after 8 weeks of the assisted SS training intervention. The authors did not measure any kinematic nor physiological variables of the movement, which does not allow for a complete analysis. Moreover, research by Ye et al. (38) has shown that using the SS in a single repetition with a load equal to 1RM of RAW value increases the velocity of the movement and the generated power. Nevertheless, using a separate 1RM measurement for the SS-assisted BP with a correspondingly greater external load, the movement speed and power generated were found to be the same regardless of whether the 1RM was obtained with or without the SS. Both of the presented studies (9,38) show contradictory results concerning previous ones involving the SS measurements, hence the resulting need for a more detailed analysis of this issue, taking into account various exercise intensities. Furthermore, the impact of the SS on endurance performance is also unclear. Although 2 studies (9,26) attempted to evaluate this phenomenon, only the number of repetitions were examined, which increased as a result of using the device. In this regard, however, there is a lack of analysis of the effects of the different loads on the efforts performed to muscle failure. The improvement in endurance can only be related to the lack of increase in external load between the RAW and SS protocols, whereas the actual impact of the SS in this regard may vary and requires an analysis in a broader context, analyzing different variables in the different characteristics of the effort.

Although a number of studies have analyzed muscle sEMG for both RAW and SS-assisted BP conditions (6,11,30,36), there is a gap in research investigating the mechanical properties of muscles. One of the methods to obtain such variables includes myotonometry, which has been shown to be a noninvasive and reliable method for assessing the biomechanical properties of particular muscles (4,7). The portable myotonometer device (Myoton) measures the mechanical response of the muscle tissue to a brief mechanical perturbation through a movable indentation probe and allows measurements in various environments, also during training. Myoton measurements not only are used to assess the mechanical properties of muscles and tendons but also show a near-linear correlation with sEMG activity and therefore provide an indirect measure of changes in the ability of muscles to generate force (4,19). Although the Myoton device has the capability to measure 5 variables (muscle tone, oscillation frequency, stiffness, decrement, relaxation time and creep), previous studies have defined only frequency of the oscillations (Hz) of the tested tissue after its deformation and stiffness (N·m−1), which determines the ability of the muscle to resist changes in its shape (16), as important factors affecting power and performance (14,18,24). However, the presented studies show ambiguous results, which do not allow to clearly determine whether the specific activity causes a decrease or an increase in stiffness and oscillation frequency. It can be assumed that the type of exercise, the intensity of the exercise, and the volume of the exercise have different effects on the mechanical properties of the muscles (14). With this in mind, it seems consistent to use myotonometric measurements to analyze the SS-assisted bench press. This will allow a more comprehensive analysis of the muscles and comparison of the obtained results with the sEMG measurements, which should indicate similar effects of the training aid. In addition, the level of fatigue measured with the Myoton device should also vary between conditions because of the unloading of the musculoskeletal system with the SS.

Considering the previously presented limitations in the research, we decided to conduct a research protocol with a separate measurement of 1RM depending on the conditions, allowing to determine the influence of the SS at different external load used and with different intensity of effort in male strength-trained athletes, taking into account the acute changes in power performance, muscle mechanical properties, and the BP kinematics. It was hypothesized that different loads and intensities will have different effects on the mechanical properties of muscles and that these will be accompanied by changes in performance.


Experimental Approach to the Problem

The subjects took part in a randomized crossover design procedure with 2 experimental sessions, preceded by a familiarization session, within 2 weeks. The experimental sessions followed the same procedure with the only difference being the lack of use (CONT protocol) or the usage of the SS (SS protocol) for each task. Both experimental sessions included the 1RM test, separately for the barbell bench press and the SS-assisted bench press. Afterward, the subjects performed the 3 sets of 2 repetitions of the bench press exercise with the load increasing every set (50-70-90% 1RM) with the fourth set performed to muscular failure with the load of 70% 1RM. All testing and familiarization trials were conducted at the same time of the day and were separated by at least a 92-hour recovery interval, and the subjects were instructed not to perform any additional exercise to avoid fatigue and keep their habitual hydration, sleep, and nutritional status. This setup aimed to investigate the influence of the SS at different external loads used and with different intensity of effort on acute changes in muscle mechanical properties, and the BP kinematics.


Twelve resistance-trained men (age = 27.1 ± 4.2 years, body mass = 90.3 ± 16.9 kg, bench press 1RM = 112.7 ± 23.1 kg) with training experience exceeding 3 years (6.9 ± 3.8) participated in the study. The subjects were free from neuromuscular and musculoskeletal disorders; 11 subjects were right handed, and one was ambidextrous (72.4 ± 36.5 laterality score from the Edinburgh Handedness Inventory [EHI]). Before commencement of the experiment, the subjects were informed about the main purpose of the study, potential benefits and risks of the study, and gave their written informed consent to participate in the experiment while being allowed to withdraw from the experiment at any moment. The research protocol was approved by the Institutional Review Board of The Jerzy Kukuczka Academy of Physical Education in Katowice (3/2021) and performed according to the ethical standards of the Declaration of Helsinki, 2013.


Familiarization Session

In the week preceding the study, each subject took part in a familiarization session. The familiarization session was used to accomplish basic measurements, complete documentation with the EHI questionnaire, and select the appropriate size of the SS together with the technical execution of the SS-assisted bench press for each subject. The subjects arrived at the laboratory at the same time of the day as the upcoming experimental sessions to diminish the effects of circadian rhythm. Initially, anthropometric measurements such as height and chest circumference together with body mass were measured. Furthermore, the SS size was established according to the body mass of the subject and manufacturer's guidelines (medium, large, and extralarge in size, each providing the same tension). The bench press grip width was standardized for each subject for all sessions and set at 150% biacromial distance with the tempo of movement defined as V/0/X/0 (denotes as volitional tempo of eccentric phase, no pause in the transition phase and maximal speed in concentric phase) (34,35). To standardize the technique of bench press attempts (without bouncing the bar off the chest or raising the hips off the bench (22)), the correct movement pattern was indicated by the resistance training coach along with the appropriate SS placement.

The subjects prepared for the effort by performing the general and specific warm-up, starting with 5-minute ergometer cycling (heart rate of 120–140 beats per minute). Afterward, the subjects performed an individual general warm-up focused on dynamic mobility exercises for the upper body. The specific part of the warm-up included 15, 10, and 5 bench press repetitions using 20, 40, and 60% of the estimated 1RM, respectively (20,22). Then, the subjects practiced the bench press as well as the SS-assisted bench press repeatedly until they felt comfortable performing the exercise (36,38). At last, the subjects performed 3–5 sets of a single bench press repetition with the SS using 80% 1RM to ensure technical proficiency (37).

Experimental Sessions

Two testing sessions with the same protocols were used in the experimental trials, except for the use of the barbell bench press (CONT) or the SS-assisted bench press (SS). The warm-up protocol, grip width, tempo of movement, and rack height remained the same as during the familiarization session. Second, the subjects accomplished the 1RM bench press test, appropriate to the protocol being performed, to assess upper-body maximal strength (21). The first testing load was set to an estimated 80%1RM and was increased by 2.5–10 kg for each following attempt until the subject would not complete the repetition. The 1RM was defined as the highest load completed within 5 attempts. The rest interval between subsequent sets was set at 5 minutes.

After the 1RM test, the subjects executed 3 sets of 2 repetitions of the bench press exercise at progressive loads of 50, 70, and 90% 1RM. Afterward, a single set was performed to momentary failure with the load equal to 70% 1RM. Muscular failure was defined as the inability to perform another concentric movement in its entire range of motion (15). The rest interval between successive attempts was set at 3 minutes. Immediately before and after each set (8 evaluations), the mechanical properties of triceps brachii long head muscle (both limbs) was assessed.

Assessment of Biomechanical Properties of the Muscle

To obtain the biomechanical properties of the triceps brachii long head, the MyotonPRO, hand-held myometer (MyotonPRO, Myoton AS, Tallinn, Estonia) was applied. Frequency of the oscillations (Hz) and stiffness (N·m−1) were subjected to further analysis. The Myoton's accelerometer was set at 3,200 Hz with an average value obtained from 5 consecutive measurements (0.4 N for 15 ms). The intraclass correlation coefficient (2-way mixed effects, absolute agreement, single rater) and coefficient of variation in this study were 0.96 (95% CI: 0.85–0.99) and 2.3% for nondominant limb and 0.92 (0.73–0.98) and 4% for dominant limb oscillation frequency, and they were 0.94 (0.80–0.98) and 4.9% for nondominant limb and 0.91 (0.71–0.98) and 5.3% for dominant limb stiffness.

Bench Press Performance Assessment

GymAware Powertool (Kinetic Performance Technology, Canberra, Australia), a linear position transducer, was used for the evaluation of peak bar velocity (PV), peak power (PP), time under tension (TUT), and number of repetitions (REP) as the reliable measuring device (3). The device was placed on the floor with the external end of the cable attached to the tip of the barbell. The velocity of the barbell was recorded at 50 Hz.

Statistical Analyses

All statistical analysis were performed using SPSS (version 25.0; SPSS, Inc., Chicago, IL) and were presented as means with SDs (±SD). Moreover, the 95% confidence intervals for mean values were also provided. Statistical significance was set at p < 0.05. The normality of data distribution was checked using Shapiro-Wilk tests. The effects of the conditions, sides, intensities, and time point (pre-post set measure) in muscle mechanical properties were examined by 4-way repeated-measures (2 conditions × 2 sides × 2 times × 4 intensities) analysis of variances (ANOVAs). The effects of the conditions and external load on the kinematic variables (2 conditions × 3 loads) were determined by 2-way repeated-measures ANOVAs. When significant, pairwise comparisons were also conducted using the Bonferroni’s test. The differences between conditions in one-repetition maximum, peak velocity, peak power, time under tension, and the number of repetitions during sets performed to failure were examined by paired sample t test. The effect size was determined by partial eta squared (η2). Partial eta squared values were classified as small (0.01–0.059), moderate (0.06–0.137), and large (>0.137). The magnitude of mean differences was expressed with standardized effect sizes (ESs); thresholds for qualitative descriptors of Hedges g was interpreted as ≤0.20 “small”, 0.21–0.8 “medium,” and >0.80 as “large.”


Performance Variables

The 2-way ANOVA (2 conditions × 3 loads) did not show a significant interaction (p = 0.209, η2 = 0.133) but showed a significant main effect of the SS for increased PP (p < 0.0001, η2 = 0.733) within the applied loads. The post hoc comparisons indicated a significantly higher PP at 50% (p < 0.001) and 70% 1RM (p < 0.001) during SS than in the CONT condition. No significant interaction (p = 0.741, η2 = 0.027) or main effect of condition (p = 0.459, η2 = 0.051) on PV was observed (Table 1).

Table 1 - Comparison of explosive performance between conditions.*
Condition 50% 1RM (95% CI) 70% 1RM (95% CI) 90% 1RM (95% CI) Failure (95% CI)
Peak bar velocity (m·s−1)
 CONT 1.29 ± 0.14 (1.2–1.38) 0.83 ± 0.13 (0.75–0.92) 0.49 ± 0.11 (0.42–0.56) 0.70 ± 0.14 (0.61–0.78)
 SS 1.31 ± 0.15 (1.21–1.41) 0.87 ± 0.11 (0.79–0.94) 0.50 ± 0.21 (0.36–0.63) 0.70 ± 0.11 (0.63–0.77)
 ES 0.13 0.32 0.06 0
Peak power (W)
 CONT 828 ± 165 (723–932) 691 ± 150 (595–786) 522 ± 126 (442–602) 586 ± 117 (512–660)
 SS 958 ± 203 (829–1,087) 807 ± 143 (716–898) 584 ± 234 (435–732) 637 ± 117 (563–711)
 ES 0.68 0.76 0.32 0.42
*CONT = control condition; SS = sling shot condition; ES = effect size.
Data are presented as mean ± SD and 95% confidence interval (95% CI).
Significant difference in comparison to corresponding set in CONT condition.

A paired sample t test revealed no significant differences between CONT and SS conditions for PP (p = 0.057) and PV (p = 0.911) during the set performed to failure (Table 1).

A paired sample t test indicated significantly higher 1RM (129 ± 26 vs. 113 ± 23 kg, p < 0.001; ES = 0.63), number of repetitions (15 ± 3 vs. 13 ± 2, p = 0.013; ES = 0.76), and time under tension (34 ± 10 vs. 29 ± 6 seconds, p = 0.017; ES = 0.59) during the set performed to failure for SS in comparison to CONT condition.

Muscle Biomechanical Properties

Oscillation Frequency

The 4-way ANOVA (2 sides × 2 conditions × 2 times × 4 intensities) did show a significant time × intensity interaction (p = 0.009; η2 = 0.291) and main effect of time (p = 0.001; η2 = 0.666) on oscillation frequency. The post hoc comparisons revealed a statistically significant increase from preset to postset measure in the second set (p = 0.038) for the dominant side and in the fourth set for both the dominant (p = 0.009) and nondominant (p = 0.023) sides in the CONT condition. Moreover, a statistically significant increase from preset to postset measure was found in the fourth set for the dominant side (p = 0.028) and nondominant side (p = 0.04) in the SS condition.


The 4-way ANOVA (2 sides × 2 conditions × 2 times × 4 intensities) did not show any significant interaction but a main effect of side (p = 0.034; η2 = 0.348) and time (p = 0.002; η2 = 0.613) on stiffness. The post hoc comparisons revealed a statistically significant increase from preset to postset measure in the fourth set for the dominant (p = 0.036) and nondominant (p = 0.022) sides during the SS condition. Moreover, a statistically significant higher stiffness was found in the dominant side compared with the nondominant side in all premeasures and postmeasures for stiffness (p < 0.05 for all; ES = 0.47–0.62), except post-second (p = 0.912) and post–fourth set measurement (p = 0.206) during the CONT condition (Figure 1).

Figure 1.:
Comparisons between pre–bench press and post–bench press percentage differences in muscle mechanical properties (A: frequency; B: stiffness) recorded for triceps brachii long head of dominant and nondominant arms with different conditions and sets. *Significant difference compared with preset measurement within the same condition; §significant difference.


This study analyzed acute changes in power performance, movement kinematics, and mechanical muscle properties of the long head of the triceps brachii muscle during the BP exercise for both RAW and the SS-assisted conditions performed at different external loads and intensities of effort. The results showed a significant increase in 1RM during the SS-assisted BP exercise compared with the RAW protocol, which affected subsequent variables. In addition, the results showed a significant effect of the SS on PP at all the intensities considered, whereas no effect on PV. The results of our study showed a significant increase in the number of repetitions and TUT with negligible influence on PP and PV for the SS-assisted BP compared with the RAW condition considering the sets performed until muscular failure, confirming our hypothesis about the different influence of the SS on movement variables in a strength endurance effort. Moreover, the mechanical properties of the triceps brachii indicated significantly higher oscillation frequency and stiffness for postset measurements compared with preset. Admittedly, there were no interactions taking into account the SS influence on the oscillation frequency and stiffness variables. However, a significantly higher stiffness was found in dominant limb than in nondominant limb during most of the measurements in the CONT condition.

This is the first study including a separate 1RM measurement for the SS condition to compare the impact of the SS elastic device on bench press performance, analyzing movement kinematics and muscle properties on both sides of the body. Most previous studies based their calculations on 1RM test conducted only under RAW conditions and may falsely suggest the benefits of using the SS (6,26,27,36). It should be taken into account that our results, as in previous publications, indicate that the maximal strength level measured with the 1RM test and peak power changed in favor of the assisted BP session. In addition, the velocity and power generated decreased with external load regardless of the BP protocol, which is consistent with the research on the bench press movement in different variations (13,17). However, results of peak velocity showed no difference between the conditions, which will be a key variable related to the kinematics of the movement. Thus, according to our results, the increase in the power generated in the SS condition is most likely only because of the increased external load while maintaining the same velocity, caused by the support of the SS material itself or the mechanically more favorable position of the elbow before reaching the sticking point (6,36). This confirms the conclusions of Ye et al. (38) and Gavanda et al. (9), in which the use of a separate 1RM measurement for the SS-assisted BP nullified the apparent benefits. These results are consistent with ours and demonstrate the need for appropriate load manipulation when using assisted BP. Combining elastic and free-weight resistance can be an effective way to increase maximal strength and power (1), but it can be critical to identify the elastic resistance support and the actual work the subject is performing. It should be remembered that the use of the SS without choosing a correspondingly higher workload may only seem to increase performance. However, the implementation of the SS, especially with loads greater than those used on a daily basis in the training routine, may result in overcoming mental barriers and fear of extremely heavy loading in trained individuals.

This study considered 2 important aspects related to the effects of the SS support during the BP exercise. Although the first is related to maximal strength and power, the second refers to strength endurance performance. In the context of effort to muscular failure, our results suggest that there are no significant differences between the conditions in velocity and power. This is in contrast to the results of Dugdale et al. (6) who was the only author to examine these variables during multiple repetitions within a set applying the SS assistance. The results of the abovementioned study show that when analyzing the PP and PV variables during 3 and 8 repetitions of the BP in particular sets, an increase in maximum velocity was found using the SS. However, it should be noted that these conclusions may not be reliable by using the same load under both RAW and SS conditions, and the SS effect itself on the power performance seems negligible. Conversely, based on our results, both more repetitions within the sets until failure and TUT under SS conditions can be mentioned despite the use of a separate 1RM. This may indicate the beneficial effect of the SS device on strength endurance efforts, which is consistent with the results of Pedrosa et al. (27) and Niblock and Steele (26), considering that they did not measure a separate 1RM level. This may be because of the greater support of the SS in the scale of the entire sets or because of the relief of the most demanding phase of the BP movement during the presticking point phase (6). Furthermore, the SS allows for a more rapid stretch during the transition from the eccentric to the concentric work of the bench press movement, improving the stretch-shortening cycle through the initial muscle stiffness and the postactivation potentiation effect within such a repetition (2,23). As research shows (8), this effect seems to be amplified when using the SS by increasing the external load. From a practical point of view, implementing increased volume training with the SS assistance seems to be more beneficial to improve local muscular endurance and hypertrophy of the upper body (15,28). In addition, increasing the TUT may be useful for enhancing strength and hypertrophic adaptations, especially considering the extended eccentric phase of the movement (35). However, it should be remembered that some of the work during the increased volume is done by the elastic components of the equipment, and it is necessary to increase the load accordingly to induce the desired adaptations.

The reported changes in oscillation frequency and stiffness partially confirmed our initial hypothesis about the influence of various intensities on the differences in mechanical properties of the muscle. Acute changes in mechanical properties of the muscle (measured by the Myoton) as a result of exercise were reported as fatigue or potentiation (acute improvement in muscle function because of previous muscle activity) (14,31,32). Previous studies have shown that increases in oscillation frequency and stiffness might be associated with fatigue and decreased performance (29,33); therefore, the reported increase in parameters independently of the condition suggests that some degree of fatigue has occurred. Apparently, an increased oscillation frequency may provide inadequate blood circulation and thus could be related to easier fatigability and overload (7). Moreover, according to the Henneman's size principle (12), these conditions required the activation of additional higher threshold motor units to reach the required muscle tension through the neuromuscular process of the central nervous system. Interestingly, there were no differences in the oscillation frequency and stiffness variables in relation to the RAW and SS conditions and also between the intensities. On the contrary, previous studies that included the measurement of muscle activity by sEMG and the use of the SS showed a significant difference between the conditions, body sides, and applied external loads (6,36,38). The authors have shown that the use of the SS acutely reduces muscle activity, which, however, increases with increasing load, and that there are also significant differences in activity between the upper limbs (37). It also seems that the SS support increases the relative involvement of the nondominant limb while decreasing the sEMG muscle activity of both limbs (37). Similarly, in this study, the dominant limb triceps brachii long head stiffness was significantly higher than that in the nondominant limb in the CONT condition, with no such differences in the SS condition. Given that an increase in stiffness may suggest fatigue, and previous indication that the use of a SS increases the involvement of the nondominant side, it may be suggested that the use of a SS could be beneficial when the aim of the training is to reduce the asymmetry between the limbs.

The lack of the influence of intensity acknowledged in our research between the SS and RAW protocols may be related to the similar involvement of the muscles with the use of a separate 1RM measurement rather than using the same absolute load. Similar conclusions were reached by Ye et al. (38) who demonstrated no significant differences in the EMG activity of the pectoralis major, anterior deltoid, and triceps brachii muscles when using the maximum load determined in the RAW and SS conditions separately. Thus, when the 1RM measurement is relative and performed separately for the SS and RAW, the work done by the muscles is comparable. Both sEMG and myotonometry variables seem to be reliable tools for diagnosing muscle properties, but neither of these methods can be used alone for a comprehensive assessment of muscle function (19). However, it is worth mentioning that the increase in the oscillation frequency and stiffness from preset to postset measurement in sets performed to muscle failure highlights the fact that the degree of fatigue has increased. The higher signs of stress that appeared could be explained by the fact that the number of repetitions during a high workload resulted in higher TUT compared with the other protocols (5). Accordingly, it seems advisable to consider the biomechanical properties of muscles simultaneously with the analysis of power performance to determine whether the difference in muscle properties can be a reliable indicator of performance potential.

The results of this study have several limitations that must be addressed. Only one muscle (triceps brachii long head) was analyzed for its mechanical properties. Because exercise involving multiple joints were performed in this study, it cannot be excluded that the results could be different for other muscles involved in the BP movement. For an even more detailed analysis of the effects of the SS on movement, biomechanical variables such as the angles in the joints or the moments of force acting on them would also have to be analyzed. Moreover, residual fatigue or potentiation could have influenced the measurements because subjects performed one set after another. In addition, residual fatigue or potentiation could impact the measurements because subjects always performed sets in the same, not randomized, order. The methodology also lacks the evaluation of long-term training on the mechanical properties of muscles, which may be different from the acutely occurring changes. Finally, the differences in comparing our Myoton results with sEMG results from previous studies suggest that it would be necessary to perform the studies with both devices simultaneously to obtain a complete evaluation of the working muscles under different conditions and loads.

Practical Applications

The results of this study point to the need for a separate assessment of maximal strength levels when using the SS device in exercise and training routines. An independent 1RM measurement and a correspondingly higher workload are required to take full advantage of the SS. Otherwise, the effects may be negligible, and the assistance that results from using the equipment is only apparent, excluding mental help in overcoming sticking points. The SS can also be used with success for increased volume to stimulate muscle endurance and local hypertrophy and increase the involvement of nondominant limb during the bench press exercise. Considering modern training solutions, the Myoton seems to be a reliable and compact tool for assessing local fatigue and muscle properties during training, especially immediately before and after specific exercises or between the sets.


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resistance training; training equipment; supportive training device; myotonometry; muscle stiffness

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