Do faster, stronger, and more powerful athletes perform better in resisted sprints?

DO FASTER, STRONGER AND MORE POWERFUL ATHLETES PERFORM 1 BETTER IN RESISTED SPRINTS? 2 3 ABSTRACT 4 This study aimed to analyze the relationships between different strength, power 5 and speed abilities and resisted sprint performance across a wide range of sled loads (10%, 6 30% and 50% body mass “BM”). Seventy-nine young physically active male sports 7 science students (age: 22.8 ± 3.4 years; BM: 74.2 ± 9.1 kg; height: 175.4 ± 8.5 cm) 8 performed two testing sessions. Session 1 consisted of 20-m sprint without any additional 9 load and with 10, 30 and 50% BM. Session 2 consisted of countermovement jump (CMJ) 10 and full squat (SQ) tests. CMJ was performed without any additional load, and with loads 11 of 30 and 50% BM, and SQ was performed with loads corresponding to 30, 50, 70 and 12 90% BM. Resisted sprint times were moderate to large correlated with unloaded sprint 13 times (r = .79 to .89), unloaded and loaded jump height (r = -.62 to -.71), and SQ 14 performance (r = -.56 to -.71). The magnitude of the relationships between velocity loss 15 induced by each sled load and jump and SQ performance increased as the magnitude of 16 the sled loads increased. In conclusion, differences in speed, strength and power abilities 17 may explain, at least partially, the individual response of each athlete during sprinting 18 towing a sled. Thus, faster, stronger and more powerful athletes require heavier relative 19 sled %BM to experience a similar exercise intensity. 20 Keywords: sprint training, sprint performance, sled towing, strength, power, speed training 21 22 23 INTRODUCTION 24 Resisted sled training (RST) is an effective approach to improve sprint 25 performance, especially over the acceleration phase (1,26). Applying external resistance 26 in form of weighted sled allows athletes to produce greater horizontal force and impulse 27 during ground contact phase (5,15), replicating to some extent the movement patterns of 28 unloaded sprints (2,8,18). This training methodology may induce specific adaptations to 29 improve sprint performance (13). Sled loading strategies have varied greatly between 30 studies (1,26). As such, some researchers have suggested sled loads of approximately 10- 31 12% of body mass (BM) to reduce alterations in sprint mechanics (2,18), whereas others 32 have reported that heavier sled loads may be necessary to provide optimal overload for 33 sprint acceleration (11,15,16,23). 34 One of the main challenges for coaches and researchers is centered on how to 35 objectively control and quantify the individual training load (i.e., sled weight). Overall, 36 in sled-based training, the load prescription is based on different percentages of BM 37 (3,25,31). However, this strategy does not account for inter-individual differences as, for 38 example, the extent to which sprint performance is affected (16). Linthorne and Cooper 39 (17) reported high inter-individual differences (up to 25%) in the rate of decrease in sprint 40 velocity under similar loading conditions (i.e., similar percentages of BM). Thus, 41 prescribing sled loading based solely on respective percentages of BM may fail to provide 42 a uniform and consistent overload among athletes. Alternatively, loading based on 43 percentages of velocity loss, relative to unloaded sprints, over a given distance (i.e. sled 44 mass that reduces sprint velocity by a given percentage) has been proposed in an attempt 45 to solve these important limitations (15,26). 46 It has been suggested that athlete´s characteristics (i.e. strength, power, speed or 47 technical ability) may influence on the extent to which sprint performance is affected 48 (7,15,24). However, the relationship between fitness characteristics (i.e. strength, power 49 or speed) and resisted sprint performance has been barely explored. Martínez-Valencia et 50 al. (22) reported significant relationships between the rate of increase in resisted sprint 51 time and countermovement jump (CMJ) height (r = -.73), CMJ relative peak power (r = 52 -.81) and squat jump relative peak power (r = -.80) in male sprinters. Thus, athletes with 53 a greater power-to-BM ratio may require greater relative sled %BM to experience a 54 similar exercise intensity (22). However, sled loads heavier than 20% BM have not been 55 analyzed in terms of correlations. 56 Recent research has suggested that very-heavy sled loads (~80% BM or ~50% 57 velocity reduction) would provide an optimal overload for sprint acceleration, as it allows 58 athletes to generate higher amounts of horizontal force and impulse while sprinting 59 (10,23). However, as above-mentioned, the relationships between resisted sprint 60 performance with heavy loads (>20% BM), and strength, power, and unloaded sprint 61 velocity have not previously been investigated. Therefore, the potential influence of 62 strength, power and speed qualities on the RST stimuli when heavy sled loads are 63 employed is unknown. A better understanding of the relationship between resisted sprint 64 performance and strength, power, and sprint velocity would enable greater insight into 65 more effective and tailored strategies for sled load prescription. Therefore, this study 66 aimed to analyze the relationships between different strength, power and speed abilities 67 and resisted sprint performance across a wide range of sled loads (10%, 30% and 50% 68 BM). As it was previously demonstrated that sprint performance is related to relative 69 strength and power outputs (9,30), and the power-to-BM ratio is related to the rate of 70 increase in resisted sprint time with sled loads <20% BM (20), we hypothesized that 71 athletes with greater relative strength-power values would perform better in resisted 72 sprints (i.e., higher velocities and lower degrees of VL), and that these relationships would 73 increase as the sled load increases. 74 75 METHODS 76 Experimental Approach to the Problem 77 A descriptive cross-sectional design was used. Testing was performed in two 78 different sessions, separated by 1 week. Session 1 consisted of 20-m sprint without any 79 additional load and with 10, 30 and 50% BM. Session 2 consisted of CMJ and full squat 80 (SQ) tests. CMJ was performed without any additional load, and with loads of 30 and 81 50% BM, and SQ was performed with loads corresponding to 30, 50, 70 and 90% BM. 82 Participants refrained from any strenuous physical activity for at least 3 days before each 83 testing session. All sessions were performed at the same time of the day for each subject 84 and under controlled environmental conditions (20-22ºC and 55-65% humidity) in a 85 research laboratory. 86 87 Subjects 88 Seventy-nine young physically active male sports science students (age: 22.8 ± 89 3.4 years; BM: 74.2 ± 9.1 kg; height: 175.4 ± 8.5 cm) volunteered to take part in this 90 study. All the subjects were regularly participating in team sports or individual sports 91 such as: athletics, soccer, basketball, handball in the 6 months before testing, 1-3 times 92 per week on average. Their performance standards ranged from recreational to regional- 93 level athletes. No physical limitations, health problems, or musculoskeletal injuries that 94 could affect the testing were reported. Once informed about the purpose, testing 95 procedures and potential risks of the investigation, all subjects gave their voluntary 96 written consent to participate. The study was approved by the Local Ethics Committee 97 and conducted in accordance with the Declaration of Helsinki. 98 99 Procedures 100 During the 2 weeks preceding this study, two preliminary familiarization sessions 101 were undertaken to improve the technique of resisted sprints, CMJ and SQ exercises. BM 102 was determined during the morning in a fasting state using a scale (Seca 710, Seca Ltd., 103 Hamburg, Germany). 104 105 Sprint testing 106 On the first day, two 20-m unloaded trials (T20) and one 20-m trial with 10, 30 107 and 50% of BM (T20-10%BM, T20-30%BM, and T20-50%BM, respectively) were 108 performed, with 3 min of recovery between each trial. The sprint times were recorded by 109 photocell gates (Witty; Microgate, Bolzano, Italy), based on a single beam radio impulse 110 transmission system and a reflection system, which were placed at 0 and 20-m. Sprints 111 were performed from a static biped starting position with the start line located 1-m behind 112 the first timing gate. Participants were encouraged to run the 20-m as fast as possible. A 113 sled (Ranking, Navarra, Spain) fixed to the subject with a belt at the approximate height 114 of the center of mass and a 3.5-m long strap attached to the sled was used. The best time 115 of the two 20-m trials was recorded for further analysis. Sprint velocity was calculated 116 as the ratio between a 20-m distance and the time interval to cover such distance. Sprint 117 velocity reduction induced by each loading condition was calculated as the decrement in 118 mean sprint velocity elicited by the sled load with respect to the unloaded condition (VL 119 = (unloaded condition – loaded condition)/loaded condition·100). The sprint test was 120 conducted on a synthetic running track in an indoor hall. All participants completed a 15 121 min standardized warm-up protocol consisting of 5 min jogging at a self-selected easy 122 pace followed by four 20-m running accelerations at 80, 85, 90 and 95% effort, 123 respectively, with 2 min rest periods between them. Lastly, a trial of 10-m running all-out 124 was performed. Intra-session test–retest reliability measured by the coefficient of 125 variation (CV) was 2.0% and the intraclass correlation coefficient (ICC) values were 0.92 126 (95% confidence interval, CI: 0.88‒0.95). 127 128 Jump measurements 129 On the second day, after a standardized warm-up, consisting of 5 min of jogging, 130 joint mobility and preparatory vertical jumps, participants performed a progressive 131 loading test in vertical jump. The CMJ was performed on an infrared timing system 132 (OptojumpNext, Microgate, Bolzano, Italy) that calculated jump height (h) through flight 133 time (t) and acceleration due to gravity (g) as follows: h = t2 · g · 8-1. The unloaded CMJ 134 was performed with both hands on the waist, while performing a downward movement 135 until about 90º-knee flexion followed by a vertical jump of maximum effort. The 136 participants were required to perform 3 trials separated by 15 s rest. The mean of the 3 137 trials was used for subsequent analyses. Just after the unloaded CMJ test, a loaded CMJ 138 test was performed with progressive loads of 30 and 50% of BM (CMJ-30%BM and 139 CMJ-50%BM). Height loss (HL) induced by each loading condition was calculated as 140 the decrement in height elicited by each load with respect to the unloaded condition. Tests 141 were performed using a Smith machine (Multipower Adam Sport, Granada, Spain), which 142 allowed a smooth vertical displacement of the bar along a fixed pathway. The athletes 143 performed two CMJ separated by 3 min rest with each load. The mean height of the 2 144 trials with each load was used for subsequent analyses. Intra-session test-retest reliability 145 for jump variables was: unloaded CMJ (ICC: 0.99; CI: 0.99-1.00; CV: 2.3%) and loaded 146 CMJ (ICC: 0.98; CI: 0.97-0.99; CV: 4.0%). 147 148 Full Squat test 149 After the jump test, an incremental loading SQ test was performed. The test was 150 conducted in the same Smith machine as the loaded jump testing. Subjects performed 3 151 repetitions against each load (30, 50, 70 and 90% of BM). The inter-set recovery time 152 was 3 min. The participants started from the upright position with their knees and hips 153 fully extended, the barbell resting across their back at the level of the acromion. Each 154 subject descended at a controlled velocity (~0.50-0.70 m·s-1) until the top of the thighs 155 got below the horizontal plane, and then they immediately reversed the motion and 156 ascended back to the upright position. The participants were always required to execute 157 the concentric phase in an explosive manner, at maximal intended velocity. Strong verbal 158 encouragement was provided to motivate participants to give a maximal effort. A linear 159 velocity transducer (T-Force System, Ergotech, Murcia, Spain) was used to register the 160 bar velocity and power. All velocity and power measures reported in this study 161 correspond to the mean values of the propulsive phase of each repetition, which is defined 162 as the fraction of the concentric phase during which the barbell acceleration was greater 163 than the acceleration due to gravity (29). Therefore, mean propulsive velocity (MPV) and 164 mean propulsive power (MPP) were analyzed. For each load, the highest MPV (SQV) 165 and MPP in both absolute terms (SQP) and relative to BM (SQPrel) were recorded for 166 further analysis. 167 168 Statistical Analyses 169 All data are reported as means ± standard deviation (SD). The distribution of each 170 variable was verified by the Shapiro-Wilk normality test. Test-retest reliability was 171 measured by the CV and ICC with 95% CI, calculated with the one-way random effects 172 model. Inter-individual variability was calculated from inter-individual CV as follows: 173 SD/mean · 100. The relationships between variables were determined using Pearson’s 174 coefficients (r) and 90% CI. The correlation coefficients were qualitatively interpreted as 175 follows: <0.1, trivial; 0.1 - 0.3, small; 0.3 - 0.5, moderate; 0.5 - 0.7, large; 0.7 - 0.9, very 176 large; >0.9 nearly perfect (14). The level of significance was set at 0.05. SPSS for Mac 177 (IBM Corporation, New York, NY, USA) (release 20.0.0) was used for all statistical 178 analyses. 179 180 RESULTS 181 The mean values and SD of the different variables assessed are shown in Table 1. It was 182 observed a great inter-individual variability in the VL induced by each percentage of BM 183 (inter-individual CV for 10%BM: 30.9%, 30%BM: 16.0%; and 50%BM: 9.5%). 184 185 ***Insert Table 1 about here*** 186 187 Relationships between sprint times and velocity loss with different percentages of body 188 mass. 189 The relationships between sprint times and VL magnitude in different unloading 190 and loading conditions are shown in Table 2. Significant correlations were found between 191 T20 and resisted sprint times (r = .79 to .87, p < .001). No significant relationships were 192 found between T20 and VL in different loading conditions (VL-10%BM, VL-30%BM 193 and VL-50%BM). 194 Significant correlations were also observed between the resisted sprint times (T20- 195 10%BM, T20-30%BM and T20-50%BM), increasing the relationship between the 196 heaviest conditions (r = .81 to .89, p < 0.001). Some significant relationships (p < 0.05 to 197 0.001) were observed between VL magnitude (VL-10%BM, VL-30%BM and VL- 198 50%BM) and resisted sprint times (T20-10%BM, T20-30%BM and T20-50%BM) (r = 199 .28 to .61). However, no significant relationships were observed between VL-10%BM 200 and T20-30%BM and T20-50%BM, neither between VL-30%BM and T20-10%BM 201 (Table 2). In addition, VL-10%BM, VL-30%BM and VL-50%BM were significantly 202 correlated with each other (r = .52 to .70, p < .001). 203 204 ***Insert Table 2 about here*** 205 206 Relationships between sprint performance in different loading conditions and lower limb 207 strength variables 208 The relationships between sprint and CMJ variables are shown in Figure 1. 209 Significant correlations were found between sprint time (T20, T20-10%BM, T20- 210 30%BM and T20-50%BM) and jump height (CMJ, CMJ-30%BM and CMJ-50%BM), in 211 both unloaded and loaded conditions. These relationships increased as the sled load 212 increased (r = -.54 to -.71, p < .001). Furthermore, jump height (CMJ, CMJ-30%BM and 213 CMJ-50%BM) also showed significant relationships (p < .001) with VL-50%BM (r = - 214 .42, -.45 and -.42, respectively). Jump height loss (HL-30%BM and HL-50%BM) 215 correlated significantly with unresisted and resisted sprint times (T20, T20-10%BM, T20- 216 30%BM and T20-50%BM) (r = .26 to .35, p < .01 - .05). However, no significant 217 relationships were observed between jump height loss and VL during running sprint 218 (unresisted and resisted conditions). 219 220 ***Insert Figure 1 about here*** 221 222 The relationships between sprint and SQ strength and power variables are shown 223 in Table 3. Performance in SQ (SQV and SQPrel) showed negative relationships with 224 14. Hopkins, WG, Marshall, SW, Batterham, AM, and Hanin, J. Progressive statistics 382 for studies in sports medicine and exercise science. 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J Strength Cond Res 21: 77-85, 2007. 435 436 FIGURE CAPTIONS 437 438 Figure 1. Correlations (90% confidence limits) between sprint and vertical jump 439 variables. T20: time in 20 m; T20-10%BM: time in 20 m with 10% of body mass (BM); 440 T20-30%BM: time in 20 m with 30% of BM; T20-50%BM: time in 20 m with 50% BM; 441 VL-10%BM: velocity loss with 10% of BM compared to the unloaded condition; VL- 442 30%BM: velocity loss with 30% of BM compared to the unloaded condition; VL- 443 50%BM: velocity loss with 50% of BM compared to the unloaded condition; CMJ: 444 countermovement jump; CMJ-30%BM: countermovement jump with 30% of BM; CMJ- 445 50%BM: countermovement jump with 50% of BM; HL-30%BM: height loss with 30% 446 of BM compared to the unloaded condition; HL-50%BM: height loss with 50% of BM 447 compared to the unloaded condition; *indicates significant correlation, p < 0.05; 448 449 450 Table 1. Values in selected neuromuscular performance variables (n = 79). T20 (s) T20-10%BM (s) T20-30%BM (s) T20-50%BM (s) 3.12 ± 0.16 3.40 ± 0.18 3.84 ± 0.22 4.37 ± 0.28 VL-10%BM (%) VL-30%BM (%) VL-50%BM (%) CMJ (cm) 8.1 ± 2.5 18.7 ± 3.0 28.5 ± 2.7 33.6 ± 5.5 CMJ-30%BM (cm) CMJ-50%BM (cm) HL-30%BM (%) HL-50%BM (%) 20.0 ± 4.0 16.4 ± 3.8 40.5 ± 5.2 51.6 ± 6.1 T20: time in 20 m; BM: body mass; T20-10%BM: time in 20 m with 10% of body mass (BM); T20-30%BM: time in 20 m with 30% of BM; T20-50%BM: time in 20 m with 50% BM; VL-10%BM: velocity loss with 10% of BM compared to the unloaded condition; VL- 30%BM: velocity loss with 30% of BM compared to the unloaded condition; VL-50%BM: velocity loss with 50% of BM compared to the unloaded condition; CMJ: Countermovement jump; CMJ-30%BM: Countermovement jump with 30% of BM; CMJ- 50%BM: Countermovement jump with 50% of BM; HL-30%BM: Height loss with 30% of BM compared to the unloaded condition; HL-50%BM: Height loss with 50% of BM compared to the unloaded condition. Table 2. Correlation coefficients (90% confidence limits) describing the relationships between sprint times and velocity loss with different percentages of body mass (n = 79). T20-10%BM T20-30%BM T20-50%BM VL-10%BM VL-30%BM VL-50%BM T20 .87*** (.82, .92) .79*** (.72, .86) .81*** (.75, .87) -.15 (-.33, .03) -.16 (-.34, .02) .02 (-.17, .21) T20-10%BM .85*** (.80, .90) .86*** (.81, .91) .36** (.20, .52) .13 (-.05, .31) .28* (.11, .45) T20-30%BM .89*** (.85, .93) .21 (.03, .39) .48*** (.34, .62) .46*** (.31, .61) T20-50%BM .19 (.01, .37) .28* (.11, .45) .61*** (.49, .73) VL-10%BM .55*** (.42, .68) .52*** (.38, .66) VL-30%BM .70*** (.61, .79) T20: time in 20 m; T20-10%BM: time in 20 m with 10% of body mass (BM); T20-30%BM: time in 20 m with 30% of BM; T20-50%BM: time in 20 m with 50% BM; VL-10%BM: velocity loss with 10% of BM compared to the unloaded condition; VL-30%BM: velocity loss with 30% of BM compared to the unloaded condition; VL-50%BM: velocity loss with 50% of BM compared to the unloaded condition. *Indicates significant correlation at p < 0.05; **Indicates significant correlation at p < 0.01; ***Indicates significant correlation at p < 0.001. Table 3. Inter-correlation matrix describing correlations between sprint, jumps, and squat velocities and power at each load. T20 T20- 10%BM T20- 30%BM T20- 50%BM VL-10%BM VL-30%BM VL-50%BM CMJ CMJ- 30%BM CMJ- 50%BM HL- 30%BM HL- 50%BM SQV-30%BM -.46*** (-.61;-.31) -.56*** (-.69;-.43) -.56*** (-.69;-.43) -.61*** (-.73;-.49) -.25* (-.42;-.08) -.23* (-.41;-.05) -.41*** (-.56;-.26) .70*** (.61;.79) .72*** (.63;.81) .73*** (.64;.82) -.30** (-.47;-.13) -.42*** (-.57;-.27) SQV-50%BM -.51*** (-.65;-.37) -.58*** (-.70;-.46) -.60*** (-.72;-.48) -.69*** (-.79;-.59) -.21 (-.39;-.03) -.23* (-.41;-.05) -.47*** (-.62;-.32) .76*** (.68;.84) .76*** (.68;.84) .79*** (.72;.86) -.28** (-.45;-.11) -.45*** (-.60;-.30) SQV-70%BM -.54*** (-.67;-.41) -.62*** (-.73;-.51) -.64*** (-.75;-.53) -.69*** (-.79;-.59) -.25* (-.42;-.08) -.26* (-.43;-.09) -.44*** (-.59;-.29) .74*** (.66;.82) .77*** (.69;.85) .80*** (.73;.87) -.28* (-.45;-.11) -.49*** (-.63;-.35) SQV-90%BM -.53*** (-.66;-.40) -.63*** (-.74;-.52) -.65*** (-.76;-.54) -.70*** (-.79;-.61) -.27* (-.44;-.10) -.27* (-.44;-.10) -.46*** (-.61;-.31) .74*** (.66;.82) .76*** (.68;.84) .80*** (.73;.87) -.35** (-.51;-.19) -.51*** (-.65;-.37) SQP-30%BM -.12 (-.06;.30) -.21 (-.39;-.03) -.21 (-.39;-.03) -.27* (-.44;-.10) -.20 (-.38;-.02) -.15 (-.33;.03) -.29* (-.46;-.12) .49*** (.35;.63) .49*** (.35;.63) .51*** (.37;.65) -.36** (-.52;-.20) -.30** (-.47;-.13) SQP-50%BM -.22* (-.40;-.04) -.27* (-.44;-.10) -.29** (-.46;-.12) -.39*** (-.55;-.53) -.13 (-.31;.05) -.15 (-.33;.03) -.36** (-.52;-.20) .58*** (.46;.70) .54*** (.41;.67) .59*** (.47;.71) -.18 (-.36;.00) -.33** (-.50;-.16) SQP-70%BM -.36** (-.52;-.20) -.44*** (-.59;-.29) -.43*** (-.58;-.28) -.51*** (-.65;-.37) -.20 (-.38;-.02) -.18 (-.36;.00) -.38** (-.54;-.22) .66*** (.55;.77) .68*** (.58;.78) .71*** (.62;.80) -.14 (-.32;.04) -.46*** (-.61;-.31) SQP-90%BM -.44*** (-.59;-.29) -.54*** (-.67;-.41) -.53*** (-.66;-.40) -.61*** (-.73;-.49) -.25* (-.42;-.08) -.22 (-.40;-.04) -.43*** (-.58;-.28) .73*** (.64;.82) .74*** (.66;.82) .78*** (.71;.85) -.31** (-.48;-.14) -.50*** (-.64;-.36) SQPrel-30%BM -.41*** (-.56;-.26) -.52*** (-.66;-.38) -.55*** (-.68;-.42) -.58*** (-.70;-.46) -.27* (-.44;-.10) -.30** (-.47;.13) -.44*** (-.59;-.29) .74*** (.66;.82) .78*** (.71;.85) .79*** (.72;.86) -.32** (-.49;-.15) -.47*** (-.62;-.32) SQPrel-50%BM -.48*** (-.62;-.34) -.55*** (-.68;-.42) -.59*** (-.71;-.47) -.66*** (-.77;-.55) -.18 (-.36;.00) -.26* (-.43;-.09) -.48*** (-.62;-.34) .77*** (.69;.85) .78*** (.71;.85) .80*** (.73;.87) -.38** (-.54;-.22) -.47*** (-.62;-. 32) SQPrel-70%BM -.54*** (-.67;-.41) -.62*** (-.73;-.51) -.64*** (-.75;-.53) -.69*** (-.79;-.59) -.22* (-.40;-.04) -.25* (-.42;-.08) -.46*** (-.61;-.31) .76*** (.68;.84) .82*** (.76;.88) .84*** (.79;.89) -.32** (-.49;-.15) -.54*** (-.67;-.41) SQPrel-90%BM -.54*** (-.67;-.41) -.64*** (-.75;-.53) -.65*** (-.76;-.54) -.71*** (-.80;-.62) -.25* (-.42;-.08) -.27* (-.44;-.10) -.47*** (-.62;-.32) .78*** (.71;.85) .81*** (.75;.87) .84*** (.79;.89) -.42*** (-.57;-.27) -.53*** (-.66;-.40) BM .33** (.16;.50) .36** (.20;.52) .37** (.21;.53) .33** (.16;.50) .08 (-.11;.27) .13 (-.05;.31) .12 (-.06;.30) -.22 (-.40;-.04) -.27* (-.44;-.10) -.25* (-.42;-.08) -.39*** (-.55;-.23) .18 (.00;.36) Correlation coefficients (90% confidence limits). N = 79. T20: time in 20 m; T20-10%BM: time in 20 m with 10% of body mass (BM); T20-30%BM: time in 20 m with 30% of BM; T20- 50%BM: time in 20 m with 50% BM; VL-10%BM: velocity loss with 10% of BM compared to the unloaded condition; VL-30%BM: velocity loss with 30% of BM compared to the unloaded condition; VL-50%BM: velocity loss with 50% of BM compared to the unloaded condition. CMJ: Countermovement jump; CMJ-30%BM: Countermovement jump with 30% of BM; CMJ- 50%BM: Countermovement jump with 50% of BM; HL-30%BM: Height loss with 30% of BM compared to the unloaded condition; HL-50%BM: Height loss with 50% of BM compared to the unloaded condition; SQV-30%BM: mean propulsive velocity (MPV) attained with 30% of BM in squat (SQ) exercise; SQV-50%BM: MPV attained with 50% of BM in SQ exercise; SQV- 70%BM: MPV attained with 70% of BM in SQ exercise; SQV-90%BM: MPV attained with 90% of BM in SQ exercise; SQP-30%BM: mean propulsive power (MPP) attained with 30% of BM in SQ exercise; SQP-50%BM: MPP attained with 50% of BM in SQ exercise; SQP-70%BM: MPP attained with 70% of BM in SQ exercise; SQP-90%BM: MPP attained with 90% of BM in SQ exercise; SQPrel-30%BM: MPP attained with 30% of BM in SQ exercise relative to BM; SQPrel-50%BM: MPP attained with 50% of BM in SQ exercise relative to BM; SQPrel-70%BM: MPP attained with 70% of BM in SQ exercise relative to BM; SQPrel-90%BM: MPP attained with 90% of BM in SQ exercise relative to BM. *Indicates significant correlation at p < 0.05; **Indicates significant correlation at p < 0.01; ***Indicates significant correlation at p < 0.001.