Journal of Applied Biomechanics, 2014, 30, 290-293 http://dx.doi.org/10.1123/jab.2013-0189 © 2014 Human Kinetics, Inc.
An Official Journal of ISB www.JAB-Journal.com ORIGINAL RESEARCH
Influence of Dorsiflexion Shoes on Jump Performance Juan J. Salinero, Cristina González-Millán, Javier Abián-Vicén, and Juan Del Coso Garrigós Camilo José Cela University, Madrid The goal of dorsiflexion sports shoes is to increase jumping capacity by means of a lower position of the heel in relation to the forefoot which results in additional stretching of the ankle plantar flexors. The aim of this study was to compare a dorsiflexion sports shoe model with two conventional sports shoe models in a countermovement jump test. The sample consisted of 35 participants who performed a countermovement jump test on a force platform wearing the three models of shoes. There were significant differences in the way force was manifested (P < 0.05) in the countermovement jump test, with a decrease in the velocity of the center of gravity and an increase in force at peak power and mean force in the concentric phase. Moreover, peak power was reached earlier with the dorsiflexion sports shoe model. The drop of the center of gravity was increased in CS1 in contrast to the dorsiflexion sports shoe model (P < .05). However, the dorsiflexion sports shoes were not effective for improving either peak power or jump height (P > .05). Although force manifestation and jump kinetics differ between dorsiflexion shoes and conventional sports shoes, jump performance was similar. Keywords: sports shoe, muscle power, perceived comfort Sports shoes can change kinetics during movement and jumping in sports which may result in an advantage or disadvantage in physical performance. In sports such as volleyball, jumping is a fundamental part of the spike, the block and the serve and is directly related to defensive and offensive success.1 The spike is one of the most important volleyball actions because its effectiveness determines the success of the volleyball attack. To obtain an effective spike, the player must hit the ball at the greatest height possible to overcome the block of the rival team. Moreover, greater jump height during the block can be an advantage in reducing the likelihood of successful spikes by the opponent while improved jump capacity during the serve may permit a more offensive trajectory. Thus, the use of sports shoes that increase jump height could be a beneficial way to improve sports performance, especially in the sport of volleyball.2 With this purpose in mind, some footwear brands provide lower heel height and elevation of the forefoot (the opposite of standard shoes) to achieve increased dorsiflexion of the ankle. The aim of this shoe design is to increase the extension of the plantar flexors which may favor muscle action and improve jump capacity. Bourgit et al3 found modifications in the motor pattern and muscle activation by increasing dorsiflexion of the ankle, particularly through augmentation of the stimulation of the calf muscles (triceps surae). This increased stimulation of the triceps surae by increased ankle dorsiflexion may induce a higher torque at the triceps surae level4 which would explain the improvement in force production found in previous studies.3,4 Pinniger and Cresswell4 found that the stretching of an activated muscle causes a transient increase in force during the stretch and a sustained, residual force enhancement following the stretch. These authors argued that this residual force enhancement may be able to be exploited for functional tasks. Juan J. Salinero, Cristina González-Millán, Javier Abián-Vicén, and Juan Del Coso Garrigós are with the Exercise Physiology and Performance Laboratory, Camilo José Cela University, Madrid, Spain. Address author correspondence to Juan J. Salinero at [email protected]. 290
Different studies have analyzed the influence of footwear designed to produce increased dorsiflexion of the ankle on jump capacity both as an acute effect when wearing these shoes and after training with them for some weeks. Examining the acute effect, Larkins and Snabb5 compared a prototype shoe with a 3.5° dorsiflexion with conventional sports shoes and obtained significant improvements in jump capacity (the average increase was 4.8 cm wearing the dorsiflexion shoes). Faiss et al6 also found improvements in jump height with the dorsiflexion shoes (4°) in the countermovement jump (on average 2 cm), squat jump (on average 3 cm) and in continuous jumping for 15 seconds (on average 4.5 cm). However, the optimal degree of dorsiflexion to induce maximal jump performance is unclear. Recent studies have not found jump improvements when comparing 2° dorsiflexion with neutral shoes (0°)7 or standard sports shoes.8 However, using a jump platform constructed to allow for a continuous range of negative and positive degrees of foot inclination (+4, 0, –2, –3 and –4), Larkins and Snabb5 found that negative conditions (eg, ankle dorsiflexion) resulted in better jump performance than nonnegative conditions (+4 and 0) with an improvement of 3.3% for –2° and 3.8% for –4° dorsiflexion in contrast to +4° dorsiflexion. Despite all these scientific data, commercial brands of dorsiflexion shoes only sell 2° dorsiflexion probably because higher dorsiflexion levels could reduce user’s perceived comfort.8 Studies which have analyzed the effect of training with shoe models that increases ankle dorsiflexion to 10° have obtained contradictory results: in one study there was an improvement in jump capacity and running speed with 10° dorsiflexion in healthy men after 8 weeks of training with dorsiflexion shoes,9 while in a later study in trained women, these gains were not found.10 Differences in the characteristics of the samples could be the reason for this discrepancy. Apart from jump performance, kinetic parameters during jump activities could be affected by the dorsiflexion shoes. Faiss et al6 found a significantly higher speed at takeoff in the countermovement jump using dorsiflexion shoes versus standard shoes, while Salinero et al8 did not find differences in this variable when comparing similar
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Dorsiflexion and Jump Performance 291
dorsiflexion and standard shoes. Both studies have reported similar peak power in this test between shoe conditions6,8; however, Salinero et al8 found differences in the force manifestation during jumping with a decreased velocity of the center of gravity and a tendency to increased force at peak power. Research in this area is still scarce and contradictory effects have been found on jump performance and force manifestation during jumping. Only a few studies have analyzed differences between dorsiflexion and conventional sports shoes and these studies have generally used small sample sizes. Thus, it is necessary to conduct more research in this area by increasing sample sizes, including other kinetic variables and comparing dorsiflexion shoes with a range of standard shoes. Evidence for the effects of dorsiflexion shoes on jump performance can be found through this experimental setting. Moreover, to gain additional knowledge that extends previous research8 and to use an ecologically valid approach, 2° dorsiflexion will be employed in the investigation because this type of shoe is the only type available for commercial sale at this time. Therefore, the aim of the current study was to compare countermovement jump performance and jump kinetics using 2° dorsiflexion sports shoes and two conventional sports shoes. We hypothesized that dorsiflexion shoes would improve jump performance due to a higher force developed at takeoff, in comparison with conventional shoes.
Methods Participants Thirty-five healthy subjects, all of whom were students at a Faculty of Physical Activity and Sports Sciences in Spain, volunteered to participate in the study (Table 1). Before the study, ethical approval was obtained from the Camilo José Cela University Review Board in accordance with the latest version of the Declaration of Helsinki and all volunteers provided their informed written consent before participation. The subjects were not informed about the characteristics of the sports shoes to be tested in the research so as not to influence their performance expectations.
Measures Countermovement Jump Test (CMJ). For this assessment, participants initiated the task in a stationary and upright position with their weight evenly distributed over both feet. Each subject placed their hands on their waist to remove the influence of the arms on the jump and the vertical force data were recorded in this position to assess the subject’s body weight. On command, the participants flexed their knees (the knee angle was not controlled) and jumped as high as possible while maintaining their hands on their waist and then landed on both feet.
Table 1 Anthropometry and age of the participants Height (cm) Weight (kg) BMI (kg/m2) Shoe Size (US) Age (years)
Min 156.0 54.5 19.5 7 19
Max 188.0 99.0 33.3 11 40
Mean 176.2 75.5 24.4 9.0 23.6
SD 6.7 9.1 3.0 1.1 4.5
Three models of commercial sports shoes were used, one with dorsiflexion and two standard shoes. Ten pairs of volleyball shoes were used with a dorsiflexion system of 2° (Springboost Model B-Voley, Switzerland); ten pairs of Kipsta indoor volleyball shoes model 300 (Decathlon, France) were used as the medium class volleyball sports shoes (CS1; 3.7° of induced plantar flexion); and ten pairs of Mizuno indoor volleyball shoes model Wave Lightning 6 (Mizuno, France) were used as the high class volleyball sports shoes (CS2; 4° of induced plantar flexion). Weights for a US size 10 were 460 g for DF, 348 g for CS1 and 354 g for CS2. The market values were similar for CS2 and DF (about $120) shoes whereas the CS1 shoes were cheaper ($40). Apart from the characteristics of dorsiflexion, the three models of sports shoes had rubber soles and EVA midsoles, and the size, shape and fastenings (laces) were similar.
Procedures The participants carried out a standard warm-up consisting of 5 minutes of continuous running followed by a shuttle run over a distance of 30 m while performing the following exercises in each half: heels to buttocks, skipping, and a jump with the help of their arms every 10 m. They then moved to the jumping area. They had been familiarized with the type of jump to be performed before the test, that is, a countermovement jump without the help of the arms. The whole procedure was performed with the participants wearing their own sports shoes. The subjects then wore the corresponding sports shoes: dorsiflexion shoes (DF) or conventional sports shoes (CS1 or CS2) and performed 3 submaximal jumps and after 3 min rest, two maximal CMJs on the force platform and their best score was recorded. After 1 min rest, the countermovement jump was repeated. When the second attempt was better than the first, they were allowed a third attempt. The same series of instructions was given to the participants before each performance.11,12 After 5 minutes of rest they changed shoes and again performed two maximal jumps on the force platform. The order of the jumps with DF, CS1 or CS2 shoes was randomized. A similar number of participants started the test with each shoe model (12 with DF, 12 with CS1 and 11 with CS2). In addition, we established two different sequences for the order of the experimental shoes (eg, DF-CS1-CS2 for the first participant and CS1-DF-CS2 for the second participant) and subsequent participants (odd and even) started the experiment with the following experimental shoes (eg, CS1-CS2-DF for the third participant and DF-CS2-CS1 for the fourth participant). For the jump test, a Kistler QuattroJump force platform (Kistler, Switzerland) was used with its associated software. Maximal leg power output during the jump was determined from ground reaction forces (F). For this calculation, the initial vertical velocity of the system was set at zero. Vertical ground reaction forces were recorded at 500 Hz and were divided by the mass of the system at each time point to determine instantaneous acceleration (ainst = Finst / mass). Acceleration due to gravity was subtracted from the calculated acceleration to ensure that only the acceleration produced by the participant during the jump was used to determine velocity. Instantaneous vertical velocity (vinst) was integrated from the acceleration. The integration constant was zero because there was no initial movement. Instantaneous power (Pinst) was calculated as the product of the velocity and force (P = Finst × vinst) at any given point. Peak power (PP) was calculated from the integration of the force-time curve during the push-off phase of the jump and normalized with the subject’s mass. At the instant at which PP was reached, force (Fpp) and the velocity of the center of gravity
292 Salinero et al.
(Vpp) were recorded. Power average from the impulse phase (the concentric part of the jump) was used for the statistical analyses. Jump height was assessed from the flight time; the flight time being the difference between the first instant of takeoff and the first instant of landing. For this measurement, we assumed that the height of the jumper’s center of mass at the instant of landing was the same as at the instant of takeoff and we used the equation proposed by Linthorne.13 The height of the center of gravity was calculated using the double integration method (work-energy).13
Statistical Analysis
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Data are presented as means ± standard deviations. The normality of the data were confirmed with the Kolmogorov-Smirnov test. The significance of the difference between means for both models of sports shoe was calculated with a one-way ANOVA for related samples. When necessary, the Bonferroni adjustment was used for paired comparisons. Statistical significance was set at P < 0.05.
All statistical analyses were performed using the SPSS for Windows 19.0 statistical package.
Results Table 2 depicts jump performance variables with all the sport shoes included in this study. DF did not improve jump height in comparison with CS1 or CS2. There were also no significant differences between DF and CS (1-2) in peak power but there were differences in the instant at which peak power was reached (later in DF vs CS1 and CS2) (Table 2), as well as in velocity at peak power and force at peak power (Figure 1). In addition, there were significant differences in mean force in the concentric phase (higher with DF vs CS1 and CS2). Moreover, there were significant differences between DF and CS1 in the drop of the center of gravity in the push-off, with a greater drop in CS1 (Table 2). Greater vertical ground reaction force at peak power was obtained with DF vs CS1 and CS2. However, a higher value was attained in the velocity of the center of gravity at peak power with CS1 and CS2 vs DF (P < .05) (Figure 1).
Discussion
Figure 1 — Velocity and force at the instant at which peak power was reached. These data represent the means ± SD for 35 participants. *Significant differences (P < .05) between DF vs CS1 and CS2. Vpp= velocity of center of gravity at peak power; Fpp = force at peak power. CS1 = conventional shoe-medium class; CS2 = conventional shoe high-class; DF = dorsiflexion shoe.
The main result of this investigation was that sports shoes inducing 2° of dorsiflexion had no effect on countermovement jump performance when compared with medium or high class conventional volleyball shoes. These results coincide with recent studies developed with shoes that induced the same ankle dorsiflexion (2°) in countermovement jump performance.7,8 These previous studies did not find differences between dorsiflexion shoes and standard sports shoes. Nevertheless, other previous studies5,6 found improvements in jump capacity using sports shoes with greater degrees of dorsiflexion (3.5° and 4° respectively) than the shoes employed in the current study (2°), so that this may be one reason why there is a discrepancy in the results. Larkins and Snabb5 found jump performance improvements with 2° of ankle dorsiflexion but used a jump platform rather than dorsiflexion shoes. From the results of these investigations, it is difficult to identify the cause of the dis-
Table 2 Jump performance with dorsiflexion (DF) and conventional sports shoes (CS1 and CS2)
H (cm) Hcg1 (cm) Hcg2 (cm) Hcg3 (cm) Vto (m/s) PP (W/kg) T(pp) (s) Fcc (N) Pcc (W/kg) Acc (N⋅s)
CS1 Mean (SD) 32.43 (5.94) –31.97 (6.67) 10.31 (1.78) 41.55 (5.82) 2.47 (0.24) 51.70 (7.44) –.064 (.007) 1460 (205) 28.54 (4.40) 386 (47.3)
CS2 Mean (SD) 32.36 (5.82) –30.82 (7.94) 10.23 (2.30) 40.48 (8.94) 2.4 7(0.24) 51.91 (8.08) –.065 (.007) 1456 (205) 28.53 (4.60) 386 (49.6)
DF Mean (SD) 32.57 (5.14) –29.39 (6.06) 10.30 (1.78) 41.68 (5.46) 2.49 (0.25) 52.05 (7.26) –.062 (.006) 1502 (211) 29.06 (4.37) 379 (49.7)
P .841 .005† .978 .650 .745 .643 .003* .021* .320 .210
*Significant differences between DF vs CS1 and CS. †Significant differences between DF vs CS1. Note. H = jump height; Hcg1 = drop of center of gravity in push-off; Hcg2 = height of center of gravity in takeoff; Hcg3 = highest point of center of gravity in the flight phase; Vto = velocity in takeoff; PP = peak power; T(pp) = instant at which PP was reached; Fcc = force mean in concentric phase; Pcc = power mean in concentric phase; Acc = acceleration boost.
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crepancies among studies. The use of different methodologies (eg, different degrees of dorsiflexion, different brands of conventional sports shoes and dorsiflexion shoes, different jump types, etc) and the different level of participants’ fitness may explain in part the differences among investigations. The small number of investigations in this area highlights the need for further research to elucidate the effects of dorsiflexion shoes on sports performance. The difference in the amount of drop of the center of gravity in the push-off in the initial phase of the jump could explain the absence of a significant difference in jump performance. It is proposed that the decrease in ankle dorsiflexion in the conventional shoes was offset by a higher drop in the center of gravity to achieve optimal ankle flexion. With DF, some part of this optimal ankle flexion is generated by the shoe, so it is not necessary to drop further. Therefore, with the three sports shoes (DF, CS1 and CS2), it is hypothesized that the participants started the concentric phase of the jump with similar ankle flexion and, consequently, jump performance was similar. The knee angle during the countermovement jump was not controlled so participants flexed their knees and ankles looking for the optimal angle for maximal performance in the same way as they would in the sports context. In addition to jump performance, jump kinetics were also examined. Increased dorsiflexion of the ankle potentially produces a modification in muscle activation.3,14 This outcome was found in this research through the changes that were present in force development in which the subjects displayed the peak power with higher force and lower velocity with the dorsiflexion shoes. Similarly, greater force in the concentric phase was obtained with dorsiflexion. However, as a practical application for sports performance, this difference did not translate into an improvement in peak power or jump performance. Faiss et al6 found higher values of velocity in the takeoff with the sports shoes with dorsiflexion (associated with improved jump performance) but we did not find significant differences in this study. Biomechanical studies of the muscle have found that a prestretch of the ankle plantar flexor musculature could increase the capacity of the muscle to generate strength.4 In the triceps surae, which is a biarticular muscle group and which is prestretched by dorsiflexion of the ankle, it has been shown that with either the knee bent or extended, optimal length for torque development corresponds to total dorsiflexion of the ankle.15 Consequently, the ankle angle induced by a sports shoe may affect force production and could improve or decrease muscle performance during sport activities. Moreover, this improvement in force production was only evident at the ankle, with no changes to the torque produced at the knee as a result of changes to the position of the ankle through dorsiflexion or plantarflexion.16 Thus, the benefits of dorsiflexion on jump performance are limited to the lower leg muscles (eg, extensor musculature of the ankle) which, in part, explains the absence of dorsiflexion effects on jump performance (a sport action that involves all the muscles of the lower limb). Differences in the mass of the sports shoes (weights for a US size 10 were 460 g for DF, 348 g for CS1 and 354 g for CS2) could be a study limitation. Nevertheless, we considered that the practical effect on jump performance was negligible. The difference between the sports shoes mass was around 110 g, and represents 0.1% of the mass mean of the participants. In summary, jump kinetics differed between dorsiflexion shoes and conventional sports shoes but a similar jump performance was found. Therefore, the use of dorsiflexion shoes did not improve sports performance in volleyball, where jumping is directly related to the success of defensive and offensive actions. More research is needed to determine the influence of dorsiflexion on sports perfor-
mance. Furthermore, future research in this area must analyze the influence of familiarization with these shoes on jump performance.
References 1. Voelzke M, Stutzig N, Thorhauer HA, Granacher U. Promoting lower extremity strength in elite volleyball players: effects of two combined training methods. J Sci Med Sport. 2012;15(5):457–462. PubMed doi:10.1016/j.jsams.2012.02.004 2. Stanganelli LC, Dourado AC, Oncken P, Mancan S, da Costa SC. Adaptations on jump capacity in Brazilian volleyball players prior to the under-19 World Championship. J Strength Cond Res. 2008;22(3):741– 749. PubMed doi:10.1519/JSC.0b013e31816a5c4c 3. Bourgit D, Millet GY, Fuchslocher J. Influence of shoes increasing dorsiflexion and decreasing metatarsus flexion on lower limb muscular activity during fitness exercises, walking, and running. J Strength Cond Res. 2008;22(3):966–973. PubMed doi:10.1519/JSC.0b013e31816f1354 4. Pinniger GJ, Cresswell AG. Residual force enhancement after lengthening is present during submaximal plantar flexion and dorsiflexion actions in humans. J Appl Physiol. 2007;102(1):18–25. PubMed doi:10.1152/japplphysiol.00565.2006 5. Larkins C, Snabb TE. Positive versus negative foot inclination for maximum height two-leg vertical jumps. Clin Biomech (Bristol, Avon). 1999;14(5):321–328. doi:10.1016/S0268-0033(98)90089-4 6. Faiss R, Terrier P, Praz M, Fuchslocher J, Gobelet C, Deriaz O. Influence of initial foot dorsal flexion on vertical jump and running performance. J Strength Cond Res. 2010;24(9):2352–2357. PubMed doi:10.1519/ JSC.0b013e3181aff2cc 7. Lapole T, Ahmaidi S, Gaillien B, Lepretre PM. Influence of dorsiflexion shoes on neuromuscular fatigue of the plantar flexors after combined tapping-jumping exercises in volleyball players. J Strength Cond Res. 2013;27(7):2025–2033. PubMed doi:10.1519/JSC.0b013e3182773271 8. Salinero JJ, Abian-Vicen J, Del Coso J, González-Millán C. The influence of ankle dorsiflexion on jumping capacity and the modified agility T-Test performance. Eur J Sport Sci. 2014;14(2):137–143. doi:10.1080 /17461391.2013.777797. PubMed 9. Kraemer WJ, Ratamess NA, Volek JS, Mazzetti SA, Gomez AL. The effect of the Meridian shoe on vertical jump and sprint performances following short-term combined plyometric/sprint and resistance training. J Strength Cond Res. 2000;14(2):228–238. 10. Ratamess NA, Kraemer WJ, Volek JS, et al. The effects of ten weeks of resistance and combined plyometric/sprint training with the Meridian Elyte athletic shoe on muscular performance in women. J Strength Cond Res. 2007;21(3):882–887. 11. Abian J, Alegre LM, Lara AJ, Rubio JA, Aguado X. Landing differences between men and women in a maximal vertical jump aptitude test. J Sports Med Phys Fitness. 2008;48(3):305–310. PubMed 12. Lara AJ, Abian J, Alegre LM, Jimenez L, Aguado X. Assessment of power output in jump tests for applicants to a sports sciences degree. J Sports Med Phys Fitness. 2006;46(3):419–424. PubMed 13. Linthorne N. Analysis of standing vertical jump using a force platform. Am J Phys. 2001;69:1198–1204. doi:10.1119/1.1397460 14. Frank K, Baratta RV, Solomonow M, Shilstone M, Riche K. The effect of Strength Shoes on muscle activity during quiet standing. J Appl Biomech. 2000;16(2):204–209. 15. Sale D, Quinlan J, Marsh E, McComas AJ, Belanger AY. Influence of joint position on ankle plantarflexion in humans. J Appl Physiol. 1982;52(6):1636–1642. PubMed 16. Croce RV, Miller JP, St Pierre P. Effect of ankle position fixation on peak torque and electromyographic activity of the knee flexors and extensors. Electromyogr Clin Neurophysiol. 2000;40(6):365–373. PubMed
An Official Journal of ISB www.JAB-Journal.com ORIGINAL RESEARCH
Influence of Dorsiflexion Shoes on Jump Performance Juan J. Salinero, Cristina González-Millán, Javier Abián-Vicén, and Juan Del Coso Garrigós Camilo José Cela University, Madrid The goal of dorsiflexion sports shoes is to increase jumping capacity by means of a lower position of the heel in relation to the forefoot which results in additional stretching of the ankle plantar flexors. The aim of this study was to compare a dorsiflexion sports shoe model with two conventional sports shoe models in a countermovement jump test. The sample consisted of 35 participants who performed a countermovement jump test on a force platform wearing the three models of shoes. There were significant differences in the way force was manifested (P < 0.05) in the countermovement jump test, with a decrease in the velocity of the center of gravity and an increase in force at peak power and mean force in the concentric phase. Moreover, peak power was reached earlier with the dorsiflexion sports shoe model. The drop of the center of gravity was increased in CS1 in contrast to the dorsiflexion sports shoe model (P < .05). However, the dorsiflexion sports shoes were not effective for improving either peak power or jump height (P > .05). Although force manifestation and jump kinetics differ between dorsiflexion shoes and conventional sports shoes, jump performance was similar. Keywords: sports shoe, muscle power, perceived comfort Sports shoes can change kinetics during movement and jumping in sports which may result in an advantage or disadvantage in physical performance. In sports such as volleyball, jumping is a fundamental part of the spike, the block and the serve and is directly related to defensive and offensive success.1 The spike is one of the most important volleyball actions because its effectiveness determines the success of the volleyball attack. To obtain an effective spike, the player must hit the ball at the greatest height possible to overcome the block of the rival team. Moreover, greater jump height during the block can be an advantage in reducing the likelihood of successful spikes by the opponent while improved jump capacity during the serve may permit a more offensive trajectory. Thus, the use of sports shoes that increase jump height could be a beneficial way to improve sports performance, especially in the sport of volleyball.2 With this purpose in mind, some footwear brands provide lower heel height and elevation of the forefoot (the opposite of standard shoes) to achieve increased dorsiflexion of the ankle. The aim of this shoe design is to increase the extension of the plantar flexors which may favor muscle action and improve jump capacity. Bourgit et al3 found modifications in the motor pattern and muscle activation by increasing dorsiflexion of the ankle, particularly through augmentation of the stimulation of the calf muscles (triceps surae). This increased stimulation of the triceps surae by increased ankle dorsiflexion may induce a higher torque at the triceps surae level4 which would explain the improvement in force production found in previous studies.3,4 Pinniger and Cresswell4 found that the stretching of an activated muscle causes a transient increase in force during the stretch and a sustained, residual force enhancement following the stretch. These authors argued that this residual force enhancement may be able to be exploited for functional tasks. Juan J. Salinero, Cristina González-Millán, Javier Abián-Vicén, and Juan Del Coso Garrigós are with the Exercise Physiology and Performance Laboratory, Camilo José Cela University, Madrid, Spain. Address author correspondence to Juan J. Salinero at [email protected]. 290
Different studies have analyzed the influence of footwear designed to produce increased dorsiflexion of the ankle on jump capacity both as an acute effect when wearing these shoes and after training with them for some weeks. Examining the acute effect, Larkins and Snabb5 compared a prototype shoe with a 3.5° dorsiflexion with conventional sports shoes and obtained significant improvements in jump capacity (the average increase was 4.8 cm wearing the dorsiflexion shoes). Faiss et al6 also found improvements in jump height with the dorsiflexion shoes (4°) in the countermovement jump (on average 2 cm), squat jump (on average 3 cm) and in continuous jumping for 15 seconds (on average 4.5 cm). However, the optimal degree of dorsiflexion to induce maximal jump performance is unclear. Recent studies have not found jump improvements when comparing 2° dorsiflexion with neutral shoes (0°)7 or standard sports shoes.8 However, using a jump platform constructed to allow for a continuous range of negative and positive degrees of foot inclination (+4, 0, –2, –3 and –4), Larkins and Snabb5 found that negative conditions (eg, ankle dorsiflexion) resulted in better jump performance than nonnegative conditions (+4 and 0) with an improvement of 3.3% for –2° and 3.8% for –4° dorsiflexion in contrast to +4° dorsiflexion. Despite all these scientific data, commercial brands of dorsiflexion shoes only sell 2° dorsiflexion probably because higher dorsiflexion levels could reduce user’s perceived comfort.8 Studies which have analyzed the effect of training with shoe models that increases ankle dorsiflexion to 10° have obtained contradictory results: in one study there was an improvement in jump capacity and running speed with 10° dorsiflexion in healthy men after 8 weeks of training with dorsiflexion shoes,9 while in a later study in trained women, these gains were not found.10 Differences in the characteristics of the samples could be the reason for this discrepancy. Apart from jump performance, kinetic parameters during jump activities could be affected by the dorsiflexion shoes. Faiss et al6 found a significantly higher speed at takeoff in the countermovement jump using dorsiflexion shoes versus standard shoes, while Salinero et al8 did not find differences in this variable when comparing similar
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dorsiflexion and standard shoes. Both studies have reported similar peak power in this test between shoe conditions6,8; however, Salinero et al8 found differences in the force manifestation during jumping with a decreased velocity of the center of gravity and a tendency to increased force at peak power. Research in this area is still scarce and contradictory effects have been found on jump performance and force manifestation during jumping. Only a few studies have analyzed differences between dorsiflexion and conventional sports shoes and these studies have generally used small sample sizes. Thus, it is necessary to conduct more research in this area by increasing sample sizes, including other kinetic variables and comparing dorsiflexion shoes with a range of standard shoes. Evidence for the effects of dorsiflexion shoes on jump performance can be found through this experimental setting. Moreover, to gain additional knowledge that extends previous research8 and to use an ecologically valid approach, 2° dorsiflexion will be employed in the investigation because this type of shoe is the only type available for commercial sale at this time. Therefore, the aim of the current study was to compare countermovement jump performance and jump kinetics using 2° dorsiflexion sports shoes and two conventional sports shoes. We hypothesized that dorsiflexion shoes would improve jump performance due to a higher force developed at takeoff, in comparison with conventional shoes.
Methods Participants Thirty-five healthy subjects, all of whom were students at a Faculty of Physical Activity and Sports Sciences in Spain, volunteered to participate in the study (Table 1). Before the study, ethical approval was obtained from the Camilo José Cela University Review Board in accordance with the latest version of the Declaration of Helsinki and all volunteers provided their informed written consent before participation. The subjects were not informed about the characteristics of the sports shoes to be tested in the research so as not to influence their performance expectations.
Measures Countermovement Jump Test (CMJ). For this assessment, participants initiated the task in a stationary and upright position with their weight evenly distributed over both feet. Each subject placed their hands on their waist to remove the influence of the arms on the jump and the vertical force data were recorded in this position to assess the subject’s body weight. On command, the participants flexed their knees (the knee angle was not controlled) and jumped as high as possible while maintaining their hands on their waist and then landed on both feet.
Table 1 Anthropometry and age of the participants Height (cm) Weight (kg) BMI (kg/m2) Shoe Size (US) Age (years)
Min 156.0 54.5 19.5 7 19
Max 188.0 99.0 33.3 11 40
Mean 176.2 75.5 24.4 9.0 23.6
SD 6.7 9.1 3.0 1.1 4.5
Three models of commercial sports shoes were used, one with dorsiflexion and two standard shoes. Ten pairs of volleyball shoes were used with a dorsiflexion system of 2° (Springboost Model B-Voley, Switzerland); ten pairs of Kipsta indoor volleyball shoes model 300 (Decathlon, France) were used as the medium class volleyball sports shoes (CS1; 3.7° of induced plantar flexion); and ten pairs of Mizuno indoor volleyball shoes model Wave Lightning 6 (Mizuno, France) were used as the high class volleyball sports shoes (CS2; 4° of induced plantar flexion). Weights for a US size 10 were 460 g for DF, 348 g for CS1 and 354 g for CS2. The market values were similar for CS2 and DF (about $120) shoes whereas the CS1 shoes were cheaper ($40). Apart from the characteristics of dorsiflexion, the three models of sports shoes had rubber soles and EVA midsoles, and the size, shape and fastenings (laces) were similar.
Procedures The participants carried out a standard warm-up consisting of 5 minutes of continuous running followed by a shuttle run over a distance of 30 m while performing the following exercises in each half: heels to buttocks, skipping, and a jump with the help of their arms every 10 m. They then moved to the jumping area. They had been familiarized with the type of jump to be performed before the test, that is, a countermovement jump without the help of the arms. The whole procedure was performed with the participants wearing their own sports shoes. The subjects then wore the corresponding sports shoes: dorsiflexion shoes (DF) or conventional sports shoes (CS1 or CS2) and performed 3 submaximal jumps and after 3 min rest, two maximal CMJs on the force platform and their best score was recorded. After 1 min rest, the countermovement jump was repeated. When the second attempt was better than the first, they were allowed a third attempt. The same series of instructions was given to the participants before each performance.11,12 After 5 minutes of rest they changed shoes and again performed two maximal jumps on the force platform. The order of the jumps with DF, CS1 or CS2 shoes was randomized. A similar number of participants started the test with each shoe model (12 with DF, 12 with CS1 and 11 with CS2). In addition, we established two different sequences for the order of the experimental shoes (eg, DF-CS1-CS2 for the first participant and CS1-DF-CS2 for the second participant) and subsequent participants (odd and even) started the experiment with the following experimental shoes (eg, CS1-CS2-DF for the third participant and DF-CS2-CS1 for the fourth participant). For the jump test, a Kistler QuattroJump force platform (Kistler, Switzerland) was used with its associated software. Maximal leg power output during the jump was determined from ground reaction forces (F). For this calculation, the initial vertical velocity of the system was set at zero. Vertical ground reaction forces were recorded at 500 Hz and were divided by the mass of the system at each time point to determine instantaneous acceleration (ainst = Finst / mass). Acceleration due to gravity was subtracted from the calculated acceleration to ensure that only the acceleration produced by the participant during the jump was used to determine velocity. Instantaneous vertical velocity (vinst) was integrated from the acceleration. The integration constant was zero because there was no initial movement. Instantaneous power (Pinst) was calculated as the product of the velocity and force (P = Finst × vinst) at any given point. Peak power (PP) was calculated from the integration of the force-time curve during the push-off phase of the jump and normalized with the subject’s mass. At the instant at which PP was reached, force (Fpp) and the velocity of the center of gravity
292 Salinero et al.
(Vpp) were recorded. Power average from the impulse phase (the concentric part of the jump) was used for the statistical analyses. Jump height was assessed from the flight time; the flight time being the difference between the first instant of takeoff and the first instant of landing. For this measurement, we assumed that the height of the jumper’s center of mass at the instant of landing was the same as at the instant of takeoff and we used the equation proposed by Linthorne.13 The height of the center of gravity was calculated using the double integration method (work-energy).13
Statistical Analysis
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Data are presented as means ± standard deviations. The normality of the data were confirmed with the Kolmogorov-Smirnov test. The significance of the difference between means for both models of sports shoe was calculated with a one-way ANOVA for related samples. When necessary, the Bonferroni adjustment was used for paired comparisons. Statistical significance was set at P < 0.05.
All statistical analyses were performed using the SPSS for Windows 19.0 statistical package.
Results Table 2 depicts jump performance variables with all the sport shoes included in this study. DF did not improve jump height in comparison with CS1 or CS2. There were also no significant differences between DF and CS (1-2) in peak power but there were differences in the instant at which peak power was reached (later in DF vs CS1 and CS2) (Table 2), as well as in velocity at peak power and force at peak power (Figure 1). In addition, there were significant differences in mean force in the concentric phase (higher with DF vs CS1 and CS2). Moreover, there were significant differences between DF and CS1 in the drop of the center of gravity in the push-off, with a greater drop in CS1 (Table 2). Greater vertical ground reaction force at peak power was obtained with DF vs CS1 and CS2. However, a higher value was attained in the velocity of the center of gravity at peak power with CS1 and CS2 vs DF (P < .05) (Figure 1).
Discussion
Figure 1 — Velocity and force at the instant at which peak power was reached. These data represent the means ± SD for 35 participants. *Significant differences (P < .05) between DF vs CS1 and CS2. Vpp= velocity of center of gravity at peak power; Fpp = force at peak power. CS1 = conventional shoe-medium class; CS2 = conventional shoe high-class; DF = dorsiflexion shoe.
The main result of this investigation was that sports shoes inducing 2° of dorsiflexion had no effect on countermovement jump performance when compared with medium or high class conventional volleyball shoes. These results coincide with recent studies developed with shoes that induced the same ankle dorsiflexion (2°) in countermovement jump performance.7,8 These previous studies did not find differences between dorsiflexion shoes and standard sports shoes. Nevertheless, other previous studies5,6 found improvements in jump capacity using sports shoes with greater degrees of dorsiflexion (3.5° and 4° respectively) than the shoes employed in the current study (2°), so that this may be one reason why there is a discrepancy in the results. Larkins and Snabb5 found jump performance improvements with 2° of ankle dorsiflexion but used a jump platform rather than dorsiflexion shoes. From the results of these investigations, it is difficult to identify the cause of the dis-
Table 2 Jump performance with dorsiflexion (DF) and conventional sports shoes (CS1 and CS2)
H (cm) Hcg1 (cm) Hcg2 (cm) Hcg3 (cm) Vto (m/s) PP (W/kg) T(pp) (s) Fcc (N) Pcc (W/kg) Acc (N⋅s)
CS1 Mean (SD) 32.43 (5.94) –31.97 (6.67) 10.31 (1.78) 41.55 (5.82) 2.47 (0.24) 51.70 (7.44) –.064 (.007) 1460 (205) 28.54 (4.40) 386 (47.3)
CS2 Mean (SD) 32.36 (5.82) –30.82 (7.94) 10.23 (2.30) 40.48 (8.94) 2.4 7(0.24) 51.91 (8.08) –.065 (.007) 1456 (205) 28.53 (4.60) 386 (49.6)
DF Mean (SD) 32.57 (5.14) –29.39 (6.06) 10.30 (1.78) 41.68 (5.46) 2.49 (0.25) 52.05 (7.26) –.062 (.006) 1502 (211) 29.06 (4.37) 379 (49.7)
P .841 .005† .978 .650 .745 .643 .003* .021* .320 .210
*Significant differences between DF vs CS1 and CS. †Significant differences between DF vs CS1. Note. H = jump height; Hcg1 = drop of center of gravity in push-off; Hcg2 = height of center of gravity in takeoff; Hcg3 = highest point of center of gravity in the flight phase; Vto = velocity in takeoff; PP = peak power; T(pp) = instant at which PP was reached; Fcc = force mean in concentric phase; Pcc = power mean in concentric phase; Acc = acceleration boost.
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Dorsiflexion and Jump Performance 293
crepancies among studies. The use of different methodologies (eg, different degrees of dorsiflexion, different brands of conventional sports shoes and dorsiflexion shoes, different jump types, etc) and the different level of participants’ fitness may explain in part the differences among investigations. The small number of investigations in this area highlights the need for further research to elucidate the effects of dorsiflexion shoes on sports performance. The difference in the amount of drop of the center of gravity in the push-off in the initial phase of the jump could explain the absence of a significant difference in jump performance. It is proposed that the decrease in ankle dorsiflexion in the conventional shoes was offset by a higher drop in the center of gravity to achieve optimal ankle flexion. With DF, some part of this optimal ankle flexion is generated by the shoe, so it is not necessary to drop further. Therefore, with the three sports shoes (DF, CS1 and CS2), it is hypothesized that the participants started the concentric phase of the jump with similar ankle flexion and, consequently, jump performance was similar. The knee angle during the countermovement jump was not controlled so participants flexed their knees and ankles looking for the optimal angle for maximal performance in the same way as they would in the sports context. In addition to jump performance, jump kinetics were also examined. Increased dorsiflexion of the ankle potentially produces a modification in muscle activation.3,14 This outcome was found in this research through the changes that were present in force development in which the subjects displayed the peak power with higher force and lower velocity with the dorsiflexion shoes. Similarly, greater force in the concentric phase was obtained with dorsiflexion. However, as a practical application for sports performance, this difference did not translate into an improvement in peak power or jump performance. Faiss et al6 found higher values of velocity in the takeoff with the sports shoes with dorsiflexion (associated with improved jump performance) but we did not find significant differences in this study. Biomechanical studies of the muscle have found that a prestretch of the ankle plantar flexor musculature could increase the capacity of the muscle to generate strength.4 In the triceps surae, which is a biarticular muscle group and which is prestretched by dorsiflexion of the ankle, it has been shown that with either the knee bent or extended, optimal length for torque development corresponds to total dorsiflexion of the ankle.15 Consequently, the ankle angle induced by a sports shoe may affect force production and could improve or decrease muscle performance during sport activities. Moreover, this improvement in force production was only evident at the ankle, with no changes to the torque produced at the knee as a result of changes to the position of the ankle through dorsiflexion or plantarflexion.16 Thus, the benefits of dorsiflexion on jump performance are limited to the lower leg muscles (eg, extensor musculature of the ankle) which, in part, explains the absence of dorsiflexion effects on jump performance (a sport action that involves all the muscles of the lower limb). Differences in the mass of the sports shoes (weights for a US size 10 were 460 g for DF, 348 g for CS1 and 354 g for CS2) could be a study limitation. Nevertheless, we considered that the practical effect on jump performance was negligible. The difference between the sports shoes mass was around 110 g, and represents 0.1% of the mass mean of the participants. In summary, jump kinetics differed between dorsiflexion shoes and conventional sports shoes but a similar jump performance was found. Therefore, the use of dorsiflexion shoes did not improve sports performance in volleyball, where jumping is directly related to the success of defensive and offensive actions. More research is needed to determine the influence of dorsiflexion on sports perfor-
mance. Furthermore, future research in this area must analyze the influence of familiarization with these shoes on jump performance.
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