Research by Faude et al., (2012) has shown that sprinting within professional football often leads to decisive moments within the game. Faude et al., (2012) research has also suggested that straight line sprints in particular are the most common action that led to goal scoring opportunities either from the goal scorer or assisting player, followed by changes of direction and jumps. Therefore, because of this, speed and power capabilities are deemed to be an essential physical trait of professional footballers (Faude et al., 2012).

A sprint can be broken down into three phases; firstly the acceleration phase, then the maximal velocity phase and finally the maintenance of top end speed (Delecluse, 1997). Research by Bangsbo (1994) has found that within professional football 96% of all sprints performed by players are less than 30m, are short in duration <6 seconds and occur every 90 seconds (Reily, Bangsbo & Franks, 2000). Sprinting within professional football matches only accounts for a very small proportion of the total distance covered by each player, around 1.4%-3.1%, depending on playing position, of which, strikers perform the most (345.5m ± 129.50m) (Andrezejewski et al., 2015). However, as stated earlier it is during these sprinting moments that most commonly leads towards a goal scoring opportunity. Research within professional football has also indicated that around 49% of sprints performed by players are less than 10 meters and often occur from a rolling start (Stølen et al., 2005). There is also evidence that would suggest that better professional football players are also faster sprinters, emphasising the importance of sprinting performance and training (Haugen, Tønnessen, & Seiler, 2013).

The importance of sprinting within sport has resulted in research focusing on the most appropriate training methods in which to improve sprint performance (Moir, Brimmer, Snyder, Connaboy, & Lamont, 2018). Research has shown the importance of force production within sprinting (Moir et al., 2018). This has subsequently led to the investigation of resistance training on sprinting performance (Moir et al., 2018). Following this, a meta-analysis by Seitz  et al., (2014) investigating resistance trainings effect on sprint performance has shown that it is an effective means to improve sprint performance, particularly distances below 30m (Moir et al., 2018).

The Phases of Maximal Speed Sprinting

Figure 1 depicts the speed-distance data from a group of elite sprinters compared to students over a 100 m sprint. The biexponential curve can be used to determine three distinct phases (Moir et al., 2018):

  1. Acceleration phase – the positive slope of the curve.
  2. Attainment of maximal speed phase – the peak of the curve.
  3. Maintenance of maximal speed phase –the negative slope of the curve.

 The identification of the different sprint phases is acknowledged within the literature, however, there is currently no consensus on the number of phases that should be included to describe maximal speed sprinting (Delecluse et al., 1995).

Figure 1. Speed-distance data for a group of elite sprinters and a group of students performing a 100 m sprint. The values in parentheses of the legend denote the average times for the 100 m in each group. Taken from Moir, G. L., Brimmer, S. M., Snyder, B. W., Connaboy, C., & Lamont, H. S. (2018). Mechanical limitations to sprinting and biomechanical solutions: a constraints-led framework for the incorporation of resistance training to develop sprinting speed. Strength & Conditioning Journal, 40(1), 47-67. From Babić, V., Čoh, M., & Dizdar, D. (2011). Differences in kinematic parameters of athletes of different running quality. Biology of Sport, 28(2).

The three phases of acceleration, maximal speed, and maintenance can be applied to all athletes, although the duration of each phase differs (Delecluse et al., 1995). Untrained athletes achieve their maximal sprinting speed much earlier than elite level athletes, due to the fact their maximal speeds are much lower (Delecluse et al., 1995). Despite the longer acceleration phase of elite sprinters, they attain approximately 80% of their maximal speed within the first 20m of a 100 m sprint (Krzysztof et al., 2013). The importance of the different sprint phases for the S&C coach is highlighted by the fact that the mechanical demands differ in each of the phases and so do the ground reaction forces (GRF) (Moir et al., 2018). A GRF is an external force provided by the ground/surface that the athlete is in contact with and represents the reaction of the support surface to the actions of the athlete, specifically the ground/support surface reactions to the athlete’s foot (Moir, 2015).

During the acceleration and maximal velocity phases of sprinting different GRF affect the athlete (Moir et al., 2018). During a sprint the average horizontal force decreases as the athlete accelerates across the acceleration phase and reaches their maximal speed, while the average vertical force increases (Moir et al., 2018). During the acceleration phase of sprinting there is a greater requirement for horizontal GRF to accelerate the athlete, which results in a forward-directed GRF (Morin et al., 2011) and during the maximal velocity phase of sprinting there is a greater requirement of the athlete to produce vertical GRF to support the athletes bodyweight and project the centre of mass (CM) into an aerial phase of sufficient duration to allow the repositioning of the swing leg when sprinting at maximal velocity (Weyand et al., 2010). These differences observed in sprinting can be seen in figure 2, during a 100 m sprint: the athlete adopts a forward lean during the acceleration phase, whilst changing to an upright position when they reach maximal velocity sprinting (Moir et al., 2018) (Figure 2).

Figure 2. The different postures attained during the early acceleration phase (a. and b.) and when sprinting at maximal speed (c. and d.). Contrast the forward lean adopted when accelerating with the upright posture attained at maximal speed. Notice the high knee lift of the swing leg at take-off during maximal speed sprinting (2d.). From Moir, G. L., Brimmer, S. M., Snyder, B. W., Connaboy, C., & Lamont, H. S. (2018). Mechanical limitations to sprinting and biomechanical solutions: a constraints-led framework for the incorporation of resistance training to develop sprinting speed. Strength & Conditioning Journal, 40(1), 47-67.

Biomechanical Analysis of Acceleration and Maximal Speed Sprinting

 Sprinting speed is achieved through alternating phases of stance and flight during each stride, a stride being defined as the event between touchdown, when the foot makes contact with the ground, of the stance leg to the next ipsilateral touchdown (DeVita, 1994). There are two stance phases, firstly when the athlete is in contact with the ground and secondly the two aerial phases, when the athlete is in flight during a sprinting stride (Moir et al., 2018). The flight phase represents the time when the leg is repositioned in preparation for the next stance phase (Moir et al., 2018). The forward-directed GRF produced during the acceleration phase of sprinting is achieved by a mechanism called the rotation-extension strategy, which is utilised during the stance phase of sprinting (Moir et al., 2018). During the stance phase of sprinting the CM is propelled horizontally by the forward rotation of the stance leg about the stance foot resulting in the rotation-extension strategy (Moir et al., 2018). The rotation-extension strategy represents a biomechanical solution to the forward-directed GRF required during acceleration whereby the forceful extension of the knee and ankle joints is delayed while the CM is rotated ahead of the resultant GRF (Moir et al., 2018) which results in horizontal propulsion of the athlete. Recent research by Mendiguchia et al., (2016) has shown the importance of the hamstring muscles in generating a forward-directed GRF when sprinting and how it is affected following hamstring muscle injury.

 Maximal speed is determined by stance distance and stance time (Moir et al., 2018). During the short stance phases associated with high sprinting speeds there is still a requirement to exert a vertical force to support the athletes bodyweight to prevent the athlete from falling (Moir et al., 2018). The vertical forces that are applied to the ground during short stance phases in order to provide the minimum aerial time to reposition the swing leg for the subsequent ipsilateral stance imposes a biomechanical limitation to maximal running speed (Moir et al., 2018). Research has shown that faster athletes exert greater vertical forces during stance compared to slower athletes (Zamparo, 2002).  The vertical GRF is achieved during maximal speed sprinting by a mechanism called the impact-limb deceleration mechanism whereby the swing leg is ‘punched’ into the ground at touchdown to create the vertical GRF, which is needed to support the athlete (Clark et al., 2014). The achievement of a large vertical GRF is achieved by the impact-limb deceleration mechanism, which appears to be a biomechanical solution to overcome the limitation associated with the inability of the stance leg extensor muscles to generate sufficient force during the brief stance phases associated with maximal sprinting speeds (Weyand et al., 2010).

 Training Methods to Improve Sprinting Speed 

 The resistance training exercises selected by an S&C coach are generally based upon the principle of specificity. Typically the S&C coach will select exercises which share biomechanical and physiological characteristics with the performance movements in order to enhance the transfer of adaptations (Siff, 2000). The specificity of these exercises increase as the athlete progresses through a training programme towards the competitive phase of the season (Moir et al., 2018). When designing a resistance training programme the S&C coach should utilise their knowledge of the mechanical limitations of sprinting and the biomechanical solutions to these, to guide the selection of the appropriate resistance training exercises (Moir et al., 2018). In order to improve sprint performance the S&C coach should select exercises which will enhance firstly the rotation-extension strategy, which is utilised to provide a forward-directed GRF during the acceleration phase of sprinting (Moir et al., 2018). Secondly, the S&C coach should select exercises than will improve the impact-limb deceleration mechanism, which is used to create a vertical GRF when sprinting at maximal speeds (Moir et al., 2018). A prerequisite for efficient use of these mechanisms are high levels of muscular strength and power (Moir et al., 2018).

 Acceleration is defined as the capacity to generate as high a force as possible in the shortest distance or time possible (Lockie et al., 2013). Through a review of the literature it is generally accepted that within field sports such as football the ability to accelerate is seen as being of the greatest significance (Murphy et al., 2003). During the acceleration phase of sprinting research has shown that the joint moments of the stance leg are required to generate force during progressively shorter stance phases (Moir et al., 2018). Research has shown a high correlation between peak power and maximum strength (r=0.77-0.94) (McLellan et al., 2011). Peterson et al., (2006) has also demonstrated a significantly strong linear relationship between muscular strength and power (P<0.01). Therefore, resistance exercises that focus on the development of strength and power of the hip, knee, and ankle joints should be included within any training programme designed to improve sprinting performance (Moir et al., 2018). Research conducted by Wisloff et al., (2004) has suggested that there is a strong relationship (r = 0.94) between an athlete’s maximal back squat strength, expressed as their one repetition maximum (1RM), and their 10m-sprint performance (Wisloff et al., 2004), revealing that athletes with higher 1RM had faster 10m sprint times. Similar relationships have also been observed between 1RM and 20-40m sprint times (Comfort, Stewart, Bloom, & Clarkson, 2014; Wisloff et al., 2004). Furthermore, this highlights the importance of an athlete’s strength as a prerequisite for sprinting speed.  Strengthening the athlete’s lower body muscles particularly the posterior chain (e.g. glute muscles, hamstrings muscles and calfs) is fundamental to any S&C programme aimed at improving sprint performance. Exercises such as Back Squats, Deadlifts, Romanian Deadlifts, Split Squats, Good Mornings, Glute Ham Raises, Nordics and unilateral versions of these exercises should form the core of an athletes strength programme (Moir et al., 2018).

The Olympic lifts, snatch and the clean and jerk, and their derivatives which utilise triple extension, extension of the ankle, knee and hip, can also be an excellent addition within an S&C programme aimed at improving sprint performance (Moir et al., 2018). These exercises promote the rapid generation of large forces throughout the movement and have dynamic correspondence towards sprinting biomechanics (Moir et al., 2018). Olympic lifting pulling derivatives such as the clean and snatch pulls, mid-thigh clean pulls, rack pulls, high pulls and jump shrugs, can also be very beneficial within the S&C programme.   These types of exercises are particularly beneficial for novice athletes, firstly as a method for developing their technique of the Olympic lifts and secondly because they may not possess the required lifting technique to perform the full lift variations (Moir et al., 2018). Therefore, the pull variations still allow novice athletes to still reap the benefits of Olympic lifting but reduce the risk associated with the full lifts (Moir et al., 2018). However, for more experienced athletes exercises such as the full clean and snatch can be used to development force while exercises such as the power clean, power jerk and power snatch can be used to develop movement velocity (Suchomel et al., 2015).

Plyometric exercises are also a very effective training modality for the development of muscular power via the development of rate of force development (RFD) and movement velocity (Markovic et al., 2010). Unloaded and loaded vertical and horizontal jumps maximise the force production of the muscles of the hip, knee, and ankle joints (Lees et al., 2004). Exercises such as, broad jumps require force generation by hip, knee, and ankle joint and  incorporate the same reciprocal activation of the hamstrings and quadriceps as observed during the rotation-extension strategy utilised during the acceleration phase of sprinting (Jones et al., 2003). These biomechanical elements make horizontal jumps essential within any training programme designed at improving sprint speed. Despite this, horizontal exercises which support the development of accelerative sprinting and are often overlooked within training programmes (Moir et al., 2018). Once the athlete is competent at performing bilateral jumping exercises the athlete can be progressed towards unilateral exercises. Initially, the athlete should perform single leg jumps onto plyometric boxes before progressing to single leg drop lands then single leg drop jumps, hops and hurdles. It is essential when performing these unilateral exercises that the athlete always lands with excellent landing mechanics, avoiding knee valgus, varus and trunk lean/rotation. An athlete should not be progressed onto more complex exercises unless they have optimal landing and plyometric technique. When the athlete is ready to tolerate greater forces at higher velocities bounding exercises can be utilised to promote the generation of power (Moir et al., 2018). The role of the joint moments of the stance leg changes from force production during the acceleration phase of sprinting towards a greater amount of force absorption, particularly at the knee and ankle joints, during the maximal speed sprinting phase (Moir et al., 2018). In order to improve the impact limb deceleration mechanism utilised within maximal speed phase of sprinting (Moir et al., 2018) the S&C coach should focus on developing  eccentric strength, leg stiffness and power (Douglas et al., 2017). The use of fast stretch shorting plyometric exercises, ground contact time (GCT) <0.250ms, such as, drop jumps and hurdle jumps have been shown to be excellent exercises to promote increased leg stiffness and develop eccentric strength (McMahon et al., 2002). Unilateral running mechanic drills, such as A/B skip, forward and lateral marches, that have been shown to replicate the ground reaction forces utilised with maximal speed sprinting while promoting ankle stiffness (Farley et al., 1999). Furthermore, because of this, these exercises may aid the development of maximal sprinting speed, by improving the athlete’s ability to use the impact limb deceleration mechanism.

Resisted sprint training is another training modality that has been shown to enhance the transfer of muscular strength and power from traditional resistance training exercises towards more biomechanical specific exercises (Moir et al., 2018). Resisted-sprint methods such as sled towing has been shown to be very effective in improving the acceleration phase of sprinting as they can help the player develop horizontal power (Moir et al., 2018). Sled towing exercises involve unilateral force production during stance and also reduced the athlete’s aerial times when sprinting which can contribute to a faster repositioning of the swing leg during stance (Moir et al., 2018). This can ultimately result in a greater development of horizontal power during accelerative sprinting (Lockie et al., 2003). Heavy sled towing (>30% decrease in sprint speed) should be utilised within an S&C programme when the focus is on the development of high force production to improve the acceleration phase, while sled towing with lighter loads (<15% decrease in sprinting speed) is more appropriate for the use of the development of maximal speed running (Petrakos, 2016). When performing resisted sprint training it is essential to use the correct sled loading in order to train at the optimal intensity. The most commonly used method to load a sled is based on the reduction in sprint speed. This has been emphasised by research conducted by Alcaraz et al., (2008) who found that a loading of 16% body mass resulted in a 10% decrease in the maximal velocity of their testing athletes. Similarly, research conducted by Makaruk et al., (2013) found that a load of 7.5% body mass resulted in a 10% reduction in sprint velocity. However, this can only be used as a guide as more research is needed to determine the optimal loading for resisted sprint training. Finally, when considering the use of resisted sled training to improve sprinting performance, the S&C needs to consider the training surface when determining the sled load, as there will be different frictional resistance between artificial grass, grass and running tracks.

Assisted-sprint training, which involves an athlete being towed via the use of an elastic cord or a mechanical winch, can also be beneficial in improving maximal sprinting speed (Moir et al., 2018).  Towing an athlete has been found to increase the vertical and braking forces associated during the early stance phase during maximal speed sprinting (Mero, 1994). This would result in a promotion of increased leg stiffness of the stance leg in a unilateral movement, promoting the development of the initial impact-limb deceleration mechanism (Moir et al., 2018).  Research by Mero (1984) has shown that the velocity of the swing leg is also increased at supramaximal velocities associated with athlete towing, aiding the development of the high knee required as part of the limb-deceleration mechanism (Moir et al., 2018). Despite this, assisted sprint training using an elastic cord or winch is not very practical in a field based setting or within team sport environment. Furthermore, there is limited research available regarding the optimal assistance needed to provide beneficial training effects (Bartolini et al, 2011).  Therefore, further research is requirement in order to implement this strategy effectively within a sprint performance programme.

To conclude, performing un-resisted sprints is another training strategy that has been shown to improve sprint performance. As this method alone can provide ample stimulus to develop the rotation-extension strategy and the impact-limb deceleration mechanism (Moir et al., 2018).   When programming un-resisted sprint sessions it is recommended that the S&C coach begins with high volume sprints at submaximal velocities, using long distance sprints or high reps of shorter distance sprints, towards short distance sprints focusing on maximal speed sprinting (Moir et al., 2018).

This has been a brief review of the literature, for a more detailed account of the biomechanics of sprinting and how to improve sprint performance please refer to the excellent work of Moir et al., (2018) ‘Mechanical limitations to sprinting and biomechanical solutions: a constraints-led framework for the incorporation of resistance training to develop sprinting speed’.

 

References

 

  1. Alcaraz, P. E., Palao, J. M., Elvira, J. L., & Linthorne, N. P. (2008). Effects of three types of resisted sprint training devices on the kinematics of sprinting at maximum velocity. The Journal of Strength & Conditioning Research, 22(3), 890-897.
  2. Andrzejewski, M., Chmura, J., Pluta, B., & Konarski, J. M. (2015). Sprinting activities and distance covered by top level Europa League Soccer Players. International Journal of Sports Science & Coaching, 10(1), 39-50.
  3. Bangsbo, J. (1994). The physiology of soccer: with special reference to intense intermittent exercise. Acta Physiologica Scandinavica, 151-156.
  4. Bangsbo, J. (2014). Physiological Demands of Football. Sports Science Exchange, 27, 1-6.
  5. Bartolini, J. A., Brown, L. E., Coburn, J. W., Judelson, D. A., Spiering, B. A., Aguirre, N. W., … & Harris, K. B. (2011). Optimal elastic cord assistance for sprinting in collegiate women soccer players. The Journal of Strength & Conditioning Research, 25(5), 1263-1270.
  6. Clark, K. P., & Weyand, P. G. (2014). Are running speeds maximized with simple-spring stance mechanics?. Journal of applied physiology, 117(6), 604-615.
  7. Comfort, P., Stewart, A., Bloom, L., & Clarkson, B. (2014). Relationships between strength, sprint, and jump performance in well-trained youth soccer players. Journal of Strength and Conditioning Research, 28, 173–177.
  8. Delecluse, C. H., Coppenolle, H. V., Willems, E., Diels, R., Goris, M., Leemputte, M. V., & Vuylsteke, M. (1995). Analysis of 100 meter sprint performance as a multi-dimensional skill. Journal of Human Movement Studies, 28(2), 87.
  9. DeVita, P. (1994). The selection of a standard convention for analyzing gait data based on the analysis of relevant biomechanical factors. Journal of Biomechanics, 27(4), 501-508.
  10. Douglas, J., Pearson, S., Ross, A., & McGuigan, M. (2017). Chronic adaptations to eccentric training: a systematic review. Sports Medicine, 47(5), 917-941.
  11. Farley, C. T., & Morgenroth, D. C. (1999). Leg stiffness primarily depends on ankle stiffness during human hopping. Journal of biomechanics, 32(3), 267-273.
  12. Faude, O., Koch, T., & Meyer, T. (2012). Straight sprinting is the most frequent action in goal situations in professional football. Journal of sports sciences, 30(7), 625-631.
  13. Haugen, T. A., Tønnessen, E., & Seiler, S. (2013). Anaerobic performance testing of professional soccer players 1995–2010. International Journal of Sports Physiology and Performance, 8(2), 148-156.
  14. Jacobs, R., & van Ingen Schenau, G. J. (1992). Intermuscular coordination in a sprint push-off. Journal of biomechanics, 25(9), 953-965.
  15. Jones, S. L., & Caldwell, G. E. (2003). Mono-and biarticular muscle activity during jumping in different directions. Journal of Applied Biomechanics, 19(3), 205-222.
  16. Krzysztof, M., & Mero, A. (2013). A kinematics analysis of three best 100 m performances ever. Journal of human kinetics, 36(1), 149-160.
  17. Lees, A., Vanrenterghem, J., & De, D. C. (2004). The maximal and submaximal vertical jump: implications for strength and conditioning. Journal of strength and conditioning research, 18(4), 787-791.
  18. Lockie, R. G., Murphy, A. J., Schultz, A. B., Jeffriess, M. D., & Callaghan, S. J. (2013). Influence of sprint acceleration stance kinetics on velocity and step kinematics in field sport athletes. The Journal of Strength & Conditioning Research, 27(9), 2494-2503.
  19. Makaruk, B., Sozański, H., Makaruk, H., & Sacewicz, T. (2013). The effects of resisted sprint training on speed performance in women. Human Movement, 14(2), 116-122.
  20. Markovic, G., & Mikulic, P. (2010). Neuro-musculoskeletal and performance adaptations to lower-extremity plyometric training. Sports medicine, 40(10), 859-895.
  21. McGowan, C. P., Grabowski, A. M., McDermott, W. J., Herr, H. M., & Kram, R. (2012). Leg stiffness of sprinters using running-specific prostheses. Journal of The Royal Society Interface, 9(73), 1975-1982.
  22. McMahon, J. J., Comfort, P., & Pearson, S. (2012). Lower limb stiffness: Effect on performance and training considerations. Strength & Conditioning Journal, 34(6), 94-101.
  23. McLellan, C. P., Lovell, D. I., & Gass, G. C. (2011). The role of rate of force development on vertical jump performance. The Journal of Strength & Conditioning Research, 25(2), 379-385.
  24. Mendiguchia, J., Edouard, P., Samozino, P., Brughelli, M., Cross, M., Ross, A & Morin, J. B. (2016). Field monitoring of sprinting power–force–velocity profile before, during and after hamstring injury: two case reports. Journal of sports sciences, 34(6), 535-541.
  25. Mero, A., & Komi, P. V. (1994). EMG, force, and power analysis of sprint-specific strength exercises. Journal of Applied Biomechanics, 10(1), 1-13.
  26. Mero, A., Luhtanen, P., & Komi, P. V. (1984). A biomechanical study of the sprint start. Scandinavian Journal of Sports Science, 5(1), 20-28.
  27. Moir, G. L., Brimmer, S. M., Snyder, B. W., Connaboy, C., & Lamont, H. S. (2018). Mechanical limitations to sprinting and biomechanical solutions: a constraints-led framework for the incorporation of resistance training to develop sprinting speed. Strength & Conditioning Journal, 40(1), 47-67.
  28. Moir, G. L., Brimmer, S. M., Snyder, B. W., Connaboy, C., & Lamont, H. S. (2018). Mechanical limitations to sprinting and biomechanical solutions: a constraints-led framework for the incorporation of resistance training to develop sprinting speed. Strength & Conditioning Journal, 40(1), 47-67.
  29. Morin, J. B., & Sève, P. (2011). Sprint running performance: comparison between treadmill and field conditions. European journal of applied physiology, 111(8), 1695-1703.
  30. Murphy, A. J., Lockie, R. G., & Coutts, A. J. (2003). Kinematic determinants of early acceleration in field sport athletes. Journal of sports science & medicine, 2(4), 144.
  31. Peterson, M. D., Alvar, B. A., & Rhea, M. R. (2006). The contribution of maximal force production to explosive movement among young collegiate athletes. Journal of Strength and Conditioning Research, 20(4), 867.
  32. Reilly, T., J. Bangsbo, & Franks, A. (2000). Anthropometric and physiological predispositions for elite soccer. Journal of Sports Science, 18, 669–683.
  33. Seitz, L. B., Reyes, A., Tran, T. T., de Villarreal, E. S., & Haff, G. G. (2014). Increases in lower-body strength transfer positively to sprint performance: a systematic review with meta-analysis. Sports medicine, 44(12), 1693-1702.
  34. Seitz, L. B., Reyes, A., Tran, T. T., de Villarreal, E. S., & Haff, G. G. (2014). Increases in lower-body strength transfer positively to sprint performance: a systematic review with meta-analysis. Sports medicine, 44(12), 1693-1702.
  35. Siff, M. (2000). Strength and the muscular system. Supertraining, 5th ed. Denver: Supertraining Institute, 1-93.
  36. Stølen, T., Chamari, K., Castagna, C., & Wisløff, U. (2005). Physiology of soccer. Sports medicine, 35(6), 501-536.
  37. Suchomel, T. J., Comfort, P., & Stone, M. H. (2015). Weightlifting pulling derivatives: Rationale for implementation and application. Sports Medicine, 45(6), 823-839.
  38. Weyand, P. G., Sandell, R. F., Prime, D. N., & Bundle, M. W. (2010). The biological limits to running speed are imposed from the ground up. Journal of applied physiology, 108(4), 950-961.
  39. Wisloff, U., Castagna, C., Helgerud, J., Jones, R., & Hoff, J. (2004). Strong correlation of maximal squat strength with sprint performance and vertical jump height in elite soccer players. British Journal of Sports Medicine, 38, 285–288.
  40. Zamparo, P., Minetti, A., & Di Prampero, P. (2002). Interplay among the changes of muscle strength, cross-sectional area and maximal explosive power: theory and facts. European journal of applied physiology, 88(3), 193-202.
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