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Nagahara, et. al. (2017) conducted a well designed study to clarify the mechanical determinants of sprinting performance during the acceleration and maximal speed phases of a single sprint, using ground reaction forces (GRFs). While 18 male athletes performed a 60-m sprint, GRF was measured at every step over a 50-m distance from the start. Variables during the entire acceleration phase were approximated with a fourth-order polynomial. Subsequently, accelerations at 55%, 65%, 75%, 85%, and 95% of maximal speed, and running speed during the maximal speed phase were determined as sprinting performance variables. Ground reaction impulses and mean GRFs during the acceleration and maximal speed phases were selected as independent variables. Stepwise multiple regression analysis selected propulsive and braking impulses as contributors to acceleration at 55%–95% (β > 0.724) and 75%–95% (β > 0.176), respectively, of maximal speed. Moreover, mean vertical force was a contributor to maximal running speed (β = 0.481). The current results demonstrate that exerting a large propulsive force during the entire acceleration phase, suppressing braking force when approaching maximal speed, and producing a large vertical force during the maximal speed phase are essential for achieving greater acceleration and maintaining higher maximal speed, respectively.

Reference

Ryu Nagahara, Mirai Mizutani, Akifumi Matsuo, Hiroaki Kanehisa, and Tetsuo Fukunaga. 2017. Association of sprint performance with ground reaction forces during acceleration and maximal speed phases in a single sprint. J Appl Biomech. Sep 27:1-20. doi: 10.1123/jab.2016-0356.

Abstract: Since sprinting involves very fast movement velocities (up to 12 m/s in the best athletes), experimental studies in this field have always been a technical challenge. While sprint kinematics and distance-time or velocity-time variables were first described by the end of the 19th century, kinetics and especially ground reaction force and mechanical power outputs have remained unexplored until the 1970s and 1980s. Cutting edge laboratory installations now allow for full-length sprint acceleration studies (single or multiple sprint protocols) with track-embedded force plates. However, a significant amount of literature and knowledge has been previously established by the use of instrumented treadmills. These were first non-motorized and not directly measuring the ground reaction force (end of the 1980s), but the most up-to-date device allows investigation of sprint mechanics and three-dimensional ground reaction force during an accelerated run (from zero to maximal velocity). In this chapter, we will present the historical development of these devices, along with their advantages and limitations, and the main experimental results obtained with the motorized accelerated treadmill. In particular, we will present the key concept of mechanical effectiveness of ground force application, and how it is related to sprint performance. Furthermore, we will discuss the muscular underpinnings of the mechanical effectiveness; specifically the role of hip extensors. Finally, we will discuss the comparison between treadmill and track sprint performance and mechanics, including data from elite sprinters, and how current and future research on this topic will allow a deeper understanding of this seemingly simple yet complex motor task.

Reference

Jean-Benoît Morin, Scott Randall Brown, Matt R Cross. 2018. The Measurement of Sprint Mechanics Using Instrumented Treadmills. DOI: 10.1007/978-3-319-05633-3_10 In book: Biomechanics of Training and Testing, pp.211-236.

Abstract and Background: The effect of hamstring flexibility on the peak hamstring muscle strains in sprinting, until now, remained unknown, which limited our understanding of risk factors of hamstring muscle strain injury (hamstring injury). As a continuation of our previous study, this study was aimed to examine the relationship between hamstring flexibility and peak hamstring muscle strains in sprinting.

Methods
Ten male and 10 female college students participated in this study. Hamstring flexibility, isokinetic strength data, three-dimensional (3D) kinematic data in a hamstring isokinetic test, and kinematic data in a sprinting test were collected for each participant. The optimal hamstring muscle lengths and peak hamstring muscle strains in sprinting were determined for each participant.

Results
The muscle strain of each of the 3 biarticulated hamstring muscles reached a peak during the late swing phase. Peak hamstring muscle strains were negatively correlated to hamstring flexibility (0.1179 ≤ R2 ≤ 0.4519, p = 0.001) but not to hip and knee joint positions at the time of peak hamstring muscle strains. Peak hamstring muscle strains were not different for different genders. Peak muscle strains of biceps long head (0.071 ± 0.059) and semitendinosus (0.070 ± 0.055) were significantly greater than that of semimembranosus (0.064 ± 0.054).

Conclusion
A potential for hamstring injury exists during the late swing phase of sprinting. Peak hamstring muscle strains in sprinting are negatively correlated to hamstring flexibility across individuals. The magnitude of peak muscle strains is different among hamstring muscles in sprinting, which may explain the different injury rate among hamstring muscles.

Reference

Xianglin, Wan, Feng Qu, William E.Garrett, Hui Liu, Bing,Yu. 2017. The effect of hamstring flexibility on peak hamstring muscle strain in sprinting, Volume 6, Issue 3, September 2017, Pages 283-289

Abstract: In this study, we sought to compare force-velocity relationships developed from unloaded sprinting acceleration to that compiled from multiple sled-resisted sprints.

Twenty-seven mixed-code athletes performed six to seven maximal sprints, unloaded and towing a sled (20-120% of body-mass), while measured using a sports radar. Two methods were used to draw force-velocity relationships for each athlete: A multiple trial method compiling kinetic data using pre-determined friction coefficients and aerodynamic drag at maximum velocity from each sprint; and a validated single trial method plotting external force due to acceleration and aerodynamic drag and velocity throughout an acceleration phase of an unloaded sprint (only). Maximal theoretical force, velocity and power were determined from each force-velocity relationship and compared using regression analysis and absolute bias (± 90% confidence intervals), Pearson correlations and typical error of the estimate (TEE).

The average bias between the methods was between - 6.4 and - 0.4%. Power and maximal force showed strong correlations (r = 0.71 to 0.86), but large error (TEE = 0.53 to 0.71). Theoretical maximal velocity was nearly identical between the methods (r = 0.99), with little bias (- 0.04 to 0.00 m s-1) and error (TEE = 0.12).

When horizontal force or power output is considered for a given speed, resisted sprinting is similar to its associated phase during an unloaded sprint acceleration [e.g. first steps (~ 3 m s-1) = heavy resistance]. Error associated with increasing loading could be resultant of error, fatigue, or technique, and more research is needed. This research provides a basis for simplified assessment of optimal loading from a single unloaded sprint.

Reference

Cross, MR, Samozino, P, Bvrown SR, and Morin, JB. 2018. A comparison between the force-velocity relationships of unloaded and sled-resisted sprinting: single vs. multiple trial methods. Eur J Appl Physiol. 2018 Mar;118(3):563-571.

Abstract and Objectives: This study investigated the relation of different previously reported preparatory strategies and musculo-skeletal loading during fast preplanned 90° cutting maneuvers (CM). The aim was to increase the understanding of the connection between whole body orientation, preparatory actions and the solution strategy to fulfill the requirements of a CM.

Methods

Three consecutive steps of anticipated 90° CMs were investigated in a 3D movement analysis setup. Pelvis orientation clustered the subjects in two groups, with minor and major pre-orientation. To understand the impact of body orientation on the specific movement strategy, joint angles, moments and energy as well as spatio-temporal parameters of the movement were analyzed.

Results

Early rotation of the body was initiated by a small step width during braking resulting in a more constant path velocity of the centre of mass and less demands on the hip- and knee surrounding muscles. Minor pre-orientation required increased work of the hip muscles to decelerate, reaccelerate and in particular to rotate the body. This resulted in an increase of contact time. While pre-orientation in combination with fore-foot striking led to a strategy where energy absorption and generation is mainly generated by the ankle plantar flexors, less pre-orientation and rear-foot striking resulted in a knee- and hip dominant strategy.

Conclusion

Step width before transition strongly determined pre-orientation and overall body position. Both strategies fulfill the requirements of a CM but induce different demands regarding muscular capacities. Pelvis orientation and step width are easy-to-use assessment parameters in the practical field.

Reference
Sina, David, Marion Mund, Igor Komnik, Wolfgang Potthast. 2018. Understanding cutting maneuvers – The mechanical consequence of preparatory strategies and foot strike pattern. Human Movement Science Volume 62, December 2018, Pages 202-210

Abstract: The capacity to rapidly generate and apply a great amount of force seems to play a key role in sprint running. However, it has recently been shown that, for sprinters, the technical ability to effectively orient the force onto the ground is more important than its total amount. The force-vector theory has been proposed to guide coaches in selecting the most adequate exercises to comprehensively develop the neuromechanical qualities related to the distinct phases of sprinting. This study aimed to compare the relationships between vertically-directed (loaded and unloaded vertical jumps, and half-squat) and horizontally-directed (hip-thrust) exercises and the sprint performance of top-level track and field athletes. Sixteen sprinters and jumpers (including three Olympic athletes) executed vertical jumps, loaded jump squats and hip-thrusts, and sprinting speed tests at 10-, 20-, 40-, 60-, 100-, and 150-m. Results indicated that the hip-thrust is more associated with the maximum acceleration phase (i.e., from zero to 10-m; r = 0.93), whereas the loaded and unloaded vertical jumps seem to be more related to top-speed phases (i.e., distances superior to 40-m; r varying from 0.88 to 0.96).

These findings reinforce the mechanical concepts supporting the force-vector theory, and provide coaches and sport scientists with valuable information about the potential use and benefits of using vertically- or horizontally-based training exercises to enhance speed performance.

Reference

Loturco I, Contreras B, Kobal R, Fernandes V, Moura N, Siqueira F, et al. (2018) Vertically and horizontally directed muscle power exercises: Relationships with top-level sprint performance. PLoS ONE 13(7): e0201475. https://doi.org/10.1371/journal.pone.0201475

Dehydration is a drag on human performance. It can cause fatigue and sap endurance among athletes, according to a 2018 study in the journal Frontiers in Physiology. Even mild dehydration can interfere with a person’s mood or ability to concentrate.

Water is cheap and healthy. And drinking H2O is an effective way for most people to stay hydrated. The National Academy of Medicine recommends that adult women and men drink at least 91 and 125 ounces of water a day, respectively. (For context, one gallon is 128 fluid ounces.) But pounding large quantities of water morning, noon and night may not be the best or most efficient way to meet the body’s hydration requirements.

“If you’re drinking water and then, within two hours, your urine output is really high and [your urine] is clear, that means the water is not staying in well,” says David Nieman, a professor of public health at Appalachian State University and director of the Human Performance Lab at the North Carolina Research Campus. Nieman says plain water has a tendency to slip right through the human digestive system when not accompanied by food or nutrients. This is especially true when people drink large volumes of water on an empty stomach. “There’s no virtue to that kind of consumption,” he says.

In fact, clear urine is a sign of “overhydration,” according to the Cleveland Clinic. And some of the latest research supports Nieman’s claim that guzzling lots of water is not the best way to stay hydrated.

For a 2015 study in the American Journal of Clinical Nutrition, researchers compared the short-term hydration effects of more than a dozen different beverages—everything from plain water and sports drinks to milk, tea, and beer, to a specially formulated “rehydration solution.” Based on urine analyses collected from the study volunteers, the researchers concluded that several drinks—including milk, tea, and orange juice, but not sports drinks—were more hydrating than plain water. (Lager was a little less hydrating than water, but a little better than coffee.)

Of course, no one’s suggesting that people dump water in favor of milk and OJ. Water is still hydrating. So are sports drinks, beer, and even coffee, to some extent. But the authors of the 2015 study wrote that there are several “elements of a beverage” that affect how much H2O the body retains. These include a drink’s nutrient content, as well as the presence of “diuretic agents,” which increase the amount of urine a person produces. Ingesting water along with amino acids, fats and minerals seems to help the body take up and retain more H2O—and therefore maintain better levels of hydration—which is especially important following exercise and periods of heavy perspiration.

“People who are drinking bottles and bottles of water in between meals and with no food, they’re probably just peeing most of that out,” Nieman says. Also, the popular idea that constant and heavy water consumption “flushes” the body of toxins or unwanted material is a half-truth. While urine does transport chemical byproducts and waste out of the body, drinking lots of water on an empty stomach doesn’t improve this cleansing process, he says.

In some rare cases, excessive water consumption can even be harmful. “In athletes or people who are exercising for hours, if they’re only drinking water, they can throw out too much sodium in their urine, which leads to an imbalance in the body’s sodium levels,” explains Nieman, who has spent a chunk of his career investigating exercise-related hydration. Doctors call this imbalance “hyponatremia,” and in some cases it can be deadly. In this scenario, sports drinks and other beverages that contain nutrients and sodium are safer than plain water.

While hyponatremia and excessive water consumption aren’t big concerns for non-athletes, there are better ways to keep the body and brain hydrated than to pound water all day long. Sipping water (or any other beverage) a little bit at a time prevents the kidneys from being “overloaded,” and so helps the body retain more H2O, Nieman says.

Drinking water before or during a meal or snack is another good way to hydrate. “Drinking water with amino acids or fats or vitamins or minerals helps the body take up more of the water, which is why beverages like milk and fruit juice tend to look pretty good in these hydration studies,” he says. Some of his own research has found that eating a banana is better than drinking a sports beverage when it comes to post-exercise recovery. And he says eating almost any piece of fruit along with some water is going to aid the body’s ability to take up that H2O and rehydrate. (These hydration rules apply to athletes as well, he says.)

The take-home message isn’t that people should drink less water, nor that they should swap out water for other beverages. But for those hoping to stay optimally hydrated, a slow-and-steady approach to water consumption and coupling water with a little food is a more effective method than knocking back full glasses of H2O between meals. “Water is good for you, but you can drown in it too,” Nieman says.

Source: August 9, 2019 Time Magazine, https://time.com/5646632/how-much-water-to-drink/. Contact editors@time.com.

This study compared the efficiency and speed of the cross-over step to backward pedaling for defensive backs when covering a receiver.

Abstract

In American football, defensive backs guarding receivers use either the cross-over (CO) run or backpedal (BP) technique, but the efficacy of these techniques is unknown. The purpose of this study was to compare linear acceleration (LA) and change of direction (COD) ability when using CO and BP. Collegiate football defensive backs participated in LA (n = 13) and COD (n = 7) testing. During LA, participants performed CO, BP, and forward sprints with split times taken between 0-3 and 3-5 yd and ground reaction forces recorded 0 and 3 yd from the start. During COD testing, participants performed the CO or BP for 3 yd and then were given a cue to sprint to a gate 5 yd away in 1 of 4 directions (downfield, midfield, sideline, or upfield). In LA, CO was faster than BP between 0-3 yd (Δ -0.20 ± 0.02 seconds, p = 0.000) and 3-5 yd (Δ -0.12 ± 0.02 seconds, p = 0.000). At the start of the movement, CO demonstrated greater propulsive forces (p = 0.017). However, 3 yd from the start, CO demonstrated greater propulsive forces and reduced braking forces (p = 0.000 & 0.003). In COD, CO was faster than BP when running in the downfield (Δ 0.21 ± 0.05 seconds, p = 0.044) and lateral directions (Δ 0.21 ± 0.08 seconds, p = 0.035), but similar in the upfield direction (Δ 0.01 ± 0.08, p = 0.986). Our results indicate that CO is superior to BP in LA, COD ability, and movement efficiency and support the use of CO for defensive backs.

Coaching Implications

Although the cross-over run and the backpedalling technique both play a role at certain times in pass coverage for defensive backs, the cross-over produces greater linear acceleration and more efficient change of direction ability than backpedaling.

Reference

Angelino, D, McCabe, TJG, and Earp, JE. Comparing acceleration and change of direction ability between backpedal and cross-over run techniques for use in American football. J Strength Cond Res. May 25, 2018.

Abstract: This study investigated the maximal sprint velocity kinematics of the fastest 100 m sprinter, Usain Bolt. Two high-speed video cameras recorded kinematics from 60 to 90 m during the men 100 m final at the IAAF World Challenge Zagreb 2011, Croatia. Despite a relatively slow reaction time (194 ms), Bolt won in 9.85 s (mean velocity: 10.15 m/s). His fastest 20-m section velocity was 12.14 m/s, reached between 70 and 90 m, by 2.70-m long strides and 4.36 strides/s frequency. At the maximal velocity, his contact and flight times were 86 and 145 ms, respectively, and vertical ground reaction force generated equalled 4.2 times his body weight (3932 N). The braking and propulsion phase represented 37% and 63% of ground contact, respectively, with his centre of mass (CoM) exhibiting minor reductions in horizontal velocity (2.7%) and minimal vertical displacement (4.9 cm). Emerged Bolt’s maximal sprint velocity and international predominance from coordinated motor abilities, power generation capacities, and effective technique. This study confirms that his maximal velocity was achieved by means of relatively long strides, minimal braking phase, high vertical ground reaction force, and minimal vertical displacement of CoM. This study is the first in-depth bio-mechanical analysis of Bolt’s maximal sprinting velocity with the segmental reconstruction.

Coaching Implications

These findings reinforce those of numerous other studies that highlighted the key factors in determining the maximum mph speed of athletes: proper form to make certain that the maximum amount of available GRF (ground reaction force) is applied at the right time, in the right direction, and in the the shortest possible GCT (ground contact time). At maximum speed, nearly 100 percent of ground force requirements are in a vertical direction to quickly minimize braking forces at ground contact, overcome the force of gravity, and propel the body back up into the air. For elite sprint performance, this requires a vertically-directed ground reaction force of over four times the body weight of athletes. Training to improve form and technique and maximum power generation (vertically-directed ground reaction force) is the key to a higher top-end speed for athletes in all sports.

Reference

Milan Čoh, Kim Hébert-Losier, Stanko Štuhec, and Vesna Babić. 2018. Kinemartics of Usain Bolt’s Maximal Sprint Velocity. Kinesiology Vol. 50, No.2 Prosinac.

Abstract: Sprinting is key in the development and final results of competitions in a range of sport disciplines, both individual (e.g., athletics) and team sports. Resisted sled training (RST) might provide an effective training method to improve sprinting, in both the acceleration and the maximum-velocity phases. However, substantial discrepancies exist in the literature regarding the influence of training status and sled load prescription in relation to the specific components of sprint performance to be developed and the phase of sprint.

Objectives
Our objectives were to review the state of the current literature on intervention studies that have analyzed the effects of RST on sprint performance in both the acceleration and the maximum-velocity phases in healthy athletes and to establish which RST load characteristics produce the largest improvements in sprint performance.

Methods
We performed a literature search in PubMed, SPORTDiscus, and Web of Science up to and including 9 January 2018. Peer-reviewed studies were included if they met all the following eligibility criteria: (1) published in a scientific journal; (2) original experimental and longitudinal study; (3) participants were at least recreationally active and towed or pulled the sled while running at maximum intensity; (4) RST was one of the main training methods used; (5) studies identified the load of the sled, distance covered, and sprint time and/or sprint velocity for both baseline and post-training results; (6) sprint performance was measured using timing gates, radar gun, or stopwatch; (7) published in the English language; and (8) had a quality assessment score > 6 points.

Results
A total of 2376 articles were found. After filtering procedures, only 13 studies were included in this meta-analysis. In the included studies, 32 RST groups and 15 control groups were analyzed for sprint time in the different phases and full sprint. Significant improvements were found between baseline and post-training in sprint performance in the acceleration phase (effect size [ES] 0.61; p = 0.0001; standardized mean difference [SMD] 0.57; 95% confidence interval [CI] − 0.85 to − 0.28) and full sprint (ES 0.36; p = 0.009; SMD 0.38; 95% CI − 0.67 to − 0.10). However, non-significant improvements were observed between pre- and post-test in sprint time in the maximum-velocity phase (ES 0.27; p = 0.25; SMD 0.18; 95% CI − 0.49 to 0.13). Furthermore, studies that included a control group found a non-significant improvement in participants in the RST group compared with the control group, independent of the analyzed phase.

Conclusions
RST is an effective method to improve sprint performance, specifically in the early acceleration phase. However, it cannot be said that this method is more effective than the same training without overload. The effect of RST is greatest in recreationally active or trained men who practice team sports such as football or rugby. Moreover, the intensity (load) is not a determinant of sprint performance improvement, but the recommended volume is > 160 m per session, and approximately 2680 m per total training program, with a training frequency of two to three times per week, for at least 6 weeks. Finally, rigid surfaces appear to enhance the effect of RST on sprint performance.

Reference
Pedro E. Alcaraz, Jorge Carlos Vivas, Bruno O. Oponjuru, and Alejandro Martinez-Rodriguez 2018. The Effectiveness of Resisted Sled Training (RST) for Sprint Performance: A Systematic Review and Meta-analysis. Sports Medicine Vol. 48, Issue 9, pp2143-2165.