Taking an Acceleration-Based Approach
Taking an Acceleration-Based Approach to Performance and Return to Play
By Derek Hansen
I had the exceptional opportunity to provide a return-to-play presentation to NFL athletic trainers as part of the PFATS (Professional Football Athletic Trainers Society) in late February 2018 at the NFL Combine in Indianapolis. The focus of my lecture was on an acceleration-based approach to hamstring return-to-play. While there are all sorts of techniques and technology directed at strengthening the hamstrings, I have always had superior results using short sprints as a means of addressing the specific strength needs of the hamstrings. The goal of my presentation was to impart basic coaching techniques for sprint and acceleration training that the athletic trainers could use as part of their in-season return-to-play protocols.
For the most part, rehabilitation professionals must return athletes to the activities required for their sport, particularly at the intensities required to be competitive. Thus, it follows that running and sprinting should be fundamental components of that return-to-play process. Because of the numerous qualities displayed in running and sprinting—high-velocity recruitment, significant force production, eccentric control, timing, coordination, active range of motion, and specific endurance requirements—it not only serves as a highly effective means of preparing athletes for the demands of their sport, but also a very useful screening tool to assess their competence across a broad spectrum of physical and neurological qualities.
Yet many people believe that basic acceleration is an inherent quality that can be worked by simply playing a sport or by developing strength in the weight room. Certainly, there are natural talents that can accelerate and sprint rapidly with very little training. However, in this day and age of watered-down natural selection, we don’t commonly witness as much high-quality sprinting and effortless-looking acceleration as we would hope to. These are skills that athletes need to develop and hone through consistent quality repetition under the watchful eye of a trained professional.
Once you have built a foundation of high-quality acceleration mechanics, athletes can much more easily attain higher running speeds. They can also easily reproduce the consistent repetition of high-quality acceleration efforts—such as those required for sporting success and longevity—from game to game, match to match, and race to race. The athletes that can find the formula to this equation will be much more effective and resilient over their careers.
The Skill Set Required to ‘Coach’ Sprinting
I have had the benefit of coaching sprinting with athletes of all ages and abilities for just over 30 years. In college, my summer job was teaching 8- to 15-year-old kids how to sprint and jump in track & field events. I gradually progressed to coaching college athletes, Olympic athletes, and professional athletes on how to improve their sprinting ability over that 30-year period. I have witnessed a lot of different approaches over three decades and I would be one of the first people to say that learning how to coach sprinting technique should be very simple and easy.
The caveat to that statement is that you need to learn how to coach sprinting from a highly competent teacher or mentor. When I say good “teacher,” I do not mean someone that was successful as a sprinter. Competence in performance has no connection to competence in coaching or teaching. A good teacher often has experience with the practical skill in question, but also understands the physics and mechanics involved in carrying out the task. More importantly, the good teacher is an exceptional communicator and a keen observer, knowing when to speak and when to take in information without unnecessarily interrupting or delaying the learning process.
In my experience, good coaches must have exceptional awareness of what is present and what is missing when it comes to an athlete under their supervision and the training program that he or she follows, particularly as they relate to performance and injury. Understanding the qualities that an athlete has in his or her possession—including levels of strength, general fitness, movement skill, and even fatigue—can influence how you guide training, determining progressions and total volumes of work. Similarly, knowing what is missing from an athlete can be critical in determining your decision-making process during the planning stages and even implementation of individual workouts.
Good sprinting performances and overall athlete health demand that all of the correct boxes are checked off in terms of identifying whether the key preconditions and elements are present throughout the training process. Similarly, each step in a sprint performance must be monitored, from the first stride to maximum velocity, in order to assemble an inventory of qualities that can either demonstrate competence or reveal significant deficiencies. The guidelines below will identify the key qualities of optimal acceleration that must be present to ensure maximal athletic performance with minimal potential for injury.
Acceleration Posture as a Strengthening Tool
The most common scenario for running-based hamstring strains is when an athlete is either transitioning to upright high-speed running or trying to maintain upright sprinting posture while running at or near top speed. The position of the upright sprinting stride creates a greater opportunity for rapid lengthening of the hamstrings under tension, eccentrically loading the hamstrings to a point of potential injury. The late-swing phase of upright sprinting—as depicted in photos 1 and 5 in Figure 1—creates a significant eccentric load on the hamstrings prior to ground contact. The combination of rapid lengthening and force production on ground contact can cause the hamstrings to strain.
This being said, an athlete that is well-prepared technically will be less apt to err in the execution of the skill, and less likely to place excess stress on the hamstrings during the cyclical action of sprinting. Additionally, an athlete that has been progressively loaded appropriately in his or her sprinting workouts and through ancillary strength work will have the capacity to handle the stresses of repeated high-speed sprinting efforts. However, while upright high-speed running is a highly specific means of preparing the hamstrings for the stresses of sprinting, there are less risky means of strengthening the hamstrings for these situations.
The simple act of acceleration provides an inherent means of strengthening the lower body dynamically and specifically for sprinting at higher speeds, while at the same time having a lower risk of injury than actual upright sprinting. An examination of the performance data on elite-level sprinters demonstrates that 70% of maximum velocity is often attained by the sixth stride (or approximately 10 meters) as indicated in Figure 2. While the velocity of running is still submaximal at this point, effort is still maximal and the output being produced by the body is intense.
Ground contact times are rapidly being reduced within the 10-meter sprinting segment, as shown in Figure 3, and stride length is increasing (Figure 4)—pointing to the fact that the rate of force development is amplifying on every step pairing.
It becomes abundantly clear that sprint acceleration training places an increasingly progressive load on the body with each stride taken throughout the acceleration repetition. The data is remarkably illustrative in this regard, and a closer examination at the mechanics of acceleration provides even more evidence of the merits of the practice.
As mentioned previously, upright sprinting—even at submaximal velocities—places a significantly greater eccentric stress on the hamstrings. Acceleration, on the other hand, involves greater knee flexion on ground contact, limiting the eccentric stress on the hamstring. The hamstring is still involved in the production of force and the stabilization of the knee joint, but it is not exposed to the same high-risk lengthening experienced in upright sprinting. Thus, an athlete can still benefit from the strengthening forces produced during acceleration through an activity that approaches, but doesn’t duplicate, the mechanics of maximum-velocity sprinting.
A closer examination of an athlete in mid-acceleration in Figure 5 demonstrates the magnitude of knee flexion over a longer ground contact period. The postural angle of the athlete during acceleration imparts the necessary vertical and horizontal force requirements for efficient and expedient acceleration on ground contact. Note that postural angle during acceleration is highly dependent on the force production abilities of the athlete, with more powerful athletes able to establish and maintain a lower angle of propulsion than less powerful individuals. This acceleration angle can also be established through various start positions and formally trained through deliberate repetition.
Figure 6 distinctly illustrates the influence of postural acceleration angle on foot placement relative to the athlete’s center of mass. If an athlete is powerful enough to establish and maintain a lower drive angle during acceleration, foot placement will be directly under or behind the center of mass during acceleration, limiting braking forces on ground contact, but also limiting eccentric forces on the hamstring complex. This becomes especially important during the hamstring rehabilitation process, when acceleration repetitions are used to strengthen the hamstring without placing undue stresses on it. The concept of strengthening, specifically without putting the athlete at risk for re-injury, allows for a more expedient return-to-play process with more durability over the long run.
Soft-Tissue Injuries and ‘Brain’ Injuries
While it is common to attribute muscle strains to a strength deficiency, it is often a much more complicated scenario. While it is a good thing to make an athlete strong, it is also very important to combine strength with skill when it comes to muscle recruitment, particularly at high velocities. High-speed sprinting involves a complex arrangement of muscle-firing patterns with a high level of dissociation between the three muscles that make up the hamstring group.
In their 2014 study, “Biceps Femoris and Semitendinosus – Teammates or Competitors? New Insights into Hamstring Injury Mechanisms in Male Football Players: A Muscle Functional MRI Study,” Schuermans et al. examined this concept in greater depth. They found that injured hamstrings demonstrated more symmetrical muscle activation patterns, causing the hamstring muscle bellies to contract less efficiently. They determined that the biceps femoris muscle compensated for the functions of the semitendinosus. Knowing that 83% of running-related hamstring injuries occur in the biceps femoris, we can infer that specific muscle groups are not firing at the correct time during the running gait cycle, creating greater risk to other structures within the hamstring complex.
Acute trauma to the hamstring muscles is of significant concern immediately following an injury, but we must also be aware of the need to re-educate the neuromuscular properties of the hamstrings. We know that upright running can place undue eccentric stress on the hamstrings, and even low-speed running in the upright position can aggravate a pre-existing hamstring injury. Acceleration mechanics, however, can provide a valuable means of stressing and strengthening the hamstrings appropriately without placing an excessive stretch on these structures. And, as demonstrated by the biomechanical data, progressively longer acceleration efforts can gradually place a greater load on the hamstrings as velocities increase and an athlete approaches the upright sprint position. Acceleration is the perfect means of progressing an athlete physically and neurologically through the return-to-play process.
Rapid, Sustained Acceleration—The Key to Top Speed
If you are an enthusiast of fast cars and motorsports, you know that the cars with the highest top speeds also have exceptional acceleration capabilities. The Hennessy Venom F5 is capable of hitting a top speed of 301 mph and can accelerate from 0 to 60 mph in 2.0 seconds with its engine delivering 1,600 horsepower. Similarly, a typical Formula 1 race car has the ability to hit a top speed of 250 mph and accelerate from 0 to 60 mph in 1.9 seconds with engines approaching the 1,000-horsepower range.
Animals and humans are no different, with horsepower and acceleration abilities closely linked to top speed. In fact, humans typically occupy the slower end of the acceleration and velocity spectrum when compared to other mammals such as horses, grizzly bears, canines, and larger varieties of felines. But biology is biology, and physics is physics. If you do not have the hardware and software to accelerate rapidly, you will not achieve relatively high speeds at the top end of your performance abilities.
Training for acceleration can often be elusive for some coaches and athletes. While we can all agree that getting stronger must be part of the acceleration solution, the specifics of how to strengthen an athlete for acceleration and sprinting are absolutely critical if you want exceptional and sustainable results. A good weightlifting program can contribute greatly to the acceleration performance of an athlete if programmed properly and provided in a manner that is supportive of an effective sprint program.
More recently, we have seen the use of heavy sleds to load athletes for horizontal force production. However, there are many downsides to relying on heavy sleds to supplement acceleration work. Figure 7 illustrates the perils of heavy sled pushing for acceleration performance, citing the excessive ground contact times—negating the desired elastic responses—and the obvious postural issues. The most significant concern relates to the misdirected use of the arms during this exercise.
Good acceleration and sprinting performances involve the proper execution of arm swing mechanics. A poor arm swing can quickly weaken posture and negatively impact the delivery of both horizontal and vertical force into the ground. Having the arms free to swing powerfully is critically important for proper sustained acceleration on the way to top speed. Good acceleration and sprinting performances involve the proper execution of arm swing mechanics. Click To Tweet
Figure 8 illustrates an example of a weak arm drive during acceleration, with a resulting posture that impairs optimal hip extension and force delivery. Conversely, Figure 9 depicts a more powerful arm drive, resulting in the dynamic delivery of force through the hip and extension leg. If the athlete is free to develop a powerful arm drive during acceleration training, this summation of multi-joint force production can lead to significant sprint performances for athletes in sports requiring rapid acceleration and high-speed running.
There are many means of loading an athlete during acceleration while keeping the arms free to powerfully assist the action of sprinting. Uphill sprints have been the traditional method of adding resistance to acceleration repetitions, while the use of dragging sleds has been more common in recent decades. New emerging technologies also provide a means of not only adding resistance to athletes, but also monitoring force production and stride symmetry during acceleration efforts. The more information we can glean from acceleration performances to help us improve efficiency and power, the more we will be able to fine-tune the training and return-to-play processes.
Acceleration Capacity, Return to Play, and Detraining Reduction
The final need with acceleration training is quantifying and monitoring changes throughout development and the competitive season. An increase in volume with a drop in intensity or speed is not building maximal capacity; it’s compromising the higher intensities necessary to prepare the athlete for competition-specific needs. Without an accurate and precise sprinting session, the threshold required for athletes may not be met, decreasing capacity and perhaps increasing risk of injury and poor performance.
As a consultant to teams regarding hamstring injuries and training, the topic of return to play comes up all the time. For those not familiar with the HiTrainer, it’s a running system that allows for deep acceleration postures and high-effort sprinting. What is provocative is that acceleration posture is not only preserved, but the cambered padding allows for a natural shoulder roll that is not available with handheld models of sled pushing.
I became aware of the system because clients want to know about all the options available for specific needs, and those that have it want to see relationships between contact times and asymmetries. Even if you don’t have baseline data, watching an athlete sprint and seeing the reporting can start the interpretation process immediately with real-time feedback. Simple measurements automated for the coach is the direction we need; instead of being servants to technology, it now finally works for us.
One area that commonly confuses coaches is the difference between fatigue and detraining, as the symptoms are similar but the causes are not always the same. Detraining occurs when training is too conservative or when injuries mount up, forcing athletes to compete on talent and not on training.
Precise and sequenced loading, also known as micro priming, transforms small doses of sprints into a very powerful stimulus for athletes if applied correctly. One example that is easy to administer in team environments during the regular season is short bursts of maximal effort sprints done periodically during the competitive season. Maximal intensity work with very low risk is a perfect fit for the NFL and other sports.
Moving Forward with Acceleration
Placing athletes in a position to accelerate powerfully and efficiently has a profound impact on their athletic abilities and their long-term durability. While simple and ubiquitous in the sporting world, there must be deliberate preparation, rehearsal, and loading to ensure athletes achieve the full potential of this type of training. Good coaching and the proper measurements must be part of the process of guiding athletes to their natural form, but there is no doubt in my mind that the road to top speed is paved with acceleration mastery.