Andrew Hamilton explains the concept of overload and intensity for strength training, and presents recent evidence demonstrating that often, less can be more… MORE
Injury prevention: stay injury free by optimising your running mechanics
There’s more to reducing the risk of sustaining a running injury than incorporating a couple of stretches and the odd weights session into your training routine. As Matt Lancaster explains, a structured approach to build ‘running robustness’ is a much better approach.
Oscar Pistorius is an athlete that made the world news headlines for all the wrong reasons. However, his ability to run 400 metres in less than 47 seconds and his determination to compete in the 2008 Beijing Olympics not only became a big story in athletics but is relevant to this article too. Pistorius was born without fibulas (the smaller of the two bones which form the lower part of the leg) and he has never walked without the aid of prosthetic limbs. He began running competitively in 2003 and after winning the 200 metres at the Athens Paralympic Games, turned his attention to competing against able bodied athletes.
The IAAF ruled this was not possible (based on an investigation by Professor Gert-Peter Brueggemann), and concluded that an athlete using the carbon fibre prosthetic blades has a more than 30 percent mechanical advantage over an athlete not using the blades. Once Pistorius reached a certain stride the blades, known as Cheetahs, behaved like stiff springs and he was able to run at the same speed as able-bodied runners using about 25% less energy. The IAAF decision and the engineered properties of the Cheetah blades tell us a great deal about both the fundamentals of running mechanics and the stresses which running places on the human body. This article draws together key aspects of running mechanics and the principles of biological robustness to explore practical ways in which you can adapt your own training to minimise the stresses and strains on your body.
Locomotion requires us to propel ourselves forwards, while at the same time counteracting the force of gravity, which is constantly pushing us down(1). In order to overcome this gravitational force, we have to push down into the ground with each foot strike, and (as anyone who recalls high school physics will know), if we are transmitting a force into the ground, an equal and opposite force is returning through our feet and ankles. This force is called a ground reaction force (GrF).
When you run, vertical GrFs can exceed three times your body weight, depending on your mass and the speed you run at (2,3). In order to move forward, we simultaneously pull our leg backwards beneath our torso, creating a horizontal GrF. The amount of muscle activity and force production required to do this increases as we run faster (4). This muscular action, along with the impact of GrFs, produces a considerable amount of stress and strain, which our tissues have to absorb if we are to avoid injury.
So, how do we accommodate these stresses while at the same time propelling ourselves upwards and forwards? Well, like Oscar Pistorius, we have springs too, only instead of being made of carbon-fibre they are composed of a complex system of muscles, tendons, ligaments and other connective tissues (5).
In simple terms, when your foot strikes the ground you absorb and dissipate energy by lowering your centre of mass (compression) before generating energy to extend the leg and propel us up and forward (recoil) (5). In this way, energy is constantly stored (largely within the tendons) and recycled using a mechanism known in biomechanics as the spring-mass model (see figure 1) (3).
However, running is not quite as straightforward as this. In addition to moving forward, up and down we also shift from side to side while our limbs and torso rotate. There are three reasons for this. Firstly, our joints are shaped irregularly and are neither perfect hinges nor spheres, meaning our movement has to occur in multiple planes. Secondly, these sideways and twisting movements help absorb GrF (Braukmann makes the case that as Pistorius does not have feet or ankle joints there is increased shock through his stumps into his knees, hips and back). And finally, if we tried to run without shifting our centre of mass from side to side, we’d almost certainly fall over! The primary purpose of running is to move forwards, but our body is subjected to stresses and strains acting in every possible direction.
Biological robustness describes the ability of a biological system to maintain its core function in the face of stresses and uncertainty occurring within the system or its environment (6,7). Another way to consider this may be to think of an organism continuing to perform despite ongoing changes and adaptations in either its components or surroundings(8). If the organism can’t adapt successfully, then disease or injury may follow (9).
Thinking about biological systems in this way draws on the principles underpinning a branch of science called complexity. Complex systems, such as our nervous system, circulatory system or indeed the entire human body, consist of a large number of components interacting together. Crucially, the overall function cannot be explained by examining the components alone(6,8,9). Put simply, the performance of a complex system is greater than its parts. For instance, a function as basic as running cannot be described simply by studying anatomy.
Developing running robustness
There is no simple or sure-fire way to avoid injury, but if we combine our basic understanding of running mechanics with the principles of biological robustness, it may give us an insight into how we can structure our training to help reduce the risk of injury. The remainder of this article considers different forms of training in relation to both a specific training goal and a robustness goal. Training types and goals are summarised in table 1 below:
Strength and modularity
Far from a single sprung structure, the human leg has three segments acting around joints – the ankle, knee and hip – which combine to produce the overall biomechanics required for running (10). We can consider each of these segments a separate module able to absorb energy, primarily around the knee and ankle, and then produce a rapid propulsive force, which is supplemented by the recoil of the springs. Within this modular architecture, the calf, thigh (quadriceps and rectus femoris) and hip extensors (gluteal and hamstring muscles) are the key running muscles (1, 10).
In biological systems, each individual module must meet the stress demands placed upon it to ensure robustness (6). Modules need to have a recovery capacity to repeat their function, which probably means that even for power-based athletes, developing local endurance properties over a period of time is important.
Interaction between modules, including their relative stiffness, also determines the distribution of stresses between the hip, knee and ankle joints (10). A module with poor capacity may increase the stress demands placed on a neighbouring module. Finally, a strong modular structure can help contain excessive stress or local damage, minimising the effects of injury on the whole system (6).
Strength training is ideally suited to increasing the specific capacity of these modules to meet the demands of running. Working against resistance, whether gravity or weight training, can be an effective method for improving the capacity of muscle-tendon units to absorb and produce force. Progression of strength exercises is usually aimed at developing more of a specific capacity, while training phases often follow a similar series progression: first endurance, then strength and then power. Examples of strength exercises for key running muscle groups are shown in figure 2.
Conditioning and fragility
There is a price to be paid for developing specific robustness, and it goes some way to explaining how highly trained athletes can still be susceptible to injury. As training and strength progress we become increasingly adapted to the stimulus our body expects. However, high levels of adaptation to a familiar stress may conversely leave you potentially fragile to an unexpected stress. And as the highly adaptable and complex being that you are, it is often tiny unexpected stresses that may prove catastrophic. This is referred to as the robustness-fragility trade off (6,7,8).
A simple way to counteract this fragility is to increase the variety of stresses your tissues are conditioned to. Conditioning training is less concerned with the specific mechanics of running than strength training. Of course, if your primary sport is running, there is little to be gained from conditioning your body to stresses as divergent as those encountered in judo or rugby. However, conditioning your body to a range of stresses that are somewhat broader than the very specific adaptations gained from running and strength training may be advantageous.
Rather than progressing repetitions or resistance of a small selection of strength exercises, the key progression here is adaptation to a wider variety of moderate stresses. This means choosing exercises that stress trunk and leg tissues in particular (see figure 3), utilising a spectrum of resistance levels and joint ranges, as well as providing multidirectional tissue challenges.
And don’t get stuck in the rut of repeating the same exercises for months at a time; adapt and then change. Remember, the goal is not necessarily to make each exercise progressively harder, but to expose tissues to a broader range of stresses than they are subject to when running. Building your condition and minimising your fragility may take time. This sort of conditioning is ideally suited to circuit training.
Coordination and system control
Within all biological systems, some form of control is essential to achieve a robust response to a particular stress (6). In humans, this control is provided by our nervous system and manifests itself as coordination. Within a complex biological system, coordination requires the mastering of all the possible movements available to us into a controllable movement pattern, such as running or jumping.
Skill involves ensuring this temporary movement pattern is resistant to any environmental stresses that may challenge its stability, and is therefore probably vital for us in developing robustness (11). Regulating modules or utilising slightly different strategies to overcome unexpected stresses requires sophisticated orchestration (6).
Traditionally, theories concerning movement control have considered particular movement characteristics to be stored within our central nervous system, ultimately leading to control and limited variation when we perform a particular skill.
However, consider what happens when we run on different surfaces. If our leg stiffness were always the same then our vertical rise and fall would be greater when we ran on a soft, compliant surface. But biomechanical experiments suggest that we actually adjust our leg stiffness quite quickly (possibly within a single step!) depending on the compliance of the surface. In other words, our legs become stiffer and compress less when we run on a soft surface to allow us to maintain a steady running pattern (3).Likewise, wearing footwear may influence our stiffness and it appears that our muscle activity is tuned to control tissue stress depending on GrFs(5,12). Far from a single movement pattern, there is constant variability in even a seemingly repetitive skill such as running. This requires coordination and it is possible that the challenge is greater still for an amputee athlete like Pistorius who does not have feedback from the feet and ankles to assist.
So how should we incorporate coordination challenges into training? Firstly, aim to perform all training components (strength, conditioning and running) with skill. The central nervous system coordinates the activity of many muscles, tendons and ligaments to set the overall ‘spring’ characteristic of running, while strength training is redundant without highly developed motor control, involving appropriate timing of movements for effective force production (3,4).
Secondly, while making a conscious effort to move well, we also need to address fragility by introducing a ‘bandwidth of variability’ in the way we run or exercise and challenge our coordination to accommodate this variability – ie taking ourselves outside our coordination comfort zone. Running drills can be an effective way of achieving this. There are too many drills to attempt to describe here, but rehearsing components of the running action and then adding variety – speed, direction, range, rhythm – is a good place to start.
Thirdly, general coordination activities that relate to running mechanics are also helpful. Skipping with a rope or simple hopping activities, such as playing hopscotch, are examples. Base the progression on the quality and breadth of your skill. Coordination will be involved in all aspects of your training but take time to challenge and develop it further.
If you want to guarantee you don’t get hurt as a result of running, don’t run. But if you do want to run while minimising the risk of injury, run smart. The volume, intensity, gradient and terrain of any training should be consistent with your capacity to cope with the stress. And here lies the inherent risk of running; in order to adapt and improve our performance, we have to expose ourselves to progressive new stresses.
Nothing enhances running robustness like running. Indeed tissues such as tendons undergo greater adaptation in response to running than other forms of exercise (13). This underpins both environmental buffering and the basis of successful coaching; it’s not the overall volume or intensity of a running program that necessarily provides the risk, but the rate of progression and the time allowed for adaptation to significant new stresses. Don’t be afraid to take things slowly in order to run faster.
Even a sensible training progression will not guarantee your protection from the small variations you may face however. By now you should be able to draw your own conclusions about how you may achieve this. Subtle variation and exposure to a slightly wider range of stresses will allow broader adaptation.
For example, based on changes in GrF and required leg stiffness, making an absolute transition from purely grass running to running only on the road is probably unwise. However, a well-balanced programme incorporating components of each may provide safe but challenging exposure to a range of stresses.
On the same basis, manipulating your running form for short periods in a controlled and skilful way could subtly broaden tissue exposure and coordination challenges. For instance, in addition to running drills, practice over or under striding, exaggerating your pelvis and hip rotation or running without any arm swing. Make measured, sensible adaptations to your training plan but within that plan train with a degree of variability.
A final word
Developing robustness requires resources, including energy and time. Going to the gym, developing a broad conditioning base and performing coordination drills may leave you less fragile, but it may also detract from the time and energy you have for running. Sacrificing too much of your running time at the expense of less specific training may leave you less adapted to the demands of running itself. And in terms of performance, it is probably your specific robustness that is most relevant. You can never remove the risk of injury completely (genetics probably has a say in this (14)), but planning and performing your training well may reduce the risk.