Plyometric training can cause tendon injury if performed incorrectly

Highly sprung – maximising tendon health for running performance

At a glance

Elite athletes have been using plyometrics to improve their performance for decades. However, many sportsmen and women overlook their tendon health, and as a result suffer preventable injuries. Keith Baar explains

In all running events over 200 metres, the outcome is primarily determined by who slows the least – in other words, by who is the most efficient or economical runner. This is where Oscar Pistorius, the South African double amputee sprinter, has the advantage over fully biological athletes. His carbon fibre Cheetah prosthetics are able to store and return up to 30% more energy than biological limbs. That extra efficiency means that he slows less as the race goes on, resulting in improved performance. What Oscar teaches us is the importance of energy return in running economy and speed. Therefore, an important question for every athlete is ‘how can I maximise this trait in my training and improve my performance?’

Muscle springs

Before we can discuss how to maximise energy return, we must first understand what it is and how it is developed. Energy return can be described as a ‘muscle spring’. Like a spring, it has nothing to do with generating force. Picture a weight on top of a spring. At rest, there is no movement. This is because a spring cannot produce energy, only store and return it. When you apply force to the weight and release it, the spring will store the force in its metal coils and return it when released. However, not all of the energy is returned: a small amount of the energy is lost as heat. The stiffer the spring, the less energy lost, and the more economical the spring.

The same is true for the human body except that in our bodies, the springs are made from muscle and tendon – not steel. Working together, the muscle and tendon store and return energy and like a steel spring, the stiffer the so-called muscle spring, the greater the energy stored and the better an athlete’s speed and economy.

This effect was best demonstrated by a group of French scientists who showed that the passive stiffness of the muscle spring correlated both with the maximal power produced by the legs and maximal running speed(1). This in turn suggests that our maximal performance will occur on the stiffest possible muscle springs.

Unlike metal springs or Cheetah prosthetics, the stiffness of our muscle springs is adjustable, both in the short term and the long term. In the short term, stiffness can be improved by activating the muscles within the spring (increasing active stiffness), while over the longer term, proper training will stiffen the elements that make up our springs (increasing passive stiffness).

Increasing passive stiffness

Unlike Oscar Pistorius, most of us don’t have carbon fibre or metal springs in our legs, so the first question is what do we use instead? In our bodies, the biochemical elements of the muscle-spring system are the protein titin (in muscle tissue) and collagen (in tendon tissue). The stiffness of these proteins determines the passive stiffness of our muscles and tendons, respectively.

Titin is the largest protein in the body. As a comparison, titin is 10-15 times bigger than myosin, the main muscle motor protein. Two titin protein molecules stretch from one end of a muscle unit (sarcomere) to the other. Each titin protein has a ‘spring’ region and the length of this region determines the passive stiffness of our muscles.

While there is only one gene responsible for making titin, there are a number of different forms of this huge muscle protein. Importantly, in fast-twitch muscle where passive stiffness is higher, titin is shorter than in slow muscle and it appears that with training we can shift towards a shorter titin, increasing the passive stiffness of our muscles. This change in titin size occurs as a result of altered gene expression induced by training.

Collagen, meanwhile, is the most abundant protein in our bodies, making up our bones, skin and tendons, among other things. Within the tendons, collagen is aligned to produce a thick rope-like connection between muscle and bone that stores and transfers energy between the two.

Unlike titin, however, the collagen within our tendons does not change its form with training. Further, because the collagen is so densely packed in tendons (making up more than 75% of the protein of the tendon), increasing the collagen content within tendons would have little effect on overall tendon stiffness. Instead, training increases the number of chemical cross links between the collagen molecules(2), and the more of these cross-links there are, the stiffer the tendon becomes(3). This also happens in all collagens with ageing and is especially obvious in babies. The reason they can suck on their toes is that their collagen is less cross-linked making it easier for them to stretch.
The long-term objectives of ‘spring training’, therefore, should be to create shorter titin molecules and more cross-linked collagen to increase the passive stiffness of the muscles and tendons that make up our springs. For athletes and coaches, the important question is ‘can training produce these changes?’ The answer to this question is ‘yes’. For example, training basketball players for eight weeks results in approximately 10% improvement in two measures of passive stiffness, which in turn translates into better jumping performance(4). This tells us that our springs can be stiffened with the appropriate training.

Increasing active stiffness

Increasing the passive stiffness of muscles and tendons is all well and good, but to gain a performance advantage, you need to translate that passive stiffness into speed. Turning passive stiffness into speed involves activating the muscles within the spring at the right time (increasing active stiffness).

When we run, our muscles start to contract before we land on the ground (see figure 1). This is important because by contracting the muscle before landing we actively increase the stiffness of our springs and maximise the amount of energy that we can store and return to the track. However, if the muscle is activated too soon, the foot is in the wrong position to absorb energy; too late, and the landing energy isn’t stored and we have to work harder to run at the same speed.

In an ideal situation, our muscles do not shorten when we run. Instead, we want our muscles to contract isometrically (ie without the muscle changing length) and our muscle spring to store energy while it lengthens before forcibly returning that energy to the track, propelling us forward. When our muscles contract isometrically, they produce more force and use less energy than when they shorten making us faster and more economical runners.

Since the stiffness of the muscle spring is controlled by precise muscle contraction, to improve the active stiffness of our springs we need to train our brains. The brain has to precisely time the activation of each muscle to maximise our spring stiffness and running economy.

During level running, the calf muscle contracts isometrically when the foot is on the ground. Since the foot goes from flexed to extended during this time, this means that the tendon stretching, not the muscle shortening, causes the movement of the foot. To make this happen, the muscle is turned on before you hit the ground (we can see this by noting the EMG activity). This tunes the muscle spring, tightening the tendon so that it can absorb as much energy as possible.

Training passive stiffness

As discussed above, when basketball players were trained for eight weeks, they increased the passive stiffness. However, not just any exercise will do this. In this particular study, individuals who performed a traditional weight-training programme did not improve their passive spring stiffness or performance. Those who did improve stiffness performed high-jerk lengthening contractions. Jerk is the rate of change of acceleration and it is highest during quick lengthening contractions or plyometric exercises. This tells us that the best way to improve the passive stiffness of the muscle spring is through plyometric exercise.

One of the best ways to start adding plyometric exercise to your training is to start running downhill. When you run downhill, you increase how much and how fast your muscles and tendons have to stretch each time you hit the ground.
The immediate result of running downhill is a small degree of muscle damage. Over the next 24-48 hours this leads to delayed onset muscle soreness, as immune cells enter the muscles and repair the damage. However, when the cells are repaired, muscles and tendons become better able to deal with subsequent downhill runs and, over time, they increase their passive stiffness, most likely by shortening the length of their titin. This increase in passive stiffness translates into an increase in the stiffness of muscle springs and improved running economy.

Another way to increase passive stiffness is ballistic or plyometric training. These are explosive strength training techniques where the amount of jerk in the legs is high. Exercises within this area include sprints, skipping, bounding, and jumping exercises such as box jumps, hurdle jumps, foot tucks, and five-jump tests.

The key when performing these exercises is quickness. The goal is not only to jump, skip, or bound as far as possible, but also to move from one jump to the next as fast as possible. Finnish researchers have shown that replacing up to 30% of endurance training with this type of explosive strength training for nine weeks decreases 5k running time in elite cross-country runners by 2.7%(6). This improvement in performance is probably due to increased passive stiffness of the muscle spring since it correlates with improved running economy.

Training active stiffness

One of the best ways to train the brain to become accustomed to activating muscles at a faster pace is to run at a faster pace. This can easily be accomplished by training on a very slight downhill slope (~1-3%). While downhill running can improve passive spring stiffness, it is also effective for improving active stiffness. The faster pace from running downhill means the brain has to increase the natural pattern of muscle activation.

The key when using downhill running training is not to use too steep a decline, to lean forward from the hips, and not to overstride. The natural response when running downhill is to open up the stride, but this not only prevents the desired adaptations within the brain, but also has a negative impact on performance. Overstriding – when the foot hits the ground too far in front of the body – results in braking forces and a loss of energy. Telling the athlete to ‘shorten their stride’ or to make sure they see ‘no more than the front half of their foot when they look down’ is often sufficient to prevent this training error.

Spring training, injury, and tendon health

Before you go out and start plyometric training you need to consider the problems with these types of exercises. Jerk is one of the most destructive forces for the musculoskeletal system. As a result, plyometrics can be associated with a high degree of injury, including spinal and knee compression and a variety of tendon injuries. However, most of these injuries occur when the plyometric exercises are performed with extra weight(7). Since the key aspect for increasing the stiffness of the muscle spring is the speed of the movement, added weight is not recommended for runners or games players; not adding weight therefore decreases injury risk. Another way to decrease injury is to limit the number of plyometric impacts per week and to make sure that these sessions are initially separated by at least 72 hours to allow full recovery.

Limiting the number of reps and weight used for plyometrics can prevent injury during the training, but the long-term effect of increasing spring stiffness is increased incidence of injury. We have recently shown that tendon is normally a non-linear elastic material(8). What this means in effect is that tendons are compliant at one end and stiff at the other (see figure 2 and box 1).

Figure 2

Box 1

Near the muscle, the tendon is quite stretchy, while near the bone the tendon is stiff(7). The elasticity of tendon near the muscle has the important effect of protecting the muscle from injury. A healthy tendon stretches a little when our muscles are loaded, resulting in a slow increase of force on the muscle fibres. In contrast, a stiff tendon rapidly shifts force to muscle fibres, resulting in more muscle damage. Therefore, while we need to increase the stiffness of our springs to return the maximal amount of energy, this can potentially lead to greater muscle injury.

The most important way you can decrease muscle and tendon injury, when incorporating downhill running and plyometrics into your training, is to add in some slow lengthening exercises at the same time. Unlike the fast lengthening contractions produced by plyometric work, slow lengthening contractions have low jerk and are good for tendon health. Because of the architecture of the muscle-tendon junction, high force, slow lengthening contractions decrease the stiffness of the muscle end of the tendon. While this may slightly decrease the stiffness of the muscle spring, it is this part of the tendon that is important for protecting our muscles from damage.

Slow lengthening exercises

The important areas to focus on when adding slow lengthening exercises are all of the most common injury sites. These include the hamstrings, quadriceps (specifically near the patellar tendon); and calves (Achilles tendon).

  • To hit the quads and the origin of the hamstrings, try step back lunges or assisted leg press;
  • For lunges, hold a weight in both hands, step back, touch the knee to the ground and step forward again. These are better focused on the lengthening contraction than the more common forward lunges;
  • For assisted leg press, have a coach help press the weight out and then lower the weight to 90 degrees on a 10 count;
  • To hit the insertion of the hamstrings, add some negative leg curls by curling a weight with both legs and letting the weight down slowly with one leg;
  • For the Achilles, negative heel raises are very effective: on a step, go up on to both toes and slowly go down on one, adding weight when this becomes too easy.

Simply adding five to 10 reps of each of these exercise once a week up until the peak phase of training will help maintain tendon function without adding any more muscle mass.

Conclusion

As effective as plyometrics can be for improving muscle springs, running economy, and speed, they can also derail training by making us more prone to injury. However, we can avoid these costly injuries by understanding the source of the problem. These injuries are likely caused by loss of tendon function near the muscle. Adding resistance exercises that focus on slow lengthening contractions can prevent this loss of tendon function and keep you on track to achieve your maximal performance.

References

1. Med Sci Sports Exerc 2001; 33:326-33
2. J Physiol 2007; 582:1303-16
3. Arch Biochem Biophys 2002; 399:174-80
4. J Exp Biol 2002; 205:2211-6
5. Science 1997; 275:1113-5
6. J Appl Physiol 1999; 86:1527-33
7. Track & Field quarterly review 1989; 89:41-43
8. J Appl Physiol 2006; 101: 1113-7

Keith Baar runs the Functional Molecular Biology laboratory at the University of Dundee where his research involves looking for genes that alter muscle and tendon function

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