Performance benefits of optimising both components of your muscle tendon units
For most coaches and athletes the main aim of a strength programme is to increase the strength of the muscles. If the programme is targeting a specific sport, then the aim is to strengthen the most relevant muscles, using movements similar to those used in the sport itself. A runner, for example, may choose to do the lunge exercise, which works the hip and thigh muscles used during running.
Take, for example, the ankle movement during running: when the foot strikes the ground, at first the ankle dorsiflexes (flexes backwards) – the lower leg and foot angle getting smaller as the body absorbs the impact and the knee bends; then the centre of mass of the runner moves forwards and the ankle joint plantarflexes (forwards), with the lower leg and foot angle increasing as the knee extends and the toe pushes off the ground.
Interestingly, research using electromyography as an indicator of calf muscle activity during running shows that the calf muscles are very active during the dorsiflexion phase and not active during the plantarflexion phase. This is contrary to what you might expect, as the theoretical role of the calf muscles is to contract concentrically (shorten) when the ankle plantarflexes. So how can the ankle plantarflex without any activity from the calf muscle? The answer is that the Achilles tendon is stretched during the dorsiflexion phase of the ankle movement, and the elastic energy stored during stretching is then released when the Achilles shortens. During running, the role of the calf muscle is to control the movement of the ankle with an eccentric (lengthening) contraction during the dorsiflexion (shock absorbing) phase, while the role of the Achilles tendon is to release energy while it shortens to plantarflex the ankle and push off the ground.
This pattern of tendon stretch followed by shortening is common to many sporting movements. For example, the trunk and shoulder tendons stretch and then shorten during throwing movements such as those used in javelin, tennis and golf; the hip flexor tendons stretch and shorten during kicking movements; and the knee and Achilles tendons stretch and shorten during jumping movements. The technical term for this kind of movement is ‘stretch shorten cycle’, or SSC. Such movements are also commonly described as ‘plyometric’.
The simple reason why this SSC pattern of movement is so common is because it is efficient. If force can be produced with the release of elastic energy, the muscles can get away with doing less work. That makes elastic energy useful either for offsetting fatigue (with distance running, for example) or enabling greater force production (eg for throwing events).
How strength training targets tendons
Having established that the muscle tendon unit has two separate components that contribute to force production, the key question for athletes and coaches is how to improve the tendon’s elastic energy release as well as the strength of the muscles. This forces us to consider strength training from a non-traditional perspective in relation to how it targets tendon rather than muscle. But before doing that we need to examine the characteristics of tendons more closely, since different training methods are beneficial for different types of tendons.
For the purposes of this article, tendons can be thought of pretty much like elastic bands(1). The energy stored by such bands is proportional to the extent to which they are stretched. Long elastic bands are good at storing energy, and thin bands are ‘compliant’ in that they can be stretched very easily, with little force. Unfortunately, though, this means they produce less force when they recoil. Thick elastic bands, on the other hand, have greater ‘stiffness’ in that they require larger forces to stretch them, but they create large forces during recoil.
Tendons may also be stiffer or more compliant and, as such, are best suited to different types of movement. If the range of motion is large and the load is light, a compliant tendon is best because it can be stretched easily under the light load through the large range, and recoil efficiently. The hip flexion movement used during sprinting is a good example: the hip flexor is stretched fully as the hip extends during push-off, and then the hip flexes as the knee comes through in front of the body.
Sprinting needs compliant tendons
Studies have shown that during sprinting the hip goes from about –20o at toe-off to +95o when the knee is at its furthest forward position. This large range of motion requires a compliant tendon to enable it to stretch easily and not limit the effectiveness of the hip extension drive. And because the leg is relatively light, the hip flexor does not need the high force recoil a stiffer tendon might create.
The shoulder movement used during the tennis serve or cricket bowling action are other examples of the benefits of compliant tendons. The tennis racquet and cricket ball are relatively light objects by comparison with the max strength of the shoulder muscles, but the muscles must be taken through a large range of motion for a fast and efficient technique, which is best achieved with compliant shoulder tendons. It is much more efficient for a tennis player or fast bowler to create speed from the tendon energy release – which can be repeated for free – than from muscle power, which is tiring.
If the range of motion required is small, a stiff tendon is best for the job, whether the load is light or heavy. Stiff tendons create big forces rapidly, which is perfect for generating power over small ranges of motion. The fact that they are also harder to stretch matters less over small ranges of motion, especially when heavy loads are involved.
The knee joint motion used during running is a good example of the effectiveness of a stiff tendon. When a runner makes contact with the ground, the knee is slightly bent at an angle of around 40o. The knee flexes to absorb the impact, which stretches the quadriceps tendon, but only to around 60o. The knee then re-extends to about 40o at toe-off. This all occurs very quickly, in about 200 milliseconds. As running speed increases, the contact time with the ground decreases (to around 100msec for élite sprinters) and the knee flexion decreases. Thus, the faster the speed the more the knee joint requires a rapid but strong extension force, which a stiff quadriceps tendon will help to create.
From these examples it is possible for coaches and athletes to see how different ranges of movements and sizes of loads are best undertaken by different types of tendons, and to design training programmes to optimise the tendon performance. A running coach, for example, would choose exercises to promote a compliant hip flexor tendon but stiff quadriceps and Achilles tendons. A tennis player might work for a compliant shoulder and wrist.
Knowledge about this kind of training is still in its infancy, but one general rule applies: heavy weight training will increase the stiffness of a muscle tendon unit, while flexibility exercises will increase compliance. (In this context, heavy weight training means lifting loads of 75-90% of one repetition maximum. Usually, athletes perform 3-5 sets of 3-6 repetitions of these kinds of loads.)
Take distance runners, for example: traditionally they have not performed much heavy weight training because they have not needed great muscular strength for their event. This is not an unreasonable stance. However, the knowledge that heavy weight training may increase the stiffness of the knee and Achilles tendons, thus making them more efficient during running, should encourage distance runners to take heavy weight training much more seriously.
Tennis players and fast bowlers may be used to performing heavy upper body weight training to increase serve and bowling power. However, if the upper body and shoulder tendons become less compliant as a result of this strength programme, some of the efficiency of the technique may be lost. In this situation, athletes who need to maintain their tendon compliance may consider plyometric training rather than heavy weight training to increase power, as this form of training will not reduce tendon compliance and may increase tendon energy release.
A word of warning here: large volumes of any type of training – endurance, heavy weights or plyometrics – are likely to increase tendon stiffness. If compliance is important, then high-quality training of moderate volume is recommended.
And another thing: do not confuse ‘stiffness’ with reduced range of motion and ‘compliant’ with increased flexibility. The two terms refer to the properties of the tendon when being stretched or recoiling from stretch and not to the extent of flexibility of the muscle tendon unit. It is possible for a muscle tendon unit to have good ‘range of motion’ and good ‘stiffness’ at the same time.
The take-home message for coaches and athletes is that they need to consider the effects of a training programme on the whole MTU, and how that will impact on performance. Consider your chosen sport and then answer the following questions:
- Do the movements involve relatively light or heavy loads on the muscles?
- Do the movements involve relatively large or small ranges of motion?
The answers to these questions will give you a basic guide as to whether tendon stiffness or tendon compliance is the MTU property you want to encourage.
Returning to the example of the distance runner, an MTU-specific programme may combine heavy weight training for the leg muscles with increased stretching for the hip flexors; this will maximise stride length and hip extensor drive and increase the spring-back from Achilles and knee tendons.
The importance of low hysteresis
The next step in tendon research is to gain understanding of the optimum levels of tendon stiffness for individual athletes in relation to their chosen sport. This is likely to form the basis for exciting developments in biomechanics and strength training theory in the coming years.
Another interesting property of tendon elastic energy release is ‘hysteresis’. This refers to the amount of energy lost between the stretch and the recoil and is independent of compliance or stiffness. Stiff tendons can have low hysteresis, which means high force recoil, with maximised stretch potential energy, or high hysteresis, which may counteract the benefits of the strong and rapid recoil. All athletes would wish to have tendons with the minimum of hysteresis, as this means that every SSC movement will be more efficient, producing more power for less muscular effort. It is very important, therefore, to perform exercises that promote reduced hysteresis, whether you are working for compliant or stiff tendons.
Research has shown that both flexibility and plyometric exercises reduce the hysteresis of a muscle tendon unit, so it is beneficial to include these exercises in any programme designed to optimise SSC movement efficiency. As mentioned earlier, plyometric training in moderate volumes will not increase the stiffness of the tendon, making it a great method for increasing the elasticity of the MTU and training the fast twitch muscle fibres without compromising tendon compliance. This would be useful for many sports – volleyball and golf, for example.
As far as flexibility exercises are concerned, given the current scientific controversy over the benefits of static stretching, I would recommend dynamic stretching as being more useful, and possibly safer. It seems logical to develop a range of motion actively as this makes it more specific to the requirements of any given sport.
The second take-home message is that strength and conditioning programmes should incorporate exercise routines for optimising tendon hysteresis. For a runner, a regular dynamic stretching routine and low-impact plyometric sprint drills session could be a very good means for achieving that end.
Remember, though, that tendons adapt slowly, so you must progress your training programme gradually, especially if you perform heavy weight training and plyometric exercises. With plyometrics, it is best to monitor the volume of the session by counting the number of leg contacts or upper body throws you perform: 50-80 contacts would count as a light session, about 100 contacts moderate, and over 140 contacts heavy.
I hope this article has made you stop and think about your event, your body and what it needs in terms of both muscular strength and tendon performance. If you can design your training to optimise both components of the MTU, you should succeed in optimising your performance.
- ‘Optimising the Tendon for Athletic Performance’. Anthony Blazevich PhD. Presentation given at the UK Athletics Strength and Conditioning Conference, Loughborough, April 2003