Raphael Brandon explains how rehab specialists can exploit our emerging knowledge of tendon properties.
The properties and behaviour of the muscle-tendon complex (MTC) is a relatively new focus for sports science research, but it is attracting a lot of interest. At last summer’s meeting of the European Sport Science Congress, the symposium sessions on muscle-tendon complex (MTC) research were filled to standing room only. Thanks to the use of ultrasound imaging, our understanding has advanced rapidly in recent years. While it is still too early to draw specific practical conclusions from the research, what follows is an introduction to the concept of the muscle-tendon complex (MTC) which highlights how the results of current research may have relevance for both sports injury therapy and prehabilitation.
Muscles do not contract in isolation; rather, movement occurs when muscles connected to tendons generate and then transmit contractile forces. Crucially, the muscles and tendons interact, with the elastic properties of the tendons contributing both to the type and quality of movement. In almost all movements, when the muscle contracts, the tendon will lengthen before it shortens, independent of any change in muscle length. This property of ‘recoil’ enables elastic energy to be stored and released, thereby increasing the efficiency of the muscular contraction. So movement is a combination of two factors within the muscle- tendon complex:
The value to the therapist or coach of thinking about the muscle-tendon complex (MTC) as a whole is that it offers the possibility of gearing training to optimise the performance of the tendon component’s interaction with its muscle.
It is helpful to think of the tendon behaving like an elastic band. The more force applied through the length of the tendon, the longer it will be stretched (until maximum strain is reached, when it snaps). When the stretch is released (the ping of the elastic band), most of the mechanical energy is recoiled, with the rest being lost as heat.
There are two key properties of tendons of relevance to the performance of the muscle-tendon complex (MTC): stiffness and hysteresis. Both can be altered with training.
Tendon ‘stiffness’ has nothing to do with our understanding of the word when we talk about flexibility, or range of motion. In this sense ‘stiffness’ is a mechanical property describing the relationship between the force applied to the muscle-tendon complex (MTC) and the change in the length of the unit. The exact equation is:Stiffness (N/m) = force/change in muscle-tendon complex (MTC) length
(N/m = Newtons per metre)
So if a greater degree of force is needed to produce a given amount of stretch, we can say the muscle-tendon complex (MTC) is stiffer. Correspondingly, the exertion of less force to produce the stretch means the muscle-tendon complex (MTC) is more ‘compliant’.
If we continue with the elastic band analogy, short and thick bands (tendons) require more force to stretch them. While this makes them stiff, it also gives them a more forceful recoil. Long and thin bands (tendons) can be stretched easily and absorb more energy, but can only recoil back smaller forces.
Muscle-tendon complex (MTC) stiffness is not necessarily a bad thing. It depends upon the joints and movements involved as to which kind of tendon property will be optimal(1). MTCs that are involved in large ranges of movement, such as many athletic hip or shoulder movements, will benefit from being compliant (longer and thinner). MTCs involved in short ranges of movement, such as the ankle and knee in running, will benefit from being stiff.
The second key property of the tendon is hysteresis: the amount of energy lost as heat during the recoil from the stretch. By minimising the hysteresis you can increase the efficiency of the movement. This is one of the main goals of a warm- up before exercise, and explains why warm weather or room temperature tend to produce better performance in power events. Tendons have both viscous and elastic properties; a rise in temperature lessens the viscosity, improving the efficiency of the tendon’s response to stretch and recoil.
We can now see, therefore, that when a therapist is designing a rehab programme it is important to take into account the different effects that different types of training will have on tendon properties.
1. Strength training
Resistance training will tend to stiffen the muscle-tendon complex (MTC). This could be advantageous for some joint movements, such as running, because of the more powerful recoil effect, which means more economical movement. Other joint movements, though, will not benefit from stiff MTCs. These are large-range movements, especially those involving low loads, such as the shoulder joint during a tennis serve. Some ongoing research by Anthony Blazevich and his team at Brunel University, Middlesex, UK, is examining whether there is any differential effect on tendon stiffness from eccentric versus concentric strength training.
The results of that study could have very interesting applications to injury rehabilitation. For instance, because compliant tendons can absorb elastic energy more easily and tend to have lower hysteresis than stiff tendons, they could be related to reduced injury risk.
However, we also want muscles to be strong. Striking the right balance between tendon compliance and stiffness is going to be finely judged, as training for one quality seems to compromise the other. So the next step in research will be to better our understanding of the effects of different types of training on tendon properties.
Another research issue which may throw up some interesting findings relates to whether there is an optimal way of achieving a sport-specific desired level of limb stiffness or stability (in order to maximise power). One way to increase this might be to train the muscle-tendon complex (MTC) for this result. An alternative approach would be to build muscle strength and develop the ‘skill’ of co-contraction around a joint, through resistance and plyometric exercises. The latter approach produces ‘active stiffness’ of the whole leg or arm.
For example, if we accept that it is useful to have a stiff ankle during running, then this functional or active stiffness could be created by good strength and skilled co-contraction of the muscles around the ankle whilst the athlete is able to retain their inherent muscle-tendon complex (MTC) compliance, in order to minimise their injury risk. However, this is all conjecture as it stands. More research is required before conclusions can be drawn as to the optimal muscle-tendon complex (MTC) state and training programmes. The Brunel research team hopes to be able to shed some light on all this.
2. Endurance training
Endurance training will increase the stiffness of the tendon. In fact, any training performed in high volume is likely to cause an increase in stiffness, probably because the training adaptation to repetitive mechanical loading increases the cross-sectional area of the tendon.
3. Flexibility training
Dynamic and static stretching has been shown to decrease both muscle-tendon complex (MTC) stiffness and hysteresis. Regular stretching could therefore counteract the stiffening effects that strength and endurance training have on the muscle-tendon complex (MTC). Athletes who require good compliance in certain joints must therefore perform regular stretching. All athletes will benefit from reduced hysteresis, another possible selling point for regular stretching. Again, though, what is important is to find the right amount of stiffness for the particular sporting movement and joint.
In the absence of other types of strength or endurance training, plyometric training has been shown to increase tendon compliance. However, as most athletes are performing plyometrics in combination with running and weight training, it is likely to have no effect in practice. Plyometric training may reduce hysteresis.
The stiffness of the muscle-tendon complex (MTC) can now be measured directly using ultrasonography.
This is a fantastic development as it allows for the analysis of the function of the muscle and tendonsin vivo, while they are active. Previously, biomechanists had to predict muscle-tendon complex (MTC) stiffness using mathematical models of joint movements and force measurements. Now, researchers can measure the exact change in length of the muscle fascicle or tendon in response to the application of a known force, and perform the simple calculation of force divided by change in length (see box below).
The potential benefits of being able to monitor changes in tendon stiffness are large. Scientists and medics have many tools and protocols to test muscle strength, either in isolation, using an isokinetic dynamometer, or in functional tests, such as the squat. However, up until now we have been unable to measure the response of tendons to rehabilitation or training. As our knowledge in this area increases, we may be able to set more sophisticated goals for rehabilitation programmes to include tendon properties as well as muscle strength and/or function.
For example, a tennis player suffering from rotator cuff tendinitis may need to strengthen the shoulder musculature, balance the rotator cuff strength and increase the compliance of the anterior shoulder MTCs. The increased strength is obvious and standard practice; but the increased compliance also has a theoretical utility. If the player’s anterior shoulder muscles can store elastic energy more easily and recoil that energy with low hysteresis then the serving movement may become more efficient, induce less muscle fatigue and potentially attenuate stress through the shoulder joint structures.
The other key benefit of current research is to be able to determine what balance of stiffness/compliance and what type of muscle architecture are optimal for different sports or movements. Hopefully, the knowledge will soon become more widespread and more applied so that the benefits can be exploited fully.
Raphael Brandon MSc is a sports conditioning and fitness specialist, working as the London region strength and conditioning coach for the English Institute of Sport