Muscle strain injury: it’s all about connections

Could connective tissue damage could play a bigger role than we thought in sport-related muscle injuries, and what are the implications for athletes, trainers and and physios?

 As any sports physio, coach or trainer will testify, muscle strain injuries are all too common in athletes. This is not an anecdotal finding; a large body of research has established that in ‘ball-sport’ athletes (soccer, rugby, basketball, football etc), muscle strain injuries are among the most common of all injuries reported^(1-3). The traditional view of muscle strain injuries is that these injuries are precipitated by excessive tissue loading, particularly during eccentric muscle contractions – ie where the muscle lengthens under load. This in turn is thought to reflect the muscle’s inability to withstand elongating forces⁽⁴⁾, and also explains why muscle strengthening and conditioning is considered the primary means of building resilience against muscle strain injury.

Is the theory flawed?

Although the mechanism of muscle strain injury outlined above seems perfectly logical, there is a major flaw in this theory. It turns out that in a number of muscles, less than 20% of the muscle fibers that make up that muscle span the entire distance between the muscle origin and its insertion (in plain English, most fibers only reach part way along the muscle). In fact, the bulk of the fibers terminate in the muscle belly (middle), connected only via a sheath of connective tissue known as the ‘endomysium’ (see figure 1)⁽⁵⁾. This strongly suggests that the connective tissue inside the muscle tissue must also play a powerful role in transmitting muscular forces generated – either within or when absorbing forces applied externally. In short, this implies that ‘fascial’ connective tissue (see box 1) mechanically assists the muscle in taking up loads –and conversely that this fascial tissue can also be damaged if the muscle loading is too high.

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Figure 1: The relation of the endomysium in the overall muscle architecture

Because most muscle fibers don’t span the entire length of the muscle, the sheaths of connective tissue known as ‘endomycium’ must also be involved in the transmission or absorption of force by muscles.

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Box 1: What is the fascia and fascial tissue?

 The fascia is made up primarily of densely packed collagen fibers, which create a system of sheets, chords and bags that wrap, divide and permeate the whole of your body - including muscles, bones, nerves, blood vessels and organs. In short, the whole of your body is protected and supported by fascial tissue, without which you wouldn't be able to maintain a taut human shape!

Because the fasical system is so complex, extensive, variable and intertwined, studying it has proved very complicated, which is why relatively little is known about it. Indeed, look at any anatomy textbook on muscle function and all you'll see are bones, muscles, tendons and ligaments - not fascia! But in recent years, scientists have begun to understand that fascial tissue is not just 'packaging' for muscles, organs etc, it's absolutely essential for proper function of the body. In particular, because the fascia wraps and encases individual organs and muscles with a strong, slippery structure, it is able to keep all these parts separate, allowing them to slide over each other easily as we move. The inherent stiffness of the fasica also provides additional resilience to the body, preventing excessive range of movements that would damage muscles and joints during sudden movements.

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Fasical tissue and muscle strain injury

The potential involvement of fascial (connective) tissue in muscle strain injuries is quite a new concept and one that has considerable implications. Until recently, the scientific data on this subject has been quite limited and circumstantial in nature. Rat muscle studies have demonstrated that when the facial tissue within certain muscles is removed, the force that can be transmitted by that muscle declines by 25%⁽⁶⁾.

In addition, research shows that fascial tissue has very similar mechanical properties to tendons (which join muscles to bones) when it comes to the maximum ability to transmit force⁽⁷⁾. However, if fascial tissue really does play an essential role in distributing muscle forces (either generated in muscles or absorbed by muscles), surely it must also be involved in muscle strain injuries – ie become damaged following an injury? And if it is, what does this means for injured athletes or those trying to treat and prevent injury?

To try and provide some definitive answers to these questions, a comprehensive review study published by German researchers in December of last year examined the data collected from a number of imaging studies, which had previously described the frequency, location, and extent of soft tissue lesions (tears) in athletes who had suffered lower-limb muscle strain injuries⁽⁸⁾. In total, 16 studies, collectively evaluating a total of 1503 muscle injuries in athletes were analyzed. These athletes included track and field athletes, Olympians, semi-professional and amateur athletes, along with professional soccer players and American football players. All but one of the studies used MRI for imaging (considered the gold standard when investigated muscle injuries), two studies used MRI plus additional ultrasound scanning, while one study used ultrasonography only. The main findings were as follows:

• The most frequently examined muscles were the hamstrings followed by the soleus muscle (of the calf).
• Around one third (32.1%) of all the injuries were found to involve fasical lesions. This compared with only 12.7% of the injuries being purely muscular (ie only muscle fiber tears were seen).
• Tendon injuries accounted for 68.4% of all injuries observed
• Where myofascial tissue lesions were observed, these were diagnosed most often in the soleus muscle of the calf (36.4%), and in the hamstring muscles (27.9%).

What these findings show is that (contrary to popular belief) a sports injury to the fascial tissue is more than three times likely than a pure muscle injury. And if we also include tendons in the connective tissue category (which they are), nearly 90% of all sporting injuries involve connective tissue damage, with very few involving muscle fiber damage only!

Fascial lesions and return to sport

A number of other studies have investigated the relationship between the extent of a fascial lesion and the return-to-play times recorded for the injured athlete. For example, a 2017 study compared the size of a fascial lesion injury (diagnosed with MRI) and the average time away from sport cause by the injury⁽⁹⁾. The results showed that in athletes injured for two weeks or more, the average magnitude of lesions was three times greater (27mm) than in those who returned to sport in under two weeks (8mm), the clear implication being that the amount of fascial damage is directly related to the time away from sport an athlete has to endure.

In another study, researchers compared return-to-play times in muscle-only injuries with those involving general connective tissue (fascia, tendon, ligaments), and concluded that athlete downtime was significantly longer for injuries involving connective tissue damage (7.6 weeks) than for muscle-only injury (3.9 weeks)⁽¹⁰⁾. Overall then, we can say that when an injury involves fascial damage, the evidence to date does seem to suggest a lengthier return-to-play time, but more studies will be need to demonstrate this conclusively.

Muscle or fascia damage?

Determining whether there is fascial tissue involvement in a muscle injury (or muscle only) and how much damage has occurred can only be reliably determined with the use of MRI imaging. Unfortunately for recreational athletes however, obtaining an MRI scan is usually time consuming and costly, so this process is unlikely to be practical for all but the most elite athletes.

But given the findings above - suggesting that an isolated muscle tissue injury (ie affecting muscle only) is present in just one in eight cases – athletes, physios and trainers should generally assume that some degree of fascial tissue damage is quite likely in most muscle injuries. This seems to be particularly the case for injuries of the soleus muscle of the calf. Therefore, if you or athletes you care for suffer a calf strain, you should automatically assume some fasical involvement. And if an MRI scan can be performed, the presence of larger fascial lesions (in excess of 20mm) will suggest a longer and more gradual rehab protocol for you/athletes in your care⁽⁹⁾.

Treatment implications

In terms of injury treatment approaches, these recent findings on the high prevalence of fasical damage in soft-tissue injuries are relevant for optimizing rehabilitation protocols. For example, where fasical damage is known to exist, the inclusion of more dynamic stretches (in addition to eccentric-type exercise, which is recommended for promoting connective tissue healing) is recommended⁽⁸⁾. (NB: see this article for a sample of relevant dynamic stretches and their proper execution)

Another factor that needs to borne in mind is that fascial tissue damage seems to produce higher levels of pain than the same size lesion in muscle tissue⁽¹¹⁾. Athletes who suffer a soft-tissue injury where significant fascial tissue damage has occurred (confirmed by MRI) will generally experience significantly more pain and discomfort than those where the damage is restricted to muscle tissue. This in turn means that any rehab protocol may need to be more gradual, and that expectations of return-to-play time should be managed accordingly.

Finally, the presence of fascial tissue damage may also have implications for you or your athlete’s nutrition during rehab, particularly his/her vitamin C status. In a 2017 study on nutrition, connective tissue synthesis and sport performance, researchers found that subjects with Achilles tendon injury who consumed vitamin C–enriched gelatine before a rope-skipping exercise, experiences substantially improved collagen synthesis in the tendon following exercise⁽¹²⁾. As we mentioned earlier (box 1), collagen forms an essential component of connective tissue such as fascia, and is synthesized in the body from proline (present in gelatine) in the presence of vitamin C. Athletes who have a sub-optimum vitamin C status can be expected to experience poorer levels of collagen synthesis. Although it’s possible to purchase proline-rich petides, gelatine (found in jelly) is an excellent source of proline, making it cheaper option for athletes with a sweet tooth!

References

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