Module 5

Sports Injury Prevention Part 2: Strength, or length? Part 1 was published in Modern Athlete and Coach January 2015 Dr Mark Brown

Mark Brown B.App.Sc(Phty); MHSc(Sports Physio); MBA; FASMF; FAIM Mark Brown is an Australian Physiotherapy Association (APA) titled Sport Physiotherapist with over 30 years’ experience in sports medicine. Currently he holds positions as the Executive Officer o f Sports Medicine Australia’s Queensland Branch, adjunct Associate Professor in the Griffith University Centre of Musculoskeletal Research and as a Member of the Oceania National Olympic Committees Medical Commission. He is a Fellow o f both the Australian Sports Medicine Federation and the Australian Institute o f Management and was the Director of Physiotherapy for the Sydney 2000 Olympic and Paralympic Games. Mark's main clinical and research interest areas relate primarily to improving safety in sport and physical activity and he has published and presented internationally in particular on: • improving the prevention and management o f medical emergencies in sport • the use of neuromuscular training programs for sports injury prevention and performance enhancement • the use of taping techniques for the prevention and treatment o f musculoskeletal conditions.

In the previous article I outlined some of the main components of an evidence informed approach to sports injury prevention, especially including the proven effectiveness of multi- component neuromuscular training programs to both reduce the number and severity of lower limb injuries in athletes, and also improve sporting performance. Neuromuscular training programs aim to improve strength and control during sports specific movements and this article will briefly examine the sometimes controversial topic of the role of flexibility training as a component of sports injury prevention programs, and whether muscle length or muscle strength are most associated with reduced sports related injuries.

Until relatively recent times the conventional wisdom amongst athletes, coaches and health professionals was that stretching exercises to increase muscle length and joint range of motion were an essential component of injury prevention programs for athletes. But a number of research studies conducted in the late 1990’s and early 2000’s produced results that caused a rethink of this concept. In particular a landmark large scale study conducted in Australia by Pope et al (1998) found there was no meaningful difference in the number of lower limb injuries in army recruits who used static stretching exercises in their warm up program compared to those whose warm up program did not include stretching.

Subseguent studies by other researchers produced similar conclusions with respect to injury prevention, while others also found that stretching before or after exercise did not reduce delayed onset muscle soreness (DOMS), or other types of exercise related pain, or measures of recovery. Around the same time other researchers found that stretching, especially static stretching, temporarily decreases muscle power which is obviously not a desirable outcome for optimal performance in most sports, especially those requiring explosive power.

But other studies looking at risk factors for sports injuries have shown that reduced flexibility or range of motion (ROM) are

associated with some types of sports injuries. For example, reduced hamstring extensibility was found to be associated with an increased predisposition to hamstring strains, and reduced ankle dorsiflexion range of motion is a risk factor for ankle injuries. But even these findings are complicated by yet other studies that show that an even greater risk factor for injury for most muscle injuries is not muscle length, but muscle strength. For example, for thigh adductor muscle strains (groin strains) adductor length or extensibility has been found to not be a risk factor for injury, however reduced adductor strength as measured on the adductor squeeze test is. Similarly, the biggest risk factor for a hamstring strain injury according to current evidence is reduced hamstring eccentric strength rather than decreased hamstring extensibility, and eccentric strengthening of the hamstring muscles in the eccentric hamstring lower exercise (often commonly referred to as “ Nordic hamstrings” ) has been found to be protective for hamstring strains.

This particular exercise has become an im portant component of many sports injury prevention programs including the FIFA 11 + injury prevention program. While this particular program is mostly orientated to injury prevention in Football many of the exercises can be readily adapted by athletics coaches and is worth a look at as the videos and other resources on the FIFA website clearly outline the exercises (http://f-m arc. com /11plus/hom e/). Currently researchers are attempting to establish minimum benchmark strength measures or strength ratios for exercises such as the Nordic hamstring curl which eventually will assist coaches and the athlete’s attending health professionals when screening athletes for injury risk factors, but at present normative data is limited.

Other research studies support the notion that strength is more im portant than length for injury prevention. Recently Lauersen et al published an article in the British Journal of Sports Medicine in 2014 that examined the effectiveness of exercise interventions to prevent sports injuries. The authors

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conducted a systematic review and meta-analysis of 25 randomised controlled clinical trials (26,610 total participants) to determine which physical activity interventions were most effective for sports injury prevention. The analysis determined that stretching did not reduce injuries, but strengthening and proprioceptive exercises did. Strength training was the most effective intervention and reduced sports injuries to less than one third (Relative risk ratio 0.315). Proprioceptive training was also found to be effective though less so than strength training (Relative risk ratio 0.550).

So based on some of the research findings it’s tempting to say that on the whole muscle strength is more im portant for injury prevention than muscle length. However, this view is overly sim plistic and ignores the fact that in some sports a certain degree of flexibility is necessary to effectively execute some of the required techniques, especially sports such as gymnastics, dance, and some martial arts disciplines but also in some track and field disciplines so a “ one size fits all” approach with regards to what sort of flexibility training is required is not appropriate. It also doesn’t take into consideration that stretching programs don’t just alter muscle length, they also have an effect on tendon elasticity which is also relevant to sports performance. It is often forgotten that the muscle should be more accurately described as a muscle tendon unit with the contractile component of the MT unit (the muscle fibres) applying a force to the boney attachments via the non-contractile components (the tendon and fascial tissue) so what sort of exercise interventions most effect tendon and muscle tissue also needs to be considered.

So how do we put all of this together? At the moment according to current research evidence it’s not a matter of “ stretching: yes or no?” but rather that stretching can be a useful part of programs if the type and tim ing of stretching programs is contextualised to the sport, and also customised to the individual differences in morphology, risk factors as identified in the screening process, and the sporting tasks required for each athlete. But, some of the factors that could be taken into consideration include:

• On the basis that the muscle tendon (MT) unit needs to be compliant enough to store and release energy effectively in the Stretch Shortening Cycle (SSC) this would suggest that more compliance in the MT unit would decrease muscle and tendon injury because the load on these tissues would be reduced. However, static and dynamic stretching immediately before activity have been found to be counter-productive to force generation, possibly through overstimulation of the stretch receptors.

• According to Kubo et al 2000 moderate or low SSC demand sports like running or cycling do not benefit from making the MT unit more compliant.

• However, sports with jumping or bouncing activities with a high intensity of SSCs require a MTU compliant enough to store and release the high amount of elastic energy required in such sports.

• Dynamic stretching produces no or little effect on muscle length but has a significant influence on tendon stiffness, which in turn increases storage and release of elastic energy in tendons which is useful in high SSC sports like jumping. But dynamic stretching is not the best technique to increase range. Kubo et al (2001) found that dynamic stretching does decrease tendon stiffness using a protocol of 2 sessions of dynamic stretching per day for 8 weeks. However, this benefit was soon lost if the stretching exercises were not maintained.

• Witvrouw et al (2007) compared dynamic stretching and static stretching and concluded that static stretching is a better technique for increasing ROM and dynamic stretching is better for increasing tendon elasticity. In their view if ROM alone is the goal or is critical to success in a particular sport or activity then static stretching as part of an overall program is indicated, though not as part of the warm up due to the temporary muscle force reduction.

• To increase muscle range of motion a large volume of static stretching is required. Marshall et al (2011) demonstrated a 20.9% increase in hamstring extensibility, but the program involved 4 different hamstring stretches, each performed 5 times a week for 4 weeks, (including 1 supervised session per week), with each stretching exercise held for 30 seconds with 3 repetitions of each.

• Konrad and Tilp (2014) concluded that static stretching did not produce a change in muscle length or structure, however people who stretch often increase range of motion due to an increased tolerance to stretch, and /or increased pain tolerance.

• Warm-up before sport also increases the visco-elasticity of the muscle tendon unit and therefore may be more appropriate than stretching immediately before sport. But as individual variation do occur different approaches to stretching and warm up for each athlete should be tested outside of competition using sports specific measures of performance.

So which stretching technique you would use and when depends on sports specific goals. Also, you need to do a lot of stretching (which costs a lot of time) to get measurable results. While that is getting complicated enough, none of the above takes into consideration the possible additional confounding variables associated with variations in joint hypo / hyper­ mobility, or the effects of age, metabolic and genetic factors on tendon tissue. But, overall for athletes with reduced flexibility there is still an argument in favour of incorporating flexibility training into their programs, but probably not immediately before sporting performance. The type of stretching and what areas should be focused on will depend on the findings by the Physiotherapist in a comprehensive musculoskeletal screening in conjunction with the coaches identification of each athlete’s training goals and sports specific role.

What is clearer is that increasing muscular power and control are important and effective in reducing injury and increasing

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performance, but gaining strength must as always take into consideration careful monitoring of the athlete’s total training load.

References:

Arnason A, Andersen T, Holme I, Engebretsen L and Bahr R. Prevention of hamstring strains in elite soccer: an intervention study. Scandinavian Journal of Medicine and Science in Sports, 2007

Konrad, A. and Tilp, M. (2014) Increased range of motion after static stretching is not due to changes in muscle and tendon structures. Clin. Biomech. 2014; 29(6):636-42.

Kubo K., Kanehisa H., Kawakami Y. and Fukunaga T. Effects of repeated muscle contractions on the tendon structures in humans. Eur. J Appl. Physiol. 2 0 0 1 ,8 4 ,1 6 2 -1 6 6 .

Kubo K., Kanehisa H., Kawakami Y. and Fukunaga T Influence of static stretching on viscoelastic properties of human tendon structures in vivo J App Physiol. 2001 90 (2), 520-527

Jamtvedt G, Herbert RD, Flottorp S, et al. A pragmatic randomised trial of stretching before and after physical activity to prevent injury and soreness. Br J Sports Med 2010;44:1002-9.

Lauersen JB, Bertelsen DM, Andersen LB. The effectiveness of exercise interventions to prevent sports injuries: a systematic review and meta-analysis of randomised controlled trials. Br J Sports Med 2014:48:871-7.

Marshall, R, Cashman A, Cheema, B. A randomized controlled trial for the effect of passive stretching on measures of hamstring extensibility, passive stiffness, strength, and stretch tolerance. J Sc. Med. Sp. 2011 14 (6) 535-540

Pope R, Herbert R, Kirwan J. Effects of ankle dorsiflexion range and pre-exercise calf muscle stretching on injury risk in Army recruits. Aust J Physiother 1998;44:65-72.

Pope RP, Herbert RD, Kirwan JD, et al. A randomized trial of preexercise stretching for prevention of lower-limb injury. Med Sci Sports Exerc 2000;32:271-7.

Witvrouw E, Mahieu N, Roosen P and McNair P. The role of stretching in tendon injuries Br J Sports Med. 2007 Apr; 41 (4): 224-226.

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