Exercise Prescription, or exercise as medicine, is a great way to ensure that we remain healthy, active and (for athletes) at the top of our game. But what if we physically can’t exercise? What if we have a debilitating injury that prevents us from exercising, or hinders our performance? It can be argued that one of the most – if not THE most – important factor in exercising for any reason is to avoid injury. However, instead of deliberately avoiding injury, it may be beneficial for individuals to focus on preventing them, through an injury prevention program. This proactive approach can potentially have profound effects not only on performance, but also on the ability to prevent injury. The problem, however, as Safran, Seaber, & Garrett (1989) argue, is that most injury prevention programs are facilitated by athletes or coaches/trainers who lack supporting scientific evidence. This often results in counter-productivity, as individuals may become more susceptible to injury by following inaccurate guidelines or suggestions. Thus, an evidence-based practice should be incorporated in to any individuals exercise regime, either from a trained professional or by the individual themselves. Furthermore, to ensure proper injury prevention, and thus the ability to continue exercising, individuals need to understand the evidence underlying certain factors. Therefore, the purpose of this paper is to review and provide the information necessary to facilitate an evidence-based approach to injury prevention.
Types of Injury
Before a proper injury prevention program can be implemented, it’s important to understand the ways in which an individual can become injured. In the field of exercise physiology, a physical injury is known as trauma. In this case, there are two main categories of injuries: macrotrauma’s and microtrauma’s.
Macrotrauma’s are classified as a sudden episode of over-load injury to a given tissue, resulting in a decrease in the tissue integrity. This type of injury can result from an individual placing too much external load (i.e. resistance) on a given muscle or structure, which can lead to damage. Macrotrauma’s are often the result of individuals who increase the intensity of a given exercise too quickly without adequate recovery between exercise sessions or sets.
Microtrauma’s are overuse injuries, resulting from repeated stress applied to a tissue over a period of time. These types of injuries are likely to occur if there is repeated, abnormal stress applied to a tissue without enough recovery time. This is often the result of a rapid increase in training volume or by improper exercise selection, particularly to the order in which exercises are performed in a single session. What makes these overuse injuries different from macrotrauma’s is that they are often an accumulation of stress placed on a joint over several training sessions, whereas macrotrauma’s occur during a single bout. Both microtraumas and macrotrauma’s can occur in several anatomical locations.
Sites of Injury
Injury can occur anywhere in the body, from the smallest, deepest cells right to the surface. However, most noticeable injuries are manifested at the super-cellular level in tissues like the bone, muscle and tendons or ligaments. Trauma to the bone can lead to either a contusion or a fracture, while joint trauma is classified as either a dislocation (complete displacement of joint) or a subluxation (partial displacement). The most common overuse (microtrauma) injury to bone is a stress fracture, which are often the result of rapid increase in training volume (Romani, Gieck, Perrin, Saliba & Kahler, 2002). Traumas to ligaments are known as sprains, and they range in degrees from a partial tear (first degree) to a complete tear (third degree). Finally, both muscle and musculotendinous trauma is grouped as either a contusion (if the trauma was direct) or a strain (if the trauma was indirect), and they can also lead to complete rupture if the load applied exceeds the limit of the tissues.
For the most part, these injuries take place in areas such as the muscle itself (muscle belly), or in the surrounding connective tissue such as the ligaments or the muscle-tendon complex (MTC). Both of these latter structures are made up primarily of the protein collagen. Typically, the collagen fibers in tendons and ligaments are significantly stronger than the muscle fibers to which they attach, so any injury is likely to happen either at the attachment, or in the muscle belly itself.
How Injury Occurs
Trauma can happen in a variety of ways; however injury to the muscle or connective tissue typically happens for similar reasons. To help explain this, it must be understood that when a muscle produces force eccentrically, or as it is lengthened, elastic energy is stored in the ligaments and tendons. In the biophysical world, this is also known as potential energy. When the eccentric muscle action is followed by a concentric action, or as it is rapidly shortened, the elastic (i.e. potential) energy is released, causing the muscle to contract. This process is collectively known as the stretch-shortening cycle (SSC; Baechle & Earle, 2008).
In the case of injury, when repeated stress (microtrauma) or overload (macrotrauma) occurs, and there is not proper recovery, damage can manifest in a variety of ways (see: Sites of Injury). For microtrauma’s, fatigue appears to play a major role in the prevalence of injury. At the cellular level, fatigue is mainly manifested in the central nervous system and particularly in a decline of the neural drive “upstream” (i.e. afferent motor nerves from the muscle to the central nervous system; Gandevia, 1998). Furthermore, Gandevia (1998) has shown that sufficiently intense contractions cause fatigue at a quicker rate than less intense contractions. As a muscle experiences fatigue over an extended period of time, the external load it can handle decreases. When this goes undetected by an individual or a trainer, then the chance of injury increases.
In contrast, macrotrauma’s - which are a result of overloading the tissue – cause injury by placing too much force on the muscle and surrounding connective tissues, causing a decrease in the tissue’s integrity (Grindstaff & Potach, 2006). In severe cases, when either the applied load is too intense or the tissue is already experiencing decreased integrity, complete rupture can occur. Using the analogy of a rubber band is a good way to explain this. As a rubber band is lengthened, there is elastic energy stored. If released, there is recoil (i.e. contraction). However, if the stretch continues over a period of time, or is either too fast or intense - the band will snap. In the human body, as previously mentioned, this tear usually occurs in either the muscle belly or the attachment of the MTC.
Factors to Prevention
Before an injury prevention program can be implemented, it is important to understand critical properties of the connective tissues (especially the MTC) that are involved in the prevention process. Three key terms that are demonstrated in the literature are: stiffness, compliance, & hysteresis.
Stiffness: The stiffness of the MTC relates to the mechanical property that describes the relationship between the force applied to the MTC, and its resulting change in length. Therefore, a “stiffer” MTC requires more applied force to stretch the structure. In tendons and ligaments, the shorter, thicker structures are typically “stiffer” than the long thin ones (Morl, Siebert & Haufle, 2015). Typically, the stiffer a structure is (i.e. the more force required to stretch it), the stronger the recoil action will be. Additionally, structures that are stiffer can typically sustain more force. This is because, as a muscle becomes stiffer, it synthesizes more of the protein collagen. As the collagen builds up, it tends to align itself in fibrils similar to that seen in muscle. The combined benefit of both additional collagen fibers and more in alignment is what makes a structure stiffer – and thus able to sustain more force. However, it is important to understand that stiffer doesn’t necessarily mean better. We want stiff structures (tendons and ligaments) around muscles that produce a large amount of force, such as that at the knee. However, in joints that can’t carry a high load, stiffness may not be a priority.
Compliance: Corresponding with stiffness, compliance relates to the amount of force needed to stretch a structure. In this sense, less applied force needed to stretch the structure means that it is MORE compliant. Relating to the SSC, a more compliant structure means that it can be stretched both easier and quicker (Cornwell, Nelson & Sidaway, 2002). However, this doesn’t necessarily mean more force production in the shortening action of the SSC. Both stiffness and compliance work together to produce an optimal amount of force generated at a given joint. Additionally, it is important to understand that in joints that can’t sustain a lot of force, such as at the shoulder, priority should be given to compliance. See “Indications” section for specific exercises prescription methods that have been shown to increase the compliance of a structure.
Hysteresis: Finally, hysteresis relates to the amount of energy lost as heat during the recoil action of the SSC, or the action following the stretch. When a structure is stretched, there is a given amount of elastic energy that is stored. The amount of stored elastic energy plays a big role in determining the subsequent force of the recoil. However, it is important to understand that following the stretch, and prior to the recoil, there is a fair amount of energy lost to heat. The exact amount of energy lost changes with each action, but the more energy that is lost to heat results in less energy available during the recoil, and thus a weaker contraction. See “Indications” for ways to limit the amount of energy lost to heat. By taking advantage of hysteresis, as well as the other key factors in injury prevention, a proper program can be implemented.
Injury Prevention (Prescription)
Indications: Indications are described as a form of treatment that is required by the rehabilitating athlete or individual (Baechle & Earle, 2008). Therefore, In the case of injury prevention, indications would be types of exercises that aim to decrease an individual’s chance of being injured. Common practices that have been shown to do this are resistance training, static and proprioceptive neuromuscular stretching, and ensuring to partake in a proper warm-up.
1. Resistance Training: Resistance Training has been shown to be an effective means of injury prevention, even among young individuals (Faigenbaum & Myer, 2010). When an individual initiates a resistance training program, one of the first physiological adaptations they experience is the ability for the motor neurons to fire more “efficiently”. It has been shown that even an untrained individual can show measurable strength gains in a matter of days as a result of these neural adaptations (Nuzzo, Barry, Gandevia, & Taylor, 2015). Specific adaptations to the nervous system can include improved synchronization of motor units firing and improved ability to recruit motor units to match the strength elicited by electrical stimulation. The latter is especially important in regards to injury prevention. Because Type II (fast-twitch) muscle fibers are recruited following Type I fiber recruitment, the Type II fibers have a higher threshold for activation. It has been shown that an increase in the ability to recruit these higher threshold Type II fibers results in greater balance recovery, especially in older individuals (Cronin, Barrett, Lichtwark, Mills & Carty, 2013). Additionally, the ability to recruit these Type II muscle fibers increases with a proper resistance training program (Pyka, Lindenberger, Charette, & Marcus, 1994).
Another beneficial adaptation of resistance training is an increase in both the mass and cross-sectional area of the muscle - a process collectively known as hypertrophy. At the cellular level, hypertrophy involves an increase in the synthesis of the contractile proteins actin and myosin. These contractile units are packed together with other structural proteins to form myofilaments, which collectively form the myofibril. When new myofilaments are formed, they are added to the periphery of the myofibril, resulting in an increase in its diameter and thus an enlargement of the fiber (i.e. hypertrophy). As an individual progresses through a resistance training program, subsequent protein synthesis and muscle growth becomes more efficient (Staron et al., 1994). It is (primarily) because of this ability for the cells to synthesize more productively that relates injury prevention to resistance training. Regardless of the extent of an injury, an individual who can synthesize these proteins more effectively will not only be better equipped to heal faster, but also to avoid injury altogether. While protein synthesis is the driving factor to injury prevention, it is important to understand that these adaptations only happen acutely post-exercise. Therefore, an individual muscle is only in this “synthesis state” for up to 48 hours. The exact length of time an individual remains in this state depends on several factors, such as training status, exercise intensity and duration, among others. Because of this, it is important for an individual to continually participate in resistance training to ensure the muscle continues the protein synthesis process. This will help ensure the muscles are continually adapting, and thus avoiding degradation or decreased integrity.
On top of the muscular hypertrophy effects, resistance training has also been shown to increase the stiffness of the MTC (Lastayo, Woolf, Lewek, Snyder-Mackler, Reich, & Lindstedt, 2003). Similarly to muscle hypertrophy, the collagen (protein) fibers in the MTC synthesize as a result of resistance training, resulting in a stiffer structure. However stiffer does not mean better, as joints that can’t handle high loads should be trained for compliance.
Endocrine Responses to Resistance Training: One of the most noticeable and important adaptations to resistance training involves changes in the endocrine system. In this case, there are several key hormones that relate to protein synthesis and injury prevention. While there are a wide variety of hormones that help in this, for the purpose of this report, the focus will be on the hormones insulin-like growth factor 1 (IGF-1), testosterone, growth hormone, and cortisol. Besides cortisol, which is classified as an adrenal hormone, the other hormones are described as anabolic hormones – or hormones that promote tissue building. Growth hormone, which is secreted by the anterior pituitary gland, has many effects on the human body. In regards to resistance training, growth hormone enhances cellular amino acid uptake and protein synthesis in skeletal muscle, which results in hypertrophy of both Type I and Type II fibers (Mccall, Byrnes, Fleck, Dickinson & Kraemer, 1999). However, it is important to understand that the hormones of the endocrine system do not work in isolation, but instead work together to promote tissue anabolism (or catabolism). For example, the presence of testosterone promotes growth hormone responses in the pituitary. Additionally, testosterone interacts with receptors on neurons to influence structural protein changes. This can enhance not only the force production potential but also the mass of the muscle. Insulin-like growth factor 1 (or IGF-1) has been the primary IGF that has been studied due to its prolific role in protein anabolism. Its role is to not only promote protein anabolism, but also to promote the effects of pituitary growth hormone (Turner, Rotwain, Novakofski, & Bechtel, 1988). Similar to protein synthesis in the muscle, the effects of these anabolic hormones work acutely and last from 24-48 hours (Shariat, Kargarfard, Danaee, & Bahri, 2015). Finally, cortisol is the primary signal hormone for the metabolism of carbohydrates and has the catabolic effect of converting amino acids to carbohydrates. A chronic level of cortisol has been shown to degrade lean tissue and catabolise muscle tissue (Crewther, Cronin, Keogh, & Cook, 2008). This is especially true in athletes who work at a higher intensity, because this is when cortisol levels are highest in blood (Kraemer et al., 1996).
Resistance Training guidelines: Resistance training has a variety of benefits for health and wellness. One of the main benefits, as previously mentioned, is an increase in muscle synthesis. Therefore, resistance training programs should follow proven guidelines to facilitate muscle growth. American College of Sport Medicine (ACSM) guidelines state that resistance training should emphasize dynamic exercises involving both concentric and eccentric muscle actions that recruit multiple muscle groups (Garber et al., 2011). Furthermore, it is recommended that two to four sets of each resistance exercise is performed, however even a single set can improve muscle size (Garber et al.). To ensure the body is always in a protein synthesis state, resistance training should be performed at least three times a week. As with all exercises, proper technique and form are critical.
2. Stretching: In general, scientific evidence on the role of stretching on injury prevention is ambiguous. Because there is a wide range of stretching protocols, as well as individual differences in response to stretching, researchers have had a hard time determining optimal stretching protocols. When it comes to structures such as the MTC, it is unclear how each component is affected by stretching. However, certain studies have been able to show that static stretching can counteract the stiffening effects of resistance training (Nelson & Bandy, 2004). It does this by helping with the reactive compliance of the structures, essentially making it easier for the structure to be stretched during exercise. Overall, stretching seems to counteract the stiffening effects of tendons and ligaments, and when combined with resistance training can help maintain a good balance of stiffness and compliance.
Stretching Indications: There are two main times when a stretching program should be implemented for injury prevention. The first is following a training session. In fact, the effects of stretching on the compliance of the MTC are best observed when the stretching protocol is initiated following exercise, especially resistance training (Nelson & Bandy). The other time when stretching has been shown to be beneficial is when it is done as its own session. In this case, proprioceptive neuromuscular facilitation (PNF) stretching appears to be the most beneficial (Sady, Wortman & Blanket, 1992). This type of stretching involves assistance from a partner, and it may be superior to other stretching methods because it facilitates muscular inhibition (Sady, Wortman & Blanket).
3. Warm-up: A proper warm up has been shown to have profound effects on a proper warm up can decrease the amount of energy lost during the recoil action of the SSC. Additionally, by increasing the internal temperature, it helps decrease the viscosity of tendon/ligament, increasing the efficiency of the movements.
Contraindications: Contrary to indications, contraindications are practices that are inadvisable due to a given injury. In the case of injury prevention, contraindications are factors that an individual should avoid in order to maintain injury prevention. However, there isn’t a lot of research on what shouldn’t be done when it comes to injury prevention. In terms of stretching, ACSM recommends contraindicating stretching (especially static) prior to exercise (Garber et al., 2011). Furthermore, in relation to resistance training, an individual should be sure to follow proper set and rep ranges, as well as work within an acceptable intensity and always maintain proper form for each technique.
Technological interventions: When it comes to technological interventions on injury prevention, the literature is sparse. This is likely due to the fact that commercial industries are constantly releasing new equipment aimed at injury prevention, but with little or no evidence to support their claims. When it comes to injury prevention, it is more beneficial for an individual to follow a proper training program that incorporates evidence-based research as its foundation.
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