Patellar tendinopathy is a knee pathology affecting the patellar tendon connecting the kneecap (patella) to the tibia. Commonly known as “jumper's knee” (4), it is widely accepted that one of its main causes is a functional stress overload caused by high-demanding jumping activities (14,68). Numerous articles have attempted to define this condition and to classify the different stages of its development (61), as well as to introduce methods to prevent and treat the condition in sports practice (3,7). One of the main challenges for appropriate tendinopathy diagnosis is that it presents similar symptoms to other knee injuries, such as bursitis, meniscal injuries, chondromalacia, or patellofemoral syndrome (15), therefore confounding proper prevention and therapeutic approaches.
Patellar tendinopathy is an injury with an increased frequency of appearance in male athletes (11,34,68). The odds of suffering from patellar tendinopathy in adult men who participate in volleyball and basketball are 2-fold when compared with women (14,34). Sport-specific loading characteristics of the knee extensor apparatus, greater body weight, and greater height are additional intrinsic and extrinsic factors of this rise in risk (69). This increased tendency in men can also be observed at high-school ages (65) with overuse or repetitive trauma injuries representing approximately 50% of all pediatric sport-related injuries (58).
Knowing the proper mechanisms for injury prevention is as important as understanding the causes. Several injury prevention strategies related to the use of different exercises have been described in past literature. Thus, concentric (5), eccentric (23,33,45), flywheel resistance (44,53), whole-body vibration (WBV) (39,54), and electrical muscle stimulation (EMS) training (37,40) have shown benefits improving tendon structure and raising lower limb strength levels. Moreover, combining these exercises in a single training program might have a potentially beneficial effect on positive tendon training adaptations while preventing overuse knee injuries. This article will focus on providing coaches and practitioners with a specific and practical set of useful exercises, designed to prevent this pathology that gives rise to a considerable functional deficit and disability in recreational and professional athletes.
INCIDENCE IN TEAM SPORTS
Several studies have been published on the frequency of sport- and exercise-related tendinopathies (17,20,36,69). However, comparison and interpretation of results are difficult because of a lack of consistent case definitions and inappropriate time loss–based injury registration methods. Most studies are conducted in select populations such as elite athletes or in participants of 1 specific sport (55). The number and the incidence of tendon injuries, in general, have increased substantially during the last few decades. It is estimated that tendon injuries, caused by overloading, account for 30–50% of all injuries that are related to sports (22,32,38).
Hägglund et al. (17) followed 51 European elite soccer clubs (2,229 players) between 2001 and 2009, observing an incidence of 0.12 patellar tendinopathies per 1,000 hours of exposure. In another injury study of the Union of European Football Associations (UEFA), Ekstrand et al. (12) found similar incidence between the 2001–2002 and 2008–2009 seasons.
Patellar tendinopathy can have a major effect on the career of many athletes, and for some, it is the reason that their careers end prematurely (25). Overuse injuries of the patellar tendon are often associated with athletes involved in some type of repetitive activity, such as jumping (volleyball, basketball, and team handball), kicking (soccer and American football), quick stops and starts with a sudden change of direction (COD), running, and weightlifting (4,14,26,41,43,52,67). Specifically, in soccer and American football, a prolonged repetitive stress of the knee extensor apparatus occurs when the ball is kicked, leading to this overuse injury of the patellar tendon (24,34).
The prevalence of jumper's knee in different team sports is mostly unknown. However, there are several studies published among volleyball players at the elite level in which the prevalence is between 40 and 50% (14,34), and 32% in elite basketball players (34). Martens et al. (41) found that volleyball and soccer were the sports in which two-thirds of all their patients with patellar tendinopathy were involved. In another study, of the 2,800 athletes in the Turku Outpatient Sports Clinic, approximately 700 male and 190 female patients had suffered a knee disorder, with patellar tendinopathy being the most prevalent tendinopathy, registering 20.8% of all registered cases (30). The highest incidences among team sports were recorded in soccer (21%), volleyball (12%), and ice hockey (7%).
Other studies have analyzed running, which is one of the most popular leisure sports activities, and is integral to the training of almost every sport. The overall yearly incidence rate for running injuries varies between 24 and 65% (18,30,60). Approximately 50–75% of all running injuries are overuse injuries because of the repetition of the same movement, with most injuries occurring in the tendons around the knee or in the Achilles tendon (31).
As we can recognize in the cited studies, the incidence of patellar tendinopathy in team sports is certainly important, and for that reason, a correct preventive approach through the use of exercise becomes necessary in this family of sports.
DEFINITION AND ROLE OF EXERCISE IN PREVENTION
Patellar tendinopathy is a patellar tendon injury, causing anterior knee pain related to the activity (59). Recent histopathological and biochemical evidence indicates that the underlying pathology of tendinopathy is not an inflammatory tendinitis, but a degenerative tendinosis (2). Nevertheless, its origin still remains unclear being considered as a syndrome (48,49). Today, it is treated as a chronic injury caused by the overuse of the knee extensor mechanism and frequently with a protracted, repetitive, and bilateral natural course (21,61). Overuse in chronic tendon pathologies occurs when tendon fascicles and fibrils have been strained repeatedly to 4–8% strain. In such conditions, the tendon is unable to endure further tension, and the injury appears (24).
During the past 10 years, much more information has become available on the causative factors of patellar tendinopathy. According to literature, several intrinsic factors such as ligamentous hyperlaxity, lack of muscle flexibility, Q-angle, patellar height (high), tenderness, pattern of force development, and extrinsic dispositions such as frequency of training, level of performance, hardness of ground, and weekly number of jumps have been identified as influential in developing this pathology (57,66). Some studies have pointed out a manifest susceptibility to patellar tendinopathy when body weight, weight training, jumping performance, and overall load are increased (34,68). In studies conducted with young volleyball athletes aged 16–18 years, a sudden increase in the volume of training, when they are approaching the senior level, is hypothesized as an important risk factor of jumper's knee (62). What is so far clear is that some predisposing factors play an important role developing this injury when exposed to training loads (34,50).
Sex is also a relevant factor in patellar tendinopathy occurrence. Despite no clear evidence to explain why men are at a greater risk, some possible explanations can be higher body mass, larger muscle mass, greater ankle dorsiflexion, trunk torque velocities at landing, and the ability to jump higher (in what some authors have called the “jumper's knee paradox”), resulting in greater tendon load (21,34,59,63). Epidemiological studies have found 3–4 times higher risk of developing tendinopathy in men who play volleyball than in women in the same discipline (62). By contrast, other knee-related pathologies, such as anterior cruciate ligament injuries, occur more frequently in the female population (9).
The role of exercise to prevent jumper's knee has been widely demonstrated in previous research. Human tendons have shown several adaptations to exercise, such as increased metabolic activity (6), increased collagen synthesis (46), tendon hypertrophy (10,56), increased compliance or stiffness (2), and increased tendon mechanical properties (5,56). Recent research has shown increases in collagen synthesis in human tendons close to 100% with just 1 bout of 60 minutes of acute exercise, and this synthesis is still stimulated 3 days after exercise (42). Some other authors propose a single bout of 10 minutes of exercise (time limit for tendon adaptation) with a 6-hour recovery period to promote collagen synthesis (47). An increase in collagen synthesis in engineered ligaments can also be observed when optimizing an intermittent stretch paradigm using extracellular signal–regulated kinase 1/2 phosphorylation (47). This fact confirms that human tendons are mechanoresponsive, therefore demonstrating their ability to improve adaptive patterns by exercise. In this direction, an increase in insulin-like growth factor (IGF-I), one of the most important peptide hormones involved in net collagen synthesis, cellular proliferation, and matrix remodeling, is involved in the upregulation process induced by a load, in both in vitro and in vivo processes (64). Mechanotransduction is also an important process in tendon adaptation. It consists of 3 steps: (a) mechanocoupling: the direct or indirect physical perturbation of the cell due to shear or compression forces; (b) cell-cell communication: refers to the leading of one stimulus from one cell to another that did not receive a mechanical stimulus; and (c) effector cell response: is the stimulus of the protein synthesis generated by a mechanical loading at the cellular level due to the activity of the integrin proteins that bridge the intracellular and extracellular regions involving the cytoskeleton, maintaining cell integrity, and distributing mechanical load (27). External loads induce autocrine and paracrine upregulation, raising the levels of local IGF-I and binding proteins leading to a cellular proliferation and matrix remodeling both, which are key processes in tendon healing (1,13,28). Besides, morphologically, tendons with a larger cross-sectional area (CSA) are less likely to suffer from mechanic stress for any given load and are less prone to develop patellar tendinopathy (8). However, tendons appear to react differently to heavy load resistance training increasing in their proximal and distal regions while presenting no changes in their midregion (29,49).
Exercise of different natures has been tested in the past to prevent or to heal tendon injuries. Eccentric training is a promising training regime to prevent tendinopathies (3,23,27,35,45). The reasons are less oxygen consumption in the eccentric phase of the movement than during the concentric regime, due to a reduced need of this molecule by tendons and ligaments when compared with skeletal muscle (35), increased synthesis of type I collagen, and changes in the thickness of the tendon associated with an improvement in the arrangement of the tendon internal fibers (2,33,45). Nevertheless, recent research has demonstrated that tendons are responsive to diverse loading regimes, diverse contraction types, and different rates of movement. When performed at low speeds, these varied training regimes are able to increase tendon compliance and break tendon fiber crosslinks (2,5). The use of high loads seems to be only effective in long-time programs over 12 weeks (5). A combination of high-load training with a low speed can be one important countermeasure to the occurrence of this condition. Flywheel resistance training has also been tested in the past to prevent and heal tendon injuries showing good results, with studies presenting increased leg strength and decreased pain perception of patients using this technology in the treatment of patellar tendinopathy (16,53). It has also been demonstrated that flywheel resistance training can cause greater effects in muscle activation than traditional resistance training (44). This improved effect is attributed to the isoinertial loading features arising from the use of this technology (maintaining a constant resistance and maximal muscle demand in every angle), resulting in better muscle hypertrophy. WBV has also shown promising results preventing tendon injuries. According to recent research, vibration training can result in increases at the proximal and mean tendon CSA, eliciting tendon hypertrophy in humans (51), even in patients with insertional pain who did not respond to eccentric training (19). Researchers also have found alteration in the trunk, knee, and ankle angular velocity during flexion when landing in volleyball players presenting with patellar pain (21). Thus, technical factors in sports involving a great number of jumps or COD, both actions that mainly increase the effect of gravity and tension to joints and tendons, may demand a greater landing and stop technique performance.
SELECTED PREVENTIVE EXERCISES
As mentioned earlier in this article, preventing an injury such as patellar tendinopathy through exercise is a complex aspect. Because the use of different types of contractions, loads, equipment, and training regimens have shown satisfactory and promising results in the past, the combination of several of these methods might result in a more successful and complete preventive approach. If the production of an injury is somewhat multifactorial, it makes sense that its prevention uses the maximum number of resources available to the athlete.
Here, a battery of exercises that meet these requirements and may be useful for coaches and practitioners trying to avoid this medical condition is presented. In it, exercises using weighted concentric and eccentric movements, suspension trainers, 30-degree slant boards, flywheel resistance devices, WBV platforms, and EMS have been included (Figures 1–10).
Coaches and practitioners can better understand the proposal by observing the provided description for each exercise (Table 1).
An example of exercise periodization is also provided in Table 2. Listed exercises can be useful at all stages of the training season but with different levels of relevance depending on the season period.
The periodization sample includes 5 different exercises for the off-season and preseason periods. Within the competition period, the number of exercises increases to 8. The main reason for this increment is the athletes' tendency to refer more discomfort in the tendon area during congested competition periods, the moment when more attention to injury prevention processes should be paid. To avoid pain while competing seems a key aspect of sports performance.
Previous research has suggested that patellar tendinopathy is a common injury in team sports, having a difficult therapeutic approach because of a high recurrence rate and an extended period of time to be resolved once it appears. For that reason, preventing the onset of this condition seems critical in maintaining athletes' health, and different nature exercises have demonstrated a great influence in this prevention management. Because reducing training and competition volume in high-standard sport is something controversial, the exercises presented here offer a good option for healthy athletes who present internal risk factors, or for those involved in disciplines frequently developing this condition and wanting to avoid it. Although the field of injury recovery and rehabilitation is not the main subject of this article, many of the proposed exercises may also be helpful in recovering athletes from patellar tendinopathies, provided that the described loads and intensities are modulated appropriately for each case.
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