Recent interest has focused on the use of Nordic hamstring strengthening exercises to prevent hamstring injuries in athletes, but evidence also supports eccentric exercise for managing tendinopathy and for prevention and rehabilitation of anterior cruciate ligament injury.
By Daniel Lorenz, DPT, PT, ATC/L, CSCS
Over approximately the last 15 years, the medical literature is increasingly focusing on eccentric exercise in rehabilitation for a host of conditions. However, the concept of eccentric exercise is not new. To the author’s knowledge, the earliest investigation of eccentric versus concentric exercise was in 1938.1 Eccentric exercise in the rehabilitation literature has been advocated primarily for management of tendinopathies, but evidence is mounting to support its use in the treatment of muscle strains, particularly hamstring injuries. Recently, eccentric training has been used to prevent anterior cruciate ligament (ACL) injury and for rehabilitation after ACL reconstruction.
During voluntary contraction of a muscle, speed of contraction and ability to exert tension are inversely related. The faster a muscle contracts, the lower the tension it is able to generate.1 Tension in muscle fibers when they are lengthening is considerably greater than when muscle fibers are shortening.2-4 During negative work (eccentric exercise), oxygen consumption rarely rises to more than twice the resting value.5-9 Studies have shown previously that when a muscle is eccentrically lengthened, the energy requirement drops substantially because adenosine triphosphate (ATP) breakdown and heat production are both slowed.8-10 Furthermore, with increased heat generation during concentric/positive work, there is a concurrent increase in cellular metabolism. Thus, more waste products will be generated, leading to chemical irritation of nerves and, eventually, pain.
Abbott et al5 measured oxygen uptake in subjects during bicycle ergometry. Positive work (concentric exercise) resulted in more oxygen consumption than negative work (eccentric exercise). Abbott and others6 then performed a follow-up study examining the relationship between oxygen consumption and work. Oxygen consumption was nearly three times higher at great force and slow speed than at low force and high speed.
Lastly, Bigland-Ritchie et al11 found the following: less muscle activity was required to maintain the same force during negative work than during positive work; fewer muscle fibers were required to exert a given force; and there was a substantial reduction in oxygen uptake when fibers were eccentrically lengthened.
These studies show that eccentric exercise results in lower oxygen consumption, more force production, and less energy expenditure than concentric exercise. Simply stated, you get much more bang for your physiological buck with eccentric exercise, both for the muscle and tendon.
Tendon injuries account for 30% to 50% of injuries in sports.12 Specifically, chronic problems caused by overuse of tendons result in 30% of all running-related injuries.13 Incidence of patellar tendinopathy is reported to be as high as 32% and 45% in basketball and volleyball players, respectively.14 Tendon pathologies lead not only to lost time and performance declines in sports, but also can result in long term damage to tendons that can affect daily function. Table 1 describes the stages of tendon injury as previously proposed by Nirschl.15
Many studies have recently substantiated eccentric exercise as an effective treatment for tendinopathies. In 1998, Alfredson16 performed, to the author’s knowledge, the first study investigating eccentric exercise on diseased tendons. The protocol utilized by those authors has since been used in most studies on eccentric training. In a prospective study of 15 athletes with chronic Achilles tendinosis, three sets of 15 repetitions of bent knee and straight knee calf raises were performed, twice a day, seven days a week, over 12 weeks. Athletes were told to work through pain, only ceasing exercise if pain became disabling. Load was increased in 5-kg increments with use of a backpack that carried the weight once exercise with bodyweight was pain free. All 15 athletes returned to pre-injury levels of activity. Additionally, they had significant decreases in pain with significant increases in strength.
Positive changes in tissue structure and mechanical properties as a result of eccentric training have been previously discovered. Shalabi et al17 evaluated 25 patients with chronic Achilles tendinopathy before and after an eccentric exercise program using the Alfredson et al16 protocol. Tendon volume and intratendinous signal were measured with magnetic resonance imaging (MRI). Eccentric training resulted in decreased tendon volume and decreased intratendinous signal, which correlated with improved clinical outcomes. Reduction in fluid content within the tendon, signified by the decreased intratendinous MRI signal, may suggest increased collagen deposition.
Langberg et al18 found that type 1 collagen synthesis measured through microdialysis increased after eccentric training in a group of 12 soccer players with unilateral Achilles tendinosis, offering a possible explanation for collagen healing. Ohberg et al19 also found a decrease in tendon thickness and normalized tendon structure, both of which correlated with less pain, in most of the participants in a group with chronic Achilles tendinosis who were trained using the Alfredson et al16 eccentric calf exercise protocol. Mahieu et al20 found that eccentric training of the plantar flexors for six weeks in healthy individuals resulted in positive changes to the mechanical properties of the plantar flexor muscle-tendon tissue. This may explain the improvements noted in the eccentric training group; specifically, that although Achilles tendon stiffness did not decrease, passive resistive torque of the dorsiflexors increased.
Several studies have supported the use of eccentric exercise for tendinopathies,21,22 and systematic reviews of literature by Wasielewski and Kotsko23 as well as Kingma et al24 examined the effects of eccentric training for reducing pain and improving strength in individuals with lower extremity tendinosis and chronic Achilles tendinopathy, respectively.
Most of the literature on muscle strains revolves around injuries to the hamstring muscle group. A review article by Hibbert et al25 explores the effectiveness of eccentric strength training for prevention of muscle strains in more detail than can be discussed here.
Hamstring strain is arguably one of the more challenging and perplexing conditions that the rehabilitation professional faces, and its management is a topic of debate. In 2006 hamstring strain was the subject of a cover story in USA Today.26 Hamstring strains are among the most common injuries in athletes.27-30 Hamstring muscle strain is currently the most common injury in professional soccer31 and accounts for 29% of injuries in track and field sprinters.32
Hamstring strains are common in sports that require maximum sprinting, kicking, acceleration, and change of direction.32 The average amount of time lost from competition and training is 18 days,33 but has been reported to vary between eight and 25 days.34 Furthermore, athletes must deal with persistent symptoms and a likelihood of re-injury, which occurs in 12% to 31% of cases.35,36 The highest risk of recurrence is in the first two weeks of returning to sport.37 Greig and Siegler38 performed a soccer-specific fatigue protocol on a treadmill and found that eccentric hamstring strength decreased with continued work time and after the halftime interval, which suggests that athletes may be at risk of injury with increased time in competition and shortly after halftime.
Muscle strength imbalances have been implicated as a potential source of hamstring strains. Sugiura et al39 performed a prospective study of elite sprinters to determine a relationship between strength deficits and subsequent hamstring injury within 12 months of testing. This was the first study to examine the concentric and eccentric isokinetic strength of the hip extensors, quadriceps, and hamstrings that reflects their actions in late swing or early contact. Testing was performed on 30 male elite sprinters. Injuries occurred in six. Eccentric weakness of the hamstrings at 60°/s was found to be a common factor among the injuries, and side-to-side comparison revealed the injury always occurred on the weaker side. Similar findings were elucidated by Orchard et al,40 who found that hamstring muscle injury was associated with low hamstring-to-quadriceps strength ratio at 60°/s on the injured side and a low side-to-side peak torque at 60°/s in a group of Australian football players.
Eccentric training for prevention
Eccentric exercise has been shown to reduce hamstring injury rates by 60% to 70% in various sports. Small et al31 investigated the effect of eccentric hamstring strengthening during soccer training. The objective was to evaluate if there was a difference in eccentric strength with fatigability and to see if eccentric training would attenuate the effects of fatigue.
Sixteen semiprofessional soccer players completed a 90-minute simulated soccer game. At halftime and at the conclusion of the game, the athletes performed isokinetic testing at 120°/s for the quadriceps and hamstrings. One group performed the “Nordic hamstring” eccentric exercise (Figures 1A, 1B) during the cool-down period while another performed the same exercises during warm-up twice weekly during the eight-week intervention program. When testing was repeated postintervention, the warm-up group had greater improvement than the cool-down group in eccentric peak torque and functional eccentric hamstring to concentric quadriceps ratio at the start of the simulated game. The cool-down group, however, had greater improvement in both measures at the halftime and postgame time points. The results suggest that eccentric exercises conducted post-training can significantly reduce the effects of fatigue.
Askling and others41 sought to find a relationship between eccentric training and subsequent injury in elite male soccer players. Thirty players were divided into two groups. One performed extra eccentric training one to two time per week for 10 weeks in addition to team training, while the control group performed team training only. Injuries were monitored over 10 months. The training group had significantly fewer injuries (3 of 15 players) than the control group (10 of 15). In addition, the training group showed greater improvements than the control group with regard to isokinetic hamstring strength and maximal running speed.
In a similar study, Petersen et al42 performed a randomized controlled trial on the use of the Nordic hamstring exercise to prevent acute hamstring injuries in professional and amateur men’s soccer players. Players in the intervention group participated in a 10-week progressive eccentric training program followed by a weekly seasonal program. Researchers found that the intervention group had reduced overall, new, and recurrent hamstring injuries compared with the control group.
Arnason et al43 compared the effects of “eccentric hamstring lowers” (Nordic hamstring exercise) and proprioceptive neuromuscular facilitation (PNF) stretching on incidence and severity of hamstring strains in professional soccer players. For two seasons, players performed one of three interventions: warm up stretching, PNF flexibility exercises, or eccentric hamstring exercises. The overall incidence of hamstring strains was 65% lower in the eccentric group than in the remaining players, and significantly lower than in the two seasons prior to the intervention. However, differences in injury severity and re-injury rates were not statistically significant.
Gabbe et al44 performed a randomized controlled trial of eccentric strengthening on prevention of hamstring strains in 220 male football players in the Victorian Amateur Football Association. One group performed the hamstring lowers as previously described43 over a 12-week period and the other performed stretching and range of motion exercises. Unfortunately, compliance was an issue in this study, but among players who completed at least two sessions, the incidence of hamstring strains was 4% in the eccentric group and 13% in the stretching and range-of-motion group.
Brooks et al45 examined the effects of eccentric hamstring lowers and stretching on incidence and severity of hamstring strains in 546 professional rugby players over the course of a rugby season. The incidence of hamstring injury in the intervention group was significantly lower than the conventional stretching/ strengthening group.
Holcomb et al46 investigated how a hamstring-emphasized resistance training program would improve the hamstring-quadriceps ratios in a group of 12 female NCAA soccer players. Although the study was used to assist with anterior cruciate ligament injury reduction, poor hamstring:quadriceps (H:Q) ratios have been implicated in the risk of hamstring strains as well as ACL injury. Subjects were tested before and after completion of a six-week training program involving two specific exercises targeting the hamstrings, the single leg deadlift, and the “good morning” exercise (a weight-lifting exercise that resembles bowing to greet someone “good morning”). The mean functional H:Q ratio comparing eccentric hamstring strength with concentric quadriceps strength increased from 0.98 to 1.08 after six weeks of training.
In another study by Greenstein and others,47 researchers investigated the effects of a closed-chain eccentric hamstring exercise program using rubber bands/loops on hamstring injuries in male professional football cheerleaders. Although there was no control group, the intervention was applied biweekly over the course of the football season and participants were encouraged to perform the two exercises at home three additional times per week. Among participants who reported hamstring-related injury pain prior to the season, the hamstring exercises significantly decreased pain during the season.
The author recommends using resistance bands or tubing for the hamstring lower/Nordic hamstring exercise initially (Figure 2). The band helps the athlete with control during descent and with returning to the start position. When the athlete has no pain and can go further into the range of motion, the band can be removed.
Injury to the ACL is the most common and significant of all knee injuries, potentially resulting in range-of-motion limitations, degenerative changes in the knee joint, and muscle atrophy.48 Muscle atrophy greater than 20% and strength loss exceeding 30% have been demonstrated during the first three months after surgery,49-54 with 10% to 20% deficits in quadriceps size and strength persisting for several years after surgery.50,51,55-65 Hamstring and gracilis muscle volume deficits of 10% and 30% were found post-ACL reconstruction with autologous semitendinosus-gracilis graft.65,66 Amelioration of these deficits continues to be a clinical challenge requiring systematically progressed strengthening protocols that encompass all components of lower extremity kinetic function.
High intensity and accelerated intervention programs have long been advocated and reported as safe following ACL reconstruction.59,67-71 LaStayo et al72 have suggested eccentric contraction training was more effective than concentric training for recovery of strength deficits post-ACL reconstruction because eccentric training promotes greater changes in neural activation and muscle hypertrophy. Investigation of higher intensity rehabilitation training (which produces increasingly greater strain on the ACL graft) versus a program producing minimal strain on the graft found no differences in knee laxity, activity level, function, patient satisfaction, and response to cartilage metabolism biomarkers.73
Although it was concluded that these higher intensity programs were safe, Beynnon et al73 emphasized caution with increasing the frequency and magnitude of quadriceps activity due to the risk of adversely increasing anterior knee laxity. Additionally, it has been demonstrated that when applied gradually and progressively, eccentric (or negative-work) exercise training is safe for postoperative patients and can be tolerated without damage.72,74-77 Eccentric knee extensor training is thought to be essential for restoration of the functional capacity of the ACL-reconstructed knee due to findings of significantly larger peak torque deficiencies in the knee extensors of involved knees compared with the knee flexors in the same knee.78
Compared with a standard post-ACL reconstruction program, progressive eccentric exercise implemented post-ACL reconstruction has also demonstrated benefit with respect to volume and cross-sectional area of the quadriceps and gluteus maximus;74,75,77 superior short term improvement in quadriceps torque,74,75,79 hopping distance, and activity level;74,75 and knee flexion/extension range of movement during gait.79 Improvements in quadriceps muscle strength and hopping distance also were significantly greater in the eccentric exercise group, as compared with the traditional exercise group, one year postsurgery.75
Eccentric exercise is showing promise as an effective intervention for a host of common conditions managed by the sports rehabilitation specialist. This approach is so promising because it is based on muscle and tendon physiology. Although positive evidence for eccentric exercise continues to surface, it is but one component of a successful rehabilitation plan. Soft tissue mobilization, pain modulation, activity modification, patient education, biomechanical assessment, risk factor modification, and regional interdependence are all integral parts of the process.
Challenges for the sports rehab specialist include identifying the ideal time to start eccentrics as well as how to manipulate training variables (load, volume, intensity, and frequency) to provide a safe yet progressive stimulus for eventual return to sport. It is hoped that future research advocating the use of eccentric exercise will explore these variables.
Daniel Lorenz, DPT, PT, ATC/L, CSCS, is a sports medicine specialist at Providence Medical Center in Kansas City, KS, and an adjunct faculty member at Rockhurst University in Kansas City, MO.
This article was adapted with permission from Lorenz D, Reiman M. The role and implementation of eccentric training in athletic rehabilitation: Tendinopathy, hamstring strains, and ACL reconstruction. Int J Sports Phys Ther 2011;6(1):27-44.
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