Combining the Copenhagen Adduction Exercise and Nordic Hamstring Exercise Improves Dynamic Balance Among Male Athletes: A Randomized Controlled Trial

Abstract

Background:

Copenhagen adduction exercise (CAE) and Nordic hamstring exercise (NHE) reduce the incidence of groin and hamstring injuries. Efficient dynamic balance can improve motor performance and reduce the risk of injuries in athletes. However, the effects of these exercises on dynamic balance have not been investigated.

Hypothesis:

CAE and NHE, as well as a combination of both exercises, would improve dynamic balance among amateur male athletes.

Study Design:

Randomized controlled trial.

Level of Evidence:

Level 1.

Methods:

A total of 200 male athletes aged 21.9 ± 2.4 years were included in the study and randomly assigned to 4 groups: CAE group (n = 50), NHE group (n = 50), CAE and NHE group (n = 50), and a control group (n = 50). A total of 177 male athletes completed the study. The primary outcome measure was the limit of stability (LoS), which was measured using the Biodex Stability System to assess the performance of the dynamic balance. The LoS of the athletes’ performance was measured pre- and postintervention after 6 weeks.

Results:

The LoS significantly improved in all treatment groups, including CAE (44.5% ± 5.3%), NHE (43.2% ± 5.3%), and CAE + NHE (48.4% ± 5.1%) groups when compared with the control group (28.3% ± 4.8%) after 6 weeks (all Ps < 0.01). The improvement of LoS was significantly greater in the CAE + NHE group compared with other groups (CAE, NHE, and control groups).

Conclusion:

There was a significant increase in dynamic balance performance postintervention among male athletes. CAE and NHE may improve injury prevention programs.

Clinical Relevance:

The results of this study provide evidence for athlete trainers and coaches to consider including the CAE and NHE as components of injury prevention programs to improve balance capacity and performance in athletes. Such improvements in balance may prevent injury risk and decrease absenteeism and injury-related financial burdens.

There is a proverb in sports, “injury is simply a part of sport.” In other words, injury in sports is inevitable. ¹⁰ The most common athletic injuries are hamstring muscle strain (HMS) and adductor-related groin pain, such as adductor muscle strain.^32,40 HMS ranges in severity from minor microscopic tearing and some loss of function (grade I) through to a full avulsion of the muscle with complete loss of function (grade III). ⁷ The biceps femoris is the most commonly injured of the hamstring muscles,^25,43 with the muscle-tendon junction and adjacent muscle fibers being the most common sites of disruption.^4,25 In many cases, HMS causes considerable time loss from training and competition ³¹ and diminished athletic performance. ⁴³ The hamstring muscles account for 12% to 37% of all muscular injuries, and the risk of reinjury is between 12% and 48%,^27,44 while the hip adductor muscles can have an injury rate of 23%. ¹⁶ Several studies^28,30,15 have shown that the etiology and risk factors of injury may include a variety of moderators, such as low flexibility, inadequate muscle strength or endurance, insufficient warm-up and stretching before exercise, fatigue, age, and low dynamic balance performance.

The Nordic hamstring exercise (NHE) seems to prevent hamstring injuries effectively in athletes, and many studies^2,5,41 have shown that adopting the NHE in isolation or combination with other injury prevention programs during training can reduce hamstring injury rates. In 2017, a meta-analysis by Al Attar et al ² reported a 51% reduction in hamstring injury (risk ratio [RR] 0.49, 95% confidence interval [CI] 0.29-0.83) in soccer players who performed injury prevention programs that included the NHE compared with teams that did not use any injury prevention measures.^1,5,41 Recently, a meta-meta-analysis ¹ conducted in 2019 reported a reduction of all injuries by 34% (RR 0.66, 95% CI 0.60-0.73) and a reduction of injuries to the lower limbs by 29% (RR 0.71, 95% CI 0.63–0.81) in soccer players who performed the Fédération Internationale de Football Association (FIFA) injury prevention program that also included NHE. Correspondingly, another systematic review and meta-analysis ⁴² conducted in 2019 found a reduction of hamstring injuries by around 51% among different sports in those who used injury prevention protocols that involved NHE.

A reduction in hip adduction strength is considered an important modifiable risk factor for groin injuries. ⁴⁵ The Copenhagen adduction exercise (CAE) was developed to strengthen the hip adductors and, thus, to prevent adductors injuries. ²⁴ The CAE protocol is a progression of eccentric training to enhance the strengthening of hip adduction and increase the EHAD:EHAB ratio. ⁵ A recent study ³⁶ examining the electromyographic activity of the hip muscles indicated that the CAE results in high adductor longus muscle activation along with other exercises such as isometric adduction with a ball between the knees and hip adduction with an elastic band.Therefore, the CAE may be suitable for groin injury prevention and rehabilitation. ³⁹ Furthermore, a randomized controlled trial (RCT) conducted found that CAE reduced the prevalence and risk of groin problems in male football players in Norway. ²⁰ Both eccentric exercise regimes, CAE, and NHE, have been shown to reduce injury incidence.

Two systematic reviews and meta-analyses conducted in 2016 ³⁸ and 2017 ¹⁹ showed that the explanation of the efficacy of an injury prevention program is related to improvements in muscular strength, proprioceptive balance, and flexibility. Emery and Meeuwisse ¹⁷ proved that introducing a neuromuscular training program including stretching, strength, agility, jumping, and balance exercises protected youth soccer players of all injuries and acute-onset injuries. Likewise, a cluster RCT by Emery et al ¹⁸ showed that a balance training program tailored for high school basketball players was effective in reducing acute-onset injury. It also demonstrated a clinically meaningful reduction of all lower extremity injuries, including ankle sprains. Thus, utilizing a proprioceptive lower limb balance exercise reported a greater reduction in the injury rate. More importantly, efficient postural balance contributes to the improvement of overall motor performance as well as reducing the risk of recurrent ankle sprain and anterior cruciate ligament injuries in athletes. ²² Although the studies showed the positive impact of neuromuscular and dynamic balance training programs in reducing the risk of injuries, no data have been published in the literature regarding the effects of the CAE and NHE on the improvement of dynamic balance. Therefore, this RCT aimed to evaluate the effect of CAE and NHE on improving dynamic balance compared to no-intervention (control) among young athletes.

Methods

Study Design and Participants

This study was an RCT comparing the effectiveness of the CAE, NHE, and both the CAE and NHE on improving the dynamic balance of young athletes to no-intervention group (control). A total of 249 male athletes aged between 18 and 27 years from different sports, teams, or clubs were invited to participate in a 6-week study during the season. The inclusion criteria included male athletes practicing at least 3 days a week. Exclusion criteria applied to athletes who had a medical history of lower extremity injury that required medical attention in the past 6 months or those who had systemic disease, cardiovascular disease, neurological disorders, bone fractures, or surgery in the previous year. The protocol of the trial was registered in the Australian New Zealand Clinical Trials Registry, registration number: ACTRN12618002036291. The study received ethical approval from the Biomedical Ethics Committee, Umm Al Qura University, approval number: HAPO02K012202010469. All participants were asked to read and sign an informed consent form that explained in detail: the purpose of the study, what participation in the study would involve, protection of privacy and confidentiality, expected outcomes, and further use of the results. The study followed the Consolidated Standards of Reporting Trials guidelines (Figure 1). All included athletes included completed a personal and demographic information form (Table 1).

Figure 1.

Consolidated Standards of Reporting Trials flow diagram. CAE, Copenhagen adduction exercise; NHE, Nordic hamstring exercise.

Table 1.

Demographics of the participants

Variable	Group	Mean ± SD
Age, y	CAE	23.63 ± 3.07
	NHE	20.88 ± 1.64
	CAE + NHE	21.50 ± 2.62
	Control	21.50 ± 1.05
	All groups	21.90 ± 2.44
Weight, kg	CAE	76.00 ± 22.05
	NHE	69.00 ± 12.80
	CAE + NHE	61.75 ± 11.30
	Control	70.00 ± 15.09
	All groups	69.13 ± 15.97
Height, cm	CAE	171.62 ± 3.34
	NHE	171.00 ± 10.03
	CAE + NHE	171.00 ± 4.66
	Control	166.83 ± 6.62
	All groups	170.33 ± 6.56

CAE, Copenhagen adduction exercise; NHE, Nordic hamstring exercise.

Randomization and Blinding

After the exclusion criteria were applied and after the consent of athletes has been obtained to participate and perform the baseline testing, they were randomly allocated into 4 experimental groups. Baseline testing consisted of weight and height measurements, and limits of stability (LoS) testing, which is explained later. Thus, a total of 200 male athletes aged 21.9 ± 2.4 years were included in the study and randomly assigned to 4 groups: CAE group (n = 50), NHE group (n = 50), CAE and NHE group (n = 50), and a control group (n = 50). Group allocation was done through an online research randomizer. The randomization process was undertaken after every enrolling athlete had been identified, thus, achieving concealed allocation.

Intervention

The CAE group was instructed to perform the CAE, which is a partner exercise in which the athlete lies on his side with one forearm as support on the floor and the other arm placed along the body. The upper leg is held at the hip height of the partner, who holds the leg with one arm supporting the ankle and the other supporting the knee. The athlete then raises his body from the field, and the lower leg is adducted so that the feet touch each other, and the body is in a straight line, the body is then lowered halfway to the ground. At the same time, the foot of the lower leg is lowered so that it just touches the floor without being used for support. The exercise is performed on both sides. ³⁶ The NHE group was instructed to perform the NHE, which is an eccentric exercise where the athlete tries to resist a forward-falling motion using his hamstring muscles to maximize loading within the eccentric phase. The athlete was required to hold their hips fixed during a slightly flexed position throughout the full range of motion, and to break the forward fall for as long as possible using their muscles. They were also asked to try keeping tension in their hamstrings even after they must “let go.” They were asked to use their arms and hands to buffer the fall, allow the chest to touch the surface, and immediately get back to the starting position by forcefully pushing with their hands to minimize loading within the concentric phase. ³² In contrast, the CAE and NHE groups were instructed to perform a combination of both exercises. The control group (no-intervention group) did not perform any specific exercise and continued their regular daily routines. The exercise prescription for the CAE and NHE is described in Table 2.

Table 2.

The exercise prescription for the CAE and NHE

Exercise	Level	Instructions	Repetition
NHE	Beginner	Kneel on a soft mat. Slowly fall forward, keeping your upper body and hips straight. Control the falling motion using your hamstrings. Use your arms to push yourself back to the starting position.	3 × 3-5
NHE	Intermediate	Kneel on a soft mat. Slowly fall forward, keeping your upper body and hips straight. Control the falling motion using your hamstrings. Use your arms to push yourself back to the starting position.	3 × 6-8
NHE	Advanced	Kneel on a soft mat. Slowly fall forward, keeping your upper body and hips straight. Control the falling motion using your hamstrings. Use your arms to push yourself back to the starting position.	3 × 8-12
CAE	Beginner	Stabilize upper leg in front of the body. Lift lower leg keep leg extended. Slow and controlled tempo.	3 × 8-16
CAE	Intermediate	The partner stabilizes the leg around the knee. Raise your body from the ground and lift your lower leg. Keep your body in a straight line. Slow and controlled tempo.	3 × 6-8
CAE	Advanced	Partner stabilizes the leg around the ankle. Raise your body from the ground and lift your lower leg. Keep your body in a straight line. Slow and controlled tempo.	6 × 6-8

CAE, Copenhagen adduction exercise; NHE, Nordic hamstring exercise.

Balance Performance Testing

The Biodex Stability System was used to test the limits of stability of the athletes’ performance, pre- and postintervention after 6 weeks. The Biodex Stability System uses a microprocessor-based actuator to adjust the stability of a suspended circular force plate. The force platform has a maximum of 20° tilt in any direction. It determines a participant’s stability based on the variance of the platform from the center during a given task using a sampling rate of 100 Hz.

Limits of Stability

The limits of stability (LoS) test challenges athletes to move and control their center of gravity (CoG) within their base of support (BoS). During each trial, athletes must shift their weight to move the cursor from the center target to the blinking target and back as quickly and with as little deviation as possible. Postural stability is defined as “the ability to control the body’s CoG within a given BoS.” However, in testing the LoS, the testing platform moves relative to the horizontal surface while the individual’s foot remains fixed. Therefore, it challenges the individuals’ ability to return the center of mass to within the BoS as the CoG approaches the limits of the BoS. This test is an indicator of dynamic control within a normalized sway envelope. ³³

The LoS test prompts participants to move a cursor, viewed on a liquid crystal display, by leaning toward a target while standing on the entirely unstable platform (level 1 of 12 levels, using current model software). Athletes were instructed to “complete the test as quickly and accurately as possible, keeping your body in a straight line, arms should be kept on hips, using the ankles as the primary axis of rotation.” The test measures the time and accuracy with which participants transfer their estimated CoG (from ground reaction force and height data), moving the cursor to intercept each of 8 successive targets on the display screen. The targets are positioned at 45° intervals around a central target that represents the participant’s center of pressure under static conditions. Each target is randomly highlighted, and the athlete reaches the target by learning and returning to the center position before the next target is selected and displayed on the screen. The test is complete when all 8 targets have been reached. Target placement was preset by the manufacturer at 50% of the LoS, based on the height of each athlete. This process considers the conversion of the angular motion of leaning to the linear movement of the CoG represented on the screen. The dependent measure extracted from the LoS test which used to assess the dynamic balance performance was the directional control (based on 100% being a straight line from the center of pressure to the intended target).

Statistical Analysis

The differences in athletes’ age, height, and weight between groups before treatment were examined using 1-way analysis of variance (ANOVA). The Shapiro-Wilk test was used to test for normality, which showed normal distribution of the data (P = 0.58). Therefore, a paired t test was used to identify the mean differences in LoS within groups to compare the means of LoS between pre- and postintervention for each group. One-way ANOVA with Tukey’s post hoc tests were used to identify the mean differences in LoS between treatment groups, which included the comparisons of the following: CAE versus control, NHE versus control, CAE + NHE versus control, CAE versus NHE, CAE versus CAE + NHE, and CAE + NHE versus NHE. The results of the current study were considered statistically significant when the P value was less than 0.05. The data were analyzed using IBM SPSS Version 24 (IBM Corp).

Results

A total of 177 male athletes completed the study: CAE group (n = 42), NHE group (n = 44), CAE + NHE group (n = 47), and a control group (n = 44).

Demographics of the Participants

One-way ANOVA showed that there was no significant difference in the mean age (P = 0.12), weight (P = 0.38), and height (P = 0.56) (Table 1).

Changes in the LoS

Paired t test was used to compare between pre- and postintervention LoS mean values for each group. The results showed that there was a significant increase in postintervention LoS mean values as compared with preintervention LoS mean values for all 4 groups (Table 3).

Table 3.

Changes in the limits of stability within intervention groups ^a

Variable		CAE	NHE	CAE + NHE	Control
Overall LoS (%)	Preintervention	25.48 ± 4.28	24.68 ± 3.99	25.55 ± 4.52	25.18 ± 4.44
	Postintervention	44.48 ± 5.34	43.18 ± 5.25	48.36 ± 5.13	28.30 ± 4.75
	P	<0.01	<0.01	<0.01	<0.01

CAE, Copenhagen adduction exercise; LoS, limit of stability; NHE, Nordic hamstring exercise.

^aResults are presented as mean ± SD. The table shows the mean difference in the LoS for each group to compare between pre- and postintervention. Significant P values are in boldface (paired t test; P < 0.05).

One-way ANOVA with post hoc tests was used to compare between groups on LoS measure. Before interventions, the results revealed that there were no significant differences between groups on LoS measure. After interventions, dynamic balance performance significantly improved in all treatment groups (P < 0.01) including the CAE group (44.48% ± 5.34%), NHE group (43.18% ± 5.25%), and CAE + NHE group (44.48% ± 5.34%) compared with the control group (28.30% ± 4.75%). The best improvement on LoS was observed in CAE + NHE group with a significantly higher LoS mean value (P < 0.01) compared with other groups (NHE, CAE, and control groups). Table 4 and Appendices 1 and 2 (available in the online version of this article) show the mean differences in LoS within and between groups.

Table 4.

Changes in the limit of stability between intervention groups ^a

Comparison (LoS)	Preintervention		P	Postintervention		P
CAE vs NHE, %	25.48 ± 4.28	24.68 ± 3.99	0.83	44.48 ± 5.34	43.18 ± 5.25	0.65
CAE vs CE + NHE, %	25.48 ± 4.28	25.55 ± 4.52	>0.999	44.48 ± 5.34	48.36 ± 5.13	<0.01
CAE vs control, %	25.48 ± 4.28	25.18 ± 4.44	0.99	44.48 ± 5.34	28.30 ± 4.75	<0.01
NHE vs CAE + NHE, %	24.68 ± 3.99	25.55 ± 4.52	0.77	43.18 ± 5.25	48.36 ± 5.13	<0.01
NHE vs control, %	24.68 ± 3.99	25.18 ± 4.44	0.95	43.18 ± 5.25	28.30 ± 4.75	<0.01
CAE + NHE vs control, %	25.55 ± 4.52	25.18 ± 4.44	0.98	44.48 ± 5.34	28.30 ± 4.75	<0.01

CAE, Copenhagen adduction exercise; LoS, limit of stability; NHE, Nordic hamstring exercise.

^aResults are presented as mean ± SD. The table shows the mean difference in LoS pre- and postintervention between intervention groups. Significant P values are in boldface (Tukey’s test, post hoc analysis; P < 0.05).

Discussion

CAE or NHE can produce significant improvements in dynamic balance among male athletes. Brachman et al ⁸ stated that including balance exercises in the training programs aims to achieve improved performance, injury prevention and optimizing motor performance. Thus, the positive effects of CAE and NHE on dynamic balance can also help reduce injury risk and decrease the injury-related financial burden on the health care system.

There have been several studies^2,21,24 that investigated the effects of NHE and CAE on injury reduction rates. It showed that implementing CAE or NHE in injury prevention programs reduces injuries incidence and leads to effective results.^12,20 For instance, a meta-analysis ¹ investigated the preventive effect of FIFA 11+ Injury Prevention Program among soccer players that included NHE indicated a reduction of all injuries by 34% (RR 0.66, 95% CI 0.60-0.73) and a reduction of 29% in lower limb injuries (RR 0.71, 95% CI 0.63-0.81). Also, acknowledged the efficiency of the simple adductor strengthening program of CAE in preventing and reducing the incidence of the risk of groin problems of semiprofessional football players in Norway. ²⁰ It is widely agreed that CAE significantly increases the eccentric hip adduction (EHAD) strength, eccentric hip abduction (EHAB) strength, and EHAD:EHAB ratio, as it showed that a lower EHAD:EHAB ratio is contributing to adductor-related injuries.^24,34 Polglass et al ³⁴ published a study in which they implemented a Modified Progressive Copenhagen Adduction (MPCA) program over 8 weeks; the program initiated with hip adductors isometric contraction and advanced into traditional CAE. They concluded that the MPCA program decreases the undesired delayed onset muscle soreness while increasing 25% of EHAD strength and 13% of EHAB strength, as well as adjusting EHAD:EHAB ratio to the appropriate level for preventing groin- and adductor-related injuries.

Literature^11,35,41 has shown that NHE leads to improvements in neuromuscular adjustments that affect injury risk factors and therefore prevent injuries. Furthermore, a recent study ²⁹ investigated electromyography and kinematic measurements during the NHE motion. They concluded that higher muscle activity was found in erector spine, internal oblique, and multifidus muscles as they stabilize the trunk and pelvis and optimizing the hamstring contraction. Accordingly, sufficient activation of these muscles is required when performing NHE and thus assisting when designing injury prevention programs for hamstring injuries and muscle imbalance. Moreover, NHE improves hamstring strength, functional hamstring-to-quadriceps torque ratio, and dynamic jump.^3,35 Thus, it is in good agreement with our findings where dynamic balance improved by CAE and NHE.

This study showed that dynamic balance performance was significantly better in athletes who received CAE and NHE programs than in those who did not receive any specific exercise program. Nevertheless, athletes who acted as positive controls in this study received neither the CAE nor NHE demonstrated significant improvement. The primary purpose of having a positive group was to show that the experimental conditions (CAE and NHE) are sensitive to intervention groups. Moreover, they could receive the training programs delivered by the coaches. Thus, the improvement in dynamic balance they showed may have occurred because of the general training program they were receiving. Additionally, they were not blinded by their peers who received intervention. Last, the learning curve could have played a role in improving the second LoS test results.

The improvement in balance control is critical for athletes and involves a combination of a range of motion, movement abilities, strength, and proprioception. ³⁷ Besides, the dynamic balance was chosen as a measurement to investigate the effect of NHE and CAE, as the athletes frequently challenging their balance dynamically, exceeding the static balance demands, such as during sprinting and jumping movements. ²² Therefore, improvements found in that assessment may translate to better athlete performance. Balance training improved performance and reduced numerous injuries among athletes.^22,23 Furthermore, a systematic review and meta-analysis by de Vasconcelos et al ¹⁵ suggested that balance training decreases the incidence of ankle sprains and improves dynamic neuromuscular control, postural sway, and the joint position sense in athletes. Daneshjoo et al ¹³ compared 2 different injury prevention programs, FIFA 11+ versus HarmoKnee program over a period of 2 months. Balance and strength exercises and other elements are the components of both prevention programs. The study showed that both intervention groups have been improving their balance and proprioception.

A systematic review and meta-analysis by Soomro et al ³⁸ concluded that injury prevention programs reduce athletes’ injury rates by 40% (IRR 0.60, 95% CI 0.48-0.75). However, whether the efficacy of the programs is attributed to proprioception, balance, muscle strength, or flexibility is inconclusive. A meta-analysis by Behm et al ⁶ hypothesized that strength training on unstable surfaces challenges the neuromuscular system, which may produce the same or a substantial improvement when compared with strength training performed on stable surfaces. Their findings showed slightly restricted effectiveness on static and dynamic balance, muscle strength, and performance in healthy individuals. Additionally, Leavey et al ²⁶ found that combining a gluteus medius strength training exercise with proprioception training compared with proprioception training only resulted in no statistically significant improvements in the dynamic balance as measured by the Biodex Stability System. Consequently, poor control, inconsistencies in the results, or increased times suggest further assessment for lower extremity strength, proprioception vestibular, or visual deficiencies may be indicated in addition to dynamic balance. ³³

There are several limitations to this study. Although the Biodex Stability System is an excellent device to evaluate the LoS, testing does not mimic the actual dynamic balance required on the field of play. The athlete is fixed in 1 position and on both feet during testing, which is not always the case in sports. Another limitation is the demographic characteristics of the sample, which included only young athletes without a history of lower extremity injury. Such athletes may already possess a good dynamic balance as baseline measures. Also, the athletes played a variety of sports, making it challenging to determine the magnitude of change per sport. Previous research^9,14 showed that dynamic balance differs based on the sport played, height, and weight. Furthermore, only the LoS was measured in this study without considering the possible changes in the hip, knee, and ankle joints range of motion, and muscular flexibility and strength of the lower extremity muscles.

The results of this study can be applied clinically. Introducing CAE or NHE or both into the training program can significantly improve the dynamic balance of young athletes.The positive changes in dynamic balance may contribute to reducing the injury rate, time-off playing, and costs of treatment.

Conclusion

In the current study, the significant increase in dynamic balance performance that was observed postintervention implies that inclusion of CAE or NHE, or both preferably, as components of programs in training young athletes is reasonable. The addition of these exercises was effective in improving balance capacity and performance in athletes.