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Adductor Strengthening Protocol - is this the next Nordic Hamstring Curl?

Author's note: I wrote this back in 2019 and need to review if there's any new evidence on this topic. Anecdotally, I often still recommend these exercises when rehabbing groin/inner thigh pain and injury.



Key Points:

  1. This is the first study examining the effects of an adductor-specific, single-exercise intervention effect on prevalence of groin injuries in athletes implemented during pre- season and competitive season.

  2. The study intervention demonstrated a reduction in prevalence and risk of groin problems in male soccer players by 41% based on Intention-to-Treat analysis and 47% based on a Per-Protocol analysis.

  3. The protocol utilized in this study can be easily implemented during pre-season and during the competitive season for field sport athletes. Additionally, the time requirements are low, no equipment is required, and dosage of protocol can be manipulated based on symptoms and timing of competitive season.


Introduction

Groin issues are a common problem among soccer (football) players. A 2015 review by Walden et al found that the rate of groin injury among club-seasonal soccer players was 0.2 2.1 per 1000 hours in men and 0.1-0.6 per 1000 hours in women, with aggregate data indicating a 2.4-times higher rate of groin injuries in male athletes. The authors state:

The proportion of groin injury during club-season play was 4-19% (aggregated 12.8%) in men's football and 2-11% (aggregated 6.9%) in women's football, suggesting that groin injury comprises a bigger proportion of the total injury burden in male footballers.

To put this rate of groin injury into perspective, if a team has 22 players on their roster with 10 hours of practice/week for 28 weeks (competitive season) that’s a total of 6,160 hours. This leads to the statistical probability that between 1-13 males and 1-4 females are dealing with groin issues on a team during a competitive season.


The authors report much of the data was based on a time-loss definition of injury [Waldén 2015]. There is nuance to this based on how “injury” is defined. Such a discussion is beyond the scope of this article; however, it is important to define our terms at the outset.


What is meant by “groin” and “injury”?


According to Doha meeting, 3 categories define clinically relevant structures possibly involved with groin pain [Weir 2015]:


  1. Groin-related structures:

    1. Adductor-related (yellow box)

    2. Iliopsoas-related

    3. Inguinal-related

    4. Pubic-related

  2. Hip-related

  3. Other


Our focus for this article will be adductor-related injury. The adductors are a group of

muscles which originate from pubic region and insert into either the femur or tibia. This

groups primary action is hip adduction, that is, moving the thigh towards midline.


Given the prevalence of groin-related issues in soccer players, and specifically the

higher prevalence in males, Harøy et al sought to assess the influence of a specific

exercise intervention on reducing groin-related injuries in male soccer athletes during a

competitive season. The authors argue that evidence is mixed on the abilities of such

interventions to reduce injury rates, and much of this may be related to how an injury is

being defined. The authors state:

One limitation of previous groin-specific prevention studies is the use of a time-loss injury definition, an inadequate approach as only about one-third of all groin problems result in time loss. Injuries causing time loss may only represent the ‘tip of the iceberg’ as a large proportion of players continue to participate despite having groin-related complaints with associated impairments or reduced performance.

In order to solve this issue, the authors sought a broader definition of injury to include

“ all physical complaints”. They utilized the Overuse Injury Questionnaire (OSTRC) to aid

with injury surveillance and classification. This approach was implemented according to

the authors, “ ...in order to capture all cases leading to pain, decreased participation or

performance, not only those resulting in time loss."


Reported Injury Risk Factors:


Whittaker et al. conducted a systematic review of risk factors associated with groin

injuries and found the following variables relevant [Whittaker 2015]:

  1. Prior groin injury

  2. Higher level of play

  3. Reduced hip adductor strength

  4. Lower levels of sport-specific training


Adductor strength is certainly a modifiable risk factor. A recent video analysis of the

biomechanical factors associated with adductor longus strain in football players found

(emphasis mine):

Acute adductor longus injuries in football occur in heterogeneous situations. Player actions can be categorised into: change of direction, kicking, reaching and jumping. Change of direction and reaching injuries were categorised as closed chain movements, characterised by hip extension and abduction. Kicking and jumping injuries were categorised as open chain movements, characterised by a change from hip extension to hip flexion and abduction to adduction. Both open and closed chain movements frequently occurred with the hip externally rotated. Despite the variety of situations, a rapid high muscle activation during rapid muscle lengthening may be considered a fundamental injury mechanism for acute adductor longus injuries.

The authors go on to state (emphasis mine):

Our findings suggest that increasing the capacity of the adductor longus to tolerate rapid loading at a lengthened state is recommended as a key element in injury prevention. Improving the ability of the muscle-tendon unit to tolerate load at a lengthened state may be achieved with eccentric training [Serner 2019].

This is an interesting takeaway. Although injury prevention is multifactorial, perhaps eccentric-focused adductor exercises are valuable components of a training program.


Purpose


Harøy et al conducted a cluster-randomized controlled trial to test the mitigating effects

of an adductor-focused exercise protocol on prevalence of groin problems in male

soccer players.


Subjects


Data acquisition occurred between February of 2016 to October of 2016. During

pre-season (February-March), semi-professional soccer teams in Norway were invited

to participate in the authors’ study. 35 teams (652 players) initially enrolled in the study

but a single team withdrew after randomization. The control group consisted of 242

athletes and the intervention group had 247 athletes, totaling 489 participants.


Methods


The authors created an intervention protocol based on the Copenhagen Adduction

exercise (CA) due to prior evidence which demonstrated, “high activation of the

adductor longus muscle, as well as considerable eccentric adduction strength gains

following standardised protocols.”


A major benefit of utilizing CA is the lack of required equipment. Furthermore, this

particular study utilized regressions of movement for symptomatic players along with a

planned manipulation of training variables (i.e., volume and intensity) of the exercise

based on whether the athlete was in pre-season or in-season.


For players randomized to the intervention group, the following levels were implemented:

  1. Level 1 - side-lying hip adduction. Figure 1A and B (easiest).

  2. Level 2 - CA but with shorter lever arm. Figure 2A and 2B (moderate).

  3. Level 3 - CA with longer lever arm. Figure 3A and 3B hardest.


Participants were instructed to initially attempt level 3, but if groin pain was experienced during the exercise (>3 on 0-10 pain scale), then regress to level 2. If level 2 presented with similar symptoms, then the athlete regressed to level 1. Regardless of selected level, the exercise was performed bilaterally.


See Table 1 for dosage recommendations of exercise intervention.

Outcome measures


The primary outcome measure was weekly prevalence of ALL groin problems

during the athletes’ 28-week competitive season . This information was recorded via

the previously-mentioned OSTRC tool. The authors also recorded secondary outcome

measures consisting of the weekly prevalence of “ substantial” groin problems during the

competitive season, as defined below.


Each week the participants completed the questionnaire via a smartphone application.

A notification was sent to each participant to complete the survey on Sunday, followed

by a short message service (SMS) notification on Monday. If a participant had not

completed the questionnaire by Thursday, they received an additional SMS notification.

If a participant still did not complete the questionnaire, then they were contacted by

phone for the information. Of note, the authors did input missing data retrospectively.

During the last four weeks of the season, the authors visited each team and had

participants complete their missing data by recall. Participants were provided with prior

questionnaire responses to help with recall.


How were “groin problems” defined?


  1. Any hip or groin symptoms reported in the questionnaire.

  2. Any symptoms to include:

    1. pain, ache, stiffness, clicking/catching or other complaints related to the groin

    2. Reduced training participation, training volume or performance due to groin problems

    3. Players categorized with substantial groin problems when reporting moderate or severe reductions in training volume or football performance, or a complete inability to participate due to groin problems.

The authors then analyzed the data for correlation between the prevalence of all

groin problems, “substantial” groin problems, and their exercise intervention. Analyses of the data were based on the 28-week timeframe during the competitive season (April - October). If players had a <75% response rate they were excluded from the final dataset (<21 weekly reports).


Primary Intention to Treat (ITT) analyses were completed as well as a secondary

Per-Protocol (PP) analysis. The PP analysis excluded players who completed <67% of

the intervention protocol pre-season or <50% during competitive season. Hopefully,

having both of these analyses will help mitigate bias and confounding factors affecting

the data and interpretation thereof.

The authors recorded 13,628 weekly reports from athletes. The intervention group averaged a weekly response rate of 74%, while the control group averaged an 80% weekly response rate.


The ITT analyses included 77% of the players who met the required 75% response rate for data inclusion. Of the players included for analysis, both the intervention group and the control group had an average weekly response rate of 90%.


Primary outcome for Intention-to-Treat Analysis:


During the 28 week competitive season, the intervention group reported an average weekly prevalence of 13.5% (95% CI 12.3-14.7%) for all groin problems. By comparison, the control group reported an average weekly prevalence of 21.3% (95% CI 20.0-22.6%) for all groin problems. The intervention group demonstrated a 41% lower relative risk of reporting groin problems comparatively to the control group.


Secondary outcomes for Intention-to-Treat Analyses:


The average weekly prevalence of “substantial” groin problems was 5.7% (95% CI 5.1-6.3%) for the intervention group and 8.0% (95% CI 7.5-8.5%) for the control group. This represents an 18% lower relative risk of reporting substantial groin problems compared to the control group, though this finding did not reach statistical significance.


Per-Protocol Analysis:


For the per protocol analysis, the authors found the intervention group demonstrated an average weekly prevalence of groin problems at 11.7% (95% CI 10.9% to 12.5%) and substantial groin problems at 4.5% (95% CI 4.1% to 5.1%).

Major Takeaways:

  1. 13,628 questionnaire responses including 2,458 reported groin problems

  2. ITT analysis demonstrated a cumulative incidence of 55% for all groin problems in the intervention group and 67% in the control group.

  3. With respect to “substantial” groin problems, 28% incidence in intervention group and 37% in control group.


Why does this article matter?


This is the first study examining the effect of an adductor-specific, single-exercise intervention on prevalence of groin problems in male football players implemented during pre-season and competitive season.


The primary findings demonstrate a reduction in both prevalence of groin problems as well as risk of reporting problems during a competitive season (41% lower for ITT and 47% lower for PP analyses).


Strengths of this study:


The adductor strengthening protocol utilized in this study can be easily implemented during pre-season and competitive season for the athletes. The time requirements appear low, and no equipment is required beyond having a teammate to help. The authors also did a great job of manipulating loading of the exercise based on symptoms and timing of competitive season (as demonstrated in Table 1). It’s also important to mention the authors did NOT exclude symptomatic players from the intervention. Given prior information discussed at the beginning of this article, we saw many players continue to participate in their sport despite symptoms.


The compliance for this study was overall high comparatively to similar intervention studies. On average, players in the ITT analysis completed 73% of the authors recommended protocol during the preseason and 70% during the competitive season (0.7 times per week with a range of 0.6-0.9). Surprisingly, 42% of players averaged a compliance rate higher than the authors recommendation during in-season. Players included in the PP analysis averaged 93% compliance during pre-season and during the competitive season completed the protocol 0.9 timers per week (range of 0.7-1.0).


Compliance matters A LOT for these situations in order to accurately assess if the intervention has an impact on athletic based injuries. The authors did a great job examining the data via an intention-to-treat and per-protocol analyses to minimize bias from drop-outs and lack of compliance to the interventions in hopes of accurately assessing the impact of their intervention.


Weaknesses of the study:


Single factor examined - The authors study really only examines one potential area with respect to groin pain: the adductor group. Recall at the beginning of this article, Weir et al discussed multiple regions that may be related to the report of groin issues (iliopsoas related, inguinal related, pubic related, hip-related, and other). However, the authors argue that the adductors are the primary area that matters: “We would argue that targeting the adductors addresses the main problem as adductor-related groin pain accounts for >2/3 of all hip and groin injuries in football.”


No assessment of adductor strength - Another important variable that was lacking from outcome measurements was hip adduction strength pre- and post- intervention. Since we have evidence that eccentric loading of the area is a correlate for groin related injuries, we would like to see if we actually affected strength as a potential explanation for the observed effects of the intervention. According to the authors, we do have evidence for other protocols assessing hip adduction strength (CA - level 3 of this program) showing a 36% increase in eccentric hip adduction strength after an 8 week intensive protocol and an 8% increase in less intensive protocols. This leads into an important unknown: dosage . We aren’t entirely sure if there is an ideal dosage to mitigate groin issues with such protocols.


Recall bias - The authors retrospectively registered missing data for the last month of the study, which introduces a risk for recall bias.


Deviations from registered protocol - The authors did deviate from their original registered protocol by removing players failing to meet the cutoff response rate standard, which potentially weakens the generalizability of this study to “real-world” situations.


Lack of injury registration methods - There was a lack of reliable methods for registering injuries based on diagnostic information. The authors argue the size of the study didn’t allow for “reliable medical follow-up”. To help with this issue, the authors offered a standard examination to any player experiencing a groin problem affecting match play for two weeks. 26 players took the authors up on their offer and underwent examination The authors state - “In a future study, the self-reported groin problems should be examined, classified and reported according to the Doha agreement on terminology and definitions.”


Generalizability of findings - this study was on male soccer players. Perhaps we can generalize to other sports like ice hockey and rugby or in female athletes but we need continued research on the topic to demonstrate further efficacy for these populations.


The authors close with this perspective:

The results from the present study suggest that the Adductor Strengthening Programme should be included in football training, among senior male football players. Whether the preventive effect from the Adductor Strengthening Programme can be generalised to female or youth-level football players, as well players at the highest professional level, is not known. Other types of athletes may also benefit from the programme as low hip adduction strength is also considered a risk factor associated with groin problems in other sports with similar movement patterns, such as ice hockey, rugby and Australian rule football.

As of right now, based on this evidence, I tend to be in agreement with this stance. Similar to what we see with Nordic hamstring curls in the mitigation of hamstring strains, it is likely we should begin prophylactically prescribing an adductor strengthening protocol for athletes in higher risk sports for groin injuries such as soccer. The protocol examined in this study has a low threshold for implementation by not requiring equipment, low time commitment (authors estimate less than 5 minutes), loading can be manipulated based on season requirements, and even progressions are outlined based on symptomatic presentation of the athlete. It would be difficult to find reasons not to implement this program. However, there is still nuance to this discussion that will need to be teased out through future study.


References

  1. Harøy J, Clarsen B, Wiger EG, et al. The Adductor Strengthening Programme prevents groin problems among male football players: a cluster-randomised controlled trial. British journal of sports medicine. 2019; 53(3):150-157.

  2. Waldén M, Hägglund M, Ekstrand J. The epidemiology of groin injury in senior football: a systematic review of prospective studies. British journal of sports medicine. 2015; 49(12):792-7.

  3. Weir A, Brukner P, Delahunt E, et al. Doha agreement meeting on terminology and definitions in groin pain in athletes. British journal of sports medicine. 2015; 49(12):768-74.

  4. Whittaker JL, Small C, Maffey L, Emery CA. Risk factors for groin injury in sport: an updated systematic review. British journal of sports medicine. 2015; 49(12):803-9.

  5. Serner A, Mosler AB, Tol JL, Bahr R, Weir A. Mechanisms of acute adductor longus injuries in male football players: a systematic visual video analysis. British journal of sports medicine. 2019; 53(3):158-164.


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