# Copenhagen Adduction Exercise to Increase Eccentric Strength: A Systematic Review and Meta-Analysis
## 4 Discussion  
The purpose of this study was to systematically review the scientific literature about the effect of CA exercise training on the EHAD test and to carry out a mini meta-analysis to determine the overall effect as mean difference. Results indicate that eight weeks of training intervention with CA exercise, 2-3 times per week, is effective to improve EHAD in football players [14; 21; 22]. Therefore, it could be advisable the inclusion of CA in preventive training programs to potentially reduce injuries in athletes and, specifically, in football players. The CA is a dynamic high-intensity exercise that includes eccentric contractions and showed high muscle activity of the adductor longus using surface electromyography [12], so it could be recommended in the prevention of adductor and/or groin injuries in athletes. Previous studies observed that additional eccentric exercise reduced the rate of overall, new and recurrent muscle injuries in professional and amateur football players [24]. The CA exercise seems to produce high peak electromyography values for abdominal muscles [12] which may explain why preventing programs based on CA exercise are appropriate against groin problems [25]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**RCT**} & \multirow{2}{*}{**CA Group**} & \multirow{2}{*}{**Control**} & \multirow{2}{*}{**Duration**} & \multicolumn{2}{c}{**Sessions**} & \multirow{2}{*}{**Reps**} & \multirow{2}{*}{**Reps**} & \multirow{2}{*}{**Reps Per**} & \multirow{2}{*}{**Season**} \\  & & & & & & & & & **Week** & **Period** \\ \hline \multirow{3}{*}{[PERSON] et al., 2018} & \multirow{3}{*}{CA resistance} & Mobility & \multirow{3}{*}{8 weeks} & \multirow{3}{*}{2} & \multirow{3}{*}{2-4} & \multirow{3}{*}{6–10} & \multirow{3}{*}{24–72} & \multirow{3}{*}{In-season} \\  & & exercises + & & & & & & & \\ \cline{1-1}  & & & & & & & 3–5 & & \\ \cline{1-1}  & & & & & & (Beginner) & & & Pre-season \\ \cline{1-1}  & & FIFA 11 (with NH) + & \multirow{3}{*}{8 weeks} & \multirow{3}{*}{3} & \multirow{3}{*}{1} & \multirow{3}{*}{1} & \multirow{3}{*}{21–30} & \multirow{3}{*}{36–45} \\  & & & & & & & & \\ \cline{1-1}  & & & & & & & 12–15 & & \\ \cline{1-1}  & & & & & & (Advanced) & & \\ \cline{1-1}  & & & & & & 2–3 & 6–15 & & \\ \cline{1-1}  & & & & & & & & & \\ \hline \multicolumn{10}{l}{RCT: Randomized Controlled Trial; CA: Copenhagen abduction exercise; RFTS: Regular football training sessions; NH: Nordic hamstring exercise.} \\ \end{tabular}
\end{table} 
Table 2: Characteristics of the interventions. Figure 2: Meta-analysis results of CA exercise intervention on eccentric hip adduction strength (EHAD). In the three studies included, the CA exercise was performed in football teams and implemented during the pre-season [22] and in-season [14; 21]. A previous study of 12-week eccentric hamstring strengthening program observed a similar magnitude of effect, irrespective of its schedule relative to the football training session. In this regard, they found an increase in strength and surface electromyography, but the adaptations differed according to the timing of the injury prevention program [26]. Therefore, future studies should explore if there is additional benefit when CA is applied in different schedules, not only in football but also in other sports, and establish the optimal volume of sets and repetitions. As can be seen in Table 2, all training programs lasted eight weeks. This means that more studies are recommended to see if the benefits can be obtained with interventions shorter than eight weeks due to the lack of time at training is one of the barriers identified to carry out evidence-based sport injury prevention programs [27], specifically in football [28]. In this regard, it is important to highlight the short time required to carry out the CA intervention, which is usually shorter than 15 min [21]. Some strengths of the CA exercise are that it seems to be well-tolerated and safe since none of the three articles reported any important problem as consequence of the intervention.