Strength imbalances and prevention of hamstring injury in professional soccer players: a prospective study. (2008)

Abstract
BACKGROUND:
The relationship between muscle injury and strength disorders remains a matter of controversy.

PURPOSE:
Professional soccer players performed a preseason isokinetic testing aimed at determining whether (1) strength variables could be predictors of subsequent hamstring strain and (2) normalization of strength imbalances could reduce the incidence of hamstring injury.

STUDY DESIGN:
Cohort study (prognosis); Level of evidence, 1.

METHODS:
A standardized concentric and eccentric isokinetic assessment was used to identify soccer players with strength imbalances. Subjects were classified among 4 subsets according to the imbalance management content. Recording subsequent hamstring injuries allowed us to define injury frequencies and relative risks between groups.

RESULTS:
Of 687 players isokinetically tested in preseason, a complete follow-up was obtained in 462 players, for whom 35 hamstring injuries were recorded. The rate of muscle injury was significantly increased in subjects with untreated strength imbalances in comparison with players showing no imbalance in preseason (relative risk = 4.66; 95% confidence interval: 2.01-10.8). The risk of injury remained significantly higher in players with strength imbalances who had subsequent compensating training but no final isokinetic control test than in players without imbalances (relative risk = 2.89; 95% confidence interval: 1.00-8.32). Conversely, normalizing the isokinetic parameters reduced the risk factor for injury to that observed in players without imbalances (relative risk = 1.43; 95% confidence interval: 0.44-4.71).

CONCLUSION:
The outcomes showed that isokinetic intervention gives rise to the preseason detection of strength imbalances, a factor that increases the risk of hamstring injury. Restoring a normal strength profile decreases the muscle injury incidence.

My comments:

This study is a follow up to the last one I had blogged on by the same authors. It’s largely the same design with largely the same results but rather than following 26 athletes with prior hamstring injuries, they followed 687 soccer players and completed information on 462 of them. That’s good research! In my file there is a lot of good research on hamstring strains apparently because soccer is such big business world wide. With the larger number of subjects, small factors that originally showed inconsistent results are now turning out to be both practically and statistically significant.

So what they found was that a professional soccer players without muscle imbalance had a 4.1% hamstring injury frequency, those with imbalances had a 4 fold increase hamstring injury frequency of 16.5%. They found that if strengthening exercises were done to normalize muscle imbalances that hamstring injury frequency dropped to 11%. If soccer players not only did their exercises but did so to the point that the imbalances were normalized, the injury frequency dropped to 5.7% which is almost as low as those with balanced strength levels to begin with.

Also interesting from the paper is that they judged an imbalance to be that in which a hamstring deficit of just 5% was seen in a side to side comparison. Concentric H/Q ratio of <0.57 or 0.55 on Cybex or Biodex testing equipment respectively, and a mixed H/Q ratio of <.98 or 1.04 on Cybex or Biodex respectively.  As with the prior study, the mixed ratio (eccentric hamstring strength at 30 degrees a second/concentric quadriceps strength at 240 degrees a second) proved most predictive of future hamstring injury. No player with a mixed ratio of 1.40 sustained a hamstring injury, which indicates stronger hamstrings than the norm, and that strong hamstrings (particularly in eccentric strength) was predictive of minimal risk of future injury.

The downside of this study, like the author’s other paper, was that it relied on isokinetic testing and training, which isn’t practical for most players. The author’s admitted that for recreational players, isokinetic testing would be too expensive and too time consuming. It would be recommended but due to the high prevalence of hamstring injuries, all players should perform manual or weight training exercises to their hamstrings.  For future research I would love to see how much players with a 1.40 mixed ratio are able to lift on leg curls (prone or seated) compared to leg extensions, and what they can lift on RDLs vs front or back squats (high bar or low). That would then give strength coaches, athletic trainers, and physical therapists more practical information to go on with regards to returning soccer players, sprinters and other athletes with hamstring strains/muscle injuries to sport as opposed to arbitrary recommendations of some number of weeks rest.

The take home message seems to be that stronger hamstrings are better, with a 1.4 mixed ratio being particularly protective, the problem being that very few places have the testing equipment to tell you what your mixed ratio is. Also worth noting is that common goals of strength recovery of 80-90% in comparison with the uninjured leg likely isn’t sufficient and goals of 95% strength recovery being more protective against injury recurrence.