Preseason Strength Imbalance Linked to Hamstring Injuries in Professional Rugby Players

Roughly one in three professional rugby players in a French cohort sustained a hamstring injury over a single season, and those who entered the year with a measurable strength imbalance in their thigh muscles faced markedly higher odds of being sidelined. The finding comes from a prospective cohort study of 134 male players from four clubs in France’s Top 14 competition, published in Frontiers in Sports and Active Living, and it adds a rugby-specific data point to a long-running and often contradictory debate about whether preseason strength testing can flag athletes at risk.

The research team, based at CHU Clermont-Ferrand and collaborating institutions, recruited players during the 2010–2011 and 2011–2012 seasons. Each participant underwent preseason isokinetic dynamometry, a laboratory technique in which a motorized computerized dynamometer measures maximal muscle torque at controlled angular velocities. The protocol included concentric contractions of the quadriceps and hamstrings at 60 and 240 degrees per second, followed by eccentric hamstring testing at 30 and 120 degrees per second, with peak torque normalized to body mass. Players also completed thigh extensibility assessments, including the straight leg raise test for hamstrings and a heel-to-buttock measurement for quadriceps, and the team recorded playing position and any history of hamstring injury.

A preseason isokinetic strength imbalance was defined as meeting at least one of three prespecified abnormalities: a side-to-side hamstring strength deficit greater than 15 percent, an abnormally low conventional hamstring-to-quadriceps ratio, or an abnormally low mixed, or functional, hamstring-to-quadriceps ratio that combines eccentric hamstring torque with concentric quadriceps torque. These thresholds were borrowed from a well-known study of professional football players by Croisier and colleagues, because rugby-specific reference values did not exist at the time of data collection. Of the 134 players, 72, or 53.7 percent, met at least one criterion and were assigned to the imbalance group, while 62 players met none.

Over 12 months of prospective follow-up, with injuries recorded by club medical staff, 40 players sustained at least one hamstring injury, a cumulative incidence of 29.9 percent, with a 95 percent confidence interval of 22.2 to 38.4 percent. The study used a deliberately strict injury definition: only clinically diagnosed injuries causing more than two weeks of time loss from training or competition counted. Injuries with shorter time loss were not captured, so the figure understates the total burden of hamstring problems in the squad.

The contrast between groups was substantial. Players without a preseason imbalance sustained at least one injury at a cumulative incidence of 21.0 percent, 13 of 62 players, whereas those with an imbalance reached 37.5 percent, 27 of 72. In a single-predictor mixed-effects model that accounted for clustering by club, the imbalance was associated with an odds ratio of 2.12, but the confidence interval of 0.93 to 4.83 crossed unity and the result did not reach conventional statistical significance, with a p value of 0.073.

After adjustment for previous hamstring injury and playing position, however, the estimate strengthened to an odds ratio of 2.56, with a 95 percent confidence interval of 1.07 to 6.12 and a p value of 0.034. The authors note that the adjustment may partly reflect an uneven distribution of injury history between the groups: previous hamstring injury was actually more common in the no-imbalance group, 30.6 percent versus 18.1 percent. Previous hamstring injury itself was independently associated with subsequent injury, with an adjusted odds ratio of 3.27. Playing position, collapsed into forwards and backs, showed no significant association.

The most striking descriptive pattern emerged when the two factors were combined. Among players who had both a preseason isokinetic imbalance and a prior hamstring injury, 69.2 percent, 9 of 13, sustained a new injury during follow-up, compared with roughly 21 percent among players with neither factor. The authors are careful to stress that this subgroup figure is descriptive and should not be interpreted as evidence of a statistical interaction, particularly given the small number of players in some cells.

Despite the adjusted association, the classification performed poorly as an individual screening tool. The area under the receiver operating characteristic curve was just 0.60, sensitivity was 67.5 percent, and specificity was 52.1 percent. The positive predictive value was only 37.5 percent, meaning most players flagged as imbalanced did not go on to injure a hamstring, while the negative predictive value of 79.0 percent meant a clean isokinetic profile offered moderate reassurance. The authors emphasize that an association between a screening measure and injury occurrence does not establish its value for predicting injury in any individual athlete, and hamstring injury is a multifactorial outcome influenced by workload, fatigue, exposure to high-speed running, and many other factors the study could not measure.

Several important limitations temper the findings. The dataset is old: it was collected during the 2011–2012 season, roughly 14 years before the manuscript was prepared, and professional rugby has since changed substantially with GPS-based workload monitoring, widespread eccentric hamstring conditioning programs, and standardized injury surveillance. Training and match exposure time were not collected, so injuries could not be expressed as exposure-adjusted rates. Injury ascertainment was not blinded, because club medical staff knew players’ baseline isokinetic results. Identified imbalances were communicated to the clubs, and corrective strength training was implemented for at least some players, an uncontrolled co-intervention whose effect could not be quantified. Testing also used two different dynamometer models without formal cross-calibration, the injury questionnaire was not formally validated, only four clubs contributed data, and the secondary comparative analyses were exploratory with no formal power calculation. The authors also disclose an ethical procedural issue: the ethics committee’s favorable opinion was issued in January 2012, after recruitment had already begun for some participants.

The authors conclude that their results should be considered exploratory and require replication in contemporary professional rugby cohorts using standardized injury definitions, exposure-adjusted analyses, and validated rugby-specific thresholds before any clinical relevance can be established. They do not present isokinetic testing as a standalone prediction tool, and they call for controlled intervention studies to determine whether correcting identified strength imbalances can actually reduce hamstring injury occurrence. For now, the study offers historical prospective evidence that preseason strength profiles and injury history together paint a concerning picture, while reinforcing the broader lesson from the sports medicine literature: no single laboratory measure reliably forecasts which individual athlete will tear a hamstring.

Subject of Research: The association between preseason isokinetic strength imbalance and subsequent hamstring injury in male professional rugby union players

Article Title: Preseason isokinetic strength imbalance and hamstring injury in male professional rugby players: a prospective cohort study

Article References: Grolier, M., Pelletier Visa, M., Julia, M., Morand, D., Pereira, B., Lambert, C., & Coudeyre, E. (2026). Preseason isokinetic strength imbalance and hamstring injury in male professional rugby players: a prospective cohort study. Frontiers in Sports and Active Living, 8, Article 1961372. https://doi.org/10.3389/fspor.2026.1961372

Image Credits: AI Generated

DOI: 10.3389/fspor.2026.1961372

Keywords: hamstring injury, isokinetic dynamometry, rugby union, injury risk factors, strength imbalance, prospective cohort study, sports medicine, injury prevention, Top 14, muscle strength testing, previous injury, athletic screening