Isokinetic Strength Imbalance Was Linked to 37.5% Hamstring Injury Rate in Rugby Cohort

A 12-month cohort study tracked whether preseason isokinetic strength testing could flag the rugby players most likely to tear a hamstring. Isokinetic strength imbalance was linked to higher risk, but only once prior injury was accounted for.

Isokinetic Strength testing has been a preseason fixture in professional sport for decades, built on a simple premise: measure the hamstrings against the quadriceps, flag the players whose numbers look lopsided, and intervene before something tears. A prospective cohort study published in Frontiers in Sports and Active Living on October 28, 2026, followed 134 professional rugby players for a full season to see how well that premise holds.

The answer is layered. Players flagged by isokinetic strength criteria did go on to sustain more hamstring injuries – 37.5% versus 21.0% – but the unadjusted association fell just short of statistical significance, and the test performed poorly as an individual screening tool.

The work was carried out by a group spanning CHU Clermont-Ferrand, where the departments of physical medicine and rehabilitation and of biometrics and clinical research are based, and CHRU Montpellier. Maxime Grolier is first author, with Emmanuel Coudeyre as senior author.

Participants were drawn from four of the 14 eligible clubs in the French Top 14, with baseline isokinetic strength assessment in the final two weeks of the 2010-2011 season or the first three weeks of 2011-2012, depending on each club schedule. Follow-up ran 12 months.

How the Isokinetic Strength Criteria Were Defined

Three thresholds placed a player in the flagged group. A side-to-side hamstring deficit greater than 15% counted. So did a conventional hamstring-to-quadriceps ratio below 0.45 at 60 degrees per second, or below 0.55 at 240 degrees per second.

The third was a mixed, or functional, ratio: eccentric hamstring torque at 30 degrees per second divided by concentric quadriceps torque at 240 degrees per second, with anything below 0.85 counted as a deficit. Meeting any single criterion was enough to be flagged, which put 72 of the 134 players in that group.

That composite approach is standard practice, and it is also one of the things the authors flag as a weakness. Grouping three quite different deficits together may blend players with distinct physiological profiles into one category.

What the Injury Numbers Showed

Across the full cohort, 40 of 134 players sustained a hamstring injury over the season, an incidence of 29.9% (95% CI, 22.2-38.4). That alone is a reminder of how routine these injuries are in professional rugby.

Split by isokinetic strength status, 27 of the 72 flagged players (37.5%) were injured against 13 of 62 unflagged players (21.0%). The single-predictor model returned an odds ratio of 2.12, but with a confidence interval of 0.93 to 4.83 and a p value of 0.073 – a near miss rather than a result.

Only after adjustment for previous hamstring injury and playing position did the isokinetic strength association reach significance: OR, 2.56 (95% CI, 1.07-6.12; p = 0.034). A sensitivity analysis treating club as a fixed effect pulled it back out again, at OR, 2.08 (95% CI, 0.90-4.81; p = 0.085).

Prior Injury Was the Stronger Signal

The most robust predictor in the model was not isokinetic strength at all. Previous hamstring injury carried an odds ratio of 3.27 (95% CI, 1.20-8.93; p = 0.020). Playing position, backs against forwards, did essentially nothing: OR, 0.90 (95% CI, 0.38-2.12; p = 0.808).

The subgroup breakdown is striking. Among unflagged players, incidence was 20.9% without a prior injury and 21.1% with one. Among flagged players it was 30.5% without a prior injury and 69.2% with one. The authors explicitly caution that this pattern should not be read as a statistical interaction.

Where Isokinetic Strength Screening Falls Down

The classification statistics are the part a strength and conditioning staff should read most carefully. The area under the curve was 0.60, barely above the 0.50 of a coin flip. Sensitivity was 67.5% and specificity 52.1%.

Positive predictive value was 37.5%, meaning roughly six in ten flagged players were not injured that season. Negative predictive value was 79.0%. The authors state plainly that the classification had limited individual-level discrimination and is not a validated screening tool.

That is the gap between a population-level association and a clinical decision. Isokinetic strength imbalance may genuinely mark elevated group risk while still being close to useless for telling a given player what will happen to them.

Why the Age of the Data Matters

The players in this cohort were tested 15 years ago, and the authors raise that themselves as a limitation. Professional rugby has changed in ways that bear directly on hamstring risk: larger players, denser fixture calendars, more high-speed running tracked by GPS, and far more systematic eccentric conditioning.

Those shifts cut in both directions. Greater exposure to sprinting may have raised baseline injury rates since 2012, while better prevention programs may have lowered them. Either way, the absolute incidence figures reported here should be treated as a snapshot of that era rather than a current benchmark.

The structural finding is more durable than the numbers. Whether isokinetic strength screening can discriminate between individuals is a question about the measurement itself, and an area under the curve of 0.60 is unlikely to have improved simply because the sport moved on. What may have changed is how clubs respond to a flagged isokinetic strength result.

How This Sits Alongside Other Hamstring Research

The finding that prior injury outranks any laboratory measure is consistent with much of the hamstring literature. Our coverage of hamstring rehabilitation and return-to-play outcomes dealt with the same awkward reality: the strongest predictor of the next injury tends to be the last one.

It also helps explain why the field has drifted away from screening and toward intervention. Pooled work on eccentric training and explosive power targets the eccentric hamstring capacity that this study measured, and structured warm-up programs built around sprint and agility work aim to raise the whole squad rather than flag individuals.

For context on how common these problems are outside elite sport, our analysis of exercise-related injuries across 12 years of US data puts recreational injury patterns alongside the professional picture.

What This Means for Athletes and Coaches

The practical reading is narrower than the headline figures suggest. An isokinetic strength deficit found in preseason is worth knowing about, but it is a piece of context rather than a prediction, and it should not be treated as a clearance decision either way.

• A clean isokinetic strength result does not make a player safe; roughly one in five unflagged players was still injured.
• A flagged result does not make injury likely; most flagged players finished the season uninjured.
• Injury history deserves more weight in planning than any single preseason strength number.
• Decisions about load, rehabilitation and return to play belong with the medical and performance staff who know the individual athlete.

Isokinetic strength testing retains a legitimate role in tracking an individual over time and in guiding rehabilitation. What this cohort suggests it cannot do is sort a squad into safe and unsafe.