Isokinetic assessment: strength, asymmetries and injury prevention

How strong is each of your legs, really? And how evenly do the muscles around the knee work together? Isokinetic assessment answers with numbers — and those numbers are directly linked to injury prevention and to a safe return to action.

What isokinetic assessment is

On the isokinetic dynamometer (Cybex), the joint moves at a constant, preset angular velocity: the harder you push, the greater the resistance you meet, but the speed does not change. In this way the muscle is assessed at its maximal effort throughout the whole range of motion, under controlled and repeatable conditions — something a simple weights test cannot offer. The result is the torque produced by each muscle group, measured objectively and comparable from one assessment to the next.

Torque and the H:Q ratio

At the knee, the two key muscle groups are the quadriceps (extension) and the hamstrings (flexion). The relationship between their strength — the H:Q ratio (hamstrings to quadriceps) — shows how well balanced the “agonist” and the “antagonist” of the movement are. The hamstrings act as a natural “brake” that protects the knee during sprints, jumps and sharp decelerations; when they fall well short of the quadriceps, that brake weakens.

Right–left asymmetries

The same assessment also compares the two limbs with each other. Small differences are normal, but larger asymmetries — which often stay “hidden” behind seemingly normal movement — deserve attention: they may be the residue of an old injury, of one-sided loading from the sport, or of incomplete rehabilitation. Without an objective measurement, such an asymmetry is rarely noticed before it shows up as a problem.

Injury prevention

The link between imbalances and injury risk is not theory. In professional footballers, Croisier and colleagues (2008) found that players who started the season with uncorrected hamstring strength imbalances had a significantly higher risk of strain — while those who restored the balance through targeted training reduced their risk to levels comparable with unaffected players. Reviews of the hamstring strain literature classify weakness and strength imbalances among the modifiable risk factors (Opar et al. 2012). In other words: they are among the few “red flags” that we can both detect early and correct.

Return to competition

After an injury or surgery — ACL reconstruction being the classic example — the question “am I ready to go back?” should not be answered by feel. Isokinetic strength is one of the most widely used objective return criteria: a systematic review by Undheim and colleagues (2015) shows that it is widely used in return-to-play decision making after ACL injury, the usual target being for the strength of the operated leg to reach at least 90% of the healthy one. Repeated measurements over the course of rehabilitation give physiotherapy measurable milestones — and give the athlete confidence grounded in data.

Who needs it

• Athletes in sprinting, jumping and change-of-direction sports — checking H:Q and asymmetries before they turn into an injury.
• Anyone returning from injury or surgery — objective criteria for progress and return, in cooperation with the treating physician and the physiotherapist.
• Teams and academies — preventive screening of the roster at the start of the season, so that strength work targets exactly where it is needed.
• Active people with recurring “niggles” — a measurable imbalance is often hiding behind them.

The assessment at our lab is performed joint by joint, with a protocol adapted to your sport and your history. The results — torques, ratios and asymmetries compared against reference norms — are recorded in your account at results.ergometrictests.com.

References

• Croisier JL, Ganteaume S, Binet J, Genty M, Ferret JM (2008). Strength imbalances and prevention of hamstring injury in professional soccer players. American Journal of Sports Medicine.
• Opar DA, Williams MD, Shield AJ (2012). Hamstring strain injuries: factors that lead to injury and re-injury. Sports Medicine.
• Undheim MB, Cosgrave C, King E, Strike S, Marshall B, Falvey É, Franklyn-Miller A (2015). Isokinetic muscle strength and readiness to return to sport following anterior cruciate ligament injury: is there an association? British Journal of Sports Medicine.