Strength profiling using isokinetic dynamometry following anterior cruciate ligament (ACL) reconstruction: Implications for rehabilitation and return to sport decision making
Authors:   , Hammond Adrian 
Published: May 2024

The evaluation of isokinetic muscle strength testing using peak torque (PT) values to identify muscle strength deficits in quadriceps and hamstrings following anterior cruciate ligament reconstruction (ACLR) at the point of return to sport (RTS) is one of the most used tests for discharge criteria. It has readily been used in previous literature to identify if athletes meet limb symmetry of the uninvolved leg following ACLR at the time of RTS. However, studies have found that the use of a single PT value lacks the reliability to highlight residual strength deficits across the full range of motion (RoM), leading to an overestimate of muscle strength function. Research has suggested that torque-angle analysis should be conducted over a full RoM to further identify residual knee extensor and flexor muscle strength deficits at the point of RTS. The aim of this thesis was to examine isokinetic strength in soccer players and other pivoting athletes using the traditional methods including PT and limb symmetry index (LSI) thresholds in addition to a strength profile which considers torque production across the full RoM tested. The use of strength profiling is proposed to provide a greater depth of analysis which may have potential implications for RTS decision-making, and subsequently, late-stage rehabilitation. The study hypothesized that athletes who ‘pass’ the traditional RTS test protocol of ≥ 90% quadriceps PT LSI will still display residual deficits when the torque-angle curve is examined. Data was collected retrospectively from twenty-five Qatari male athletes who had undergone primary ACLR and played level 1 professional sport in Qatar. With inclusion criteria ensuring all athletes had to achieve ≥ 90% quadriceps PT LSI across repetitions 2,3 and 4. Isokinetic strength assessment for quadriceps (concentric) and hamstrings (concentric and eccentric) were performed on both the involved and uninvolved limbs at 60˚/s (Biodex). Average torque values across each six 10˚ window (20˚-29˚, 30˚-39˚, 40˚-49˚, 50˚-59˚, 60˚-69˚ and 70˚-79˚) were used to form an LSI % to inform of potential strength deficits. Hamstring to Quadriceps (H:Q) ratios were also conducted across all the torque-angle windows using an average torque and compared against the traditional methods. The results for LSI of average torque across the six 10˚ windows on all testing protocols show that only 28% (7/25) participants achieved a ≥ 90% mean torque LSI across all six 10˚ segments at all three testing protocols. Therefore, 72% (18/25) who were deemed safe to RTS based on a ≥ 90% quadriceps PT LSI showed residual deficits in at least one 10˚ window of knee flexion when assessing residual deficits in knee extension and flexion muscle strength. The study also highlights of those that have had a hamstring (HS) graft only 8% achieved ≥ 90% 5 LSI across all 10˚ segments at all three testing protocols compared with 46% (6/13) who achieved this after having bone-patellar tendon-bone (BPTB) graft. Only one significant main effect was found in the 10˚ segment variable found in all three measures: Hamstring concentric F (1.76, 115) =51.47, P<.001, r = 0.83, Hamstring eccentric F (1.46, 33.65) =67.11 P<.001, r = 0.57 and Quadriceps concentric F (5, 115) =173.51 P<.001, r = 0.63. This study highlights further research needs to be conducted into the use of torque-angle analysis across a full RoM using average torque values, as opposed to the sole use of a PT value for identifying ‘safe’ RTS after ACLR.