Underlying Factors of Neural Activity that Regulate Torque Development after Anterior Cruciate Ligament Reconstruction

Abstract

To quantify a loss in strength after anterior cruciate ligament reconstruction (ACLr), researchers have often only utilized discrete measures of quadriceps force production such as peak torque. Yet, analyzing other characteristics of the waveform, such as rate of torque development (RTD) and time to peak torque (TTP), can tell us about several other factors that regulate force development such as alterations in muscle firing rate and alpha motor neuron recruitment, respectively. PURPOSE: To determine if alterations in the RTD, TTP, and peak torque exists between limbs after ACLr at the time when patients are cleared to return to activity. METHODS: 10 control subjects (4 female, 6 male; age 23.5±3.26yrs; mass 71.79±9.40kgs) and 88 patients (40 female, 48 male; age 21.1±6.05yrs; mass 75.59±15.84kgs) who had undergone ACLr and had been cleared to return to activity (7.53±1.34months) participated. Quadriceps strength was quantified via an isokinetic dynamometer at 60°/sec in concentric mode. The extension phase of the isokinetic trials were analyzed. TTP was calculated by the seconds it took from the onset of the torque value, threshold at 7.5 Nm, to the peak torque value of the trial. RTD was calculated as the slope of the torque from the threshold to the peak torque value, taken every 10ms, and normalized to body weight. One way repeated measures ANOVAs were used to compare limbs for both the ACLr and control groups and P≤0.05 was accepted as significant. RESULTS: As expected, the control group exhibited no difference between limbs in the TTP, RTD, or peak isokinetic torque (P>0.05). However, inter-limb differences were present in the ACLr group, where the involved leg demonstrated longer TTP (.546±.175s vs .505±.153s, P=0.01), lower RTD (2.822±1.52 Nm/kg/s vs 4.346±1.96 Nm/kg/s, P=0.00) and lower peak torque values (105.8±38.7Nm vs 152.0±52.2Nm, P=0.00) as compared to the uninjured leg. CONCLUSIONS: Post-surgery, ACLr individuals’ exhibit inter-limb quadriceps dysfunction in force production that is not similarly present in a healthy population. The lower RTD may be a cause of a decrease in firing rate in the quadriceps of the involved limb. Whereas the increase in TTP may be indicative of a decrease in the number of motor neurons being recruited. Supported by NIH Grant K08 AR053152-01A2 and NCATS Grant UL1TR000433