Mechanisms of quadriceps muscle weakness in knee joint osteoarthritis: the effects of prolonged vibration on torque and muscle activation in osteoarthritic and healthy control subjects

Abstract

Introduction: A consequence of knee joint osteoarthritis (OA) is an inability to fully activate the quadriceps muscles, a problem termed arthrogenic muscle inhibition (AMI). AMI leads to marked quadriceps weakness that impairs physical function and may hasten disease progression. The purpose of the present study was to determine whether γ-loop dysfunction contributes to AMI in people with knee joint OA.

Methods: Fifteen subjects with knee joint OA and 15 controls with no history of knee joint pathology participated in this study. Quadriceps and hamstrings peak isometric torque (Nm) and electromyography (EMG) amplitude were collected before and after 20 minutes of 50 Hz vibration applied to the infrapatellar tendon. Between-group differences in pre-vibration torque were analysed using a one-way analysis of covariance, with age, gender and body mass (kg) as the covariates. If the γ-loop is intact, vibration should decrease torque and EMG levels in the target muscle; if dysfunctional, then torque and EMG levels should not change following vibration. One-sample t tests were thus undertaken to analyse whether percentage changes in torque and EMG differed from zero after vibration in each group. In addition, analyses of covariance were utilised to analyse between-group differences in the percentage changes in torque and EMG following vibration.

Results: Pre-vibration quadriceps torque was significantly lower in the OA group compared with the control group (P = 0.005). Following tendon vibration, quadriceps torque (P < 0.001) and EMG amplitude (P ≤0.001) decreased significantly in the control group but did not change in the OA group (all P > 0.299). Hamstrings torque and EMG amplitude were unchanged in both groups (all P > 0.204). The vibration-induced changes in quadriceps torque and EMG were significantly different between the OA and control groups (all P < 0.011). No between-group differences were observed for the change in hamstrings torque or EMG (all P > 0.554).

Conclusions: γ-loop dysfunction may contribute to AMI in individuals with knee joint OA, partially explaining the marked quadriceps weakness and atrophy that is often observed in this population.

Figure 1

Schematic diagram of the γ-loop. During voluntary muscle contraction, supraspinal centres coactivate the α-motoneuron and γ-motoneuron pools. The γ-motoneuron pool in turn innervates muscle spindle endings via fusimotor nerve fibres, enhancing their firing. Muscle spindles provide a tonic excitatory input to the homonymous α-motoneuron pool via Ia sensory nerve fibres.

Figure 2

Experimental setup used during vibration of the infrapatellar tendon.

Figure 3

Pre-vibration quadriceps and hamstrings peak torque (Nm) in the osteoarthritis and control groups. MVC, maximum voluntary isometric contraction at 90° of knee flexion. *Significant difference between groups (P = 0.005). Data are means and standard deviations.

Figure 4

Change in quadriceps and hamstrings peak torque following vibration. Percentage change in quadriceps and hamstrings peak torque (Nm) following vibration in the osteoarthritis and control groups. *Significant difference between groups (P = 0.011). **Significant change from zero (P < 0.05). Data are means and standard error of the means.

Figure 5

Change in quadriceps surface electromyography amplitude following vibration. Percentage change in quadriceps surface electromyography (EMG) amplitude following vibration in the osteoarthritis and control groups. RMS, root mean square of EMG signals; VL, vastus lateralis; VM, vastus medialis. *Significant difference between groups (P ≤0.005). **Significant change from zero (P ≤0.001). Data are means and standard errors of the means.