Heightened flexor withdrawal responses in subjects with knee osteoarthritis

Patients with osteoarthritic (OA) knee pain often present with impaired muscle activation and function that may be attributed in part to hyperexcitability of flexion withdrawal reflexes (FWRs). The aim of this study was to investigate alterations in the excitability of FWR in individuals with knee OA and its potential associations with impaired quadriceps activation (QA) and subjective reports of pain. Twenty subjects with and 20 without knee OA (age, 45 to 75 years) participated. Impaired QA was determined in OA subjects during maximal volitional contraction of the quadriceps. FWRs were tested in isometric conditions using electrocutaneous stimulation applied at the medial foot at 1x and 2x FWR threshold and surface electromyographic recordings obtained from tibialis anterior (TA) and biceps femoris (BF). Joint torques at the hip, knee, and ankle were calculated and normalized to body mass. FWR threshold was significantly diminished in OA versus the control group (P < .01). In addition, FWR latencies were consistent with spinally mediated responses, with significantly earlier responses in OA versus control subjects of TA at threshold (P = .002) and BF at suprathreshold stimulation (P = .0006). Impaired QA was found in 4 of 20 OA subjects but was not correlated to FWR threshold or pain levels. Peak joint torques were diminished in the OA versus the control group (P < .0006).

Increased excitability of FWRs was evident in subjects with chronic knee osteoarthritis, including those subjects without resting pain, but was not associated with impaired QA. Management strategies for this patient population must consider altered pain processing in addition to addressing impairments found at the knee.