Is muscle strength in a painful limb affected by knee pain status of the contralateral limb? - Data from the Osteoarthritis Initiative.

Contralateral knee pain has been suggested to be associated with muscle weakness in a pain-free knee, potentially through a mechanism of central nervous inhibition. Whether contralateral knee pain also affects muscle strength in a painful knee, however, is unknown. Here we study the extent to which isometric muscle strength differs between matched painful limbs of people with unilateral knee pain vs. matched painful limbs people with bilateral knee pain. To that end, 163 participants with unilateral knee pain were identified from the Osteoarthritis Initiative. Unilaterally painful (UP) limbs were defined as having numerical rating scale (NRS) ≥4/10, infrequent/frequent pain in the painful limb, while contralateral pain-free limbs were defined by NRS=0-1, no/infrequent pain and Western Ontario and McMaster Universities Arthritis Index (WOMAC) ≤1. The comparator group were matched bilaterally painful (BP) limbs. Maximum isometric muscle strength (N) was compared between 1:1 matched BP and UP limbs. Extensor strength was found to be lower in BP limbs than in UP limbs, (-2.9%; p=0.39) but this difference was not statistically significant. Extensor strength was significantly lower in the UP vs. contralateral pain-free limbs (-6.2%; p<0.001). No differences were observed between BP and contralateral painful limbs (0.6%; p=0.87). In conclusion, the current results identify a slight reduction of maximum knee extensor strength in a painful limb, when the contralateral knee is also painful. In contrast to pain-free limbs, this effect did not reach statistical significance, but the overall findings support the concept of central nervous inhibition of muscle strength by contralateral knee pain.