Changes in motor unit recruitment strategy during pain alters force direction

Abstract: Motor unit (MU) recruitment is altered (decreased discharge rate and cessation of discharge in some units, and recruitment of new units) in force-matched contractions during pain compared to contractions performed before pain. As MU’s within a motoneurone pool have different force direction properties we hypothesised that altered MU recruitment during experimental knee pain would change the force vector (total force (F _T): amplitude and angle) generated by the quadriceps. Force was produced at two levels during 1×60-s and 3×10-s isometric contractions of knee extensors, and recorded by two force transducers at right angles. This enabled calculation of both F _E (extension force) and F _T. MU recruitment was recorded from the medial and lateral vastii with four fine-wire electrodes. Pain was induced by hypertonic saline injection in the infra-patella fat pad. Nine subjects matched F _E and six subjects also matched both medial and lateral forces (F _T) before and during pain. Changes in MU discharge pattern (decreased discharge rate (P <0.001), complete cessation of firing, and recruitment of new units) during pain were associated with a ∼5° change in absolute force angle. As force angle changed in both directions (left/right) for individual subjects with pain there was no change in average F _T amplitude between conditions. When both medial and lateral forces were matched MU discharge rate decreased (P <0.001) with pain, but, fewer units ceased firing or were newly recruited during pain. Change in motoneurone recruitment during pain alters direction of muscle force. This may be a strategy to avoid pain or protect the painful part. [ABSTRACT FROM AUTHOR]