Consistency of motor-unit identification during force-varying static contractions

Due to inter-operator variability, two operators were used to assess the co nsistency of motor unit (MU) identification during ramp contractions, by th e comparison of semi-automatic decompositions of the same recordings. Stati c shoulder abduction was performed against a force transducer in a position with the upper arms vertical and elbows flexed to 90 degrees. The subjects followed an 8-s force trajectory: 30% maximum voluntary contraction (MVC, 2 s), a reduction in force from 30% to 0% MVC (2 s), 0% MVC(1 s), an increa se in force from 0 to 30% MVC (2 s), and 30% MVC(I s). Muscle activity was recorded from the supraspinatus muscle with a quadripolar needle. From six recordings of 8 s duration, a total of 2527 MU firings were identified by b oth operators, and 93% of these were identified identically into 31 MUs. Bo th operators identified 8 of these MUs as continuously firing, 5 as only br ing active either before or after the 1 s at 0% MVC, and 18 as being de-rec ruited during force decreases and recruited during force increases. Both op erators agreed that 16 of these 18 MUs were de-recruited at a higher force level than that at which they were recruited, which may be due to the elect romechanical delay. The coefficient of variation for double determination o f the results obtained by operators A and B was 8.5% for the number of MU f irings, 4.5% for the MU mean firing rate, and 8.4% for the MU action potent ial (MUAP) amplitude. Therefore, the operator interactive decomposition met hod was considered to be valid for studying recruitment and de-recruitment as well as firing rate and MUAP amplitude during static, force-varying ramp contractions.