Methods
Nine healthy female students performed 5 double clicks on a computer mouse with 2 s intervals. Before and after a ramp contraction of index + middle finger extension up to 10 % MVC was performed. During these contractions MU activity was recorded with a quadripolar needle electrode (Dantec) inserted into m. extensor digitorum communis, EDC. MU recruitment pattern was analyzed by the Precision Decomposition Technique disregarding the norm value for firing probability inherent in this program. All inter firing intervals, IFI, below 20 ms were defined as double discharges, also termed doublets. The concentric MU action potential was used for MU properties: MUAP amplitude, duration, and area. The computer mouse was instrumented with a custom build trigger for synchronizing mouse clicks with MU recordings.

Results
A total of 38 double clicks on the computer mouse were recorded from the 8 of the subjects. In one subject no MU was identified during double clicking but only during ramp contractions; this subject was omitted here. In 5 subjects MU activity was found only in close connection to the lifting phases of the index finger after pressing the keys. However, in 3 subjects also a large number of the MU showed activity in the 2 s periods between the double clicks performed. In 28 double clicks at least one of the recorded MU showed doublets. In 30 of a total of 43 MU - corresponding to approx. 70 % - one or more doublets were identified. Doublets were registered in double click recordings for all 8 subjects but not during ramp contractions. The majority of the 72 identified doublets were found in the high acceleration phases of lifting the index finger between the two mouse clicks and the remaining after the second mouse click. The IFI for the doublets ranged from 1 to 20 ms with a mean of 11ms. The properties of MU recruited during ramp contractions and double clicking, respectively, showed similar mean values for MUAP amplitude, duration, and area: .642 mV, 4.3 ms, and .430 mV*ms during double clicking and .860 mV, 4.4 ms, and .660 mV*ms during ramp contraction.

Conclusions
Low threshold MU seem to be recruited during computer mouse clicking based on the MU properties identified. These MU are generally considered to have slow twitch characteristics. The large number of doublets identified in this study may be due to the mechanical requirements of high rate of force development or shortening velocity, that can only be fulfilled by these MU implementing special motor control patterns, e.g. double discharges. To which extend such firing pattern may predispose for the development of muscle disorders remains to be elucidated.