I should have known you used simi by the UK flag. You don't see that system too much here in the states. Do you have the simi hardware or just the software? I've only used simi once and I know the hardware setup had an A/D converter where you could plug in analog signals like EMG. If you don't have the simi hardware you'll need an A/D converter and some interface software to time sync the signals. The simi motion capture software is probably capable of this interface (I'm not sure) but if not you could use Labview or even write a macro in Visual Basic.