ElectraStim Sensavox EM140

It starts as a faint tingle and climbs, one notch at a time, into a deep pulsing throb you feel from the inside out. Because each of the two channels has its own dial, you and a partner can sit at completely different intensities off the same box - or plug in the microphone or your music and let the current pulse in time with the beat, where the louder it gets, the harder it hits.

Ninety-nine intensity levels per channel are spread over more than four full rotations of the digital dials, so finding your level is a gradual climb rather than a jolt. Nine pre-programmed patterns can each be modified for speed and pulse width, a boost button adds a short 25% surge, and the unit always restarts at zero when you switch on or change programs.

The truly isolated dual outputs run up to two bi-polar or four uni-polar electrodes at once; electrodes are sold separately, and anything with 2mm pin connectors will fit. It works from the included worldwide mains adapter or a 9V battery, with a power-saving mode that dims the LED display, and it travels in a hard carry case with the connecting wires, microphone, line-in cable and adapter.

Keep the unit dry and wipe the control panel clean rather than washing it, and never place electrodes on the nipples or anywhere across the chest. It carries a one-year warranty, extendable to three years when you register the purchase with ElectraStim.