Try this as the code:

// last measured value
var measured = msg.payload;

// calculate average (as comparison) with previous values
var alpha = 0.05; // how much weight do we give the last measured value in our average?
var avgPrev = flow.get('avgPrev')||0;
var avgNow = alpha*measured + (1-alpha)*avgPrev;
flow.set('avgPrev', avgNow);


// compare measured value to average
var sens = 0.2; // sensitivity (try 0.2 for motion, 0.02 for pulse)
if (measured-avgNow>sens*avgNow) { // more than 20% larger? defines threshold event
var send = { payload : "Step! Threshold was crossed"}; // output value from counter
return send;
}