He'd always thought it was an expression instead of an involuntary response.

I couldn't even believe I had stopped; it was an involuntary response.

An odd involuntary response, but there you have it.

An involuntary response caused him to jerk back from it.

The indignation was twice as heavy to make up for the first involuntary response.

His mouth was hard and hot, burning her lips with an intensity of desire that brought an involuntary response.

What you're trying to do is provoke an involuntary response.

All these are normal, involuntary physical responses to internal emotions.

He could see the pilot looking down, one hand against his helmet in an involuntary response to a radio call.

Deliberately, he spoke her name, and watched her involuntary response.