A bit of a stretch.

There is some evidence that a bit of a stretch is as effective as one held three times as long, provided you repeat to achieve equivalent overall stretching time. Those of us who shy away from the discomfort of stretching may hold a stretch for as little as 10 seconds and then duplicate it, instead of holding each for at least 30 seconds, as previously thought.

Researchers randomly assigned subjects into one of two hamstring stretching groups: a 10-second stretch duration, and a 30-second stretch duration. The 10-second stretch was repeated six times for a total of one minute; the 30-second protocol was repeated twice for a total of one minute. Stretching was performed twice daily, for a total of two minutes each day, for six weeks. Gains in hip flexion and range of motion were equally significant between groups. The key to improvement, then, may be total daily stretch time, and not the duration of each stretch.

Remember that stretching in smaller intervals does not mean "bouncing." Bouncing can trigger the muscle spindle reflex, causing a muscle to contract when it senses rapid changes in tension. This is not only the opposite of the goal of lengthening the muscle, it can lead to tearing. But holding for 10 seconds then repeating several times allows you to reset the stretch, going deeper and deeper with each pass. As long as your total stretching time equals what you'd be doing in longer bouts, taking shorter stretches appears to remain beneficial to muscles.

(J. Strength & Cond. Res., 2003, Vol. 17, No. 2, pp. 274-278)