Most of us grew up hearing that we should warm up with a stretch. Strike and hold a pose, such as touching your toes, for 30 seconds or more, we were told, and you'll be looser, stronger and injury-proof.

But anyone who follows fitness science - or this column - knows that in recent years a variety of experiments have undermined that idea. Instead, researchers have discovered, this so-called static stretching can lessen jumpers' heights and sprinters' speeds, without substantially reducing people's chances of hurting themselves.

Now, two new studies are giving us additional reasons not to stretch.

One, a study being published this month in The Journal of Strength and Conditioning Research, concluded that if you stretch before you lift weights, you may find yourself feeling weaker and wobblier than you expect during your workout. Those findings join those of another new study from Croatia, a bogglingly comprehensive re-analysis of data from earlier experiments that was published in The Scandinavian Journal of Medicine and Science in Sports. Together, the studies augment a growing scientific consensus that pre-exercise stretching is generally unnecessary and likely counterproductive.

Many issues related to exercise and stretching have remained unresolved. In particular, it is unclear to what extent, precisely, subsequent workouts are changed when you stretch beforehand, as well as whether all types of physical activity are similarly affected.

For the more wide-ranging of the new studies, and to partially fill that knowledge gap, researchers at the University of Zagreb began combing through hundreds of earlier experiments in which volunteers stretched and then jumped, dunked, sprinted, lifted or otherwise had their muscular strength and power tested. For their purposes, the Croatian researchers wanted studies that used only static stretching as an exclusive warm-up; they excluded past experiments in which people stretched but also jogged or otherwise actively warmed up before their exercise session.

The scientists wound up with 104 past studies that met their criteria. Then they amalgamated those studies' results and, using sophisticated statistical calculations, determined just how much stretching impeded subsequent performance.

The numbers, especially for competitive athletes, are sobering. According to their calculations, static stretching reduces strength in the stretched muscles by almost 5.5 percent, with the impact increasing in people who hold individual stretches for 90 seconds or more. While the effect is reduced somewhat when people's stretches last less than 45 seconds, stretched muscles are, in general, substantially less strong.

They also are less powerful, with power being a measure of the muscle's ability to produce force during contractions, according to Goran Markovic, a professor of kinesiology at the University of Zagreb and the study's senior author. In Dr. Markovic and his colleagues' re-analysis of past data, they determined that muscle power generally falls by about 2 percent after stretching.

And as a result, they found, explosive muscular performance also drops off significantly, by as much as 2.8 percent. That means that someone trying to burst from the starting blocks, blast out a ballistic first tennis serve, clean and jerk a laden barbell, block a basketball shot, or even tick off a fleet opening mile in a marathon will be ill served by stretching first. Their performance after warming up with stretching is likely to be worse than if they hadn't warmed up at all.

A similar conclusion was reached by the authors of the other new study, in which young, fit men performed standard squats with barbells after either first stretching or not. The volunteers could manage 8.3 percent less weight after the static stretching. But even more interesting, they also reported that they felt less stable and more unbalanced after the stretching than when they didn't stretch.

Just why stretching hampers performance is not fully understood, although the authors of both of the new studies write that they suspect the problem is in part that stretching does exactly what we expect it to do. It loosens muscles and their accompanying tendons. But in the process, it makes them less able to store energy and spring into action, like lax elastic waistbands in old shorts, which I'm certain have added significantly to the pokiness of some of my past race times by requiring me manually to hold up the garment.

Of course, the new studies' findings primarily apply to people participating in events that require strength and explosive power, more so than endurance. But "some research speaks in favor" of static stretching impairing performance in distance running and cycling, Dr. Markovic said.

More fundamentally, the results underscore the importance of not prepping for exercise by stretching, he said. "We can now say for sure that static stretching alone is not recommended as an appropriate form of warm-up," he said. "A warm-up should improve performance," he pointed out, not worsen it.

A better choice, he continued, is to warm-up dynamically, by moving the muscles that will be called upon in your workout. Jumping jacks and toy-soldier-like high leg kicks, for instance, prepare muscles for additional exercise better than stretching. As an unscientific side benefit, they can also be fun.