Doing a dynamic warmup of exercises like leg swings increases quadriceps strength and hamstring flexibility, whereas a warmup of static stretching doesn't produce those gains, according to a study published in the Journal of Strength and Conditioning Research.
Researchers at the University of North Carolina measured subjects' leg strength, power, flexibility and vertical leap, then had them do one of three routines before restesting those measures. Some people did a dynamic warmup, some did static stretching and some did nothing. When retested, those who did the dynamic warmup scored significantly better on quadriceps strength and hamstring flexibility than they had in the pre-warmup test. Those who did static stretching or nothing between the two rounds of tests neither improved nor lessened their strength and flexibility.
This study supports what elite runners and coaches are increasingly doing--using a dynamic warmup to increase muscle activity before hard workouts. Aswe reported earlier this month, some studies find that the traditional pre-run static stretching decreases performance by lowering muscles' ability to contract rapidly and powerfully. Static stretching is best done after a workout as a means of lengthening muscle fibers.