Most of us have probably learned that stretching prior to a workout is critical. While very true, it’s actually the type of stretching that’s even more important. There is static stretching (holding a stretch for 20 to 30 seconds) and then there is dynamic stretching (using momentum to propel the muscle in a certain range of motion). Static stretching was the recommended form of stretching for years, but recent research has shown that athletes generate less force from their muscles after static stretching when compared to not stretching at all! In addition, other studies have found a 30% decrease in muscle strength during exercise, after extensive static stretching.
The reason for this is because static stretching results in a neuromuscular inhibitory response. Basically the muscle becomes less responsive, for up to 30 minutes after static stretching, making it weaker. While on the other hand, dynamic stretching actually increases power, flexibility, and range of motion without inhibiting the muscular response.
A proper warm-up will loosen your muscles through range of motion exercises, while also increasing body heat and blood flow to warm up the body in preparation for your workout. This can include warm-up light jogs, light calisthenics, or brisk walking. A warm-up should not be done too early (before you start your exercise) nor should it be done at too intense of a level.
This is not to say that static stretching is harmful or unnecessary. The best stretching regimen includes a majority of dynamic movements prior to exercise, and then incorporating some static stretches after your workout. Research shows that it is beneficial to warm up large muscle groups via dynamic stretching prior to beginning a workout, and then follow up with static stretches to increase flexibility. The following article explains more on the topic: