When sports season gets into full swing, many young athletes set new goals and push their bodies to the limit. Many of these sports require a great deal of flexibility and strength.

But it’s important for young athletes, coaches, and parents to understand that those attributes can’t be acquired overnight, says athletic trainer Jason Cruickshank, ATC, CSCS.

“It usually takes a good six to eight weeks of training to see any kind of change in a muscle, whether it be lengthening muscle through stretching or making it stronger through strength training,” Mr. Cruickshank says.

During the first six to eight weeks of training, the muscles “learn” how to stretch under a load, and begin to retain their shape.

“A good rule of thumb is that anything that is gained quickly is not going to stay,” he says. So consistency with a slow and steady regimen is the way to go.

Take a break when injury occurs

When muscles are overloaded by being forced into a stretch or into a range of motion that they’re not ready for, a muscle strain injury is the likely result, Mr. Cruickshank says.

If you put more stress on strained muscles repeatedly over time, the injuries can become far more serious.

“Think of a muscle as similar to a spooled cable. When injury occurs, you’ve broken some of those fibers that are inside the cable. This is what causes swelling, and muscle aches and pains,” Mr. Cruickshank says.

Do the right kind of stretching

For athletes who want to increase their strength and flexibility in a safe manner, it’s important to know what to do and what not to do.

Bouncing or ballistic stretching is not recommended, Mr. Cruickshank says. This kind of stretching actually inhibits the muscle from performing better and can cause microscopic injuries or worse.

Static stretching, which involves holding a position, is best for increasing flexibility long-term, Mr. Cruickshank says. However, static stretching is not a great performance stretch.

For most athletes, dynamic stretching is a good option.

“Dynamic stretches like butt kickers, lunges, or side-steps, where the leg is going out as far as it can, take the muscles through natural ranges of motion with activity. This increases the blood flow, and recruits all the muscle fibers to work together,” Mr. Cruickshank says.

Not every athlete’s body is the same. So what works for one person might not work for another, Mr. Cruickshank says. Athletes should listen to their bodies and never push themselves to the point where they feel pain.