Muscle injury occurs when the muscle fibers are stretched too far, too fast. The strain or tear usually starts at the weakest part of the contractile unit. And the injury is most likely to occur during an eccentric contraction.
Eccentric contraction means the muscle is shortened or already contracted and is now lengthening. An example of this is the biceps muscle in the upper arm. Making a fist and bending your elbow as much as possible is a concentric contraction of the biceps muscle. Now, as you lower your hand and straighten the elbow, the biceps muscle is contracting eccentrically (lengthening or stretching out).
The weakest point of the muscle (where a strain is most likely to occur) is at the myotendinous junction. This is the transition zone between the muscle fibers and the tendon that attaches the muscle to the bone. The muscle is soft and pliable. The tendon is more like tough connective tissue and less resistant to sudden force.
Some muscles are more prone to injury because of the location or fiber type. For example, muscles that attach across two different joints are under increased force from different joint angles and movement. The hamstrings behind the thigh cross the hip and knee. The gastrocnemius (calf) muscle crosses the knee and ankle.These are two of the most commonly injured two-joint muscles.
Athletes with muscle strength imbalances appear to be at increased risk of a primary (first) muscle strain. That’s one reason why a preseason screening program is advised for all competitive athletes. Training errors and poor biomechanics are two other possible risk factors for muscle imbalances that can lead to a muscle strain.
For older adults who are active in sports or other physical activities, muscle atrophy and loss of tendon flexibility (contractility) are major risk factors. There’s some evidence that stretching before physical activity may help prevent muscle strains. But this is currently a controversial area with some studies showing no preventive effect of stretching before exercising.
It makes sense that warm muscles are more flexible and possibly less prone to strain injury. Warm-up activities and stretching are still advised by some as part of strain injury prevention. Until further studies can clear up confusion around this subject, it can’t hurt to include warm-ups in any training program and it may help.