The briefest summary of how a strain occurs is when the amount of force put on a muscle is greater than the ability of the muscle to generate an opposing force then the muscle will strain. This most often occurs during high speed movement; however, it can occur during sustained contractions.

This means that strains have a higher frequency of occurring:

in people with previous strains because of existing muscle weakness
after static stretching muscles because of the inhibition of muscles spindles which control the stretch-shorten cycle by regulating muscle length
in older populations because the muscles become less pliable as age increases
in muscles with poor flexibility and mobility because the muscle cannot elongate very far without straining especially as a person fatigues
near the end of workouts because muscles have less ability to maintain adequate force output to prevent straining as fatigue increases
in weak people because weak muscles are easier to strain
with impingement where motor nerve output is decreased for whatever reason thus leading to decreased force output of the muscle and increased likelihood to strain
Now you may have heard that muscle damage, especially factors related to the damage such as delayed onset muscle soreness, is caused by eccentric muscle contraction. This is true. The same thing occurs with strains. They always occur during the eccentric contraction of the muscle.

Even in cases where a strain occurs during a ‘concentric’ contraction are actually during an eccentric right at the time when the concentric motion is switching to the eccentric contraction. For instance, if you are one who tends to arch/crane your neck by looking up instead of keeping head neutral during deadlifts, squats, and pullups, the impingement of the nerves that come out from the cervical plexus may inhibit force output during the movement. Thus, as the movements move into the eccentric phase, the body may not have enough muscular force to exert to slow the movement down and the muscles may strain. This is one of the major reasons for correctly executed these techniques as well as the fact that craning the neck often gives people pain and tight muscles as well as headaches.

Therefore, knowing what we know now it is important that if you are getting older, having poor technique, poor flexibility and mobility, tight muscles, or are weak and do intense workouts you may be more susceptible to strains.

Obviously, if you have previous medical issues or strains you will know that you need to be proactive. If you are doing intense workouts at high speeds, save the static stretching for afterwards unless you have flexibility detriments that need to be addressed prior to workouts to ensure safety during the workout. You can find more about the the when and why of static stretching in this previous published article.

Common areas that strain

As we discussed strains always occur during eccentric loading of the muscles.

This means that during sprinting in the hamstrings they will tend to occur during the phase where the foot is starting to receive the ground as that is where the muscle is at its longest. Groin strains (at least adductor magnus) tend to be similar. The other half of groin strains – the hip flexor strains, adductors longus and brevis and pectineus – occur on the opposite side of the stride where the foot is starting to leave the ground to come forward.

In kicking sports strains will tend to occur after kicking the ball extremely hard as the leg travels up and forward in front of you.

Strains during lifting such as the back tend to occur when technique breaks down and the muscles lose their isometric contraction such as during squats and deadlifts when the back starts rounding. Likewise, in the upper body they will occur on the descent of pullups, the descent of the dips, and lowering the weight half heartedly or jerky.

This is why it is important to keep strict form especially when you are getting fatigued because that is when they are most likely to occur.