Most of the time we do not use our hamstrings very much. The action of these muscles is hip extension and knee flexion. To facilitate significant recruitment of these muscles in normal life you either need to pick-up fairly heavy things (for hip extension) or run forward or climb something (for knee flexion). These are things that we do not do very often in modern live and in all likelihood the most work you do for your hamstrings is getting into and out of the car followed by getting in and out of bed. Compared to the rest of the lower body the hamstrings get almost no work. This leads to the potential for problems.
The two main issues that come out of this are muscle strength imbalances and muscle recruitment deficiencies.
When you have muscles that are imbalanced, the antagonist muscle group is able to contract faster and with more force; in this case the quads for knee extension. If the force is too great the hamstrings can get pulled or torn. The knee and hip joints may also suffer alignment or tracking problems; given that antagonist muscle pairings do offer structural support to the joints making them more stable.
When you have recruitment deficiencies you lack complete control over a muscle. Consciously you may not be able to contract it at will, or with very little force. Unconsciously when you move, the fibers will not contract is efficiently as they could. For example, 50% of the fibers may fire when 35% are needed or 80% fire when 95% of them are needed.
Unless you live with knee or hip pain caused by weak hamstrings you are not likely to notice anything until that rare occasion that you have a very sudden movement that is well outside the realm of normal. For example a car crash were you need to undo your seat belt and crawl you way out of a flipped vehicle or when you suddenly run very fast and your hamstrings are too weak to slow knee extension. New repetitive movements may aggravate patella tracking issues that are caused by hamstring weakness.
Considerations when training hamstrings muscles:
They are primarily fast twitch fibers so they need to be trained quickly on the concentric phase of a movement and the rep ranges should be no more than 10.
You recruit more fibers when you put a muscle on length. A great example of this is allowing your hips to drift back before you start to go down with squatting or when you start to lower the weight with dead lifting. Doing is effectively tilts your hips forward relative to the legs which stretches the hamstrings.
Hamstring muscles adapt very quickly so you need to perform a variety of different movements to train them effectively. Lying, standing, one leg, and Swiss ball curls for knee flexion. Romanian dead lifts, dead lifts, glut/ham raises, reverse hyper extension for hip extension.
Recruit these muscles when you are training your quads by deliberately trying to contract them – by driving the force through the heels with leg press, you transfer some of the effort onto the hamstrings.
If your hamstrings are weak, or if you believe they are, take the time to give them extra attention on leg day. They will catch up quickly if you focus on it and this will improve performance and decrease the risk of injury.