Understanding muscle physiology and biomechanics can make a world of difference to a person training but also to their recovery when injured. These concepts apply to any muscles but with the prevalence of hamstring strains being one of the highest this might hit home with more people. Since they’re fairly large ideas we’ll start with my favorite concept the length tension relationship.
Length Tension curve
This should ring a bell with anyone who has studied exercise physiology. The concept explain how and why a muscle can produce different amounts of force as it progresses from long to short and short to long. Your fingers can be a great example of how the many small fibers of any muscle interact; a key foundation of the concept. Interlock your fingers (like the photos below) and imaging thousands of tiny hooks on the fingers of one hand that will connect with thousands of hooks on the other hand. The greater the area of contact between fingers then greater the number of hooks that connect and give you the greatest strength. This follows a simple pattern: as you slide your fingers together the area of contact increases to a peak (middle image) then slowly decreases as your fingers overlap too far and slide behind your hand (third image). The curve below shows how that area of contact directly affects that muscle’s strength. I’ve added dotted lines to show the area of overlap, notice how that area of overlap directly relates to the strength curve below.
So now apply that concept to your hamstrings but this time we’ll run the concept in reverse order, short to long. Imagine lying flat on your back and think about the length of your hamstrings. In that position they are pretty short. Probably close to that fully shortened position from the pictures above (on the right). Now imagine one leg being lifted straight in the air until your foot is directly above your hips. As your leg moved from the floor to straight up in the air it passed from near fully shortened (too much muscle fiber overlap), through mid length (optimal overlap and max strength) to near full length (minimal fiber overlap and weakness). The key is that you are not equally strong at all lengths. You have the best strength and lowest likelihood of injury in that middle 50% of the movement.
So here is the real magic. Hopefully this turns a light bulb on. Let’s get back to tight hamstrings. If yours are tight you have probably realized that you can’t actually lift your foot all the way above your hips. If you could you’d have a fairly large range of that motion where you are acceptably strong (highlighted in green). Instead, inflexible one, you have a very narrow window (shown on the right image in green) where you are usefully strong surrounded by two equally large ranges where you are weak and susceptible to injury.
Improving your length and strength
When you get right down to performance or recovery you want a larger range of motion through which you have useful strength. Adding length to a muscle through rigorous stretching will give you greater range but all you’ve really done is spaced out those hooks. You’ll also introduced new ranges of motion that your nervous system hasn’t experienced so you can expect to be fairly uncoordinated. Proper improvements come from slowly increasing the length of a muscle while performing strengthening exercises that progressively challenge that new range of motion. This will encourage your muscle to build more hooks and train your nervous system to control this new found range.