In this chapter we're going to talk about flexibility and improving your flexibility through stretching. After this lesson, you should better understand the factors that affect flexibility, know how we measure flexibility, and also be able to apply the FITT formula just like we did for cardiorespiratory fitness and muscular fitness, but in this case to apply that formula to a stretching program to increase your flexibility.
Flexibility is one of the components of health-related physical fitness, and by flexibility we simply mean the ability to move your joint or a group of joints through their complete range of motion, and importantly, flexibility is very joint-specific, so the flexibility you have in your knee is virtually unrelated to the flexibility that you have in your shoulder joint, and so when we talk about flexibility, we're going to talk about flexibility at a particular joint, or when we talk about measuring flexibility at a particular joint.
So why do we care about flexibility? Well flexibility is helpful in performing many different activities of daily living. Think about all of the bending and reaching and stretching you do throughout your day. All of those things require some form of flexibility. Flexibility may also help to reduce the risk of injury and can improve performance, whether we're talking about physical performance in your physical activity routines or sport performance, certainly, as well.
Over the next few slides we'll talk about three main factors that affect the flexibility of our joints. The first of these is joint structure, or the way that the joint is built and designed, and that structure really determines the direction of movement, and also partially determines the range of movement. So, for example, you see two different joints listed here. The first is a hinge joint, and the hinge joint only allows movement in one plane. So examples of this are your knee and your elbow. When you compare this to the ball-and-socket joint, the ball-and-socket joint has rotation in almost all joint angles. So examples of this are hip and our shoulder, and if you imagine all the different movements that your hip and shoulder can make compared to the movements that your elbow and your knee could make, there's obviously a much wider range of motion in the hip and shoulder. And all of these factors in the type of joint affects the flexibility and the range of motion that you'll have at each joint, and this is part of the reason that we talk about flexibility being very joint-specific.
The soft tissues in our body also influence our flexibility or range of motion, and these tissues can include muscles, tendons, fat, among many others. Our muscles are a very key part to flexibility and our range of motion. Simply moving our muscles through their range of motion by being physically active generally enhances flexibility. The temperature of our muscle also affects flexibility, so if you think about when people tell you not to stretch cold muscles or not to stretch immediately, to warm up maybe before you stretch, this is why, because warm muscles are more elastic and thus more flexible and easier to stretch as well, and we'll talk about that later. The elasticity and the compliance of our muscles decreases with age, and so this is part of the reason that we see people become less flexible as they age, but as I said earlier, remember that some of those age-related declines can be attenuated with being physically active, so you can lessen some of those age-related declines of flexibility by maintaining a physical activity routine throughout your lifetime.
The third factor that influences flexibility is nervous system activity, or the signals that are sent to your central nervous system by proprioceptors in the muscles and in the tendons that attach muscles to bones. The stretch receptors, also called muscle spindles, are in the thick belly of the muscle and are very sensitive to quick and forceful stretching of the muscle. If this quick and forceful stretching occurs, as a protective mechanism to prevent overstretching, the spindles will send a signal to cause muscle contraction. So in order to prevent overstretching in the muscle, the muscle spindles cause that muscle contraction.
The reflexive muscle contraction that we just talked about on the previous slide is called the stretch reflex. The Golgi tendon organs, located in the tendons and not the muscles here, but the tendons, help to prevent excessive strain in the muscle and to protect the muscle from injury in the case of rapid and forceful contractions, and these Golgi tendons do this by causing relaxation of the muscle. So to summarize, the three main factors that affect flexibility are joint structure, soft tissues and nervous system activity. But these aren't the only things that affect physical activity. Some other factors that can influence your flexibility include your age; so as we age, again, we talked about losing some of that compliance and elasticity in your muscles, which generally reduces your flexibility as you age. Gender can also affect flexibility; women are general more flexible than men. And finally, as we talked about before, your level of physical activity; the more physical activity you do, the more likely it is that you're moving your joints through their range of motion, and generally that can enhance your flexibility.
There are multiple different ways to assess flexibility, but regardless of the measure that we use or the method that we use to assess flexibility, all of these tests are specific to the joint of interest. So just like flexibility is joint-specific, as we talked about throughout the lecture, obviously if something is joint-specific we want to measure it by a particular joint. And so you could be really flexible, let's say, in your hip joint, but not very flexible in your shoulder joint, and so it's entirely joint-specific. And there are really two main ways that we can assess flexibility. The first are established tests, and one example of this might be the sit and reach test, a pretty common test, and you can see the young girl in the top picture here sitting on the ground, legs extended straight out in front of her, and she's reaching forward onto a box that's measuring how far she can reach towards her toes or past her toes in this case. The second way we can measure flexibility is through a goniometer, and here you can see on the bottom, a goniometer is a very simple ruler, protractor-type device that is anchored on a fulcrum, and it can really just measure the joint angle of any joint that you want to measure. So this can be used across the body in all your different joints to assess your flexibility.
When we think about designing a program to improve flexibility, there are different types of stretching exercises that you can perform. The first are passive stretching exercises, and here this is a natural stretch, also could be called relaxed stretching, where you're not applying any additional force to create that stretch, so you're not trying to move the muscle beyond its normal range of motion. So an example here would be bending over from a standing position to touch your toes and just hanging in that position, nice relaxed position to stretch your hamstrings. If you compare that to active stretching, active stretching involves taking a muscle beyond its normal range of motion by having some sort of assistance. So in the example of stretching your hamstrings, this time you might sit on the floor, legs extended out in front of you, reaching towards your toes. That first part is a passive stretch, but then if you had a trained partner come behind you and press you forward to move you just past your normal range of motion—not until you have pain, but past your normal range of motion—that would become active stretching.
The next type of stretching is called static stretching, and this simply involves holding the muscles in a stretched position, lengthening out your muscles, which habituates the stretch receptors in your muscles to that stretched out, or lengthened, position. When you do this, over time you reduce the signaling of those stretch receptors, and if you remember, those stretch receptors, or the muscle spindles that are in your muscle, cause a reflexive contraction if they're pulled too forcefully. So this is why we like to stretch slowly and carefully, and eventually, if you reduce the signaling of these muscle spindles, you allow greater lengthening of the muscles. And so this is why both getting into the position slowly and also only having mild tension or mild discomfort, not pain, and holding that stretch, is very important.
A very different type of stretching from static stretching that's not often recommended due to the increased chance of injury, especially for people who are less physically active, is called ballistic or dynamic stretching, and here you're using momentum to move the joint past its normal range of motion. For example, leg swings, or as you see in the picture, you see this young woman doing high leg kicks. Again, she's moving her leg through its normal and past its normal range of motion by using momentum. Activities or ballistic stretching is often good for warming up the body for athletic movements, but you have to remember that if that stretch is too forceful or too rapid, this type of stretching could certainly activate the stretch receptors, or the muscle spindles, inside the muscle, and then that would really oppose the goal of the activity. So instead of lengthening of the muscle you would have contraction of the muscle.
The next type of stretching exercise is called proprioceptive neuromuscular facilitation, or PNF stretching, and here you're really taking advantage of the Golgi tendons that, remember that they cause muscle relaxation when they detect rapid and forceful contraction, and you're using them to help increase flexibility. And so the way this method would work—we'll use the hamstrings as an example as they're pictured here—you have two people, one person lying on the ground that's being stretched, and then you have a partner that has the person's left leg up on their shoulder, and they're really helping their partner on the ground to do this stretching exercise. You can also think of PNF stretching as kind of a hold-and-relax method. So here the hamstring is originally stretched for about 10 seconds, and the partner helps the person on the ground to do that, and then the person on the ground does a max isometric, meaning their leg is not going to move, they're going to press against their partner for about 6 seconds. And again, that partner's job is to make sure that the leg is not moving, so we're not having an isotonic contraction, but we're just having an isometric contraction where the hamstrings are contracting and the partner is providing resistance. Again, you do that for about 6 seconds, then the person on the ground that's being stretched relaxes their hamstrings and the partner slowly pushes them a little further for about 30 seconds into a hamstring stretch again, and this time, the leg should go a little bit further, and you see a little increase in flexibility and range of motion of the hamstring muscle. So really the key here is that PNF stretching is taking advantage of some of those reflexive responses in your tendons to allow for greater muscle flexibility and range of motion at the joints.
So now that you're familiar with the different types of stretching exercises available, we'll apply the FITT formula to design a stretching program that works for you to improve flexibility. The first category is frequency, or how often should you stretch. It's recommended that 2-3 days per week you stretch all of the major muscle groups, and you really can do this up to 3-5 days a week if you want to see greater progress and greater benefits.
For the intensity of stretching, you really want to make sure that you're not causing pain while you're stretching. Slight tension and mild discomfort are okay, but pain should never be felt while you're stretching. It's also important that for relaxation purposes, to maintain a relaxed state in your muscle, and in your overall body, that you continue to breathe throughout the stretch and that you're not holding your breath while you're trying to stretch your muscles.
Stretching can often take quite a bit of time, as static stretches should be held for 15-60 seconds. And again, we talked about wanting to hold that stretch so that eventually you habituate your muscles and the spindles in your muscles to send fewer signals, and so the length of time is important here. We also talked about PNF stretching, where you should use that 6-second contraction, followed by a 10-30 second assisted stretch after the contraction when the person is relaxing their muscle again. And so if you think about spending this amount of time for each joint angle, maybe multiple sets at each joint angle, and trying to do your entire body, stretching is something that you definitely have to make time for in your exercise routine.
The type of stretching, we've talked about active and passive, static, dynamic, and PNF stretching, but also available to you are activities such as yoga and tai chi, which are heavily focused on improving flexibility, and also have some other goals, as you see here on the slide, but those are great activities that can also help you to build your flexibility. And so I'll leave you with this thought: just remember that flexibility declines as we age, but maintaining an active lifestyle can reduce some of the aging-related declines of flexibility. So whether you choose to do yoga, tai chi, or simply static stretching on your own, flexibility is such an important key component of your health-related fitness to focus on.
