Understanding Stretching and How it Impacts the Development of Flexibility
In a previous post, we introduced mobility and how it’s not just a passive process but an active one - and it requires both flexibility AND strength (read that post here). I've studied joint mobility and flexibility considerably. From research articles, seminars, workshops to practical experience. Based on my studies, I am convinced that we can ALL improve joint function, flexibility and active range of motion (ROM). This may come as a surprise to many (often I hear coaches and players saying that "they've never been flexible" or that they were "born stiff"). The truth is, like any training quality, achieving more active and passive ROM is simply a matter of deliberately providing the desired tissues with a training stimulus, allowing time for recovery/adaptation and repeating this process.
As the second post on this topic, we will primarily focus on flexibility (a component of mobility). That includes the science of stretching, whether short-term & long-term stretching can decrease injury risk and an example of how to attain more range of motion WHILE AT THE SAME TIME gaining strength in that range.
If you haven’t read the previous post on this topic, I advise you to do so as it will help you better understand the info presented here.
Let’s start by getting a few things clear. Stretching is a form of flexibility training. There are different ways to stretch - this includes dynamic stretching, static stretching, ballistic stretching. These could be further subdivided into either active or passive components. What does that mean exactly? Let’s use static stretching as an example - as this will be the focus of the post. If I hold a stretch for a certain length of time and that stretch is being assisted by gravity or some sort of external load (Image 1), that would be a passive-static stretch. In this example, that means I’m not actively contracting the stretched area. The opposite would be an active process whereby you’d contract anything and everything to get that leg back as far as possible (Image 2).
Ok, so many of us are familiar with static stretching. The question remains, why do we do this form of stretching and does our desired goal line up with the ACTUAL outcomes? To answer this question we must first understand 2 concepts.
Concept 1 - Viscoelasticity of Muscle Tissue
Muscle, along with fascia, tendon, ligament, 80% of nerve, bone, capsule etc, are ALL made up of connective tissue (Spina 2017). This means that when you stretch, all of these tissues are affected. According to research (Magnusson 1996), muscle is both viscous and elastic (i.e. viscoelastic). Viscous meaning that when a force is applied it can permanently change shape and elastic meaning that when we apply a force (& change it’s shape), then get rid of that force, it regains it’s original shape (like an elastic band of some sort). That brings us to the topic of flexibility training - if I hold a stretch for 30 seconds (or 60 or however long), do I actually lengthen the muscle? The answer seems to be no. This change in shape is transient - in other words, it lasts for a very short period of time. Researchers have validated this (Magnusson 2000) - after several weeks of regular stretching, subjects became MORE flexible, WITHOUT any change in the shape of the muscle. If nothing happens to the muscle, how do we increase flexibility? This brings us to our second concept, the stretch reflex.
Concept 2 - Flexibility and the Stretch Reflex
As Dr. Andreo Spina of Functional Anatomy Seminars (expert in mobility/flexibility training) put its, most people believe they can’t achieve a certain range of motion - (like the splits for example - because that’s as long as a muscle can go before ripping. This, however, is not true. Mechanoreceptors in muscle, called muscle spindles, monitor length changes. If you exceed a certain length your muscle isn’t used to, you activate the stretch reflex (i.e. the nervous system no longer allows you to go any further, so it sends a signal to contract the involved muscles, which effectively stops the stretch).
That means that when we practice a particular stretch for several weeks and find an increased ROM in that area, it’s our nervous system allowing us to go further. Because we’re continuously telling the nervous system, it’s ok, I won’t get injured in this range, the nervous system responds by giving us a bit more range. If I never tell my groin muscles to get into the splits, they won’t! But if I practice stretching my groin region and continuously work on increasing that range, the body says "ok, I’m not injured so I’ll let you go a little further". Until one day, you can do the splits (or at least have more range in that specific area).
However, the nervous system is quite smart. If you don’t have the strength in a particular range, it’ll be more reluctant to give you access to that range. You must provide the nervous system with a stimulus to access those ranges. This is the ACTIVE process of flexibility training. Before we get into the specifics of how to stretch to improve both range and strength, let’s talk about stretching for injury prevention.
Stretching for Injury Prevention
Many athletes stretch because they believe it will decrease the likelihood of injury. This, however, is a broad statement. Do they mean that static stretching before training or competition decreases injury? Or that stretching after training (or at other times) decreases injury? And what about the length of the stretch? Does it matter whether I stretch for 20 seconds or 5 minutes? This, along with what injury truly is, needs to be defined.
How Does Injury Occur?
According to Dr. Spina, injury is a result of the load being placed on a tissue exceeding the load bearing capacity of that tissue. Think of it this way, every time you serve, there are micro tears that occur at the cellular (tissue) level. The more you serve, the greater these tears become. If at some point, the tissues can no longer handle the amount of serving you’re doing (because they’re not resilient at these loads), injury will ensue. Here’s what the formula looks like:
Injury = Load > Capacity
Notice something? Injury doesn’t really have anything to do with the ROM of a particular tissue. It simply says don’t exceed what the tissues can handle, and you won’t get injured. So let’s say you have the ROM that’s required for a big tennis serve but you don’t have the strength in those ranges, what do you think will happen? Whether that’s tendinopathy, impingement, tears...I don’t know exactly, but the odds of an eventual injury are highly probable.
Stretching Before Training or Competition
Back to stretching for injury prevention. Studies have shown that pre-exercise static stretching may actually increase the chances of injury. Why? Well you’ve just temporarily increased the ROM of a particular joint or muscle group WITHOUT increasing strength in that range...and now you’re about to go play. Say you’re lunging out for a wide ball and you’ve given your tissues that extra range...but your groin isn’t strong in that range. What happens? Well the load is now greater than the capacity of the groin muscles...so perhaps you pull your groin. Makes sense doesn’t it?
Some more recent evidence (Behm 2015) suggests that static stretching before sport might decrease injury risk slightly. But what the authors failed to outline was whether these stretches were simply passive or active. But this could make sense if the sport requires an athlete to get into a particular range. For example, modern tennis often forces players to get into a splits position (not full but partial at least). Prior to playing, I may want to stretch the groin area (and inner thighs) in a way that not only increases the range, but activates all motor units in that range - basically I’m telling my nervous system that I do in fact have strength in that range, here it is. This will not only help me get into a better splits position but it’ll also allow me to express strength in that range...mitigating injury.
Stretching at Other Times
In terms of stretching at other times (not prior to training or competition), there seems to be no clear evidence pointing in favour of injury prevention. That said, in terms of increasing flexibility, chronic (i.e. stretching several - 3-5 days/wk or more - over weeks/months) does improve ROM. There is also no real evidence that suggests there is a change in muscle stiffness properties (in fact, Ichihashi, 2013, saw no change in these properties after long-term stretching). It probably has more to do with the stretch-reflex. In other words, your tolerance to stretch improves, hence, improved ROM.
Why don’t long-term stretching protocols decrease injury risk? We have to go back to our load/capacity equation. If we’ve increased ROM over a specific time period, but we haven’t increased strength in these ranges, then we haven’t improved these tissues ability to handle additional loads...loads that might occur during sport. There is a shred of poor evidence (Garrett 1989) that argues that long-term passive-static stretching improves the force absorbing quality of muscle through a decrease in muscle-tendon stiffness...but since being originally published in the late 80s, this evidence has not been substantiated.
Using Isometric Training to Increase Range
So how do we both increase ROM and increase the strength in those ranges? That’s where isometric contractions come into play. Remember isometrics? It’s when you contract a muscle (or muscle group) and there is no change in muscle length (i.e. no shortening or lengthening like with concentric/eccentric contractions). An example would be someone pushing against a wall. The wall doesn’t move but you’re creating tension in the body.
According to Dr. Spina, when it comes to increasing ROM, we must hold a stretch for around the 2 min mark. This can fluctuate based on the individual but there seems to be good evidence (Langevin 2005) that cells begin responding at around the 2 min mark. So I hold a stretch passively for 2 min. After that period, I contract isometrically in a particular direction for about 10-20 seconds. At this point I’m telling the nervous system, “look, I can produce force at this range and I’m not causing damage, give me some more range”. And what does the nervous system do? It grants your wish by providing more range when you relax into the stretch. Then, you perform another isometric contraction in the opposite direction, with the goal being to actively move deeper into the stretch (you may not actually move but that is the intent). You then relax again. You’ll now (temporarily) have access to more range. I say temporarily because to increase ROM and solidify those ranges, we must continuously stimulate the desired tissues. If I want to increase internal shoulder ROM, I have to stretch that area repeatedly over days, weeks, months etc. Look at the shoulder example below.
Why Does Isometric Training Work?
There are several ways isometric training can improve ROM. First, according to Dr. Spina, isometrics can help us override the stretch-reflex. This essentially convinces the nervous system we have active control over a particular range, so it grants us more range. Second, and more importantly in terms of gaining strength in a particular range, isometrics maximize motor unit activation (read more about that here). This is referred to as the size principle (the heavier the load, the more motor units are called upon - if the load is immovable, like in isometrics, we will activate all available motor units). Also, remember the force velocity curve, well that curve was related to concentric contractions, however; eccentric and isometric contractions can also be represented on a force and velocity graph (Figure 1).
As you can see, isometric contractions can produce more force than concentric contractions. One caveat. When it comes to isometrics, you’re only increasing strength at the specific angle of contraction! In other words, if I maximally contract my external rotators isometrically at an angle of 45 degrees, most of my strength adaptations will occur at that angle. I say most because there seems to be this band of adaptation that gives me strength within 15 degree intervals - so there is some adaptation outside that 45 deg...but you won’t gain strength at 90 deg!
Shoulder Internal Rotation and Tennis
We’ll get into other important joints for tennis in later articles but because we’ve been spending a lot of time talking about the shoulder on this blog, let’s continue with that topic. If you recall from an earlier post, tennis players with internal rotation deficits are more inclined to shoulder/elbow injuries than players with ‘normal’ range. If we go one step further, Ellenbecker found that the biggest risk factor is actually ACTIVE range in the internal rotators. So there we see it again, having range doesn’t mean much if it’s simply passive range. We must be able to produce force in that range to mitigate injury. Practically, I see SO MANY juniors with a 'loose' shoulder joint - in other words, they have lots of range (as much as twice the amount you see in the images below). But (and it's a big but), they have absolutely no strength in this range. While performing isometrics isn't the only type of strength required (they also need ballistic strength with the use of plyo balls, dynamic strength via strength training, specific strength via serving etc), it can help close the gap between active and passive strength and ROM in various key joints (shoulders, hips in tennis, for instance).
An Example - The Sleeper Stretch
Hopefully this article has cleared up some of the science of stretching. I strongly believe stretching is very important for the modern tennis player for a variety of reasons, including (but not limited to) injury prevention. In another post, we'll look at stretching and performance as well as other forms of mobility training you can incorporate into your training program.