SCIENCE OF THE STRETCH

Flexibility, as it is most easily defined, is the range of motion about a joint.  It’s the degree to which our joints move freely, in their various directions and planes.  Just as not all joints are biomechanically or anatomically the same, nor is their collective flexibility, necessarily.  Therefore, you might be able to touch your toes, but that doesn’t guarantee that you can touch your elbows behind your back:  Flexibility is joint-specific.

Anyone can improve his or her flexibility.  It is true that genetics plays a small role, but like most things, practise makes perfect.  Most fitness buffs either leave out the flexibility portion of their training programs altogether, or they leave two minutes at the end of the workout to do some quick side bends before booking it to the juice bar.  This is not the way to improve flexibility.  Stretching for a warm-up or cool-down is not the same as stretching to improve flexibility; we must put just as strong a focus on the flexibility component of training as we do, say, resistance training or cardiovascular work.

The Physiology of Stretching To fully understand what happens when we stretch, it’s important to understand what happens when our muscles contract. Let’s start with the Sarcomere.

The Sarcomere When you observe muscle cells under a microscope, you’ll see that they contain a striped pattern, called striations (which you can often see in extremely lean and dehydrated bodybuilding competitors).  The striations are formed by a series of units called sarcomeres.  Think of a sarcomere as a single piece of hay. They are arranged in a stack throughout the muscle tissue (imagine a bale of hay).

Every single muscle cell can be comprised of thousands of sarcomeres. Each sarcomere has various proteins which vary in length, which is what causes the length of each muscle to change.  The sarcomeres contain proteins, which are made up of actin (thin) and myosin (thick) filaments.  When the actin and myosin react together within the sarcomere, it shortens, causing a muscle to contract.  Enter the Sliding Filament Theory.

The Sliding Filament Theory In 1954, scientists published two groundbreaking papers describing the molecular basis of muscle contraction1. These papers described the position of myosin and actin filaments at various phases of contraction within muscle fibers and proposed how this interaction produced contractile force. Researchers observed changes in the sarcomeres as the muscle tissue shortened.

The sliding filament theory states that the sliding of actin past myosin generates muscle tension. Actin literally slides beside myosin and ‘grabs on’, leaving the entire sarcomere, and therefore muscle, shorter; contracted.  Because actin is attached to structures located at the lateral ends of each sarcomere, any shortening of the actin filament length would result in a shortening of the sarcomere and thus the muscle.

So now that you understand how the muscle contracts, or shortens, let’s explore the opposite reaction: relaxation, or lengthening of the muscle.
As the sarcomere begins to relax, there is a decreased overlap between actin and myosin, allowing the muscle fiber to elongate.  Once all the sarcomeres are fully stretched, the muscle fiber reaches its maximum rested length.  If we continue to stretch at this time, there is increased tension and the connective tissue takes up the ‘slack’.

When a muscle is stretched, some of its fibers lengthen, but other fibers may remain at rest. The length of the muscle depends upon the total number of stretched fibers.  The more fibers that are stretched, the greater the length developed by the stretched muscle.
Now let’s explore what happens when the muscle has reached its maximum length.

The Stretch Reflex When the muscle is stretched, so is the muscle spindle. Muscle spindles are sensory receptors (in the belly of the muscle), which detect changes in the length of the muscle.  The spindle then records the change in length of the muscle from the stretch, and sends signals to the spine2.  This triggers the stretch reflex which then tries to resist the change in muscle length by causing the stretched muscle to contract. The more rapid the change in muscle length, the stronger the muscle contractions will be (think plyometrics).   This basic function of the muscle spindle helps to both maintain the tone of the muscle as well as to protect said muscle from injury.

When we take our place on the mat to stretch (whether via static, dynamic or proprioceptive neuromuscular facilitation [PNF] stretching) there are acute or short term effects on the muscle.

The acute effect of stretching on flexibility is pretty clear: increased range of motion – which is ultimately what we’re seeking.  This range of motion, on average, will persist for sixty to ninety minutes3.  Holding a stretch for the oft-prescribed 20 to 30 seconds is a good standard because most of the stress relaxation occurs in the first 20 seconds.

The reason that our collective range of motion is enhanced is because of increased stretch tolerance; as we sit and hold a stretch, the soft tissue becomes accustomed to the lengthening, and it ‘relaxes’.

Stretching to Improve Flexibility
As mentioned above, the standard prescription of holding a stretch for 20-30 seconds a few times a week will elicit acute and short term increases in range of motion. But what about those of us looking to improve our long term flexibility? What if we want to increase flexibility, say, permanently? It is essential to incorporate stretching into our training programs regularly throughout the week.  We must devote the time and attention to the flexibility portion of our routine just as we do to resistance training and cardiovascular work.

One of the reasons for holding a stretch for an extended period of time is that by doing so, the muscle spindle becomes accustomed to the new length and reduces its signaling to the spine (remember ‘Stretch Reflex’). Eventually, you can train your stretch receptors to allow greater lengthening of the muscles, thereby increasing flexibility.

Various reviews of research on stretching a variety of muscle groups report significant improvements in range of motion with 3 to 6 weeks of training3,4.

Does stretching decrease muscular performance?
Clearly, stretching can improve both short and long term flexibility, and for that reason is a key component in the training regime.  However, overstretching a muscle or muscle group can cause performance problems in the weight room

If you recall the Sliding Filament Theory from above, you will remember that the stretch reflex counters the contraction that occurs within the sarcomeres of the muscle.  So, basically, we stretch in opposition to contraction.  It follows, then, that too much of a stretch in a muscle would inhibit, or certainly lessen, the contractile properties of the muscle in question.  It is clear that from the standpoint of maximizing muscular performance, stretching creates an acute decrease in performance, therefore significant stretching should not normally occur prior to exercise,  but be programmed during the cool-down after resistance training.

FACTS and FALLACIES: When should I stretch?
It is a still-heard, decades old debate: Warm up before a workout? Stretch before a workout? Or just get right into it?  Well, as you’ve read here, and here’s Lesson One: stretching prior to a workout can decrease your performance.  But please don’t underestimate the importance of stretching after a workout session.  And the science is also very clear on the importance of being warm before stretching.

I always use the analogy of the rubber band in the freezer.  If you pull it out after being in there all night and try to stretch it, it’s going to break.  But if you roll it between your hands, allowing it to warm up and then gently try to manipulate it, it will stretch without rupturing.  Lesson Two? Warm up before you stretch. So now that we have that out of the way, what can we consider a warm-up? Basically, it’s anything that can actually increase the temperature of your soft tissue.  Any dynamic movement will work – walking, cycling, jumping jacks… you name it.

References
1 Clark, M. Sliding filament model for muscle contraction. Muscle sliding filaments. Nature Reviews Molecular Cell Biology 9, s6–s7(2008). 2 Huxley, H. E. & Hanson, J. Changes in the cross-striations of muscle during contraction and stretch and their structural interpretation. Nature 173, 973–976 (1954). 3 Knudson, D. The Biomechanics of Stretching. Journal of Exercise Science & Physiotherapy, Vol. 2: 3-12, 2006. 4 Duchateau, J. and N. Guissard. Effect of static stretch training on neural and mechanical properties of the human plantar-flexor muscles. Muscle Nerve 248-55 (Feb. 2004).

SUPPLEMENT FACTS

SERVING SIZE 1 SCOOP (30 G)

SERVINGS PER CONTAINER 19 (CHOCOLATE AND TROPICA) 21 (OTHER FLAVORS)