In part one we talked about what the actual measurement as it pertained to the passive properties. For the most part they remain constant throughout exercise, as it would not be beneficial if tendons and ligaments got more elastic during a workout. What does change over the course of a workout is the active component of our muscles. So diving into a little physiology to answer this question...
Our muscles have small moving parts called sarcomeres, these parts are the actual movers of muscle and when these shorten they cause a small microscopic decrease in the muscle length and after the summation of thousands of these little sarcomeres you see muscle shortening, or in most common terms "contracting". So at this small microscopic level these sarcomere have myosin and actin filaments. These little guys act together to cause shorting. Muscles work, on the microscopic level, very similar to how we would climb a rope. Your body is the myosin filament and the rope is the actin filament. In our bodies we need two things that cause muscle contraction: ATP and Calcium. ATP is simply energy and Calcium in layman's terms is motivation. So take the climbing a rope example. In order to first grab the rope you need motivation to grab it right? You just are not going to climb every rope you see. (Unless you are a monkey, if so this analogy might not be best suited for you).
So you now want to grab the rope for whatever reason you are now motivated to climb that rope. This is Calcium activating the actin making it more appealing for myosin to attach to it. The next step is you reach up and grab the rope. You then pulldown and pull yourself up, you then have to make the choice to do it again so you still need motivation now you need more energy (ATP) to do it over again. So as long as you have energy (ATP) and motivation (Ca+) you will continue to climb the rope. You get to the top and now you have climbed the rope successfully pulling yourself to the end of the rope and in essence shorten that distance between you and the end of the rope basically shorting muscle. Now take that analogy and multiply it by a million and you have what goes on in each muscle in the body. You have thousands of myosin climbing the actin ropes in each muscle ultimately shortening the sarcomere and causing muscle contraction. **Caution this is a very simple analogy it is way more complex than this*** So just like you climb the rope in a workout, you get tired right? You either lack energy or motivation and you can’t climb the rope anymore. Same thing happens in muscle for a contraction to stop: either you run out or ATP or Ca+ is no longer present.
Well a lot of stuff happens during workout that causes fatigue , one is the accumulation of acid (commonly H+). One thing about our bodies is that it does a great job at buffering acids, but it takes time. This time to recover from the buildup of these substrates is our recovery time. This is what we are measuring with the ROM test. When we build up all these substrates they mess up/reduce our ability to produce energy and or block or stop Ca movement.
These substrates may block the body's release of calcium in the muscle making our muscle not want to contract and/or they can stop Ca from being uptaken by the system or by not allowing for them to unbind from the Actin. Essentially this keep up the motivation (Ca) in the system making you reach up and grab the rope. This act of grabbing the rope causes tension in the rope. This new found tension in the rope changes its passiveness or flexibility of the rope. So the rope's relaxed state is taught because you are grabbing it, you have the motivation to grab it but not the energy so you just hold on. This holding on at the sarcomere level causes tightness in the muscle making your decrease in ROM and thus validating the test. Then once your body is recovered and has pumped all the Ca back to where it is supposed to be, your body relaxes and the energy substrates are shuttled out of the muscle you are recovered and can go back to getting strong. This also works if your workout is not fatiguing enough... your body will maintain the status quote and be able to relax. Muscle contractions drive this reaction, the more and the harder the contractions the more substrate build up. If you don't build up enough you didn't work hard. So work hard and build up those substrates; then rest and let your body recover and then get back to it. Which can all be measured in this simple test.