Intensity....The Great Debate!

Intensity is another word thrown around by fitness buffs throughout the industry but I think many lack a fundamental definition of this buzzword. Intensity, at it's most basic level is defined as Power. Power is defined by the amount of "work" done per unit "time". These are physics definitions, however everything in this world can be boiled down to physics, especially in our specific case- kinematic movements- or exercise. Let's continue down this path, what is work?

At its most simple level, work is the amount of force applied to an object (barbell, dumbell, heavy box, implement, etc) multiplied by the distance the object is moved. And to clarify, force is weight. To keep things simple, we will use the SI (metric) system here. A 100kg dumbell has a mass of 100 kilograms. It's weight is calculated by the mass multiplied by the local acceleration due to gravity which is about 9.81 meters/second/second. What that means is the Earth has a constant acceleration that is dependent on local altitude and other factors. Enough about that, let's continue on this examply. The 100kg dumbell requires a smidgen over 981 Newton-meters (Joules) of force (100kg x 9.81m/s/s) to move it off the ground. Why more than 981 Newton-meters (Joules)? Well if the dumbell weights 100kg and the acceleration due to gravity is 9.81 meters/second/second then the force the dumbell is exerting on the floor is 981 Newtons. So to lift it vertically, 981.1 Newton-meters (or Joules) would be needed to get it moving. Similarly, 1000 Newton-meters (Joules) could be exerted by the person lifting the dumbell off the floor and the only difference would be "how fast" the dumbell is lifted off the floor. This leads us back to power and intensity :-). By the way, Newtons is the term used in the SI system of measurement of work. In English terms, it is foot-lbs, which makes sense because when we think of weight, we think of pounds. And force is weight multiplied by distance, which in the English system of measurement is feet, so foot-pounds is akin to Newtons. Do not confuse torque for force. Although they are measured in the same way (ft-lbs) a torque is the "force" used to cause rotation, i.e. rotational work. Enough about that, onto power and intensity....

So if we know the work that is required to move the object, in this case a dumbell, then how do we figure out power. Well Power is work per unit time. So we can extrapolate this out to work/time, or Newton-meters (Joules)/second. By the way, 1 Joule is equal to 1 Newton-meter, and most scientists and physicists use Joules by convention. So how much power is is required to lift the 100 kg dumbell 2 meters vertically? Well since we know "work" is equal to force multiplied by distance, and force is mass times acceleration let's get some known quantities figured out here. A 100kg dumbell at 9.81meters/second/second local gravity that is on the ground exerts a force of 981 Newtons on the floor and if the person doing the exercise is lifting it 2 meters vertically, the distance is 2 meters. This gives us the work total, so in this case work, which is equal to force times distance is 981 Newtons x 2 meters= 1962 Newton-meters (or 1962 Joules). So the work is 1962 Joules, and the lifter is doing this "work" all in 1 second. This fits nicely into our power equation. Power is work per unit time, or Work/time. In this case we have 1962 Newton-meters/ 1 second, which in Power is 1962 watts. So 1 Newton-meter per 1 second = 1 watt.

Let's put power into a more identifiable term, horsepower. 1962 watts (the power it takes to move a 100kg dumbell 2 meters vertically in 1 second) is equal to about 2.6 horsepower! This should make you respect your 300 horsepower sports car a little more. Now let's get back to the exercise relevance of all this. You can measure your output by figuring out how much power (work/time) you've exerted during a workout. All you need to know is how much weight you've moved, how far you've moved it, and how long it took you to do it. Thus, you can see real gains in your capacity to do work by figuring out your power output. Power is synonymous with Intensity. That is the more power you exert, the more intense the workout was. Let's do an example for clarification.

Say your workout is 100 pullups as fast as you can. To measure your weight, simply stand on a scale and get your weight in kilograms. If you can only get pounds, divide the weight by 2.2 and you will be pretty darn close. So let's say you are 60kg (remember that is a mass, not a weight). So the force you are applying to the pullup bar is 60kg x gravity's acceleration. So 60 x 9.81 (acceleration due to gravity) is about 589 Newtons for every pullup. To figure out the distance you have to exert that "force" over, you need to measure the distance your center of gravity travels during a pullup. Interestingly enough, this is fairly simple. Your center of gravity (CoG) is located somewhere between your bellybutton and your groin. However, even if you seriously miss the mark on measuring this, as long as you use the same point for reference you won't have a very large error in your data. Ironically, most compound movements' (pullup, pushup, squat, overhead press, etc) distances are the length of a particular limb. For instance, in a pullup the distance your CoG moves is approximately the length of your arm, since when you go from the bottom of a pullup (full extension) to the top (head above the bar) you brought your shoulders to your hands, in effect, your entire arm (from shoulder to hand) is the distance you travel in 1 rep of a pullup. So we have the force you're exerting (589 newtons) and a distance (the average arm is .8 meters long for men). This gives us the opportunity to find the work for 1 rep of exercise. 1 pullup for a person who is 60kg (589 Newtons) and has a .8 meter arm requires 471.2 Newton-meters of "work" to complete. Now since the person is doing 100 reps, he needs to expend 47120 Newton-meters to complete the task (471.2 Newton-meters x 100 reps). Let's say it took 4 minutes to complete this task. So that's 4 minutes x 60 seconds= 240 seconds. This plugs into our power formula, which is work/time. So 47120N-m/240s~ 196 watts. This is approximately 0.2 horsepower!

Let's say the same athlete does the workout at a later date and did it in 3 minutes even. He still weighs the same and has the same arm length (hopefully). So now we still have the same work being done- 471.2 N-m per rep x 100 reps= 47120 N-m total but divided by 180 seconds, instead of 240 seconds like the first go around. This gives us an output of approximately 262 Watts, or 0.4 horsepower! That's double the power and intensity by shaving a minute off his time! This is real improvement, and the intensity was very high for this athlete! So let's put it all together.

High intensity exercise has vast benefits for the human body, such as increased muscle growth, increased fat loss, increased bone density, increased cardiorespritory fitness, increased muscular capacity (endurance), increased strength, among many others. In order to increase the intensity of a workout, you must either increase the force output (weight lifted) or decrease the time in which the force was applied (time to complete the task). You can imagine a situation where a person is squatting 225 lbs 50 times. Let's say it takes him 10 minutes to do this (slow-btw haha). If the person wants to increase the intensity then he should want to do the same workout at a later date in less than 10 minutes, or increase the weight to 250lbs and not increase his time to such an extent that his power output suffers, more on that delicate balance in the next post. What we can surmise from all this physics and numbers is that if we do lower our time for a given set of tasks, then we got stronger, have a higher capacity for work, and probably all the other good things that we get from high intensity exercise, and we ARE improving. However if there is no improvement, or we get slower then we know our program isn't working to help us be better in any single measurable facet of fitness.

So, hopefully after reading that you get the sense that measuring your intensity is worth the time and can yield better results in the long term. Now get those calculators out and get to work!