This post helps to clarify some of the physics and math put into a new app I developed called CarbHacker, which is a carb calorie burning estimator. It may seem like a little bit of hocus pocus, but there is a good amount of math and logic backing the app. Plus it doesn’t hurt that some fitness gurus like Tim Ferriss and Mark Sisson have covered the topic similarly, although without the math behind it. This post covers just one part of the app’s assumptions regarding work and muscular contraction.
Not all muscle contractions are the same. There are actually three of them with regards to skeletal muscle: isometric, concentric and eccentric. Isometric contractions occur when the muscle’s length is or joint angle stays the same. A good example is when you are holding yourself against a wall. Concentric contractions occur when the muscle is shortening. An example is during the lifting phase of a bicep curl. Eccentric contractions occur as the muscle is lengthening, an example of which is the downward phase of the bicep curl.
Plyometrics makes use of the elastic force in muscles along with eccentric muscle contractions as muscle lengthening has been shown to provide larger gains in hypertrophy than concentric contractions.
Typically, most exercises have at least one of each type of muscle contractions. We will take the standard squat as an example.
Most often, the squat begins with a short up phase to lift the bar off of the weight bench. Then the squatter will hold the weight in the standing position. This is the first isometric phase as stabilizing muscles are working to hold the body in an upright position. The down phase is the eccentric phase of the squat. In this case the glutes and hamstrings are lengthening. The quadriceps muscles are mostly contracting, but are acting as stabilizers.
At the bottom of the squat you have a second isometric phase as there is a shift between eccentric contraction / downward motion to concentric contraction and upward motion. And for you math geeks, you should be able to recognize the sine curve and the biomechanics behind the squat.
What is important about the various types of contractions is that work is occurring during both phases. There is active contraction in all phases, however by doing a fully repetition you are effectively lifting your weight over two distances. In terms of calculating calories expended, this means that since work = force * distance and force in our case is effectively weight (mass * gravitational force), total work = force * distance * 2.
Questions? Leave them in the comments.