Exploring a Novel Non-invasive Method of Obtaining Muscle Composition

In a previous article (Two-set prediction of 1 Rep Max), I presented Matt Brizky’s two-set method for 1RM prediction. It is indeed a much better metric for physical performance assessment than all those calculators based on averages. I would like to explore to what extent muscle composition can be extracted from the KG vs. REPs plot.

People with a high proportion of Slow Twitch (ST) muscle fibers tend to have a soft slope, while those with a high proportion of Fast-Twitch (FT) muscle fibers have steeper slopes.

The slope alone isn’t a good metric for FIBERS RATIO cause for a 50kg difference (between 1RM and fatigue weight) you have the same slope whether your 1RM is 100kg or 1000kg.

100kg 1RM and 50kg 12RM represents a lower ratio of ST than
1000kg 1RM and 950kg 12RM because in the later case, 12RM corresponds to 95% of 1RM (which is extraordinarily good) while for the former it is obtained at 50% (which is bad).

FIBERS RATIO, which represents the muscle composition, determines whether a muscle is more designed to succeed in endurance-like activities or power activities. It doesn’t solely determine one’s ability to succeed at a given activity but is an important metric to take into consideration.

Despite its importance, the metric is much neglected in the world of sports. The only way so far to estimate the fibers ratio it accurately is biopsy, meaning extracting a sample of your muscle and looking at it under the microscope. There is at least one method, called tensiomyography, that is non-invasive but patented and likely to be expensive to use.

Muscle contraction time is statistically highly correlated with the ratio of fast and slow muscle fibers. Depending on its value, we can classify the measured muscles as slow (tonic) or fast (phasic.)

A non-intrusive FIELD technique would be interesting for use by the community.

Some studies claim that few month of resistance training do not change the fiber composition of skeletal muscles. It is however unknown whether over a longer period significant changes arise or not. (See page 547 of Exercise physiology  By William D. McArdle, Frank I. Katch, Victor L. Katch).

Regardless of the answer, this finding is relevant. If the muscle composition can change in controllablefashion, it would allow people without a predisposition for a given type of activity to develop it with time and exercise. And if the ratio will not change with exercise, it is still valuable information as it may allow to focus on activity best suiting given one’s composition.

I am looking for a way to process the performance plot (KG vs. REP) so as to obtain a number between 0 and 100 representing the ratio of one type of fibers with respect to the total.

The slope is expressed in kg/rep, and is not sufficient as explained above. But the idea is the higher it is the more FT you have, the lower it is the more ST you have.

To tend to something closer to the expected metric, I tried removing the relativity to 1RM by dividing the slope by the 1RM. The metric makes more sense, it is equal to zero when the plot is flat (lots of ST, ZERO FT). It is high when the curve is nearly vertical (lots of ST, ZERO FT) but it still goes above 100.

Ultimaltely, looking at the simple equation :

Ax + By + C = 0

It it obvious that A is linear with the number of ST and that B is linear with the number of FT. I hope to find quickly a theoretically acceptable metric, to then attempt to validate it by looking at real data (biopsis + workout log).

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