Estimated Fuel Level - EFL


The ‘Fuel’ that muscles use for exercise is predominantly made up of Glycogen. However, other muscle constituents that contribute to energy production, such as protein, carnitine and creatine can also be regarded as part of this Fuel. Like glycogen, these elements are also bound to water and can, potentially, be seen on the ultrasound scan used to estimate a muscle’s Fuel Level (See ‘Rationale’ below).

Estimated Fuel Level (EFL) is one element of what we call Muscle Energy Status (MES)

Our approach is unique in that EFL provides real time, individualized measures of muscle fuel levels reflecting the physiologically established fact that such levels will vary on a day to day and even hour to hour basis, according to a variety of factors including muscle size, fitness, nutrition, fatigue, and training status.

Underlying Principles  

To estimate the capacity of any ‘container’, its maximum and minimum levels must first be established. We apply this same principle for scanned muscles using a ‘Fuel Tank’ concept. Maximum fuel levels are typically obtained by scanning muscles pre-exercise with individuals that are ‘rested and fed’. Minimum fuel levels are typically obtained by scanning muscles post-exercise, when fuel levels are at their lowest. The difference between these Maximum and Minimum fuel levels provides an estimate of the muscle’s ‘Fuel Tank’ capacity.


We represent the individual’s highest (Maximum) value as an Estimated Fuel Level (EFL) of 100%, and their lowest (Minimum) value as an EFL of 0%. All future MuscleSound scores for this muscle are then expressed as a percentage EFL of that initial ‘Fuel Tank’ range.

As more data is collected, the maximum and minimum fuel tank levels for that same muscle may change. For example, if the muscle achieves a new highest score, this will become the new estimated maximum capacity (100% EFL) for the muscle’s fuel tank. In the same way a new lowest score will become the new estimated minimum capacity (0% EFL). This automatic updating process enables the muscle tank’s estimated capacity to become more and more compatible with the physiological uniqueness of the person being scanned. The EFL of a muscle will always be reported as a percentage of its current fuel tank capacity and changes will reflect the always fluctuating fuel status of muscles.

NOTE: this may have implications for research – See ‘Using MuscleSound Data’ below

Rationale for EFL

The basis of this approach is that it provides a more flexible, subjective and relevant ‘point in time’ estimate of muscle status, that also adapts to individualized changes in muscle status. To achieve this, we have refined our protocols and our terminology to incorporate the new findings of our ongoing research as to what ultrasound can ‘see’. This new approach now encompasses a more expansive view of ‘Muscle Fuel’ which incorporates not only glycogen but other intra muscular constituents, themselves associated with muscle fluid, and are essential components of the muscle’s capacity for energy production, such as Protein, Carnitine, Creatine, and Free Water.

The unique advantage and utility of this new scoring methodology is that EFL values are standardized. Valid comparisons can now be made between muscles, between devices and between individuals. For example an EFL of 65% in one player can be compared to an EFL of 65% in a different player. NOTE: while each player’s muscle is equally ‘full’ the absolute amount of fuel may be different. We are comparing the relative vs the absolute quantity of fuel.

Other related information and assumptions

Data Collection

If the data collected is the first assessment for that individual’s muscle, and only one scan assessment is available/possible, maximum and/or minimum levels will be estimated from our global database using scans taken from the same ultrasound device. The global fuel tank will be referenced on the first session only. When the second assessment has been conducted, the first assessment’s values will be re-calculated using the muscle’s maximum and minimum now that they are available.

Based on observation and analysis from our extensive database of more than a dozen scientifically designed and controlled studies involving hundreds of subjects collected over the last 4 years, we will estimate the fuel level as 80% when the fuel tank size is relatively small.

Calculating Fuel Tank size

The size of a muscle’s fuel tank is always calculated from the same ultrasound device. This is important because ultrasound devices produce (render) images differently depending on their manufacturer. These differences particularly impact the high and low scores of the muscle image. If the initial range of a muscle’s high and low scans is small, it will result in a much smaller size fuel tank. This will have a major impact on how the fuel tank is calculated. It will also significantly reduce any consequent Estimated Fuel Levels within this range. In addition, if the tank is small, we cannot properly estimate a completely full (100%), nor a completely empty (0%) level. In such cases we again use our default maximum level assumption of 80% and the actual minimum MuscleSound score. As more scans of this muscle are taken, its fuel tank range (and capacity) will expand, together with the Estimated Fuel Levels. Once the fuel tank range is large enough, we can start to estimate the fuel level from (0% to 100%).

To obtain a more accurate Maximum Fuel estimate, sessions should be scheduled where the individual being scanned is ‘rested and fed’ (see ‘Underlying Principles, above). In this situation there is high confidence that muscle fuel levels would be high. Similarly, to obtain a more accurate Minimum Fuel estimate, sessions should be scheduled where the individual being scanned has just completed a fuel depleting session, for example immediately after high intensity/long duration exercise. In this situation there is high confidence that muscle fuel levels would be low.

If you would like a better sense of what is possible (true maximum and minimum fuel levels), we suggest you reference the Fuel Rating measures.

Using MuscleSound data

In general, at MuscleSound, we have clients with two different requirements:

  1. Sports performance/training and health professionals want continuous monitoring of outcomes. Results are viewed in a serial fashion ‘as they happen’. While an initial baseline can certainly be used a reference point, it is less important than how the actual measures change - and are seen to change - on a day to day or event to event basis.
  2. Researchers, on the other hand, want a fixed, consistent pre-intervention baseline measure against which to compare post-intervention outcomes collected over the duration of the study. Results are analyzed only after all data has been collected. This aspect of protocol design is a standard feature of research studies.

To address this second requirement, we are including a specific ‘Data Collection’ Assessment in the MuscleSound app designed specifically for researchers and the research environment. Here’s how it works in overview:

All data for the study is collected in the cloud normally. Once the study is completed, our algorithm collates and normalizes data against the initial baseline measure, essentially ‘fixing’ this initial baseline score, so that future intervention-produced EFL changes can be compared against it. This will soon be included as an automatic feature of our app. In the meantime it can be provided on request.

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