When to measure muscle fuel

MuscleSound determines the fuel levels of a muscle by assessing the brightness of its scanned image. The brightness of this image is determined by the muscle’s water content (Hill & San Millán, 2014; Nieman et al., 2015) which fluctuates both during and post exercise (Sjogaard & Saltin, 1982; Hackney et al., 2012). This can potentially introduce artifact into the scan producing erroneous assessments of actual Fuel levels.

Muscle Fuel is composed mainly of glycogen, but may also include other water-bound constituents such as protein, carnitine and creatine. Research has shown that, because of the fluctuations of muscle fluid during and following exercise, identifying optimal time frames for scan sessions is essential for producing valid and reliable results.

Optimal Times to Assess Muscle Fuel

MuscleSound can be used pre- and immediately post-exercise

As indicated by the validation studies, MuscleSound provides a good, indirect assessment of glycogen utilization pre-post exercise. However, the MuscleSound value is   unique to the individual. Person-to-person and muscle-to-muscle values may differ considerably within the same sport/exercise session. This is due to the fact that, while fluid is distributed relatively evenly throughout the whole muscle, Glycogen is distributed unevenly both in its location and depth. It is also used at different rates depending on its fiber type, location in the muscle, intensity of the exercise and the fitness level of the individual.

MuscleSound can be used several hours after the end of moderate to high intensity/long duration steady state exercise (such as cycling) that does not involve extensive eccentric contractions

After this time muscle fluid levels are likely to have equilibrated and fuel/glycogen can be more accurately assessed. If the session involves extensive eccentric contractions, MuscleSound should be postponed.

MuscleSound can be used one to two days or more after high intensity/long duration sports such as soccer, football, rugby, and basketball

The 1-2 days minimum period allows time for muscle damage to be repaired, edema to subside, fluid levels to equilibrate and fuel/glycogen to be more accurately assessed.

MuscleSound can be used one to two days before a competition to make sure fuel is adequate. If fuel levels are low there is adequate time to replenish and recover using appropriate nutritional and workload strategies.

Sub-Optimal Times to Assess Muscle Fuel

MuscleSound should not be used within several hours of the end of moderate to high intensity/long duration steady state exercise

During this post-exercise period, muscle fluids that shift during exercise are still in flux and have not equilibrated to pre-exercise levels. As mentioned above, the glycogen to water ratio in the recovery phase could be as high as 1:17 and could also include fluid  not bound to glycogen. This additional fluid will appear as more hypoechoic on any MuscleSound scans taken at that time, and will give an exaggerated assessment of Fuel/glycogen content.
If exercise also includes prolonged or repeated eccentric contractions (e.g. downhill running, multiple set heavy weightlifting), it will cause increased micro-damage inside the muscle, producing additional fluid in the form of intramuscular swelling (edema). This process typically begins soon after exercise ceases but under certain circumstances (Marathon/ultra-distance running, high intensity weightlifting) can begin even during activity. If this occurs, it will give an exaggerated assessment of Fuel/glycogen usage or replenishment that may take several days to normalize.

MuscleSound should not be used the day after high intensity/long duration competition in sports such as soccer, football, rugby, and basketball

Rapid changes of direction are an integral part of many sports and are a major source of eccentric contractions. This can result in extensive micro damage accompanied by intramuscular swelling (edema) and can transiently increase the volume of fluid inside the muscle. The image will be more hypoechoic and  MuscleSound scans taken at that time will have an exaggerated assessment of fuel/glycogen replenishment.
Allowing at least one day post-game provides time for edema to subside, fluid levels to equilibrate and glycogen to be more accurately assessed.


MuscleSound scans display the ‘echogenicity’ (Brightness) of a muscle image which is based on the speed at which sound waves reflect back from different tissues within the muscle. Connective tissue is very dense and the sound waves quickly reflect back to the transducer. Images of this type of tissue appear brighter (hyperechoic) on the scan. Water, on the other hand, allows the sound waves to pass through without resistance and so they are not reflected back to the transducer. Images of scan areas containing water appear darker (hypoechoic): the higher the water content of the muscle, the darker (hypoechoic) the image will be (Hill & San Millán, 2014).

Each gram of glycogen is tightly bound to three grams of water (Olsson and Saltin, 1970; Fernández-Elías, et al.,2015). When the muscle contains more glycogen, it also contains more water, producing a darker  (more hypoechoic) image. During exercise, as glycogen is used up, the water associated with it leaves the muscle. This exposes the muscle fibers, which are denser than water. This enables the sound waves to be more easily reflected back producing a brighter (more hyperechoic) image. In predictable situations, therefore - as explained in the following paragraphs - the darker areas of the image can be assumed to contain more glycogen. Note: other energy-producing constituents of muscle such as protein, creatine and carnitine are also tightly bound to water, and may well also contribute to the darkness of the image.

“Fluid Dynamics”

Considerable shifts in muscle fluid occur during and after exercise as part of the metabolic process of energy production. During this process, water shifts into and out of different compartments of the muscle (Sjogaard & Saltin, 1982; Hackney et al., 2012). This includes both “free water” as well as the “bound water” released when muscle glycogen, and other muscle elements (protein, creatine, carnitine) are broken down for energy.

The type of exercise performed can also impact the amount of fluid inside muscle. For example, eccentric contractions (an essential component of many types of sports and exercise) can produce micro-damage resulting in post-exercise swelling (Proske & Morgan, 2001). This may also increase intramuscular fluid content. At certain times, such fluid shifts can introduce artifacts in MuscleSound scans that mask the water actually bound to glycogen. This will produce dark images that could be interpreted (erroneously) as showing greater levels of muscle fuel/glycogen than is actually the case. Identifying these situations will differentiate between optimal and suboptimal times to use MuscleSound. See below.

Despite the fluid shifts accompanying exercise of different types, research has confirmed that hydration levels, per se, do not impact the accuracy of MuscleSound scans. The glycogen/water bond is very strong and, even when the body is dehydrated, the water molecules will remain attached to glycogen until it is broken down for energy. Only when this happens is the bound water released and made available to the rest of the body, in effect contributing to hydration status. While dehydration does not impact normal muscle glycogen breakdown for energy, it does increase its rate of breakdown. Because of this, in dehydrated conditions glycogen stores are used up much faster and the muscle becomes fatigued a lot sooner (Logan-Sprenger, 2015).

Under certain circumstances some research has indicated that there is a supra-physiological amount of fluid being retained by the muscles. For example, research has shown that in the 1-4 hours post exercise, glycogen synthesis (refueling) is at its highest rate. However, in some conditions research shows that the water taken into the muscle is very high. An apparent ratio of 1:17 has been reported, compared to the commonly reported ratio of 1:3 (Fernández-Elías, et al., 2015). Not all this water may be bound to glycogen, however, some may be ‘unbound’ water freely distributed inside the muscle (Peters & Lavietes, 1933). More research is needed in this area. Regardless, as referenced above, this may well create artifact in MuscleSound scans, since the transient increases in muscle water volume will produce an over assessment of Fuel/glycogen.

Both internal and external research has shown that these periods of extra-normal muscle fluid movements and volumes primarily occur soon after and/or several days after exercise, depending on the intensity, duration, and eccentric nature of the exercise.


The foundation of our fuel scoring system is that elements of muscle fuel (glycogen, creatine, carnitine, protein etc.) are tightly bound to relatively large proportions of water. This water is displayed as darker areas on the ultrasound muscle scan, and our current algorithm assesses a “Fuel Score” based on the relative darkness (echogenicity) of the pixelated image. On occasion however, as explained above, some individual scores in these data sets actually appear to increase post exercise. In other words, the post exercise image is shown as darker than the pre-exercise image. We have seen examples of such results in running, cycling and, more recently, in high-volume strength training.

Next Steps

Absent scanning issues, our research has shown that such paradoxical occurrences are the result of artifact related to exercise related fluid shifts within the muscle. While the excess fluid erroneously produces an apparently elevated fuel score, it also impacts architectural components of the muscle. For example, as muscle fluid increases so, too, do scores for muscle pennation angle, muscle thickness and muscle fascicle length. We can now measure and track these fluid-influenced changes in muscle architecture, which may allow us to quantify the extent of the fluid change and, ultimately, to control for its impact on fuel scores. Adopting this approach using pennation angle has already produced some promising preliminary results. We will be reporting more on this phenomenon as our investigations progress.


Costill DL., et al. Muscle water and electrolyte distribution during prolonged exercise. Int J Sports Med 2(3):130-134, 1981

Fernández-Elías, VE. et al., Relationship between muscle water and glycogen recovery after prolonged exercise in the heat in humans. Eur J Appl Physiol 115:1919–1926, 2015

Hill, J. C., and San Millán, I. Validation of musculoskeletal ultrasound to assess and quantify muscle glycogen content. A novel approach. The Physician and Sportsmedicine, 42(3), 45-52, 2015

Hackney, KJ. et al. Skeletal muscle volume following dehydration induced by exercise in heat. Extreme Physiology & Medicine 1:3, 1-9, 2012

Ivy, JL et al., Muscle glycogen synthesis after exercise: effect of time of carbohydrate ingestion. J. Appl. Physiol. 64(4): 1480- 1485, 1988.

Ivy, et al., Muscle Glycogen Storage after Different Amounts of Carbohydrate Ingestion. J. Appl. Physiol. 65(5): 2018-2023- 1485, 1988

Lundvall, L., et al. Fluid Transfer between Blood and Tissues during Exercise. Acta Physiol Scand. 85: 258-269, 1972.

Mora-Rodríguez, R. Skeletal muscle water and electrolytes following prolonged dehydrating exercise. Scand J Med Sci Sports 25: e274–e282, 2015.

Neufer, D. Hypohydration does not impair skeletal muscle glycogen resynthesis after exercise. J. Appl. Physiol. 70(4): 1490-1494, 1991

Nieman, David C., et al. Ultrasonic assessment of exercise-induced change in skeletal muscle glycogen content. BMC Sports Sci Med Rehab 7.1, 2015

Peters, JP and Lavietes, PH. The Nature of "Preformed Water". J Clin Investig 12:695–712, 1933.

San Millán, I. et al. "Measurement of skeletal muscle glycogen status in critically ill patients: a new approach in critical care monitoring." Critical Care 19.1, 2015

Shiose, K. et al. Segmental extracellular and intracellular water distribution and muscle glycogen after 72-h carbohydrate loading using spectroscopic techniques. J Appl Physiol 121: 205–211, 2016.

Sjogaard, G and Saltin, B. Extra- and intra- cellular water spaces in muscles of man at rest and with dynamic exercise. Am. J. Physiol. 243 (Regulatory Integrative Comp. Physiol. 12): R271-R280, 1982

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