When to measure muscle fuel

MuscleSound assesses ‘Muscle Energy Status’ (MES) - a source of muscle energy production that is composed of muscle glycogen and associated muscle fluid. However, research has shown there are temporal limitations to accurately assessing MES and, consequently, optimal and sub-optimal times to use MuscleSound.

Optimal Times to use MuscleSound

  1. MuscleSound can be used pre- and immediately post-exercise for the following reasons:
    1. 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.
  2. 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 for the following reasons:
    1. After this time muscle fluid levels are likely to have equilibrated and MES/glycogen can be more accurately assessed. If the session involves extensive eccentric contractions, MuscleSound should be postponed as indicated below.
  3. MuscleSound can be used one to two days or more after high intensity/long duration sports such as soccer, football, rugby, and basketball for the following reasons:
    1. The 1-2 days minimum period allows time for muscle damage to be repaired, edema to subside, fluid levels to equilibrate and MES/glycogen to be more accurately assessed.
  4. MuscleSound can be used one to two days before a competition to make sure MES is adequate. If MES levels are low there is plenty of time to replenish and recover using appropriate nutritional and workload strategies.

NOTE: More than 50 professional sports team including football, soccer, cycling and hockey have adopted this last approach. Using MuscleSound 1-3 times a week, they monitor and assess readiness and recovery of their athletes.

Sub-Optimal Times to use MuscleSound

  1. MuscleSound should not be used within several hours of the end of moderate to high intensity/long duration steady state exercise for the following reasons:
    1. 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 MES/glycogen content.
    2. If exercise also includes prolonged eccentric contractions (e.g. downhill running) it will cause increased micro-damage inside the muscle. This produces additional fluid in in the muscle in the form of intramuscular swelling (edema). This process begins soon after exercise ceases. If this occurs it will give an exaggerated assessment of MES/glycogen replenishment that may take several days to normalize.
  2. MuscleSound should not be used the day after high intensity/long duration competition in sports such as soccer, football, rugby, and basketball for the following reasons:
    1. 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 darker and   MuscleSound scans taken at that time will have an exaggerated assessment of MES/glycogen replenishment.
    2. Allowing at least one day post-game provides time for edema to subside, fluid levels to equilibrate and glycogen to be more accurately assessed.

Background

MuscleSound scans display the ‘echogenicity’ (Brightness) of a muscle image which is based on the speed at which sound wave reflects 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 tissue appear hyperechoic (brighter) 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 this tissue appear hypoechoic (darker) on the scan. The higher the water content of the muscle therefore, the darker the image will be.  

Each gram of glycogen is tightly bound to three grams of water. When the muscle contains more glycogen therefore the image is darker. 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 and so the image will become brighter. In predictable situations therefore – as explained in the following paragraphs - the darker areas of the image can be assumed to contain more glycogen.

Considerable shifts in muscle fluid occur during and after exercise as part of the metabolic process of energy production. Water shifts into and out of different compartments of the muscle. Intramuscular fluid shifts occur in addition to the bound water that leaves the muscle when glycogen is 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 (edema). This may also increase intramuscular fluid content. At certain times such fluid shifts can introduce artifacts into MuscleSound scans that mask the water actually bound to glycogen. This will produce dark images that appear (erroneously) to show greater levels of muscle glycogen than is actually the case. Identifying these situations will differentiate between optimal and suboptimal times to use MuscleSound.

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 the rate of glycogen breakdown. Because of this, in dehydrated conditions glycogen stores are used up much faster and the muscle becomes fatigued a lot sooner.

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. Not all this water is bound to glycogen, some is ‘unbound’ water freely distributed inside the muscle. As referenced above, this creates artifacts for MuscleSound scans, since the transient increases in muscle water volume will produce an over assessment of glycogen and MES.

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.


Resources

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