Distribution Of Muscle Throughout The Body and Metabolic Flexibility

Oct 11, 2024

4 min read

Muscle plays a key role in glucose storage, utilization and consequently metabolic flexibility and health. This is a big deal for those who want to pre-hab their health and avoid metabolic health problems or those who are already dealing with metabolic problems such as pre-diabetes, type 2 diabetes, fatty liver disease, central obesity, high blood pressure and others.

The description of muscle use of energy below uses a few descriptions that need some explanation first:

In the cell, glucose, fatty acids, and ketone bodies (and some others) are metabolized into energy molecules called ATP - adenosine triphosphate. This molecule with 3 phosphate molecules is split to release the energy binding the molecules together and use that energy for cell energy. This will reduce the molecule to adenosine diphosphate (ADP). As required, the molecule can be further split to adenosine monophosphate (AMP) which is the final division and last level of contributing energy. An AMP/ATP ratio represents the amount of energy available: “no energy left to give/full energy to give”.

Muscle contraction enhances glucose uptake in skeletal muscle cells primarily through the activation of the GLUT4 (glucose transporter type 4) pathway. This process allows muscle cells to absorb glucose from the blood, a critical function during exercise or physical activity when energy demands increase.

Here’s a step-by-step description of how muscle contraction activates glucose absorption:

1. Muscle Contraction and AMP/ATP Ratio Increase

During muscle contraction, ATP (adenosine triphosphate), the cell’s main energy molecule, is rapidly consumed.
This causes an increase in AMP (adenosine monophosphate), the low-energy form of ATP, signaling the need for more energy.

2. Activation of AMPK (AMP-Activated Protein Kinase)

The rise in AMP levels activates AMPK, which serves as an energy sensor in cells.
AMPK, once activated, plays a key role in regulating energy homeostasis and enhancing glucose uptake to meet the increased energy demands.

3. Translocation of GLUT4 to the Cell Membrane

In resting muscle cells, the majority of GLUT4 transporters are stored inside the cell within vesicles.
Upon AMPK activation during contraction (and also in response to insulin), GLUT4 vesicles are mobilized and translocated to the plasma membrane.

4. Glucose Uptake via GLUT4 Transporters

Once GLUT4 is on the cell surface, it facilitates the entry of glucose into the muscle cell from the bloodstream.
The increased presence of GLUT4 on the membrane dramatically enhances glucose uptake, providing the muscle cell with the necessary fuel (glucose) to produce more ATP.

5. Glucose Utilization for Energy

Inside the muscle cell, glucose is either used immediately for ATP production via glycolysis (especially during intense activity) or stored as glycogen for future energy needs.

This process occurs independently of insulin during exercise, though insulin can also trigger GLUT4 translocation. Muscle contraction ensures glucose uptake even in cases where insulin sensitivity is reduced, such as in type 2 diabetes.

If muscle activation plays a key role in glucose absorption by muscle and regulating glucose in the bloodstream, it would be usefull to know what muscle groups have the greatest ability to do this. To illustrate where the greatest amount of muscle mass is located on the human body, I will describe the major muscle groups that contribute the most to overall muscle mass.

Here are the areas with the greatest muscle mass:

1. Upper Body:

Pectoral muscles (Chest): Large muscles covering the upper chest.
Latissimus dorsi (Back): These broad muscles span the sides and back, connecting the spine to the upper arms.
Trapezius (Upper back and neck): Extending from the neck to the mid-back, helping with shoulder and arm movements.
Deltoids (Shoulders): Large, triangular muscles that shape the shoulder and allow arm rotation.
Biceps and triceps (Arms): Located in the front and back of the upper arm, respectively.

2. Core:

Rectus abdominis (Abs): The "six-pack" muscles that extend along the front of the abdomen.
Obliques (Sides of abdomen): Muscles on the sides of the waist, important for rotation and side bending.
Erector spinae (Lower back): A group of muscles running vertically along the spine, essential for posture and lifting.

3. Lower Body:

Gluteal muscles (Buttocks): The gluteus maximus is the largest muscle in the body, responsible for hip extension and stabilization.
Quadriceps (Front thighs): A large group of muscles on the front of the thigh, crucial for knee extension and leg movement.
Hamstrings (Back thighs): Located at the back of the thighs, they help in knee flexion and hip extension.
Calf muscles (Gastrocnemius and soleus): Located in the lower legs, essential for foot movement and posture.

These muscle groups together make up most of the muscle mass in the human body, especially in areas like the legs, chest, back, and buttocks.

The distribution of muscle mass throughout the human body varies between individuals, but general estimates based on average anatomy studies can be broken down as follows:

Summary Breakdown:

Upper Body Total: ~20-26%

Core Total: ~7-11%

Lower Body Total: ~38-48%

The legs (especially the quadriceps, glutes, and hamstrings) contain the greatest amount of muscle mass, while the upper body and core have smaller individual percentages.

1. Upper Body:

Pectoral muscles (Chest): ~5-7% of total muscle mass.

Latissimus dorsi (Back): ~6-8% of total muscle mass.

Trapezius (Upper back and neck): ~2-3% of total muscle mass.

Deltoids (Shoulders): ~2-3% of total muscle mass.

Biceps and triceps (Arms): ~3-5% of total muscle mass combined (2% biceps, 2-3% triceps).

2. Core:

Rectus abdominis (Abs): ~2-3% of total muscle mass.

Obliques (Sides of abdomen): ~2-3% of total muscle mass.

Erector spinae (Lower back): ~3-5% of total muscle mass.

3. Lower Body:

Gluteal muscles (Buttocks): ~8-10% of total muscle mass.