Anatomy and physiology in EMS training

Functional components of a striated muscle fiber

• Striated muscle makes up about 40% of body weight.
• Functions: support and holding function as well as movement (together with tendons, bones, and joints).
• The muscle consists, depending on size, of several muscle bundles (fascicles).
• The fascicles consist of several muscle fibers.
• The muscle fiber represents the muscle cell.
• The striation of a muscle fiber results from the regular arrangement of the contractile proteins actin and myosin.
• The myofibrils are divided into approx. 2 μm long sarcomeres by Z-discs.
• The sarcomere is the smallest contractile unit of a muscle.
• The sarcolemma is the cell membrane that surrounds the muscle fiber and continues into the tendons.

Contraction mechanism – sliding filament mechanism

• Myosin head attaches to G-actin (cross-bridge formation) as the myosin binding site is exposed by calcium.
• Myosin head tilts from a 90° to a 45° position – shortening of the sarcomere.
• The myosin head is detached from actin by an ATP molecule and "preloaded" like a spring.
• ATP is split into ADP and Pi by ATPase.
• The cycle starts again.

Main fiber types of skeletal muscle

Type I: Slow, red fiber type (ST fibers, Slow Twitch Fibres)

• Slow contraction time, approx. 80 ms.
• Higher myoglobin content, rich in capillaries and mitochondria.
• Predominantly aerobic/oxidative metabolism.
• Low ATPase activity.
• For endurance work, e.g., postural muscles.
• Activated first during classical loading (before FT fibers).
• Optimally controllable via EMS up to 30 Hz.

Type II: Fast, white fiber type (FT fibers, Fast Twitch Fibres)

• Short contraction time, approx. 30 ms.
• Divided into oxidative-glycolytic (Type 2a) and glycolytic (Type 2x) fibers.
• Predominantly anaerobic metabolism.
• High ATPase activity.
• For fast movements.
• Easily electrically excitable due to the low threshold of motoneurons.
• Optimally controllable via EMS between 50 and 100 Hz.
• Controllable during EMS training even before the fatigue of ST fibers.

Energy supply

• The breakdown of ATP to ADP is the basis for every muscle contraction.
• Aerobic metabolism: CO₂ and H₂O are produced in the mitochondria.
• Anaerobic metabolism: Glycogenolysis outside the mitochondria produces lactate, which leads to intracellular acidification and reduces performance.

Muscle soreness

Weakness

• Damage to the Z-discs within the myofibrils.
• Osmotic effects lead to water retention in the muscle.
• Increase in CK levels as an expression of muscle damage.
• Creatine kinase (CK) is excreted through the kidneys in the urine.

Pain

• Breakdown of destroyed substances leads to pain-causing substances (e.g., histamine).
• Reflexive tension increases the pain.
• Drink plenty of water to avoid muscle soreness and greater damage, especially during EMS training, as many muscle groups are trained simultaneously.

Transmission of stimuli from nerve cells to muscle fibers

• The starting point of the contraction process is the coupling of electrical nerve impulses to mechanical contraction.
• Motor neurons originate in the anterior horn cells of the spinal cord.
• The totality of muscle fibers innervated by a motor neuron is called a motor unit.
• The transmission of excitation occurs via the motor endplate.

Resting potential

Every cell has a resting potential (about 60–100 mV in muscle and nerve cells). The inside of the cell is negatively charged, the outside positively charged. The electrical resting potential arises from an unequal distribution of ions (potassium⁺, sodium⁺, and chlorideˉ).

Conclusion

The red Type I endurance muscle is best trained with EMS devices at a frequency of up to 30 Hertz. These muscles are used during cardio training and appear more athletic.

The fast-twitch, large-area white skeletal muscle of Type II is best trained with a frequency of 50 to 100 Hertz. These muscles appear bulky and are activated during intense exertion.