Summary
Highlights
The number of motor units recruited depends on the muscle type. Gross muscles (e.g., in legs) involve one nerve fiber stimulating many muscle fibers, while fine muscles (e.g., in hands) have fewer muscle fibers stimulated per nerve fiber, allowing for more precise movements. Electrostimulation must consider these differences in recruitment based on the muscle's inherent characteristics.
Different fiber types require different discharge frequencies and recovery times. Slow-twitch fibers need longer pauses between stimulations, while fast-twitch fibers can be stimulated more rapidly. The speed of muscle contraction also varies; slow fibers contract gradually, while fast fibers show quicker contractions. This influences how pulse and pause times are set for effective electrostimulation.
Muscle contraction comprises three phases: latency (initial stimulation causing ion migration and depolarization), active (muscle contraction), and repolarization (elements returning to their resting state). These phases constitute the refractory period, during which the muscle cannot be immediately re-stimulated. A new pulse should only be applied after this refractory period ends to ensure effective stimulation.
Electrostimulation is categorized into neuromuscular (for intact nerve-muscle connections) and muscular (for denervated muscle fibers due to nerve damage). Neuromuscular electrostimulation, the focus here, considers intensity and polarity for effective treatment.
Intensity is crucial, adapted to the muscle's condition (atrophy, strength) to achieve depolarization. It’s determined by rheobase (threshold level) and chronaxie (duration from stimulation onset to refractory period end). Polarity matters for electrode placement; the cathode, inducing greater stimulation, is placed on the motor point (most sensitive area) as per the law of polar actions, which states that stimulation is strongest at the cathode's closing.
For successive contractions, a frequency of 8-10 Hz is recommended for visible and quality contractions. Frequencies between 25-80 Hz can cause tetanization (sustained muscle contraction). At 1000 Hz or higher, an inhibition effect (Wedensky inhibition) occurs, canceling contractile effects because muscle fibers cannot be stimulated without respecting their refractory period, leading to no response.