IIaspiervezels

IIa spiervezels, also known as type IIa muscle fibers or fast oxidative glycolytic (FOG) fibers, are a class of skeletal muscle fibers that combine both fast contractile speed and substantial endurance capacity. They are fast-twitch fibers that rely on a mix of oxidative and glycolytic metabolism and express myosin heavy chain type IIa.

Physiologically, IIa fibers have a relatively high mitochondrial content, abundant myoglobin, and dense capillary networks, giving them greater oxidative capacity than the fastest glycolytic fibers (type IIx) and better fatigue resistance than those fibers. They contract more quickly than slow-twitch type I fibers, allowing rapid force production, while maintaining a level of fatigue resistance suitable for sustained efforts.

Functionally, IIa fibers are recruited for activities that require a balance of speed and endurance, such as middle-distance running, cycling at moderate to high intensity, or repeated sprints with recovery. They play a key role in many daily and athletic movements where both power and endurance are advantageous.

Training and adaptation influence IIa fibers. Endurance training tends to enhance their oxidative capacity and fatigue resistance, while resistance training can increase their cross-sectional area and force output. Type IIa fibers are considered plastic; their properties can shift with training and aging, and their relative proportion varies among individuals and muscles.

Clinically and in aging, declines in oxidative capacity and fiber size can affect performance and mobility, highlighting the relevance of maintaining IIa fiber health through balanced training.