The first digit of the forearm is called the thumb. The thumb is a unique structure in the human hand that allows for various gripping and grasping actions. It is opposable to the other fingers, meaning it can touch each of the other digits, enabling humans to perform delicate and intricate tasks.

**Types of Muscles in the Forearm:**
The forearm is composed of various muscles that are responsible for the movement and control of the hand and wrist. These muscles can be broadly classified into two groups:

1. **Extrinsic Muscles:** These muscles originate from outside the forearm and extend into the hand. They are responsible for controlling the movement of the wrist and fingers. Examples include the flexor muscles (flexor carpi radialis, flexor carpi ulnaris) and extensor muscles (extensor carpi radialis longus, extensor carpi radialis brevis).

2. **Intrinsic Muscles:** These muscles are located entirely within the forearm and are responsible for controlling the movement of the fingers and thumb. Examples include the flexor digitorum profundus, extensor digitorum, and abductor pollicis brevis.

**Muscle Fibre Types:**
Muscles are composed of individual muscle fibers that contract to produce movement. There are three main types of muscle fibers:

1. **Type I (Slow Twitch) Fibers:** These muscle fibers are efficient at producing energy through aerobic metabolism. They are fatigue-resistant and are well-suited for activities requiring prolonged contractions, such as endurance exercises. In the forearm, type I fibers are predominant in muscles involved in maintaining posture, such as the extensor muscles.

2. **Type IIa (Fast Oxidative) Fibers:** These muscle fibers have a moderate capacity for aerobic metabolism. They are relatively fatigue-resistant and can generate more force and speed compared to type I fibers. In the forearm, type IIa fibers are found in muscles involved in activities requiring both strength and endurance, such as the flexor muscles.

3. **Type IIb (Fast Glycolytic) Fibers:** These muscle fibers have a high capacity for anaerobic metabolism. They can generate a large amount of force but fatigue quickly. In the forearm, type IIb fibers are present in muscles involved in rapid and powerful movements, such as the muscles controlling finger and thumb flexion and extension.

In conclusion, the first digit of the forearm is called the thumb. The forearm is composed of extrinsic and intrinsic muscles responsible for the movement and control of the hand and wrist. Muscle fibers in the forearm can be classified into type I (slow twitch), type IIa (fast oxidative), and type IIb (fast glycolytic) based on their metabolic properties.