The basic functional unit of muscle is the called the Muscle-Tendon unit, which attaches to the bones across one or two joints. For example, the biceps femoris (a hamstring muscle) crosses both the hip and knee joints, and is described as a two-joint (biarticular) muscle. The vastus lateralis (a quadriceps muscle) only crosses the knee joint (uniarticular). Muscle strains primarily occur in two-joint muscles such as the hamstrings and calf. This occurs because the muscle can be stretched at both ends by movements at two different joints.
The muscle cell is called a Muscle Fiber. Fibers are arranged into groups called fasicles. Some muscles such as the gastrocnemius (calf) have more than 1 million fibers, while some of the small muscles of the inner ear have less than a thousand fibers. There are several types of muscle fiber, but the simplest way to categorize muscle fibers is define them as slow-twitch or fast-twitch. Muscles responsible for controlling posture, such as the soleus and erector spinae muscles, are primarily slow-twitch and are resistant to fatigue. Muscles responsible for intermittent rapid-force development, such as the gastrocnemius and hamstrings during sprinting, are primarily fast- twitch, and will fatigue rapidly. Muscle hypertrophy refers to the increase in muscle-fiber cross- sectional area that occurs with resistance training. Strength gains are primarily due to hypertrophy with minimal increase in the number of muscle fibers (hyperplasia).
Each muscle fiber is made up of a collection of smaller fibers called Myofibrils. Muscle hypertrophy is due to an increase in the number of myofibrils within each fiber. Each myofibril extends the full length of the fiber, and is made up of a longitudinal arrangement of units called sarcomeres. The sarcomere is the structure responsible for the actual contraction. Each sarcomere consists of thick (myosin) and thin (actin) protein filaments that lie adjacent to one another horizontally, but are slightly separated longitudinally. Contraction occurs when the thick filaments attach to the thin filaments (crossbridge) and pull the thin filaments toward the center of the thick filament. The number of sarcomeres in series along a myofibril depends on the length and architecture of the specific muscle. The sartorius, which is the longest muscle in the body, has more than 100,000 sarcomeres in series, while a shorter muscle, such as the soleus, has closer to 10,000 sarcomeres in series.