Pennate muscles include triangular-shaped (e.g., adductor longus), unipennate or semipennate (e.g., flexor pollicis longus), bipennate (e.g., rectus femoris), multipennate (e.g., deltoid), and circumpennate (e.g., tibialis anterior) muscles.

Some muscles are bipennate and their fascicles join a single central tendon; multipennate muscles have more than one tendon coursing through the muscle substance. Fascicles may also present a curved spiral arrangement between the origin and the insertion (e.g., pectoralis major, supinator). Several muscles are composed of a single belly or may have a complex internal architecture made up of multiple heads with a different origin (e.g., two heads for the biceps brachii and the biceps femoris; three heads for the triceps brachii and the triceps surae) and join together to generate a distal tendon (Fig. 2a–d).

From the biomechanical point of view, during muscle contraction, the force is transmitted to the skeleton by the tendon or aponeurosis and may or may not result in joint motion. Skeletal muscle contractions may be divided into three types: isometric, when the muscle contracts, but there is no change in its length; isotonic or concentric, when the muscle contracts and simultaneously shortens; and eccentric, when the muscle contracts and at the same time lengthens.

When a muscle is activated by the nervous system and it is required to lift a load which is less than the maximum tetanic tension the muscle can generate, the muscle is able to shorten. Contractions that permit the muscle to shorten are referred to as concentric or isotonic contractions. An example of a concentric contraction is the lifting of a weight during a biceps curl. In concentric contractions, the force generated by the muscle is always less than the muscle’s maximum force. As the load the muscle is required to lift decreases, contraction velocity increases. This occurs until the muscle finally reaches its maximum contraction velocity.

In eccentric contractions the muscle actively lengthens. During normal activity, muscles are often active while they are lengthening. Classic examples of this are walking, when the quadriceps (knee extensors) are active just after heel strike while the knee flexes, or setting an object down gently (the arm flexors must be active to control the fall of the object).

As the load on the muscle increases, it finally reaches a point where the external force on the muscle is greater than the force that the muscle can generate. Thus even though the muscle may be fully activated, it is forced to lengthen due to the high external load. This is referred to an eccentric contraction. There are two main features to remark regarding eccentric contractions. First, the absolute tensions achieved are very high relative to muscle’s maximum tetanic tension generating capacity (you can set down a much heavier object than you can lift). Second, the absolute tension is relatively independent of lengthening velocity. This suggests that skeletal muscles are very resistant to lengthening. The basic mechanics of eccentric contractions are still a source of debate, since the cross-bridge theory, which so nicely describes concentric contractions, is not as successful in describing eccentric contractions. Many muscle injuries and delayed soreness are often associated with eccentric contraction. Moreover, muscle strengthening may be greater using exercises that involve eccentric contractions. It has been recently demonstrated that eccentric contractions that normally determine an injury may stimulate the activation and proliferation of satellite stem cells to guide skeletal muscle regeneration. For this reason therapeutic strategies to combat disuse muscle atrophy may be implemented by the use of eccentric contractions in a rehabilitative setting (Valero et al. 2012). During isometric contraction the muscle is activated, but instead of being allowed to lengthen or shorten, it is held at a constant length. An example of an isometric contraction would be carrying an object in front of you. The weight of the object would be pulling downward, but your hands and arms would be opposing the motion with equal force going upwards. Since your arms are neither raising nor lowering, your biceps will be isometrically contracting. The force generated during an isometric contraction is dependent on the length of the muscle while contracting. Maximal isometric tension is produced at the muscle’s optimum length, where the length of the muscle’s sarcomeres is on the plateau of the length-tension curve.

Sometimes, a fourth type of muscle “contraction,” known as passive stretch, is described. As the name implies, the muscle is being lengthened in a passive state (i.e., not being stimulated to contract) (Fig. 3a–d). An example of this would be the pull one feels in their hamstrings while touching their toes. The usefulness of stretching is still a controversial topic. Several studies investigated optimal stretching techniques causing confusion among researchers and clinicians. Further investigations may be useful to determine how passive and active muscular stretch may improve muscular function as a whole and eventually prevent musculoskeletal derangements (Williams et al. 2013). Examples of muscular contractions on ultrasound (US) examinations are demonstrated in three video clips (VIDEO 1–3) which can be accessed and downloaded via www.​extras.​springer.​com. Representative snapshots of the video clip for isometric contraction are given in Fig. 4.