What is an Accessory Muscle?

Accessory muscles, in the context of human anatomy and physiology, refer to muscles that assist the primary muscles responsible for a particular movement. While not the prime movers, these muscles play a crucial role in augmenting force, providing stability, and refining the precision of actions. Understanding accessory muscles is fundamental to comprehending the intricate biomechanics of movement, whether in everyday activities or specialized athletic endeavors. Their involvement is particularly pronounced during strenuous activities or when the primary muscles are fatigued or compromised.

The Respiratory Accessory Muscles

The respiratory system, primarily driven by the diaphragm and intercostal muscles, can enlist the aid of accessory muscles, especially during periods of increased respiratory demand. These situations can include intense physical exertion, respiratory distress, or conditions that impede normal breathing.

Muscles of Inspiration

During normal, quiet breathing, the diaphragm contracts and flattens, and the external intercostal muscles lift the rib cage, expanding the chest cavity and drawing air into the lungs. However, when a deeper or more forceful inhalation is required, accessory muscles of inspiration come into play.

• Sternocleidomastoid: This prominent muscle of the neck, originating from the sternum and clavicle and inserting on the mastoid process of the temporal bone, is a key accessory inspiratory muscle. When its attachments are stabilized, it can elevate the sternum and clavicles, thereby increasing the volume of the thoracic cavity. This action is readily observable as the neck and upper chest move upwards during deep breaths.

• Scalenes: Located in the side and front of the neck, the scalene muscles (anterior, middle, and posterior) originate from the cervical vertebrae and insert onto the first and second ribs. They assist in elevating the upper ribs, further expanding the thoracic cage during forced inhalation. Their contribution is significant in situations where breathing is compromised, such as in individuals with chronic obstructive pulmonary disease (COPD).

• Pectoralis Minor: This smaller muscle lies beneath the pectoralis major. When the shoulder girdle is fixed, the pectoralis minor can elevate the ribs to which it attaches, contributing to an increase in thoracic volume. This is often seen when someone is using their arms to support their body weight, aiding in breathing.

• Serratus Anterior: This fan-shaped muscle originates from the outer surfaces of the upper eight or nine ribs and inserts along the entire length of the medial border of the scapula. While primarily known for its role in scapular protraction and rotation, it can also assist in forced inspiration by elevating the ribs.

Muscles of Expiration

Normally, exhalation is a passive process driven by the elastic recoil of the lungs and chest wall after the inspiratory muscles relax. However, during forceful exhalation, such as coughing, sneezing, or strenuous exercise, accessory muscles of expiration become active.

• Abdominal Muscles: The rectus abdominis, external and internal obliques, and transversus abdominis are the primary accessory muscles of forced exhalation. By contracting, they decrease the volume of the abdominal cavity, pushing the diaphragm upwards and compressing the thoracic cavity, thereby expelling air from the lungs with greater force. This is why you might feel your abdominal muscles working intensely when you cough forcefully.

• Internal Intercostals: While the external intercostals are involved in inspiration, the internal intercostals contract during forced exhalation. They pull the ribs downwards and inwards, reducing the volume of the thoracic cavity.

Accessory Muscles in Limb Movement

Accessory muscles also play a vital role in facilitating and refining movements of the limbs, particularly the arms and legs. They often work in synergy with the prime movers to achieve a desired action.

Shoulder and Arm Movements

The shoulder joint is a highly mobile ball-and-socket joint, allowing for a wide range of motion. This mobility relies on a coordinated effort of many muscles, including numerous accessory muscles.

• Deltoid: While the anterior, middle, and posterior fibers of the deltoid are primary movers for shoulder abduction, flexion, and extension respectively, their collective action can also contribute to other shoulder movements, acting as accessory muscles. For instance, during a bicep curl, the anterior deltoid assists the biceps brachii.

• Trapezius: This large, superficial muscle of the upper back and neck has upper, middle, and lower portions, each with distinct actions. While its primary actions include elevating, depressing, retracting, and rotating the scapula, it also assists in shoulder abduction and neck extension. For example, during overhead pressing movements, the upper trapezius assists the deltoids.

• Rhomboids: Located deep to the trapezius, the rhomboid major and minor muscles are primarily responsible for retracting the scapula. However, they also assist in stabilizing the scapula during arm movements.

• Rotator Cuff Muscles (Supraspinatus, Infraspinatus, Teres Minor, Subscapularis): While their primary role is to stabilize the glenohumeral joint and facilitate rotation, these muscles can also act as accessory muscles during various arm movements, particularly in providing fine control and preventing unwanted motion. The supraspinatus, for instance, initiates abduction, assisting the deltoid.

Leg and Foot Movements

Similar to the upper limb, the lower limb also utilizes accessory muscles for efficient and controlled movement.

• Quadriceps Femoris Group: The vastus lateralis, vastus medialis, vastus intermedius, and rectus femoris collectively extend the knee. However, during movements like walking or running, other muscles may act as accessory muscles. For example, the iliopsoas, a primary hip flexor, can assist in initiating leg swing.

• Hamstring Group: The biceps femoris, semitendinosus, and semimembranosus primarily flex the knee and extend the hip. During movements like squatting, they work in concert with the gluteal muscles and quadriceps. The gastrocnemius, a calf muscle, can also assist in hip extension and knee flexion to a limited degree.

• Ankle and Foot Muscles: Numerous small muscles in the lower leg and foot assist in stabilizing the ankle and providing fine motor control for walking, running, and maintaining balance. For example, the tibialis anterior, primarily responsible for dorsiflexion, can also assist in controlling foot drop during the swing phase of walking.

The Importance of Accessory Muscles

Accessory muscles are not merely secondary players; they are integral components of our musculoskeletal system, contributing significantly to our ability to move efficiently and effectively.

• Enhanced Force Production: By working in conjunction with prime movers, accessory muscles can amplify the force generated, allowing for more powerful movements. This is critical in activities requiring bursts of strength.

• Stabilization and Control: Accessory muscles provide crucial stability to joints, preventing excessive or unwanted movement. This is particularly important for complex joints like the shoulder and hip, ensuring precision and preventing injury.

• Fatigue Management: When prime movers become fatigued, accessory muscles can take over some of their workload, allowing an activity to continue. This is a vital mechanism for endurance.

• Fine-Tuning of Movement: Accessory muscles allow for subtle adjustments and refinements in movement, contributing to agility and dexterity. This is evident in sports requiring intricate motor skills.

• Adaptability to Demands: The recruitment of accessory muscles allows the body to adapt to varying levels of physical demand. From a gentle stroll to an all-out sprint, the body can call upon additional muscular support as needed.

When Accessory Muscles Become Problematic

While generally beneficial, the over-reliance on or dysfunction of accessory muscles can indicate underlying issues.

• Respiratory Distress: In respiratory conditions, accessory muscles of inspiration may become visibly hypertrophied due to the constant effort to breathe. Their increased use during normal breathing signifies a problem with the primary respiratory muscles or the airways.

• Muscle Imbalances and Pain: If accessory muscles are consistently overused due to weakness or inhibition of prime movers, it can lead to muscle imbalances, strain, and pain. For example, tightness in the neck and shoulder accessory muscles can contribute to headaches and neck pain.

• Injury: An injury to a primary muscle group can lead to compensatory overuse of accessory muscles, potentially leading to secondary injuries.

Understanding the role and function of accessory muscles is essential for professionals in fields such as physical therapy, sports medicine, and kinesiology. It provides deeper insight into biomechanics, injury prevention, and rehabilitation strategies, ultimately contributing to a more comprehensive understanding of human movement.