Aug 18, 2026

Health and Wellness

Ideal posture is not only determined by proper spinal alignment and curvature but also by the ability of muscles to maintain that alignment efficiently. Muscles play an essential role not only in maintaining static posture but also in producing functional movement.

To understand muscle function in ideal posture, it is important to understand the major classifications of muscle systems that have been used to describe posture and movement.

Tonic and Phasic Muscles

Traditionally, muscles have been classified into tonic muscles and phasic muscles.

Tonic muscles are primarily composed of Type I muscle fibers and are responsible for low-intensity, sustained activity. They have high endurance and contribute to postural stability. For example, the soleus muscle remains continuously active during standing and helps maintain balance against gravity.

Phasic muscles are mainly composed of Type II muscle fibers and are responsible for producing larger and faster movements. Although they generate greater force, they fatigue more quickly compared to tonic muscles. The gastrocnemius muscle, for example, produces powerful propulsion during walking and running but has lower endurance.

However, Kolar emphasized that all muscles contain both tonic and phasic motor units in different proportions. The key difference is not simply fiber type, but how the nervous system controls these muscles during posture and movement. Both systems must work together in a balanced manner to achieve efficient movement.

Postural and Phasic Muscles (Janda’s Classification)

Janda focused more on functional characteristics rather than muscle fiber composition.

Postural muscles tend to become overactive, tight, and shortened. They are easily activated, especially during stress, pain, fatigue, or injury. Their increased activity may contribute to muscle imbalance, altered posture, and joint dysfunction.

Phasic muscles tend to become inhibited, weakened, and atrophied. They often show reduced activation during movement dysfunction and are commonly associated with poor motor control.

According to Janda, overactive muscles have a lower activation threshold, meaning they are recruited more easily. This can lead to dominance of certain muscle groups and inhibition of their antagonists, contributing to abnormal movement patterns.

Local and Global Muscle Systems (Bergmark)

Bergmark introduced the concept of local and global muscle systems to explain spinal stability.

The local muscle system consists of muscles that attach directly to the lumbar spine and provide segmental stability. These muscles include the multifidus, interspinales, intertransversarii, medial fibers of the lumbar erector spinae, and medial fibers of the quadratus lumborum. Their main role is to control spinal alignment and maintain segmental stability.

The global muscle system consists of larger muscles that generate movement and manage external loads. These include the thoracic erector spinae, abdominal muscles, and lateral fibers of the quadratus lumborum. Their role is to control larger movements and transfer external forces so that the local system can maintain spinal stability.

In simple terms:

External load → Global system → Reduced load transfer → Local system controls spinal stability

Both systems must work together to maintain efficient movement and protect the spine.

Stabilizers and Mobilizers

Comerford and Mottram expanded the concepts of local/global systems by introducing stabilizers and mobilizers.

Stabilizers provide controlled support and maintain joint alignment, while mobilizers generate larger movements. However, Kolar emphasized that muscles cannot be strictly categorized as only stabilizers or mobilizers because the same muscle may perform different roles depending on the task, posture, and external demands.

Movement and stability develop together and continuously interact.

Systemic Local Muscle System (SLMS) and Systemic Global Muscle System (SGMS)

A more comprehensive classification integrates structural and functional concepts: the Systemic Local Muscle System (SLMS) and the Systemic Global Muscle System (SGMS).

Systemic Local Muscle System (SLMS)

The SLMS is closely related to normal postural reflex mechanisms and provides the foundation for posture, stability, and controlled movement.

These muscles are generally:

• Located deep within the body
• Smaller in size
• Attached close to joints
• Rich in muscle spindles for sensory feedback
• Responsible for precise postural adjustments

The SLMS provides internal stability and controls movement around the body’s central axis. Examples include the transversus abdominis, multifidus, diaphragm, and pelvic floor muscles.

These muscles work together to create intra-abdominal pressure (IAP), which contributes to spinal stability.

Systemic Global Muscle System (SGMS)

The SGMS consists of more superficial muscles that respond to external forces and larger body movements.

These muscles are:

• Strong and powerful
• Easily activated
• Dominant during large movements
• More likely to become tight and shortened when overused

Examples include the larger abdominal muscles, erector spinae, and other superficial trunk muscles.

When the SGMS becomes dominant, it may reduce the contribution of the SLMS and contribute to abnormal movement patterns, muscle imbalance, and pain.

Conclusion

Optimal posture and movement require a balance between the Systemic Local Muscle System (SLMS) and the Systemic Global Muscle System (SGMS).

The SLMS provides the foundation for postural control, stability, and precise movement, while the SGMS provides strength and movement efficiency. Neither system works independently; their interaction allows the body to maintain alignment, adapt to external demands, and perform functional movements efficiently.

Therefore, clinical assessment should not focus only on individual muscles but also evaluate movement patterns and the interaction between these two systems. Identifying imbalance and restoring proper coordination between the systems are essential for improving posture, movement quality, and reducing pain.