Causes of Weakness in the Teres Minor and Triceps: The Role of Sarcolemma Dysfunction and ACTN3 Gene Mutations

By SWA


Muscle weakness can arise from a combination of localized injuries, nerve damage, systemic disorders, and even genetic factors. Two muscles that often experience weakness under these circumstances are the teres minor and the triceps brachii. The teres minor, part of the rotator cuff, plays a key role in stabilizing and externally rotating the shoulder, while the triceps is the primary extensor of the elbow. Both muscles can be affected by structural issues at the cellular level, particularly through sarcolemma dysfunction and ACTN3 gene mutations. This article explores the various causes of weakness in these muscles and how cellular and genetic factors contribute to muscle performance.

Understanding the Teres Minor and Triceps: Their Roles and Vulnerabilities

Teres Minor:

The teres minor is one of the four rotator cuff muscles responsible for external rotation of the shoulder and stabilization of the glenohumeral joint. It works alongside the infraspinatus and is particularly vulnerable to injury due to its role in repetitive overhead activities and shoulder stability.

Triceps Brachii:

The triceps is a large muscle on the back of the upper arm, consisting of three heads (long, lateral, and medial). Its primary function is elbow extension, but it also assists in shoulder stabilization, particularly the long head. Weakness in the triceps can significantly affect pushing movements, overhead tasks, and arm strength.

Common Causes of Weakness in the Teres Minor and Triceps

1. Nerve Impingement or Injury:

  • Teres Minor: The axillary nerve innervates the teres minor. Conditions like quadrilateral space syndrome can compress this nerve, leading to weakness and atrophy of the muscle.

  • Triceps: The radial nerve supplies the triceps. Radial nerve palsy, which can result from humeral fractures, prolonged compression, or nerve entrapment, leads to weakness in elbow extension.

2. Rotator Cuff or Tendon Injuries:

  • Teres Minor: A partial or full-thickness tear in the rotator cuff, although more common in the supraspinatus, can involve the teres minor. This results in pain and reduced shoulder strength, particularly in external rotation.

  • Triceps: Triceps tendonitis or tendon rupture can cause localized pain and weakness, making it difficult to extend the elbow fully.

3. Disuse Atrophy and Muscle Imbalance:

  • Disuse Atrophy: Prolonged immobilization following injury or surgery can lead to muscle wasting. Lack of use causes muscle fibers to shrink, resulting in generalized weakness in both the teres minor and triceps.

  • Muscle Imbalance: Overdevelopment of antagonistic muscles (e.g., internal rotators overpowering the teres minor or biceps overpowering the triceps) can lead to chronic weakness due to imbalance in muscle forces.

4. Systemic Conditions and Aging:

  • Inflammatory Myopathies: Autoimmune conditions like polymyositis or dermatomyositis can cause inflammation and weakness in various muscles, including the teres minor and triceps.

  • Aging and Degeneration: Sarcopenia, the age-related loss of muscle mass and strength, can affect both muscles. Degenerative changes in the shoulder and elbow joints also contribute to muscle weakness over time.

The Role of Sarcolemma Dysfunction in Muscle Weakness

The sarcolemma is the cell membrane that surrounds muscle fibers, playing a vital role in muscle contraction, structural integrity, and signal conduction. Damage to the sarcolemma disrupts the ability of muscles to contract effectively, leading to weakness and atrophy.

Conditions Linked to Sarcolemma Dysfunction:

  1. Muscular Dystrophies (DMD and BMD):

    • Duchenne Muscular Dystrophy (DMD) and Becker Muscular Dystrophy (BMD) are caused by mutations in the dystrophin gene, a key protein that stabilizes the sarcolemma. The absence of dystrophin results in repeated muscle fiber damage, leading to progressive weakness. While larger muscles are affected first, over time, muscles like the teres minor and triceps become involved.

  2. Limb-Girdle Muscular Dystrophy (LGMD):

    • LGMD affects muscles in the shoulder and pelvic girdle, with many subtypes linked to mutations in sarcolemma-associated proteins. This leads to progressive muscle weakness, including in the rotator cuff and upper arm.

  3. Inflammatory Myopathies:

    • Conditions like polymyositis and dermatomyositis can cause immune-mediated attacks on muscle fibers, damaging the sarcolemma and resulting in muscle weakness.

  4. Sarcolemmal Channelopathies:

    • Disorders such as hypokalemic periodic paralysis involve ion channel dysfunction in the sarcolemma, causing episodic muscle weakness that can affect various muscles, including the triceps.

  5. Trauma or Ischemia:

    • Direct trauma or ischemic injury can damage the sarcolemma, leading to localized weakness in muscles like the teres minor and triceps.

ACTN3 Gene Mutations and Muscle Weakness

The ACTN3 gene encodes alpha-actinin-3, a protein found in fast-twitch (type II) muscle fibers, which are responsible for generating rapid, powerful contractions. While ACTN3 mutations are not directly linked to muscle diseases, they significantly influence muscle performance and strength.

Function of the ACTN3 Gene:

  • Structural Role: Alpha-actinin-3 helps anchor actin filaments in the Z-discs of fast-twitch muscle fibers, providing structural stability during contraction.

  • Influence on Muscle Power: The presence of alpha-actinin-3 enhances the muscle’s ability to generate explosive power and strength.

ACTN3 R577X Polymorphism:

A common genetic variation in ACTN3 is the R577X polymorphism, which affects the production of alpha-actinin-3.

  • R Allele (Functional Alpha-Actinin-3):
    Individuals with one or two R alleles (RR or RX genotypes) produce functional alpha-actinin-3. This is associated with greater muscle strength and power performance.

  • X Allele (Non-functional Alpha-Actinin-3):
    Individuals with the XX genotype lack alpha-actinin-3 due to a premature stop codon. This leads to reduced fast-twitch muscle efficiency and favors endurance performance over explosive strength.

Impact of ACTN3 Mutations on the Teres Minor and Triceps:

  • Teres Minor: As part of the rotator cuff, the teres minor relies on fast-twitch fibers for dynamic shoulder movements, especially in sports or activities requiring rapid external rotation. Individuals with the XX genotype may experience reduced power in these movements, potentially leading to functional weakness over time.

  • Triceps: The triceps, especially during activities like pushing and lifting, relies on fast-twitch fibers for explosive elbow extension. A lack of functional alpha-actinin-3 can result in reduced strength and power, affecting performance in strength-based tasks.

When to Seek Medical Attention

Persistent weakness in the teres minor, triceps, or other muscles should be evaluated by a healthcare provider, especially if it is progressive, bilateral, or accompanied by other systemic symptoms. While ACTN3 mutations alone do not cause disease, muscle weakness related to sarcolemma dysfunction or other underlying conditions warrants further investigation.

Diagnostic Approaches:

  • Electromyography (EMG): To assess the electrical activity of muscles and detect nerve or muscle disorders.

  • Muscle Biopsy: To evaluate for structural abnormalities, such as sarcolemma damage or dystrophic changes.

  • Genetic Testing: To identify mutations in genes like ACTN3, dystrophin, or other sarcolemma-associated proteins.

  • Imaging (MRI/Ultrasound): To visualize muscle atrophy, fatty infiltration, or structural damage in the shoulder and upper arm.

Conclusion

Weakness in the teres minor and triceps can result from a range of causes, including nerve injuries, tendon tears, disuse atrophy, and systemic conditions like inflammatory myopathies. Sarcolemma dysfunction plays a significant role in muscle weakness, particularly in conditions like muscular dystrophy and channelopathies. Additionally, ACTN3 gene mutations influence muscle performance by affecting the function of fast-twitch fibers, leading to variations in power and endurance capabilities. Recognizing these factors and seeking appropriate medical evaluation can help guide treatment strategies and improve muscle function and overall quality of life.

Reference:
The Effect of ACTN3 and VDR Polymorphisms on Skeletal Muscle Performance in Axial Spondyloarthropathies https://pubmed.ncbi.nlm.nih.gov/34456969/ 

SPP1 genotype is a determinant of disease severity in Duchenne muscular dystrophy
https://pubmed.ncbi.nlm.nih.gov/21178099/

More: 89 citations for rs1815739 https://www.ncbi.nlm.nih.gov/snp/rs1815739#publications






© 2000-2025 Sieglinde W. Alexander. All writings by Sieglinde W. Alexander have a fife year copy right. Library of Congress Card Number: LCN 00-192742 ISBN: 0-9703195-0-9

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