Post-Burn Hand Muscle Imbalance: Understanding Flexor and Extensor Tension in the Hand
- Vita Yu-Hsien TU
- 2 hours ago
- 5 min read
Vita Yu-Hsien Tu, Occupational Therapist and Project Manager
Sunshine Social Welfare Foundation
Dealing with hand burns presents unique challenges due to the hand’s complex structure and intricate balance of muscles, tendons, and ligaments. Post-burn issues such as finger contracture, stiffness, or abnormal postures are not solely caused by scarring; they often stem from an imbalance between flexor and extensor muscles. This post-burn hand muscle imbalance can ultimately lead to functional limitations, making daily activities more challenging. Understanding the underlying mechanisms of muscle imbalance—why and how it develops—is essential for therapists to properly assess hand function and develop effective intervention strategies. This article explores the structure of the hand’s flexor and extensor muscles, the impact of burns on muscle tension balance, and how this post-burn muscle imbalance contributes to abnormal finger postures.
Anatomy of the Hand: Flexor and Extensor Muscles
Finger movement relies on the coordinated function of two major muscle groups: flexors and extensors. These muscles work as antagonists—when the flexors contract, the extensors must relax, and vice versa. During flexion movement, flexors act as the prime movers, while extensors serve as antagonists; during extension, extensors are the prime movers, and flexors become the antagonists.
The resting position and movement of each finger are determined by the interplay of forces on both the flexor and extensor sides. These forces include both dynamic contributions from muscles and static influences from ligaments. At rest, each finger naturally assumes a slightly flexed posture, with flexion progressively increasing from the index to the little finger. This positioning reflects the predominance of flexor forces over extensor forces, maintaining the hand’s natural balance.
The fingers’ flexor and extensor muscles and their main functions are as follows:
Finger Flexors
FDP (Flexor Digitorum Profundus)
The only muscle that directly flexes the DIP joint, also involved in PIP and MCP flexion.
FDS (Flexor Digitorum Superficialis)
Primarily responsible for PIP flexion, also assists MCP flexion.
Lumbricals & Interossei (intrinsic muscles)
The primary contributors to MCP flexion, particularly in fine motor tasks. They also aid in PIP and DIP extension.
Although multiple muscles contribute to MCP flexion, the dominant source of force varies significantly depending on wrist position. Strong gripping force is exerted mainly at the DIP and PIP joints and, when the wrist is extended, also at the MCP joints.
Finger Extensors
EDC (Extensor Digitorum Communis), EIP (Extensor Indicis Proprius), EDM (Extensor Digiti Minimi)
These muscles control active MCP extension but contribute less to PIP and DIP extension. EIP specifically extends the index finger, while EDM extends the little finger.
Lumbricals & Interossei (intrinsic muscles)
These intrinsic hand muscles provide extension force for the PIP and DIP joints but also act as MCP flexors.
Whether the hand is at rest or making movements, it relies on the balance between flexors and extensors, which must also be maintained by ligaments. Under normal circumstances, the MCP joint relies on the balance between the EDC and the intrinsic muscles (I); the PIP joint relies on the balance across the EDC, the intrinsic muscles and the flexor digitorum superficialis (FDS); the DIP joint relies on the balance across the EDC, the intrinsic muscles and the flexor digitorum profundus (FDP). When muscle tension becomes imbalanced, it affects finger movement and may result in abnormal hand postures.
Main Causes of Post-Burn Hand Muscle Imbalance
After a burn, several factors can contribute to post-burn muscle imbalance between the flexors and extensors, including:
Scar Contracture
Scar formation during wound healing can lead to contractures. If scars develop on the palmar side, they may cause finger flexion contractures, making extension difficult. Conversely, scars on the dorsal side can result in extension contractures, making flexion difficult. This leads to tendon and ligament tightness on one side, disrupting the balance between flexor and extensor tension.
Edema
Post-burn swelling in the hand can cause intrinsic muscles to become tight and less elastic. Over time, this can lead to excessive MCP flexion and hyperextension of the PIP and DIP joints, resulting in Intrinsic Tightness.
Prolonged Immobilization
If the hand remains immobilized for an extended period, it must be positioned correctly to ensure safety. Incorrect positioning can lead to muscle tightness or laxity, further exacerbating imbalance.
Tendon/Ligament Injury
If burns directly damage tendons or ligaments, or if wounds remain exposed, muscle movement will be affected. The most common scenario is dorsal PIP joint injury, leading to central slip rupture and subsequent deformity.
Nerve Damage
Ulnar nerve injuries due to burns can lead to Claw Hand, where intrinsic muscle weakness causes an imbalance between flexors and extensors. Median nerve injuries can result in Ape Hand, affecting thumb opposition.
Abnormal Hand Postures and Deformities From Post-Burn Hand Muscle Imbalance
Below are the common postures and deformities resulting from post-burn hand muscle imbalance. To learn more about post-burn deformities, check out our other blog article.
(Related article: Three useful cheat sheets so you'll never be confused about post-burn hand deformities)
Intrinsic Tightness
Mechanism: The lumbricals and interossei become excessively tight, creating an imbalance where the MCP joints are pulled into flexion, while the PIP and DIP joints remain extended (Intrinsic Plus Hand). This restricts PIP and DIP flexion, especially when the MCP joint is extended. The imbalance is often caused by scar contractures, edema, or prolonged immobilization.
Abnormal Posture: MCP hyperflexion, PIP & DIP hyperextension (Intrinsic Plus Hand posture).
Clinical Signs: When MCP is flexed, PIP extends normally; when MCP is extended, PIP flexion is limited (Bunnell-Littler test is positive).
Claw Hand
Mechanism: Muscle imbalance arises from weak intrinsic hand muscles (lumbricals and interossei), often due to dorsal scar contractures. Without intrinsic muscle function to flex the MCP joints and extend the PIP/DIP joints, the extrinsic flexors (FDP, FDS) and extrinsic extensors (ED) dominate, leading to MCP hyperextension and PIP/DIP flexion.
Abnormal Posture: MCP hyperextension, PIP & DIP hyperflexion.
Clinical Signs: Passive MCP flexion allows PIP/DIP to extend more easily (opposite of intrinsic tightness).
Swan-Neck Deformity
Mechanism: Muscle imbalance occurs when the PIP joint becomes hyperextended, often due to ligament laxity or an abnormal extensor mechanism. As the PIP extends excessively, the flexor digitorum profundus (FDP) compensates by pulling the DIP joint into flexion, creating the characteristic PIP hyperextension and DIP flexion pattern.
Abnormal Posture: PIP hyperextension, DIP hyperflexion, sometimes with MCP flexion.
Boutonnière Deformity
Mechanism: Muscle imbalance arises from a weakened or ruptured central slip, which normally extends the PIP joint. As a result, the flexor digitorum superficialis (FDS) pulls the PIP into flexion, while the lateral bands of the extensor mechanism shift volarly and become more effective in extending the DIP joint, leading to DIP hyperextension — which is the opposite of swan-neck deformity.
Abnormal Posture: MCP hyperextension, PIP hyperflexion, DIP hyperextension.
Conclusion
Post-burn muscle imbalance between flexors and extensors can lead to various deformities and abnormal postures, significantly impacting daily function. It is crucial for therapists to recognize and address this issue early through preventive interventions and timely rehabilitation to minimize functional impairments and improve patients’ quality of life.
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