Why does spasticity predominantly affect the flexor muscles in the upper extremity and extensor muscles in the lower extremity?

From the Guidelines

Spasticity predominantly affects the flexor muscles in the upper extremity and extensor muscles in the lower extremity due to the predominance of antigravity muscles in these areas. In the upper extremity, flexor muscles (like biceps and wrist flexors) are primarily responsible for antigravity movements, such as lifting objects or bringing food to the mouth, and are more active in daily activities, thus developing more spasticity when neurological control is impaired 1. In the lower extremity, extensor muscles (like quadriceps and calf muscles) are the main antigravity muscles, essential for standing and walking, and are constantly engaged to maintain posture and resist gravity, making them more prone to spasticity in neurological conditions 1. Some key points to consider in managing spasticity include:

• The use of targeted injection of botulinum toxin into localized upper limb muscles to reduce spasticity, improve passive or active range of motion, and improve dressing, hygiene, and limb positioning, as recommended by the American Heart Association/American Stroke Association guidelines 1.
• The consideration of oral antispasticity agents, such as baclofen (10-25 mg, 3-4 times daily) or tizanidine (2-4 mg, up to 3-4 times daily), for generalized spastic dystonia, although they may result in dose-limiting sedation or other side effects 1.
• The potential benefits of physical modalities, such as NMES or vibration applied to spastic muscles, to improve spasticity temporarily as an adjunct to rehabilitation therapy 1.
• The importance of tailoring treatment to the individual's specific needs and the severity of spasticity, as spasticity is correlated with activity limitations associated with hygiene, dressing, and pain, and can increase caregiver burden and reduce quality of life 1.

From the Research

Spasticity and Muscle Affected

• Spasticity is a disorder of the sensorimotor system characterized by a velocity-dependent increase in muscle tone with exaggerated tendon jerks, resulting from hyperexcitability of the stretch reflex 2
• In the upper extremity, spasticity typically affects the flexor muscles, leading to features such as shoulder adduction/internal rotation, forearm pronation, and elbow, wrist, and finger flexion 3
• The exact reason for the predominance of flexor muscle spasticity in the upper extremity is not explicitly stated in the provided studies, but it may be related to the hyperexcitability of the alpha motoneuron pool at the segmental level 2
• In contrast, spasticity in the lower extremity typically affects the extensor muscles, although this is not directly addressed in the provided studies

Pathophysiology of Spasticity

• Spasticity is caused by a central nervous system injury, which leads to a hyperactive reflex pathway 4
• The hyperexcitability of the stretch reflex is thought to occur through a variety of mechanisms, including changes in the rheologic properties of the involved and neighboring muscles 2
• Chronic spasticity can lead to changes in muscle tone, stiffness, contracture, atrophy, and fibrosis, which can interact with pathologic regulatory mechanisms to prevent normal control of limb position and movement 2

Clinical Implications

• The distinction between resistance due to spasticity and that due to rheologic changes is important for therapeutic implications 2
• Diagnostic nerve or motor point blocks and dynamic or multichannel EMG are useful to distinguish the contributions of spasticity and stiffness to the clinical problem 2
• Surgical management of spasticity involves tendon lengthening, release, and transfer, as well as selective neurotomy, in an effort to reduce spastic muscle hypertonicity, restore balance, prevent further contracture, and improve posture and function 3