Duchenne muscular dystrophy (DMD) is a severe X-linked recessive genetic disorder that profoundly affects skeletal muscle throughout the body, with particularly devastating consequences for the lower limbs. Caused by mutations in the dystrophin gene, DMD results in the absence or severe deficiency of dystrophin protein, which is essential for maintaining muscle fiber integrity during contraction. The lower limb pathology in DMD follows a characteristic pattern of progressive weakness, contractures, and functional decline that fundamentally alters ambulation and quality of life.
Early Lower Limb Manifestations
The effects of DMD on the lower limbs typically become apparent in early childhood, usually between ages two and five. Initial signs often include difficulty with activities requiring lower limb strength and coordination, such as running, jumping, and climbing stairs. Boys with DMD characteristically develop a waddling gait pattern, known as Trendelenburg gait, resulting from proximal muscle weakness in the hip abductors and gluteal muscles. This distinctive walking pattern occurs as the child compensates for weak hip stabilizers by shifting their trunk over the stance leg with each step.
Another hallmark early sign is the Gowers’ sign, where affected children use their hands to “walk” up their own legs when rising from the floor. This maneuver compensates for profound weakness in the quadriceps, hip extensors, and other proximal lower limb muscles. The proximal-to-distal gradient of weakness is a defining characteristic of DMD, with hip and thigh muscles affected earlier and more severely than distal leg and foot muscles in the disease’s initial stages.
Pseudohypertrophy of the calf muscles represents another distinctive lower limb feature of DMD. Despite appearing enlarged and muscular, these calves are actually infiltrated with fat and connective tissue rather than containing functional muscle fibers. This false hypertrophy creates a deceptive appearance of strength while the underlying muscle tissue is degenerating.
Progressive Muscle Weakness and Degeneration
As DMD progresses, the lower limb muscles undergo relentless degeneration. Without functional dystrophin to protect muscle fibers from contraction-induced damage, repeated cycles of injury and inadequate repair lead to progressive muscle fiber necrosis. The body attempts to regenerate damaged fibers, but this compensatory mechanism ultimately fails, and muscle tissue is gradually replaced by adipose and fibrous connective tissue.
The quadriceps femoris muscles, crucial for knee extension and maintaining upright posture, are among the most severely affected muscle groups. Progressive quadriceps weakness makes standing from a seated position increasingly difficult and eventually impossible without assistance. The hip extensors, particularly the gluteus maximus, also deteriorate significantly, compromising the ability to maintain hip extension during standing and walking. This weakness forces affected individuals to adopt compensatory postural strategies, including increased lumbar lordosis (swayback) to shift their center of gravity posteriorly and maintain balance.
The hip flexors, ankle dorsiflexors, and plantarflexors also weaken progressively, though typically at a slower rate than proximal muscles initially. The tibialis anterior muscle, responsible for lifting the foot during the swing phase of gait, weakens substantially, leading to foot drop and an increased risk of tripping. As plantarflexor muscles weaken, the ability to rise onto tiptoes and provide push-off during walking diminishes.
Contractures and Skeletal Deformities
One of the most debilitating consequences of DMD in the lower limbs is the development of progressive contractures. As muscles weaken and boys become less mobile, soft tissues around joints—including muscles, tendons, and joint capsules—shorten and become fibrotic. The hip flexors, knee flexors (hamstrings), and ankle plantarflexors are particularly prone to contracture development.
Ankle equinus contractures, where the foot becomes fixed in a plantarflexed position, are nearly universal in DMD. These contractures develop because the gastrocnemius and soleus muscles are relatively preserved compared to the weakened dorsiflexors, creating muscle imbalance. Additionally, when boys spend more time sitting or lying down as the disease progresses, gravity and positioning favor plantarflexion. Equinus contractures severely compromise standing ability and necessitate the use of ankle-foot orthoses to maintain a plantigrade foot position.
Knee flexion contractures develop as the hamstring muscles become shortened and fibrotic. These contractures make standing with extended knees impossible, forcing affected individuals to stand and walk with bent knees, which dramatically increases the energy cost of ambulation and accelerates functional decline. Hip flexion contractures similarly prevent full hip extension, contributing to the characteristic crouched posture seen in ambulatory boys with advanced Duchenne muscular dystrophy.
These contractures create a vicious cycle: as contractures worsen, ambulation becomes more difficult and energy-intensive, leading to reduced mobility, which in turn promotes further contracture development. Without aggressive stretching, physical therapy, and sometimes surgical intervention, contractures can progress to the point where the lower limbs are fixed in severely flexed positions.
Functional Decline and Loss of Ambulation
The cumulative effects of muscle weakness, pseudohypertrophy, and contractures lead to progressive functional decline in the lower limbs. Boys with Duchenne muscular dystrophy typically lose the ability to run and jump by age seven or eight. Climbing stairs becomes impossible without support shortly thereafter. As proximal muscle weakness advances and contractures worsen, the energy expenditure required for walking increases dramatically, making ambulation increasingly exhausting.
Most boys with Duchenne muscular dystrophy lose independent ambulation between ages seven and thirteen, with a median age of around nine to ten years. This loss of walking ability represents a devastating milestone for affected individuals and their families. The transition to wheelchair dependence is often gradual, beginning with wheelchair use for long distances and eventually progressing to full-time wheelchair use.
Following loss of ambulation, the lower limbs continue to be affected by Duchenne muscular dystrophy. Without weight-bearing and active use, bone density decreases, contractures worsen, and muscle tissue is almost entirely replaced by fat and fibrous tissue. The lower limbs become thin and atrophic in appearance, contrasting sharply with the pseudohypertrophied calves sometimes still present in younger, ambulatory boys.
Conclusion
Duchenne muscular dystrophy profoundly impacts the lower limbs through a devastating combination of progressive muscle weakness, pseudohypertrophy, contracture development, and functional decline. The characteristic pattern of proximal weakness, compensatory gait abnormalities, and eventual loss of ambulation significantly diminishes quality of life and independence. While current treatments including corticosteroids, physical therapy, and emerging genetic therapies can modestly slow disease progression and prolong ambulation, Duchenne muscular dystrophy remains a severely disabling condition. Understanding the specific lower limb pathology in Duchenne muscular dystrophy is essential for developing targeted interventions and providing comprehensive care to affected individuals throughout their disease course.