As an orthopaedic surgeon working closely with pediatric patients—both typically developing and those with motor impairments like cerebral palsy—understanding muscle power in the lower limbs is key to planning interventions, rehabilitation, and surgical decisions.

When we speak of muscle power in Newtons, we're usually referring to the force-generating capacity of the muscle, not to be confused with "power" in the physics sense (which includes velocity). Clinically, we often assess this through isokinetic dynamometry, or more practically, through manual muscle testing, gait analysis, or functional movement scoring.

1. Average Lower Limb Muscle Force in Typically Developing Children:

In healthy, typically developing children, lower limb muscle force varies widely with age, body weight, and level of activity, but as a general reference:

• Quadriceps muscle force in healthy children (ages 5–12) ranges between 150 to 300 Newtons (N).
• Hamstrings may generate 80 to 200 N, depending on the joint angle and testing method.
• The plantar flexors (calf muscles)—important for push-off during walking—can generate around 250–500 N, particularly in older children or adolescents.

These values are higher in athletic or highly active children and increase significantly with growth, puberty, and neuromuscular development.

2. Lower Limb Muscle Force in Children with Hemiplegic Cerebral Palsy (CP):

In children with hemiplegic cerebral palsy, we see a marked reduction in voluntary force output on the affected side. Research and clinical observations suggest:

• Muscle force on the affected limb is typically 30–50% lower than in age-matched typically developing peers.
• For example, the quadriceps may generate as little as 60–120 N, depending on the severity (GMFCS Level I–III).
• Plantar flexor strength, which is critical for walking efficiency, is often compromised, contributing to gait abnormalities like toe-walking or crouch gait.
• The unaffected side (non-hemiplegic side) may show compensatory overuse and sometimes approaches normal force values but can still lag slightly due to overall neuromotor inefficiencies.

These deficits are not only due to reduced muscle mass but also due to impaired motor control, spasticity, and reduced voluntary activation. Importantly, muscle weakness in CP is not just a strength issue—it’s a complex interplay of neurological inhibition, poor motor unit recruitment, and altered muscle-tendon mechanics.

Clinical Implications:

Understanding these differences is critical in:

• Rehabilitation goal setting (e.g., designing strength training programs)
• Orthotic prescription
• Surgical planning (e.g., tendon lengthening or SEMLS—Single Event Multi-Level Surgery)
• Gait training and physiotherapy

In children with hemiplegia, targeted strength training—especially eccentric and functional strengthening—can significantly improve functional outcomes, although gains are often modest due to the underlying neurological impairment.

In summary:

• Healthy children: Lower limb muscle force averages between 150–500 N, depending on muscle group, age, and activity level.
• Children with hemiplegic CP: Force on the affected side is often reduced by 30–50%, typically 60–250 N, depending on muscle and severity.

While these are average ranges, the real measure of progress in children with CP often lies in functional gains rather than raw numbers—how far they can walk, how confidently they can climb stairs, and how independently they can move.

Dr.Gitonga