What is the difference in muscle power between hemiplegia and hemiparesis?

Muscle Power Difference Between Hemiplegia and Hemiparesis

Hemiplegia represents complete paralysis (0/5 motor power) of one side of the body, while hemiparesis indicates partial weakness (1/5 to 4/5 motor power) with preserved voluntary movement on the affected side.

Defining the Power Distinction

The fundamental difference lies in the degree of voluntary motor control:

• Hemiplegia involves complete loss of voluntary movement with no antigravity effort or movement at all (corresponding to NIHSS motor scores of 3-4, indicating no antigravity effort or no movement) 1
• Hemiparesis encompasses a spectrum from minimal movement to near-normal strength, with preserved ability to generate some voluntary muscle contraction 2

The NIHSS motor assessment scale operationalizes this distinction: scores of 0-2 indicate hemiparesis (ranging from no drift to some antigravity effort), while scores of 3-4 indicate hemiplegia (no antigravity effort or complete paralysis) 1

Clinical Manifestations and Functional Impact

Hemiplegia presents with:

• Complete inability to perform voluntary movements of the face, arm, and leg on the affected side 3
• Flaccidity in acute stages, potentially progressing to spasticity 1
• Severe functional limitations requiring maximal assistance for all activities of daily living 1
• Wheelchair dependence for mobility 4

Hemiparesis demonstrates:

• Preserved voluntary movement with varying degrees of weakness, abnormal synergies, and loss of dexterity 2
• Ability to generate some antigravity movement, though coordination and speed are impaired 5
• Potential for functional ambulation, though energy expenditure may be elevated up to 2 times that of able-bodied persons 1
• Variable independence in activities of daily living depending on severity 1

Underlying Muscle Contractility

A critical distinction often overlooked: the intrinsic contractile capacity of muscle tissue remains largely preserved in both conditions 6. When directly stimulated electrically, the affected muscles in hemiplegia generate near-normal force compared to the unaffected side 6. This demonstrates that the primary deficit is failure of rapid, coordinated adjustment of graded high-frequency motor neuron firing in purposeful complex synergies, not muscle weakness per se 6.

Rehabilitation Implications

The power difference fundamentally alters rehabilitation strategies:

For hemiplegia (complete paralysis):

• Functional electrical stimulation is specifically recommended to facilitate muscle contraction and prevent disuse atrophy 4
• Multichannel FES for gait impairment may be considered, though evidence remains insufficient for routine recommendation 4
• Wheelchair mobility training becomes essential for longer distances 4
• Aggressive passive range of motion to prevent contractures is critical 4

For hemiparesis (partial weakness):

• Task-specific motor training interventions should be prioritized to induce neuroplasticity 4
• Constraint-Induced Movement Therapy requires minimum motor criteria: 20-degree wrist extension and 10 degrees for each finger 4
• Patients with some voluntary movement in the hemiparetic shoulder have higher rates of shoulder joint tissue injury on MRI, suggesting more physical activity promotes injury 1
• Balance training and gait retraining can achieve functional community ambulation 4

Common Assessment Pitfalls

Do not confuse the Fugl-Meyer score with comprehensive motor assessment - it primarily captures abnormal synergies and weakness but not loss of dexterity 2. Patients with subacute stroke have worse reaching dexterity than chronic patients even when matched on Fugl-Meyer scores, indicating these are dissociable components 2.

Avoid assuming that resistive exercise training is the primary solution - since directly stimulated muscle strength is preserved, the central disability is failure of coordinated motor control, not muscle weakness 6. Rehabilitation should focus on motor control strategies rather than pure strengthening 6.

Recognize that impaired joint individuation, not just weakness, explains reaching deficits - the inability to isolate movement to single joints accounts for most variance in reach path curvature and endpoint error in hemiparesis 5. Strength primarily explains reaching velocity, while spasticity contributes minimally to functional deficits 5.

Prognostic Considerations

Energy expenditure during gait varies with the degree of weakness, spasticity, training, and bracing 1. Hemiplegic patients demonstrate markedly elevated oxygen cost of walking compared to able-bodied subjects, with mechanical efficiency reduced up to 50% 1. This creates a vicious circle of decreased activity and greater exercise intolerance, leading to secondary complications including reduced cardiorespiratory fitness, muscle atrophy, and osteoporosis 1.

Between 25% and 50% of stroke survivors require at least some assistance with activities of daily living, with half experiencing severe long-term effects such as partial paralysis (hemiparesis) or complete paralysis (hemiplegia) 1.