An increase in height of the medial longitudinal arch of the foot that does not flatten on weight bearing (i.e., a distinctly hollow form of the sole of the foot when it is bearing weight).

Weakness of the muscles of the legs.

Decreased intensity of the Achilles reflex (also known as the ankle jerk reflex), which can be elicited by tapping the tendon is tapped while the foot is dorsiflexed.

Fasciculations are observed as small, local, involuntary muscle contractions (twitching) visible under the skin. Fasciculations result from increased irritability of an axon (which in turn is often a manifestation of disease of a motor neuron). This leads to sporadic discharges of all the muscle fibers controlled by the axon in isolation from other motor units.

A type of decreased nerve conduction velocity that affects the motor neuron.

Spinal muscular atrophy (SMA) is characterized by muscle weakness and atrophy resulting from progressive degeneration and irreversible loss of the anterior horn cells in the spinal cord (i.e., lower motor neurons) and the brain stem nuclei. The onset of weakness ranges from before birth to adulthood. The weakness is symmetric, proximal > distal, and progressive. Before the genetic basis of SMA was understood, it was classified into clinical subtypes based on maximum motor function achieved; however, it is now apparent that the phenotype of SMN1-associated SMA spans a continuum without clear delineation of subtypes. With supportive care only, poor weight gain with growth failure, restrictive lung disease, scoliosis, and joint contractures are common complications; however, newly available targeted treatment options are changing the natural history of this disease.

Sudden and involuntary contractions of one or more muscles.

A lack of strength of the proximal muscles.

Reduced ability to walk (ambulate).

Reduced ability to run.

A lack of strength in the triceps muscle, which normally is responsible for extending (straightening) the elbow and mediating certain shoulder movements.

Muscular atrophy affecting the lower limb.

Evidence of chronic denervation on electromyography.