ALS causes apparent muscle weakness and atrophy, which is caused by the upper and lower motor neuron degeneration. ALS sufferers usually have difficulty starting and controlling voluntary movements of muscles throughout the body. But commonly, the bladder muscles, anal sphincter, and muscles that move the eyeball are not affected. Cognitive function is often not impaired, but 5% of cases are accompanied by frontotemporal dementia. Sensory and autonomic systems are usually normal. Early symptoms of ALS include marked weakness and muscular atrophy. Other signs that arise include muscle fasciculation (twitching), cramps, or stiffness of the affected muscles, slurred speech, or nasal irritation. 75% of cases of weakness involving the extremities, both upper limb, or inferior limb. 25% of cases involve bulbar muscles.

 Based on clinical characteristics, ALS is divided into:

1. Type of UMN
LMN Features are the loss of agile movements, decreased motor strength, spasticity, hyperreflexia, and the presence of pathological reflexes. The loss of agile movements, causing the patient to be slow, stiff can be seen with pronation tests and hand supination. Weakness motoric UMN type, superior limb extensor muscles, and lower limb flexor muscles are weak compared to the antagonist's muscles due to disinhibition of control of the vestibulospinal and reticulospinal tracts. Muscle loses flexibility, resistance increases, resulting in the clasp-knife phenomenon.
What is unique from ALS is usually superficial reflexes, such as the superficial abdominal reflex and the normal cremaster reflex. Pseudobulbar palsy, UMN-type bulbar muscle weakness, and spinal mass reflex (tonic flexor spasm) are signs of UMN spinal cord lesions. If there is only a weakness of the UMN type, it is called primary lateral sclerosis.

2. LMN type
The characteristic of LMN is a severe weakness in motor power. The dropped head is typically seen in ALS, because of weakness in the neck extensor muscles. The presence of motor unit lesions, causing muscle atrophy that can be seen at the beginning of onset, muscle loss in volume and size. Other characteristics are flaccid and hypotonic muscle weakness. Fasciculation and muscular ram often occur. Muscle cramps occur not only in the calf but also in the muscles of the thighs, arms, stomach, neck, and jaw. If there are only LMN characters, it is often called progressive muscular atrophy.

3. Bulbar type (paretic bulbar palsy) Deficits of N VII, IX, X, and XII. Symptoms include dysphagia, drooling dysarthria, and if severe aspiration can arise. In ALS, a type of bulbar muscle weakness, usually a mixture of UMN and LMN. If it only affects the bulbar muscle, it is called progressive bulbar palsy.

4. Respiratory Disorders
Paralysis that affects the respiratory muscles, usually at the terminal stage, often causes complications in patients with ALS.

5. Musculoskeletal disorders
Pathognomic signs often seen on the hands and feet. Claw hands arise due to the weakness of the lumbricus and interosseus muscles. In some literature, the weakness in the abductor, adductor, thenar, and dorsal interosseous muscle atrophy is called the 'skeletal hand' or 'cadaveric hand.' The metacarpophalangeal joint hyperextension while the distal joints are flexed. Atrophy is evident in the thenar muscle (weakness in the abductor pollicis brevis muscle). Droop foot, often seen in ALS. The final stage of these extremity muscle fragility is contractures.

6. Very rare accompanying disorders
In the beginning, 120 years ago, Charcot, defined that ALS was a purely motor disorder (typical ALS), but at present, in some cases accompanied by sensory disturbance, dementia, ocular palsy, and urinary disorders and bowel movements. Monomelic amyotrophy, the weaknesses of UMN and LMN types in one limb, usually in the superior limb. This is a variant of ALS too.