Why does ALS make muscles both stiff and wasted at the same time? · Grey Matter

Because it kills two kinds of motor neuron at once.

Why does ALS make muscles both stiff and wasted at the same time?

Because it kills two kinds of motor neuron at once. The lower motor neurons in the spinal cord and brainstem connect directly to the muscles, and when they die the muscle loses its nerve supply, twitches and wastes away. The upper motor neurons in the cortex control those spinal neurons from above and hold back the spinal reflexes, so when they die the reflexes become brisk and the limbs stiff. Finding both kinds of sign in the same person is what points a neurologist to ALS.