Amyotrophic lateral sclerosis (ALS) is the most frequent adult-onset motor neuron disease characterized by degeneration of upper and lower motor neurons (MNs), generalized weakness and muscle atrophy. The "neurocentric" view of ALS assumes that the disease primarily affects motor neurons, while muscle alterations only represent a consequence, in the periphery, of motor neuron loss. However, this outlook was recently challenged by evidence suggesting that non-neural cells such as microglia, astrocytes, peripheral blood mononuclear cells (PBMCs) and skeletal muscle fibres participate in triggering motor neuron degeneration, and this stressed the concept that alterations in different cell types may act together to exacerbate the disease. In this review, we will summarize the most recent findings on the alterations of skeletal muscle fibres found in ALS, with particular attention to the relationship between mutant SOD1 and skeletal muscle. We will analyze changes in muscle function, in the expression of myogenic regulatory factors, and also mitochondrial dysfunction, SOD1 aggregation and proteasome activity.