Objective To test the hypotheses that decomposition electromyography (dEMG) motor unit action potential (MUAP) amplitude and firing rate are altered in SMA; dEMG parameters are associated with strength and function; dEMG parameters are correlated with traditional electrophysiological assessments. Methods Ambulatory and non‐ambulatory adults with SMA on nusinersen and healthy controls were enrolled. MUAPs were decomposed from multielectrode surface recordings during 30‐s maximum contraction of the abductor digiti minimi (ADM). Isometric strength, upper limb function, patient‐reported function, and standard electrophysiologic measures of the ADM (compound muscle action potential CMAP, single motor unit potential SMUP, motor unit number estimation MUNE) were collected. Results dEMG MUAP amplitudes were higher in ambulatory versus control and non‐ambulatory groups and were higher in controls versus non‐ambulatory SMA. In contrast, dEMG firing rates were higher in ambulatory versus non‐ambulatory and control groups but similar between non‐ambulatory and control. dEMG parameters showed moderate to strong positive correlation with strength and function whereas CMAP and MUNE better correlated with function than strength. SMUP did not correlate with strength, function, or dEMG MUAP amplitude. dEMG parameters show overall good test–retest reliability. Interpretation dEMG provided reliable, noninvasive measure of MUAP amplitude size and firing rate and revealed divergent patterns across disease severity in adults with SMA. Firing rate enhancement, as seen in milder SMA, may provide a therapeutic avenue for improving function in more severe SMA, where firing rates appear preserved. MUAP amplitude size and firing rate, quantified with dEMG, may be promising monitoring biomarker candidates for noninvasive assessment of SMA.