Background <p>Spinal muscular atrophy mainly affects motor neurons in the spinal cord, leading to progressive muscular atrophy and weakness. This study aims to carefully analyse the effect of body parameters on motor function and fine motor skills, measured at one-year intervals, in patients with spinal muscular atrophy.</p> Methods <p>The study included 38 participants diagnosed with SMA. Patients were divided into three groups based on their level of motor function: non-sitters (<i>n</i> = 9), sitters (<i>n</i> = 23), and walkers (<i>n</i> = 6). Anthropometric measurements (weight, height/length) were taken in all participants, and BMI and BMI-z-score were calculated. The motor function was assessed using validated scales: CHOP-INTEND for the non-sitters group and HFMSE scale for the sitters and walkers group. The RULM scale assessed fine motor skills for the sitters and walkers group.</p> Results <p>At the 12-month follow-up, the CHOP-INTEND scale showed improvement in most non-sitter patients (6/9). The sitter’s group was more heterogeneous on the HFMSE scale, with 10 out of 23 patients showing deterioration in function, 8 showing improvement, while the walker’s group showed the least variability. In the non-sitters group, the dominant category of BMI z-score was underweight, while half of the sitters in the group were underweight, although there were just as many healthy weights. The strongest group, the walkers, was mostly healthy. We have observed a certain tendency that as BMI increases, motor function increases in all groups, while fine motor skill decreases in the most varied sitters group.</p> Conclusions <p>As BMI increases in SMA patients, motor function improves. Fine motor skill deteriorates as BMI increases in the sitter’s group. The assessment of individual components of motor function and fine motor skills reveals the actual changes that occur over time in patients with SMA.</p>