The relationship between surface EMG (sEMG) and muscle force is complex, depending on factors like motor unit recruitment and firing rates, muscle fiber type composition, fatigue, and dynamic conditions. In general, sEMG amplitude and muscle force increase proportionally with recruitment and higher firing rates. However, the exact shape of the sEMG-force relationship can vary between linear and non-linear depending on the individual muscle and contraction conditions. Dynamic contractions and muscle fatigue can further impact this relationship. While sEMG can provide an estimate of relative muscle force, many factors must be considered for accurate quantification.