Observational information on muscle activity is related to sEMG activity expressed as a percentage of the maximum voluntary contraction (MVC). This is the case for the Garg and Moore Strain Index and the Hand Activity Level (HAL) proposed by the ACGIH. This latter involves the assessor recording the levels of MVC in 10% intervals. This degree of sensitivity certainly calls for the use of surface electromyography (sEMG) rather than just observation. Furthermore sEMG signal can contribute to assess muscle fatigue and force with the modern instruments for signal frequency analysis; earlier limitations have been overcome and dynamic assessment can now be done.