Figure 1. Schematic representation of the forces at play in different conditions. Panel on the left: free swimming; central panel: full tethered swimming; panel on the right: semi-tethered swimming. In all cases, a fraction of the force generated by the muscles (F_TOT) is wasted to give water kinetic energy (e.g. is not useful for propulsion). The fraction of useful force (F_P) to total force is termed propelling efficiency (F_{P /} F_TOT) and is assumed to be the same in all conditions. The useful force is then used to overcome drag in free swimming (F_D) and/or the external load in full tethered swimming (F_T) and semi-tethered swimming (F_ST).