In our calculations, we estimated the resistive force on an ellipsoid body that moved through the water according to the kinematics of a swimming frog. A second explanation is an underestimation of the propulsive forces of swimming due to our calculation method. It is possible that during swimming high lateral forces occur,causing the resultant force to be of similar magnitude for both swimming and jumping. First, since we based our calculations of the propulsive impulse on the kinematics of the center of mass of the frog, we took into account only force components parallel to the direction of motion. This difference in impulse was explained by four major hypotheses, which are not necessarily mutually exclusive. In a previous study on the comparison of the kinematics of jumping and swimming of these semi-aquatic frogs, we were able to show that the propulsive impulse is much smaller during swimming( Nauwelaerts and Aerts, 2003).

In spite of these dramatic differences between aquatic and terrestrial environments with respect to several physical properties( Denny, 1993 Vogel, 1994), many animals succeed in using their limbs to locomote both in water and on land. The changes in force generation are expected to be most extreme for species that move through different physical environments, such as water vs land( Biewener and Gillis, 1999). These resulting changes originate from the alteration of the force development during locomotion. Obviously, animals must modulate their locomotor behaviour to adjust for changes in the external environment, resulting in changes in direction, speed or posture ( Biewener and Corning,2001 Irschick and Jayne,1998). However, many animals move through complex and heterogeneous physical environments at varying speeds and using different modes of locomotion. Much of what we know about animal locomotion is derived from studies that examined animals moving within single, homogeneous environments( Biewener and Gillis, 1999). This difference is caused by a difference in inter-limb coordination. During swimming, maximal force is reached early in the extension phase, 20% into it, while during jumping, peak force is attained at 80% of the extension phase. The difference between the force profiles,apart from a slightly higher peak force during jumping, lies mainly in a difference in shape.

This means that the difference in impulse between jumping and swimming in frogs is even larger than previously stated. The force profile obtained from the impulse–momentum approach exposed an overestimation of drag by 30% in the drag–thrust calculations.

This means that it is not only the acceleration of water backwards that provides thrust, but also that the deceleration of water flowing towards the frog as a result of recovery accelerates the centre of mass prior to leg extension. On the kinematic profiles of swimming, leg extension precedes propulsion. The new approach to quantifying the instantaneous forces during swimming was tested and proved to be a valid method for determining the external forces on the feet of swimming frogs. The force profile obtained was compared with force profiles obtained from drag–thrust equilibrium of the centre of mass and with the force profiles generated during jumping. To explore this difference further, we determined the instantaneous forces during propulsion in swimming using an impulse–momentum approach based on DPIV flow data. A comparison of the kinematics of swimming frogs in a previous study revealed a difference in propulsive impulse between jumping and swimming. Frogs are animals that are capable of locomotion in two physically different media, aquatic and terrestrial.

Impulsive force models