Although most terrestrial animals that run, hop, or trot across the ground need to spend more metabolic energy to go faster, the hopping tammar wallaby can go faster without little or no increases in energetic cost. Furthermore, a female tammar wallaby can carry the heavy load of the infant “joey” in her pouch without increasing her cost of locomotion. These remarkable feats are likely due to the storage and recovery of elastic energy by the large springy tendons in the wallaby’s hind legs. During the leaping, aerial phase of the hop cycle, the wallaby’s forward movement represents kinetic energy and the gravitational pull back to the ground is a form of potential energy. These energies transform into elastic strain energy of stretching tendons when the foot hits the ground. That energy can then be recovered in the elastic recoil of those tendons that helps propel the wallaby back off the ground. As much as 90% of the energy stored in the tendon can be recovered for such reuse. The key to this energy recovery is that muscles attached to the tendons are stiff enough so that their length changes little as they generate force. If the muscles changed in length a lot, they might absorb and dissipate the tendon’s elastic energy, making it unavailable to power the next hop.
The faster the wallaby goes and the heavier the load, the more elastic energy gets stored and recovered, hence the cost of locomotion can be unchanged with speed or load over a normal range of speeds. The use of tendons and elastic energy is also found in many other large animals that run (such as horses and turkeys), but to a much less dramatic extent in terms of energy savings as those observed in kangaroos and wallabies. It is as yet unclear exactly why these macropods experience such high savings in energy compared with other animals. The general strategy of elastic energy storage as a means of increasing locomotor efficiency is also observed in a variety of swimming animals, from squid to dolphins.
The use of elastic energy storage could be considered in the human design of all sorts of moving structures to increase energy efficiency. “Spring loaded locomotion” has been used in the design of the pogo stick and some prosthetic legs.
“Moderate to large macropodids can increase their speed while hopping with little or no increase in energy expenditure. This has been interpreted by some workers as resulting from elastic energy savings in their hindlimb tendons. For this to occur, the muscle fibers must transmit force to their tendons with little or no length change. To test whether this is the case, we made in vivo measurements of muscle fiber length change and tendon force in the lateral gastrocnemius (LG) and plantaris (PL) muscles of tammar wallabies Macropus eugenii as they hopped at different speeds on a treadmill. Muscle fiber length changes were less than +/-0.5 mm in the plantaris and +/-2.2 mm in the lateral gastrocnemius, representing less than 2 % of total fiber length in the plantaris and less than 6 % in the lateral gastrocnemius, with respect to resting length. The length changes of the plantaris fibers suggest that this occurred by means of elastic extension of attached cross-bridges. Much of the length change in the lateral gastrocnemius fibers occurred at low force early in the stance phase, with generally isometric behavior at higher forces. Fiber length changes did not vary significantly with increased hopping speed in either muscle (P>0.05), despite a 1. 6-fold increase in muscle-tendon force between speeds of 2.5 and 6.0 m s-1. Length changes of the PL fibers were only 7+/-4 % and of the LG fibers 34+/-12 % (mean +/- S.D., N=170) of the stretch calculated for their tendons, resulting in little net work by either muscle (plantaris 0.01+/-0.03 J; gastrocnemius -0.04+/-0.30 J; mean +/- s.d. ). In contrast, elastic strain energy stored in the tendons increased with increasing speed and averaged 20-fold greater than the shortening work performed by the two muscles. These results show that an increasing amount of strain energy stored within the hindlimb tendons is usefully recovered at faster steady hopping speeds, without being dissipated by increased stretch of the muscles’ fibers. This finding supports the view that tendon elastic saving of energy is an important mechanism by which this species is able to hop at faster speeds with little or no increase in metabolic energy expenditure.” (Biewener et al. 1998:1681)