Our work demonstrates tuned, pre-landing EMG activity in antagonistic elbow muscles in hopping toads. In long hops, these muscles exhibit more intense activity just before landing than during short hops, suggesting that toads prepare for impact in a distance-dependent manner. Longer hops lead to greater impact forces in jumping anurans (Nauwelaerts & Aerts 2006
), and different levels of forelimb muscle recruitment are probably important for managing different degrees of impact. In addition, onset timing in m. anconeus
changes with hop distance in a manner that parallels the timing of forelimb touch-down (b
). This strongly suggests that the onset of m. anconeus
is modulated so as to create a fixed interval (approx. 90 ms in these animals) between muscle activation and landing, as in various limb muscles in humans and other mammals (Santello 2005
). The onset of m. anconeus
activity occurs before take-off in hops shorter than approximately 30 cm, indicating that this modulation or tuning can take place even before the animal's feet leave the ground.
Unlike in m. anconeus, the onset of m. coracoradialis activity always occurs before take-off and at a roughly fixed interval from the start of the hop (a). Its onset closely corresponds to the time when the forelimbs leave the ground, shortly after which they are pulled forward to prepare for landing (electronic supplementary material, video). The onset timing of m. coracoradialis suggests its importance in these early forelimb movements, while its intensity modulation just before landing suggests that it is also important in forelimb stabilization on impact.
Previous work on pre-landing activation patterns in various mammals (e.g. humans, monkeys and cats) has shown prescient modulation of recruitment timing and intensity in limb muscles important for resisting impact during jumping or falling (Prochazka et al. 1977
; Dyhre-Poulsen & Laursen 1984
; Santello & McDonagh 1998
). However, to our knowledge our work represents the first direct evidence of this phenomenon in an anuran, which, like jumping mammals, must modulate limb muscle stiffness to absorb the impact of hops covering a range of distances, or suffer the consequences (e.g. poorly coordinated transitions between hops or even injury). Here we show that pre-landing recruitment intensity (in m. coracoradialis
and to a lesser extent in m. anconeus
) and onset timing (in m. anconeus
but not m. coracoradialis
) are tuned to hop distance in cane toads. The extent to which visual, vestibular and/or proprioceptive feedback are important for these anticipatory responses remains to be tested (electronic supplementary material), but all three mechanisms have been implicated in mammals (McKinley & Smith 1983
; Avela et al. 1996
; Santello et al. 2001
). How widespread this ability is among other anurans (e.g. those that more commonly jump into water than on land) is currently unknown, but cane toads in some sense, seem to ‘know’ how far they are hopping, even before they take off, and prepare for landing accordingly.