Undulatory locomotion
Undulatory locomotion is the type of motion characterized by wave-like movement patterns that act to propel an animal forward.
Although this is typically the type of gait utilized by limbless animals, some creatures with limbs, such as the salamander, forgo use of their legs in certain environments and exhibit undulatory locomotion.
[2] When the animal swims in a fluid, two main forces are thought to play a role: At low Reynolds number (Re~100), skin friction accounts for nearly all of the thrust and drag.
For those animals which undulate at intermediate Reynolds number (Re~101), such as the Ascidian larvae, both skin friction and form force account for the production of drag and thrust.
[2] In animals that move without use of limbs, the most common feature of the locomotion is a rostral to caudal wave that travels down their body.
Snakes can exhibit 5 different modes of terrestrial locomotion: (1) lateral undulation, (2) sidewinding, (3) concertina, (4) rectilinear, and (5) slide-pushing.
In general, the amplitude of the lateral undulation and angle of intervertebral flexion is much greater during terrestrial locomotion than that of aquatic.
A typical characteristic of many animals that utilize undulatory locomotion is that they have segmented muscles, or blocks of myomeres, running from their head to tails which are separated by connective tissue called myosepta.
In addition, some segmented muscle groups, such as the lateral hypaxial musculature in the salamander are oriented at an angle to the longitudinal direction.
A higher initial angle of orientation and more dorsoventral bulging produces a faster muscle contraction but results in a lower amount of force production.
This phenomenon results in an architectural gear ratio, determined as longitudinal strain divided by fiber strain (εx / εf), greater than one and longitudinal velocity amplification; furthermore, this emergent velocity amplification may be augmented by variable architectural gearing via mesolateral and dorsoventral shape changes, a pattern seen in pennate muscle contractions.
[8][10] Simple bending behavior in homogenous beams suggests ε increases with distance from the neutral axis (z).
Furthermore, it has been hypothesized that muscle fibers recruited for a particular task must operate within an optimal range of strains (ε) and contractile velocities to generate peak force and power respectively.
[8][10] Siren lacertina, an aquatic salamander, utilizes swimming motions similar to the aforementioned fishes yet contains hypaxial muscle fibers (which generate bending) characterized by a simpler organization.
[8] Brainerd and Azizi found that longitudinal contractions of the constant volume hypaxial muscles were compensated by an increase in the dorsoventral dimensions.
Azizi et al. discovered that the initial hypaxial fiber α trajectory in the EO is greater than that of the IO.
Therefore, variability in AGR within the hypaxial musculature of the Siren lacertina counteracts varying mesolateral fiber distances and optimizes performance.
[11] This hypothesis has been studied further by examining the oxygen consumption rates in the snake during different modes of locomotion: lateral undulation, concertina,[13] and sidewinding.
The reason that lateral undulation has the same energetic efficiency as limbed animals and not less, as hypothesized earlier, might be due to the additional biomechanical cost associated with this type of movement due to the force needed to bend the body laterally, push its sides against a vertical surface, and overcome sliding friction.
One such example is the half center oscillator which consists of two neurons that are mutually inhibitory and produce activity 180 degrees out of phase.
In addition, the phase relations can be established by asymmetries in the couplings between oscillators or by sensory feedback mechanisms.
[15] Although the couplings between neurons spans six segments in both the anterior and posterior direction, there are asymmetries between the various interconnections because the oscillators are active at three different phases.
The lamprey moves using lateral undulation and consequently left and right motor hemisegments are active 180 degrees out of phase.
Based on biologically hypothesized connections of the central pattern generator in the salamander, a robotic system has been created which exhibits the same characteristics of the actual animal.
[16][17] Electrophysiology studies have shown that stimulation of the mesencephalic locomotor region (MLR) located in the brain of the salamander produce different gaits, swimming or walking, depending on intensity level.
