A physiological analysis of walking in the American lobster (Homarus americanus)

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Abstract
The normal, unrestrained, forward walking of the lobster was studied with a closed-circuit television system and a video-tape recorder. A frame-by-frame analysis was undertaken and measurements made of unilateral stepping sequences, contralateral and ipsilateral phase relations between pairs of legs, the movements at the leg joints primarily involved in stepping and their differences in each of the four pereiopods. The order of stepping was expressed in terms of the probability of any leg following any other leg and it was found that while there is a preferred order, there is considerable variation from the dominant pattern. The commonest deviations from the dominant gait are those involving the simplest types of re-ordering of the sequence. Pairs of contralateral legs show a strong tendency to alternate but all phase relations can occur. Similarly, while the ipsilateral legs show preferred phase relations, all possible relations do occur. The four pereiopods from anterior to posterior were found to have respectively, a pulling action, a combined pulling and rowing action, a rowing action and a combined pushing and rowing action. The same parameters of stepping were recorded from animals walking on a transparent, driven treadmill and, as no significant differences were found in comparisons with results from freely moving animals, subsequent results were obtained from animals walking on the treadmill where more detailed study and manipulation could readily be made. The stepping action of the third pereiopod during forwards walking involves major movements about two joints whereas the other pereiopods move about three joints. Detailed study of the intra-leg activity was therefore confined to the third pereiopod where the simpler action considerably simplified the problems involved in collecting and analysing data. Measurements were made of the angles swept out by the joints of the third pereiopod during movement. Electromyograms were recorded from the six muscles primarily responsible for these movements and from movement transducers placed at the joints. The duration of the movements and of the bursts of activity in the muscles and the interrelation between different muscle bursts were measured and a computer-aided analysis made to determine the characteristic features of the inter-burst relations during stepping. While there is considerable variability from step to step, the overall activity is relatively phase-constant over a wide range of stepping frequencies. When some of the key parameters of normal walking had been characterized, changes designed to alter the sensory input to the system were imposed. Changes in the duration of the power stroke and return stroke fractions of the stepping cycle were found when the animal was induced to pull or carry a load. Because of this result, a series of modifications were applied to unloaded animals. These included splinting joints at abnormal angles, attaching prosthetic legs, harnessing legs up off the substrate and partial or total amputation. It was found that the normal phasing of leg movement is altered if the support function of any other leg is impaired and that the change is in a direction which would compensate for the loss of support. This result was investigated further by loading animals in several different ways so that the power stroke or return stroke fraction of the cycle would be loaded differentially. Comparisons with unloaded animals demonstrated some previously undescribed changes in output in response to load. There is a marked reduction in the variability of the output irrespective of where a cycle is loaded, so that the burst structure during a loaded step becomes highly phase constant in most aspects. Other changes in overall burst structure are dependent on where the load is applied. The significance of the results, and particularly of the previously undescribed responses to load, were discussed in the context of the role of sensory input in lobster walking.