Motor patterns for horizontal and upside down walking and vertical climbing in the locust

Abstract

The motor patterns of the locust's flexor tibiae and metathoracic subcoxal joint muscles were compared during unrestrained horizontal walking, vertical climbing and walking upside-down hanging from a branch. Combining anatomical and structural data with the results from myographic recordings revealed the role of these muscles during walking and their functional adjustments to different loads and gravity effects caused by changes in walking conditions. Motor patterns are remarkably constant during a given walking situation even at quite different walking speeds. In all walking situations, changes in step duration correlate strongly with changes in the duration of retraction but only weakly with the duration of protraction. Different motor units of one muscle can be distinguished by their spike amplitude. They may be active simultaneously or they may alternate and then fire in different phases of the step cycle. For example, during horizontal walking, the small unit of the first abductor (M125) is active during protraction and the large one during retraction. During retraction, the coxal muscles serve two functions: (1) the joint has to be brought back to the posterior extreme position by retractor muscles; (2) the joint has to be stabilized, that is held 'stiffly', by co-activity of functional antagonists. During protraction, it is moved to the anterior extreme position by powerful contractions of protractor muscles. The muscular activity patterns show marked differences depending on the walking situation. Some large motor units spike only sporadically during horizontal walking but burst during vertical climbing and upside-down walking (muscle M121p, the large units of muscles M126 and M120). During upside-down walking, muscles M120 and M121 and the large unit of muscle M125 are active during opposite phases of the step cycle when compared with horizontal walking and vertical climbing (for example, during walking and climbing, muscle M120 is active at the transition between retraction and protraction, whereas during upside-down walking, activity occurs during late protraction and early retraction). The results describe how motor patterns are adjusted to the different requirements of various walking situations.