Muscle activity in cycling: the double burst

The period of a muscle's activity during cycling can be described by the onset and offset of electrical activity. Certain muscles can exhibit two distinct periods of activity (a ‘double burst'), reflecting different muscular functions during a pedal revolution. Our aims were to determine (1) the consistency of the double burst pattern in the Gastrocnemius Laterals (GL) muscle during cycling and (2) by how much the reliability of the offset of GL is improved by taking the double burst into account. 11 cyclists volunteered to participate. Participants cycled for 6 min at 265 W and 90 rev.min-1 on three separate days. Using surface electromyography, an ensemble-average of GL was constructed from a 30 s segment. Onset and offset threshold was set at 25% of the peak activity of the ensemble-averaged pattern to determine the crank angle of onset/offset. Two onsets/offsets were recorded for double bursts. Reliability of the primary offset was determined using the Standard Method (SM), that is, the offset of the final activity; and an Alternative Method (AM), which incorporated the muscular function, with the most frequent offset across days being regarded as the ‘primary' offset. A double burst was consistently detected for only 7 of the 11 cyclists. AM showed better reliability for the offset than SM (Standard Error of Measurement: 6.8° vs 21.3°, Intra Class Correlation: 0.82 vs 0.47). The double burst pattern is commonly observed in GL while cycling in trained cyclists. However, detection across several days is more difficult and could lead to erroneous conclusions about the (double) burst pattern. When analysing muscles with a possible double burst pattern, researchers could incorporate the muscle's function. This decreases the impact of an inconsistent double burst pattern across measurements.