Behavior of human gastrocnemius muscle fascicles during ramped submaximal isometric contractions

Precise estimates of muscle architecture are necessary to understand and model muscle mechanics. The primary aim of this study was to estimate continuous changes in fascicle length and pennation angle in human gastrocnemius muscles during ramped plantar flexor contractions at two ankle angles. The secondary aim was to determine whether these changes differ between proximal and distal fascicles. Fifteen healthy subjects performed ramped contractions (0-25% MVC) as ultrasound images were recorded from the medial (MG, eight sites) and lateral (LG, six sites) gastrocnemius muscle with the ankle at 90 degrees and 120 degrees (larger angles correspond to shorter muscle lengths). In all subjects, fascicles progressively shortened with increasing torque. MG fascicles shortened 5.8mm (11.1%) at 90 degrees and 4.5mm (12.1%) at 120 degrees, whereas LG muscle fascicles shortened 5.1mm (8.8%) at both ankle angles. MG pennation angle increased 1.4 degrees at 90 degrees and 4.9 degrees at 120 degrees, and LG pennation angle decreased 0.3 degrees at 90 degrees and increased 2.6 degrees at 120 degrees. Muscle architecture changes were similar in proximal and distal fascicles at both ankle angles. This is the first study to describe continuous changes in fascicle length and pennation angle in the human gastrocnemius muscle during ramped isometric contractions. Very similar changes occurred in proximal and distal muscle regions. These findings are relevant to studies modeling active muscle mechanics.