Two forms of isometric muscle function: Interpersonal motor task supports a distinction between a holding and a pushing isometric muscle action

In sports and movement sciences isometric muscle function is measured by pushing against a stable resistance. However, subjectively one can hold or push isometrically. Several investigations suggest a distinction of those forms. The aim of this study was to investigate, whether or not these two forms of isometric muscle action can be distinguished by objective parameters in an interpersonal setting. 20 subjects were grouped in 10 same sex pairs, in which one partner should perform the pushing isometric muscle action (PIMA) and the other partner executed the holding isometric muscle action (HIMA). The partners were coupled by an interface including a strain gauge and an acceleration sensor. The mechanical oscillations of the triceps brachii (MMGtri) muscle, its tendon (MTGtri) and the abdominal muscle (MMGobl) were recorded by piezoelectric-sensor-based measurement system (mechanomyography (MMG); mechanotendography (MTG)). Each partner performed three 15s (80% MVIC) and two fatiguing trials (90% MVIC) during PIMA and HIMA, respectively (tasks changed in the couple). Regarded parameters to compare PIMA and HIMA were (1) the mean frequency, (2) the normalized mean amplitude, (3) the amplitude variation, (4) the power in the frequency range of 8 to 15 Hz and (5) a special power-frequency ratio and the number of task failures during HIMA or PIMA (partner who quit the task). A "HIMA failure" occurred in 87.5% of trials (p<0.000). No significant differences between PIMA and HIMA were found for the mean frequency and normalized amplitude. The MMGobl showed a significantly higher values of the amplitude variation (15s: p 0.013; fatigue: p=0.007) and of the power-frequency-ratio (15s: p = 0.040; fatigue: p = 0.002) during HIMA and a higher power in the range of 8 to 15 Hz during PIMA (15s: p=0.001; fatigue: p=0.011). MMGtri and MTGtri showed no significant differences. Based on the findings it is suggested that a holding and a pushing isometric muscle action can be distinguished objectively, whereby a more complex neural control is assumed for HIMA.