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Are there two forms of isometric muscle action? Results of the experimental study support a distinction between a holding and a pushing isometric muscle function

PainSci » bibliography » Schaefer et al 2017
Tags: muscle, movement, neat

One article on PainSci cites Schaefer 2017: The Role of Eccentric Contractions in Rehab

PainSci commentary on Schaefer 2017: ?This page is one of thousands in the bibliography. It is not a general article: it is focused on a single scientific paper, and it may provide only just enough context for the summary to make sense. Links to other papers and more general information are provided wherever possible.

Is there a difference between preventing an object from moving and pressing on a fixed object with the same force? You wouldn’t think so, if the amount of force required is truly the same, but apparently there is. This experiment clearly shows that a “holding” contraction is more exhausting than pushing equally hard on a stable object, demonstrating that “there are probably two forms of isometric muscle action.” Isometric contraction is contraction without movement. Both of these contractions are isometric — there’s no movement, and the forces are the same — but one of them is more tiring than the other.

It’s the difference between pushing on a wall versus trying to stop a moving wall … which sounds like a silly analogy, but the Star Wars trash compactor scene is a perfect example: “The walls are moving!”

The authors suggest some possible reasons for the difference, such as “complexity of neural control strategies” — in other words, adjusting your force to match an externally applied force may be more of a neuromuscular juggling act than applying a steady force to a stable object.

Although there were measurable differences, I think it might be overstating it to declare that there are two different “types” of contraction. This seems more like evidence that the same kind of contraction is simply more exhausting in one context than another.

~ Paul Ingraham

original abstract Abstracts here may not perfectly match originals, for a variety of technical and practical reasons. Some abstacts are truncated for my purposes here, if they are particularly long-winded and unhelpful. I occasionally add clarifying notes. And I make some minor corrections.

BACKGROUND: In isometric muscle function, there are subjectively two different modes of performance: one can either hold isometrically — thus resist an impacting force — or push isometrically — therefore work against a stable resistance. The purpose of this study is to investigate whether or not two different isometric muscle actions — the holding vs. pushing one (HIMA vs PIMA) — can be distinguished by objective parameters.

METHODS: Ten subjects performed two different measuring modes at 80% of MVC realized by a special pneumatic system. During HIMA the subject had to resist the defined impacting force of the pneumatic system in an isometric position, whereby the force of the cylinder works in direction of elbow flexion against the subject. During PIMA the subject worked isometrically in direction of elbow extension against a stable position of the system. The signals of pressure, force, acceleration and mechanomyography/-tendography (MMG/MTG) of the elbow extensor (MMGtri/MTGtri) and the abdominal muscle (MMGobl) were recorded and evaluated concerning the duration of maintaining the force level (force endurance) and the characteristics of MMG-/MTG-signals. Statistical group differences comparing HIMA vs. PIMA were estimated using SPSS.

RESULTS: Significant differences between HIMA and PIMA were especially apparent regarding the force endurance: During HIMA the subjects showed a decisively shorter time of stable isometric position (19 ± 8 s) in comparison with PIMA (41 ± 24 s; p = .005). In addition, during PIMA the longest isometric plateau amounted to 59.4% of the overall duration time of isometric measuring, during HIMA it lasted 31.6% (p = .000). The frequency of MMG/MTG did not show significant differences. The power in the frequency ranges of 8-15 Hz and 10-29 Hz was significantly higher in the MTGtri performing HIMA compared to PIMA (but not for the MMGs). The amplitude of MMG/MTG did not show any significant difference considering the whole measurement. However, looking only at the last 10% of duration time (exhaustion), the MMGtri showed significantly higher amplitudes during PIMA.

CONCLUSION: The results suggest that under holding isometric conditions muscles exhaust earlier. That means that there are probably two forms of isometric muscle action. We hypothesize two potential reasons for faster yielding during HIMA: (1) earlier metabolic fatigue of the muscle fibers and (2) the complexity of neural control strategies.

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