The present results provide findings, which partially support the hypothesis of two different forms of isometric muscle action. Overall, the authors assume the findings to be sufficient to distinct between the holding and the pushing isometric muscle action (HIMA vs. PIMA), respectively. Nevertheless, one also has to consider, that there are some items, which do not support the differentiation between such two forms.
The parameters, which show statistical differences between HIMA and PIMA, are the force endurance, the power in the frequency range of 8 to 15 Hz and 10 to 29 Hz of the MTG of the triceps tendon, the mean amplitude of the MMG of the triceps muscle at exhaustion and potentially the higher variability of the MMG of the abdominal external oblique. Therefore, the findings are generally consistent with them of Enoka and Rudroff et al. [
6,
7,
9] as in the present study a longer force endurance appeared during PIMA. However, in contrast to their findings, the present study shows a significant difference between the HIMA and the PIMA regarding the endurance time even under higher intensities (80% of MVC) and with a vertically positioned forearm. One has to consider primarily, that their measurements were made in a different setting (no use of pneumatics, different forearm position), which could explain the different outcomes.
The main question for us to discuss is how the lower force endurances during the HIMA, compared to the PIMA, can be explained. This also includes the findings concerning the oscillation characteristics (frequency and amplitude), since they may be interdependent. In general, different physiological processes could be responsible for faster muscular fatiguing. Weineck [
22] lists several reasons, for example a central fatigue or a fatigue caused by reduced hormone production. These two possibilities can be excluded due to the short measuring time. The measurement duration also speaks against a faster synaptic fatigability in the HIMA. Furthermore, studies have shown that the EMG-activity is lower during eccentric muscle action (e.g., [
23‐
25]). Assuming that the HIMA is closer to the eccentric muscle action (see below), this would also speak against a synaptic fatiguing. Thus, two potential sources of the faster yielding at submaximal static force endurance remain:
(1)
Metabolic fatigue of the muscle fibers (e.g., oxygen supply, removal of lactate)
(2)
Complexity of the neural control strategies.
The subsequent discussion is based on the hypothetical idea, that during the holding isometric muscle action (HIMA) the neuronal control strategies and muscle physiological aspects could be similar to the lengthening mode – analog to the eccentric activity – but without performing a change in length or joint angle. Vice versa, the mechanisms of the pushing isometric muscle action (PIMA) could be similar to the shortening mode – comparable to the concentric activity – without making a change in length or joint angle. On the one hand, this hypothesis is based on the subjective feeling during the performance of either the HIMA or the PIMA. On the other hand, this assumption could also be supported by the findings of Hunter et al. [
9], who found greater AEMG at exhaustion during PIMA compared to HIMA. It is well known that during concentric muscle action, the amplitude of EMG is greater than during the eccentric one. So perhaps this could be a link between the proposed isometric muscle actions and the common eccentric and concentric ones. Garner et al. [
4] also proposed this hypothesis. Furthermore, the study of Grabiner et al. [
26] showed a higher muscle activation (EMG) if the subject expects a concentric contraction compared to if it anticipates an eccentric one. These different a priori activations of the CNS might have also occurred in the current investigation. The subjects were instructed to either hold the external resistance without allowing the push rod to overcome them (i.e., they were expecting an external resistance, which could force the subject into eccentric action, if the impacting force was not resisted sufficiently). Or the subjects were instructed to push against the push rod, which included the task to contract the muscle as in concentric muscle action, but not more than to a specific level of force. There are different approaches, which could support the hypothesis that HIMA is closer to eccentric muscle action, while PIMA corresponds to the concentric one instead. The following discussion grounds on this hypothesis.
For the first mentioned aspect as a potential reason for the lower force endurance during HIMA – the metabolic fatigue (1) – a local lack of metabolic exchange under the conditions of intramuscular compression during isometric action could be presumed. This consideration is speculative as the current investigation did not measure substrate transport.
According to several authors [
28‐
30] an ischemia of muscles can occur at intensities as low as 5–30% of MVC because the muscle is compressing its own blood vessels. Jensen et al. [
31] reported a reduction of oxygenation at 30–40 mmHg, which corresponds to a contraction level of 20% of MVC. With intensities of 80% of MVC it is most likely that the blood vessels are also compressed, resulting in a probable ischemia. In this case it is plausible that not only the oxygenation, but also the exchange of substrates might be limited or not possible at all. The inadequate oxygenation due to the muscles’ compression would rather implicate an anaerobic metabolism during the present muscle tasks. Thereby, inter alia, lactate would be produced and would increase during the isometric muscle action. According to Katz et al. [
32] this can amount up to 15 times of the initial value of lactate. Under these tensed up conditions during static muscle action, one has to ask, how an exchange and transport of substrates can be enabled at all over an interval of 50–60s? And why does the duration of force maintenance differ between HIMA and PIMA, even though the generated force, and thus the compression of muscle fibers, should remain the same? One possible explanation on how the exchange and transport of substrates is maintained could be based on the neuromuscular oscillations.
In the present study, the amplitudes of the MMG of the triceps muscle show higher values at exhaustion during PIMA compared to HIMA. EMG and MMG are different, but causally linked, methods with similar behavior at exhaustion (i.e., amplitude increase at higher intensities [
11,
33‐
38]). Thus, one could expect related results. The investigations of Rudroff et al., Hunter et al. and the present study all show higher values at exhaustion concerning the mean amplitude during PIMA compared to HIMA in the AEMG [
6‐
9] and in the MMG of triceps muscle, respectively (present study). The amplitude of the MMG reflects the fine mechanic oscillations of the muscle fibers. We presume that they are possibly necessary to maintain any of the exchange and transport of substrates during isometric muscle action. Because under particular circumstances the transport of fluids can be supported by vibration [
39,
40]. The muscle tissue may possibly use a similar mechanism during reduced flow of substrates due to compression. Therefore, we hypothesize that higher oscillations – equivalent to vibrations – in the MMG of the triceps brachii muscle during the PIMA would probably support the substrate flow between the cell and interstitium as well as the flow within these areas. Conversely, lactate would probably be removed slower, and therefore would accumulate more during HIMA. Even though the lactate production is lower during eccentric muscle action [
41,
42], the ratio between production and removal could potentially be more detrimental during the HIMA. Thus, the muscular fatigue could begin sooner during the HIMA and consequently the subject could yield earlier. Further investigation including measurements of lactate or O2 remain.
In fact, the hypothesis of exchange and transport of substrate could be a fundamental physiological reason for the obligatory requirement of muscular oscillations during isometric action in general. This could be an area of future research.
In the present measurements, it was impressively apparent, that during HIMA the position could not be maintained stable by the subjects for as long as during PIMA. The holding position was interrupted earlier by sudden, quick, but short motions of the push rod (and thus, short-time lengthening of the muscle). But immediately after these abrupt releasing actions, the position could again be maintained at the same level of force, but with a slightly changed elbow angle. This was merely a subjective observation, but maybe the exchange and transport of substances could be expedited by the jerky yielding of muscle tension (compared to a very short intensive oscillation). In this way, the metabolic environment might be improved again for a short time.
In view of these considerations it has to be taken into account, that only the MMG of the triceps brachii muscle shows a higher mean amplitude at exhaustion. The other sensor locations do not. Since the abdominal external oblique muscle is probably only utilized to stabilize the trunk, the different tasks of HIMA and PIMA may not be reflected there.
The question remains, why the triceps tendon does not show the same output as its muscle? Even though both methods, the MMG and MTG, measure mechanic oscillations, the mechanisms behind these oscillations are different. The working muscle generates oscillations actively, whereas the tendon just resonates in a passive way during force transmission. From this point of view, it is conceivable that at higher intensities, the muscle (motor) vibrates more intensively. In contrast, a more tightened tendon (like a passive rope) would swing with lower amplitudes (and possibly higher frequencies).
Almost no basic knowledge exists about the MTG. Based on our investigation we know that the oscillations of the MTG and MMG show similar frequency ranges and are able to generate coherent behavior during isometric muscle action [
43,
44]. The authors are not aware of any other studies about the MTG. Thus, the MTG as a method is not yet established in science. Since the same sensors are used for MMG and the tendon is a passive structure which directly is connected to the muscle, the oscillations of the muscle theoretically has to be transmitted to the tendon. This is what we saw in previous investigations [
43,
44]. Anyhow, we suppose that the tendon of triceps muscle occupies a kind of superposition, since all three heads, and a large number of motor units of the triceps muscle come together and form the oscillations. This, in turn, could be an explanation for the different outcomes regarding the power of the MTG resp. MMG signals. The MTG of the triceps brachii muscle show significantly higher power in the frequency range of 8 to 15 Hz (as well as 10 to 29 Hz) during HIMA compared to PIMA. We remember that the mean amplitude of MMG of triceps brachii was higher during PIMA at exhaustion. Normally, higher amplitudes indicate higher power. In this case one may ask, how these findings can be brought together?
Complexity of the neural control strategies as a possible reason for lower force endurance during HIMA
Among others, Semmler et al. [
5] have shown that the synchronization of motor units (EMG) is higher during eccentric muscle action than during the concentric one. Possibly – and still under the assumption that the HIMA is closer to the processes of eccentric muscle action – a higher synchronization would enable the muscle fibers to oscillate in a narrower frequency range than during the PIMA. This could be the reason for the higher power of the MTG of the triceps in the low frequency range of 8 to 15 Hz (and 10 to 29 Hz) during the HIMA. While the PIMA is potentially wider spread, including other frequencies, and therefore results in lower power in the specific frequency range of 8 to 15 Hz. Rudroff et al. [
7] found contrary results concerning the EMG. Their findings show that the power during the force task (similar to PIMA) was higher in 10 to 29 Hz than during the position task and not vice versa as found here.
Fang et al. [
25] investigated the movement-related cortical potential (MRCP) via EEG comparing eccentric and concentric muscle action. They found that – although the EMG in their study showed lower amplitude during eccentric muscle action – the MRCP still resulted in greater values. The investigators postulated that eccentric muscle action is more difficult to perform compared to the concentric one, and therefore results in the higher brain activity. These findings are supported by the review of Enoka [
27], who summarized, that the eccentric muscle action involves more complex control strategies of the nervous system than the concentric one. This was underpinned by Duchateau & Baudry [
45], who added, that the unique mechanisms of eccentric muscle action are still unknown.
Bringing all of this together, the first paradoxically appearing result of significantly higher power in small frequency ranges during HIMA in the MTG, and conversely the greater amplitude at exhaustion during PIMA in the MMG of the triceps muscle could rely on the assumption, that eccentric muscle action is based on more complex neural control strategies. Thus, the present investigation indicates that HIMA could be more difficult to perform compared to PIMA due to a more complex adjustment of the neuromuscular system. Indicators for this are the shorter endurance time and the higher power of the MTG oscillations in a small frequency range of 8 to 15 Hz as a hint for a greater synchronization of muscle activation during HIMA. In turn, this possibly influences the fatigue mechanism, but further research is needed to test this hypothesis.