Chapter 19 Do Lengthening Contractions Represent a Case of Reversal in Recruitment Order?

doi:10.1016/S0079-6123(08)62858-7

Progress in Brain Research

Volume 123, 1999, Pages 215-220

https://doi.org/10.1016/S0079-6123(08)62858-7 Get rights and content

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The central nervous system (CNS) recruits spinal motoneurons in an orderly fashion under most of the tested experimental paradigms. Experimental observations suggesting selective excitation/recruitment of larger motor units during the electrical stimulation of cutaneous nerves, electrical stimulation of the rubrospinal tract, and lengthening contractions have been reported. The results utilizing synchronous electrical stimulation of cutaneous nerves, or the red nucleus, have to be interpreted as nonphysiological. Instead, under physiological activation of these pathways, the preferential inhibition of small motoneurons by these pathways would change the slope of the recruitment curve instead of causing selective recruitment of large motoneurons. The implications and mechanisms for the selective recruitment reported during lengthening contractions have to be reconsidered. This chapter presents the data obtained with two different experimental paradigms, both of which result in lengthening of active muscles. These observations do not support selective recruitment of fast twitch motor units during lengthening contractions. Two main papers where single motor unit data have been produced and interpreted to suggest selective recruitment of large motor units are central to the discussion in the chapter.

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Cited by (30)

Effects of ageing on motor unit activation patterns and reflex sensitivity in dynamic movements
2010, Journal of Electromyography and Kinesiology
Citation Excerpt :
This would indirectly support Hennemans size principle on the orderly recruitment of MU’s (Henneman et al., 1965a,b), as the selective recruitment of larger units in ECC should have increased the mean spike amplitude either through new, larger, spikes, or through the derecruitment of spikes with smaller amplitudes. The preservation of size principle has been found in ECC contractions in several studies (Bawa and Jones, 1999; Garland et al., 1996; Kossev and Christova, 1998; Pasquet et al., 2006; Sogaard et al., 1996), while some evidence of a reversal of recruitment exists (Howell et al., 1995; Nardone et al., 1989). The present results of similar mean spike amplitudes in all contraction types would not imply a previously reported lower threshold of MU recruitment in dynamic contractions (Ivanova et al., 1997; Linnamo et al., 2003; Tax et al., 1989).
Both contraction type and ageing may cause changes in H-reflex excitability. H reflex is partly affected by presynaptic inhibition that may also be an important factor in the control of MU activation. The purpose of the study was to examine age related changes in H-reflex excitability and motor unit activation patterns in dynamic and in isometric contractions. Ten younger (YOUNG) and 13 elderly (OLD) males performed isometric (ISO), concentric (CON) and eccentric (ECC) plantarflexions with submaximal activation levels (20% and 40% of maximal soleus surface EMG). Intramuscular EMG data was analyzed utilizing an intramuscular spike amplitude frequency histogram method. Average H/M ratio was always lowest in ECC (n.s.). Mean spike amplitude increased with activation level (P < .05), whereas no significant differences were found between contraction types. Both H-reflex excitability, which may be due to an increase in presynaptic inhibition, and mean spike frequency were higher in YOUNG compared to OLD. In OLD the mean spike frequency was significantly smaller in CON compared to ISO. Lack of difference in mean spike amplitude and frequency across contraction types in YOUNG would imply a similar activation strategy, whereas the lower frequency in dynamic contractions in OLD could be related to synergist muscle behavior.
Distinct brain activation patterns for human maximal voluntary eccentric and concentric muscle actions
2004, Brain Research
Citation Excerpt :
A different motor unit recruitment pattern may reflect a unique nervous system control strategy for eccentric movements, and that strategy may need greater cortical activity to carry it out. However, other studies [6,45] suggested that selective recruitment of high-threshold motor units does not occur during human wrist flexor eccentric contractions. Studies [1,41] using transcranial magnetic stimulation (TMS) of the brain to examine excitability of corticospinal neurons also support the notion of different control strategies for eccentric and concentric muscle actions.
Eccentric muscle contractions generate greater force at a lower level of activation and subject muscles to more severe damage than do concentric actions. A recent investigation has revealed that electroencephalogram (EEG)-derived movement-related cortical potential (MRCP) is greater and occurs earlier for controlling human eccentric than concentric submaximal muscle contractions. However, whether the central nervous system (CNS) control signals for high-intensity or maximal-effort eccentric movements differ from those for concentric actions is unknown. The purpose of this study was to determine whether the MRCP signals differ between the two types of maximal-effort contractions. Eight volunteers performed 40 maximal voluntary eccentric and 40 maximal voluntary concentric elbow flexor contractions on a Kin-Com isokinetic dynamometer. Scalp EEG signals (62 channels) were measured along with force, joint angle, and electromyographic (EMG) signals of the performing muscles. MRCP-based two-dimensional brain maps were created to illustrate spatial and temporal distributions of the MRCP signals. Although the level of elbow flexor muscle activity was lower during eccentric than concentric movements, MRCP-indicated cortical activation was greater both in amplitude and area dimension for the eccentric task. Detailed comparisons of individual electrode signals suggested that eccentric movements needed a significantly longer time for early preparation and a significantly greater magnitude of cortical activity for later movement execution. The extra preparation time and higher amplitude of activation may reflect CNS activities that account for the higher risk of injury, higher degree of movement difficulty, and unique motor unit activation pattern associated with maximal-level eccentric muscle actions.
Recruitment of single human low-threshold motor units with increasing loads at different muscle lengths
2004, Journal of Electromyography and Kinesiology
We investigated the recruitment behaviour of low threshold motor units in flexor digitorum superficialis by altering two biomechanical constraints: the load against which the muscle worked and the initial muscle length. The load was increased using isotonic (low load), loaded dynamic (intermediate load) and isometric (high load) contractions in two studies. The initial muscle position reflected resting muscle length in series A, and a longer length with digit III fully extended in series B. Intramuscular EMG was recorded from 48 single motor units in 10 experiments on five healthy subjects, 21 units in series A and 27 in series B, while subjects performed ramp up, hold and ramp down contractions. Increasing the load on the muscle decreased the force, displacement and firing rate of single motor units at recruitment at shorter muscle lengths (P<0.001, dependent t-test). At longer muscle lengths this recruitment pattern was observed between loaded dynamic and isotonic contractions, but not between isometric and loaded dynamic contractions. Thus, the recruitment properties of single motor units in human flexor digitorum superficialis are sensitive to changes in both imposed external loads and the initial length of the muscle.
Motor unit activation patterns during isometric, concentric and eccentric actions at different force levels
2003, Journal of Electromyography and Kinesiology
Motor unit activation patterns were studied during four different force levels of concentric and eccentric actions. Eight male subjects performed concentric and eccentric forearm flexions with the movement range from 100° to 60° in concentric and from 100° to 140° elbow angle in eccentric actions. The movements were started either from zero preactivation or with isometric preactivation of the force levels of 20, 40, 60 and 80% MVC. The subjects were then instructed to maintain the corresponding relative force levels during the dynamic actions. Intramuscular and surface EMG was recorded from biceps brachii muscle. Altogether 28 motoneuron pools were analyzed using the intramuscular spike-amplitude frequency (ISAF) analysis technique of Moritani et al. [1]. The mean spike amplitude was lower and the mean spike frequency higher in the isometric preactivation phase than in the consequent concentric and eccentric actions. When the movements started with isometric preactivation the mean spike amplitude increased significantly (P<0.001) up to 80% in isometric and concentric actions but in eccentric actions the increase continued only up to 60% (P<0.01). The mean spike frequency in isometric preactivation and in concentric action with preactivation was lower only at the 20% force level (P<0.01) as compared to the other force levels while in eccentric action with preactivation the increase between the force levels was significant (P<0.01) up to 60%. When the movement was started without preactivation the mean spike amplitude at 20% and at 40% force level was higher (P<0.01) in eccentric action than in concentric actions. It was concluded that the recruitment threshold may be lower in dynamic as compared to isometric actions. The recruitment of fast motor units may continue to higher force levels in isometric and in concentric as in eccentric actions which, on the other hand, seems to achieve the higher forces by increasing the firing rate of the active units. At the lower force levels mean spike amplitude was higher in eccentric than in concentric actions which might indicate selective activation of fast motor units. This was, however, the case only when the movements were started without isometric preactivation.
Muscle fibre activation and fatigue with low-load blood flow restricted resistance exercise—An integrative physiology review
2020, Acta Physiologica
Phosphorylation of B-crystallin and its cytoskeleton association differs in skeletal myofiber types depending on resistance exercise intensity and volume
2019, Journal of Applied Physiology

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Chapter 19 Do Lengthening Contractions Represent a Case of Reversal in Recruitment Order?

Publisher Summary

Brain Res.

Brain Res.

Frequency response of human soleus muscle.

J. Neurophysiol.

Firing patterns of human flexor carpi radialis motor units during the stretch reflex.

J. Neurophysiol.

Voluntary and reflexive recruitment of flexor carpi radialis motor units in man.

J. Neurophysiol.

Are there important exceptions to the size principle of alpha-motoneurone recruitment?

Voluntary motor commands in human ballistic movements.

Ann. Neurol.

Motor unit activity during isometric and concentric-eccentric contractions of human first dorsal inter-osseous muscle.

J. Neurophysiol.