Page 151 - 《运动与健康科学》(英文)2024年第2期

TaggedAPTARAFigure
TaggedAPTARAEndMotor units’ adaptations to disuse and active recovery 273
TaggedAPTARAPWe hypothesized that as a consequence of peripheral
impairment of the muscle fibers mechanical contractile proper-
4
ties after disuse, MUs are recruited at lower absolute intensi-
ties. At the same time, the reduced DR for the lower-threshold
MUs introduces an additional neural limitation to force
production. By increasing contraction intensity and recruiting
higher-threshold MUs, the higher-threshold motoneurons with
preserved functionality may in principle try to compensate for
the loss of force caused by impaired lower-threshold moto-
neurons (already recruited and sustaining force production); in
practice, however, they would have little or no effect due to
mechanical constraints of muscle fibers. 4,37 This last hypoth-
esis suggests that DR reduction of lower-threshold MUs,
alongside impaired muscle fibers mechanical contractile prop-
4
erties and other factors, could be responsible for the reduction
in muscle force commonly observed after disuse.TaggedAPTARAEnd

TaggedAPTARAH24.2. Reduced DR, but not DR modulation, might be
responsible for reduced muscle forceTaggedAPTARAEnd
TaggedAPTARAPTo further investigate the changes in MUs DR, we analyzed
the DR modulation (ΔDR RT ) of single MUs. In the total pool
of MUs, the DR modulation was reduced for lower- but not
higher-threshold MUs. However, in the same pool, lower-thre-
shold MUs were recruited at a higher intensity in relative terms
(i.e., the relative RT of the MUs decomposed at 10% and 25%
MVC at LS10 was higher than at LS0). Therefore, we
concluded that DR modulation was only partially affected by
ULLS and that the observed difference was mainly a conse-
quence of a differently balanced population of MUs decom-
posed at LS10 compared to LS0. This was also confirmed in
the pool of tracked MUs, where no significant difference in
DR modulation was observed. It should be noted, however,
that tracked MUs showed a trend similar to that observed in
the total pool, suggesting that significant alteration of DR
modulation might become apparent with longer-term
unloading or more severe models of disuse, as previously
5
5
suggested. Indeed, the work of Duchateau and Hainaut on
hand muscles reported a larger reduction in DR modulation for
lower- compared to higher-threshold MUs after 68 weeks of
hand cast immobilization. This finding suggests that the prefer-
ential impact of disuse on the motoneurons innervating lower-
threshold MUs should be expected for both smaller and larger
muscles, despite different utilization of MUs recruitment strat-
egies and DR modulation to sustain force production (i.e.,
compared to large muscles, smaller muscles are expected to
achieve the full MUs recruitment at lower intensity of contrac-
tion and to rely more on DR modulation to achieve the

based on the classification of lower- and higher-threshold (i.e., recruited
below or above 25% MVC). Individual MUs are represented by dots and clus-
tered by subject. Summary data are presented as mean § SEM, and the direc-
tion of the changes is highlighted by a connection line. Significance levels are:
Fig. 6. Swarm plots representing the MUs properties obtained from the pool * p < 0.05, ** p < 0.01, *** p < 0.001. AR = active recovery; DERT = dere-
of tracked MUs. (A and C) MUs RT and DERT are presented in both absolute cruitment threshold; DR = discharge rate; LS = limb suspension; MUs = motor
and (B and D) relative terms (as percent of the MVC). MUs DR is shown at units; MVC = maximal voluntary isometric contraction; N = Newton;
(E) recruitment, (F) derecruitment, and (G) during the steady-state phase, at pps = pulses per second; RT = recruitment threshold; SEM = standard error of
the 3 data collection points. From left to right, MUs properties are presented the mean.TaggedAPTARAEnd