Page 150 - 《运动与健康科学》(英文)2024年第2期

TaggedAPTARAFigure
TaggedAPTARAEnd272 G. Valli et al.
might drive the development of rehabilitation protocols specifi-
cally designed to target these motoneurons (low intensities of
exercise) and suitable to be carried out shortly after the cause
of disuse (e.g., injury or surgery).TaggedAPTARAEnd

TaggedAPTARAH24.1. MUs DR is affected by ULLSTaggedAPTARAEnd
TaggedAPTARAPOur data indicate a difference between the ULLS-induced
changes in absolute RT and DERT, observed both in lower-
and higher-threshold MUs, and the changes in DR that differed
for lower- and higher-threshold MUs. Notably, all these
changes were confirmed in the pool of tracked MUs, where
there are no confounding effects due to different populations
of decomposed MUs, 11,22 thereby strengthening the reliability
of the results.TaggedAPTARAEnd
TaggedAPTARAPThe strong correlation between absolute RT and DERT
suggests that the order of recruitment and derecruitment was
not altered by disuse; in other words, short-term disuse seems
36
to not cause deviations from the Henneman’s size principle.
Additionally, the correlation between MVC and absolute RT
and DERT highlights a synergic change of the 2 parameters
and indicates that the reduced RT observed at LS10 at all
contraction intensities should be a consequence of internal
muscular impairments equally affecting all MUs. Indeed,
recent evidence suggests that short periods of bed rest and
ULLS are sufficient to impair muscle fibers contractility by
4
altering intracellular calcium handling and muscle fibers
specific force. 37 Interestingly, these changes were identified
evenly in slow and fast muscle fibers. In this case, the recruit-
ment of MUs with altered contractile properties will generate
lower contractile force and, therefore, all MUs will be
recruited at lower absolute intensities.TaggedAPTARAEnd
TaggedAPTARAPRegarding the different effect of ULLS on DR of lower-
and higher-threshold MUs, we hypothesized the possibility of
alterations specific to the motoneurons with a lower depolari-
Fig. 5. Plots of the repeated-measures correlation describing the common within-
individual association between (A) MVC and absolute RT, (B) MVC and absolute zation threshold (which are expected to innervate smaller MUs
DERT, as well as (C) absolute RT and absolte DERT across the different data possibly composed by a prevalence of slow-type muscle
collection points. From left to right, correlations are presented at the 3 different fibers 22,36,38,39 ). Indeed, it was demonstrated that 3 weeks of
submaximal contraction intensities (i.e., 10%, 25%, and 50% MVC). The r value is
knee immobilization induced a phenotype shift from slow to
reported in the upper left of each figure. Significance level is p < 0.001 for all corre- 40,41
lations. DERT = derecruitment threshold; MVC = maximal voluntary isometric fast fibers. This phenomenon might be related to, and
anticipated by, a different functional preservation of lower-
contraction; N = Newton; RT = recruitment threshold.TaggedAPTARAEnd
and higher-threshold motoneurons. In this case, although
higher-threshold motoneurons might retain largely unaltered
of Duchateau and Hainaut 5 on small hand muscles and DR activity (at least for 10 days), muscle fibers are unable to
suggests that the preferential impact of disuse on the moto- effectively translate the neural signaling in force production
4
neurons innervating lower-threshold MUs is a general due to impaired contractile properties. TaggedAPTARAEnd
phenomenon. In other words, this response appears as a funda- TaggedAPTARAPIn addition, mechanisms such as the recurrent inhibition 42
mental mechanism independent from the size or the anatom- (i.e., motoneuron inhibition mediated by the Renshaw cells) or
ical location and characteristics of the considered muscle. 8,35 TaggedAPTARAEnd synaptic noise 4345 (i.e., random disturbing synaptic activity
TaggedAPTARAPUnveiling the effects of unloading on the neural control of in neurons that can alter the common input to the moto-
vastus lateralis, a large component in the group of knee neurons) could contribute to the modulation of the lower-thre-
extensor muscles that are fundamental for daily life motor shold motoneurons’ activity. Indeed, the reduced force
tasks and with key metabolic roles, may have relevance for steadiness that we observed at LS10 at 25% MVC suggests the
developing practices aimed at preventing or recovering disuse- presence of synaptic noise induced by ULLS, in light of recent
induced neuromuscular impairments. For example, knowing evidence suggesting that altered synaptic noise might be
the preferential deterioration of the motoneurons innervating responsible for altered steadiness at lower intensities of
lower-threshold MUs, and the rapid onset of this alteration, contraction. 4345 TaggedAPTARAEnd