Biology subjects

Franchi, M. V.

Publications and source records attributed to Franchi, M. V..

6 recordsLinked to original sources

bioRxiv2026

Responses of Muscle Volume, Strength, and Motor Unit Properties to Bed Rest and Retraining in Young Males and Females

Sex differences in motor unit (MU) properties to simulated microgravity are unknown. Young males and females completed a 21-day head-down bed rest followed by a 21-day retraining. Bed rest reduced knee extensor strength and volume without sex differences. MU firing rate decreased in both groups, more in females, alongside increased serum C-terminal agrin fragment. After retraining, these measures returned to baseline, whereas strength only partially recovered, remaining below baseline.

physiology↗

bioRxiv2025

Motor unit behaviour adaptations across the lifespan: sex differences in young, middle-aged and old adults

Ageing is associated with neuromuscular decline, and emerging evidence suggests that sex may influence the time course of motor unit adaptations. This study examined age- and sex-related differences in motor unit firing behaviour across young (YG), middle-aged (MA), and older adults (OLD), by integrating high-density EMG motor unit analysis with muscle morphology and daily physical activity measurements. The analysis of single motor unit activity during submaximal isometric contractions of the vastus lateralis revealed that older adults had lower firing rates and a reduced capacity to modulate discharge frequency during force-increasing contractions. In the YG and MA groups, females showed higher motor unit firing rates and variability than males, while in OLD these sex differences were no longer present. Females also demonstrated a steeper decline in firing rate modulation between MA and OLD. Reductions in muscle cross-sectional area and thickness were similar between sexes. Physical activity levels declined with age in both sexes. These findings reveal distinct, sex-specific trajectories of neuromuscular ageing, with females showing greater motor neuron function decline between MA and OLD, in the absence of sex-related differences in the rate of morphological deterioration. The attenuation of sex differences in older age suggests a convergence of neuromuscular profiles with ageing. While physical activity may contribute to the observed sex-specific patterns, other mechanisms related to hormonal shifts warrant further investigation. These insights underscore the importance of considering age and sex in the study of motor control and in the development of targeted interventions to preserve muscle function across the lifespan.

neuroscience↗

bioRxiv2025

Changes in motor unit conduction velocity after unilateral lower limb suspension and active recovery correlate with muscle ion channel gene expression

The effects of muscle disuse on the propagation of action potentials along muscle units, a key process for effective muscle activation and force production, remain poorly understood. This study aimed to investigate changes in action potential propagation and to identify biological factors influencing these changes following unilateral lower limb suspension (ULLS) and active recovery (AR). Eleven young male participants underwent 10 days of ULLS followed by 21 days of AR based on resistance exercise. Maximal force of the knee extensor muscles (MVC), High-Density surface EMG recordings and muscle biopsies of the vastus lateralis muscle were collected before ULLS, after ULLS and after AR. EMG recordings collected during submaximal isometric contractions were decomposed to estimate single motor unit conduction velocity (MU CV). Muscle biopsies were used to measure muscle fibre diameters via histochemical analysis and ion channel transcriptomic profiles via mRNA-sequencing. MVC decreased after ULLS by 29% and fully recovered after AR. MU CV decreased after ULLS and fully recovered, up to exceeding baseline values after AR. Muscle fibre diameters did not change across the interventions and showed no correlation with MU CV. Conversely, a feature importance analysis revealed that mRNA expression levels of specific ion channel genes, particularly those involved in K+ transport, correlate with MU CV at baseline and across the interventions. This study highlights the crucial role of K+ ion channels in influencing MU CV in humans, offering new insights into MU CV modulation and the mechanisms of muscle force changes after disuse and active recovery. Key pointsO_LIMuscle disuse, such as in unilateral lower limb suspension, leads to a decrease in motor unit conduction velocity (MU CV), a critical factor for muscle activation and force production. C_LIO_LIActive recovery through resistance exercise results in the full recovery of MU CV, even exceeding baseline levels. C_LIO_LIMuscle fibre diameters do not change significantly after limb suspension or active recovery and show no correlation with MU CV. C_LIO_LIConversely, ion channel mRNA expression, particularly of those related to K+ transport, correlates with MU CV and its changes following disuse and recovery. C_LIO_LIThese findings highlight K+ ion channels as a key factor in regulating MU CV in humans and provide new molecular determinants of the changes in muscle force after disuse and recovery. C_LI

physiology↗

bioRxiv2022

An increase in serial sarcomere number induced via weighted downhill running improves work loop performance in the rat soleus

Increased serial sarcomere number (SSN) has been observed in rats via downhill running training due to the emphasis on active lengthening contractions; however, little is known about the influence on dynamic contractile function. Therefore, we employed 4 weeks of weighted downhill running training in rats, then assessed soleus SSN and work loop performance. We hypothesized trained rats would produce greater net work output during faster, higher-strain work loops due to a greater SSN. Thirty-one Sprague-Dawley rats were assigned to a control or training group. Weight was added during downhill running via a custom-made vest, progressing from 5-15% body mass. Following sacrifice, the soleus was dissected, and a force-length relationship was constructed. Work loops (active shortening followed by passive lengthening) were then performed about optimal muscle length (LO) at 1.5-3-Hz cycle frequencies and 1-7-mm strains to assess net work output. Muscles were then fixed in formalin at LO. Fascicle lengths and sarcomere lengths were measured and used to calculate SSN. Intramuscular collagen content and crosslinking were quantified via a hydroxyproline content and pepsin-solubility assay. Trained rats had longer fascicle lengths (+13%), greater SSN (+8%), greater specific active forces (+50%), and lower passive forces (-45-62%) than controls (P<0.05). There were no differences in collagen parameters (P>0.05). Net work output was greater (+101-424%) in trained than control rats for the 1.5-Hz loops at 1, 3, and 5-mm strains (P<0.05) and showed relationships with fascicle length (R2=0.14-0.24, P<0.05). These results suggest training-induced longitudinal muscle growth may improve dynamic performance.

physiology↗

bioRxiv2021

JNK activation in TA and EDL muscle is load-dependent in rats receiving identical excitation patterns

ABSTACTO_ST_ABSAimC_ST_ABSAs the excitation-contraction coupling is inseparable during voluntary exercise, the relative contribution of the mechanical and neural input is poorly understood. Herein, we use a rat in-vivo strength training setup with an electrically induced standardized excitation pattern previously shown to lead to a load-dependent increase in myonuclear number and hypertrophy, to study acute effects of load per se on molecular signalling. MethodsAnaesthetized rats were subjected to unliteral identical electrically-paced contractions of the TA and EDL muscles under a high or low load for a duration of 2, 10 or 28-minutes. Muscle soluble proteins were extracted, and abundance and specific phosphorylations of FAK, mTOR, p70S6K and JNK were measured. Effects of exercise, load, muscle and exercise duration were assessed. ResultsSpecific phosphorylation of S2448-mTOR, T421/S424-p70S6K and T183/Y185-JNK was increased after 28-minutes of exercise under the high- and low-load protocol. Elevated phosphorylation of mTOR and JNK was detectable already after 2 and 10 minutes of exercise, respectively, but greatest after 28-minutes of exercise. T183/Y185-JNK and S2448-mTOR demonstrated a load-dependent increase in phosphorylation in the exercised muscles that for mTOR depended on muscle type. The abundance of all four kinases was higher in TA compared to EDL muscle. FAK and JNK abundance was reduced after 28 minutes of exercise in both the exercised and control muscle. ConclusionThe current study shows that JNK and mTOR activation is load-driven, and together with muscle-type specific mTOR and p70S6K effects it may drive muscle-type specific exercise and load-responses.

physiology↗

bioRxiv2021

Biceps femoris long head sarcomere and fascicle length adaptations after three weeks of eccentric exercise training

PurposeEccentric exercise is widely used to increase muscle fascicle lengths and thus decrease the risk of muscle strain injuries. However, the mechanisms behind this protection are still unknown. The aim of this study was to determine whether Biceps femoris long head (BFlh) fascicle length increases in response to three weeks of eccentric exercise training are the result of addition of in-series sarcomeres within muscle fibres. MethodsTen recreationally active participants (age: 27 {+/-} 3 years, mass: 70 {+/-} 14 kg, height: 174 {+/-} 9 cm) completed three weeks of Nordic hamstring exercise (NHE) training. We collected in vivo sarcomere and muscle fascicle images of the BFlh in two regions (central and distal), utilising microendoscopy and 3D ultrasonography. These images allowed us to estimate sarcomere length, sarcomere number and fascicle lengths before and after the training intervention. ResultsEccentric knee flexion strength increased after the training (15%, P < 0.001, {eta}p2= 0.75). Further, we found a significant increase in fascicle (21%, P < 0.001, {eta}p2 = 0.81) and sarcomere (17%, P < 0.001, {eta}p2 = 0.9) lengths in the distal but not in the central portion of the muscle. The estimated number of in series sarcomeres did not change in either region. ConclusionFascicle length adaptations appear to be heterogeneous in the BFlf in response to three weeks of NHE training. An increase in sarcomere length, rather than the addition of sarcomeres in series, appears to be underlying this adaptation. The mechanism driving regional increases in fascicle and sarcomere length remain unknown, but we speculate it may be driven by regional changes in the passive tension of muscle or connective tissue adaptations.

physiology↗