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Scientific Programme

Biomechanics & Motor control

OP-BM05 - Neuromuscular physiology II

Date: 03.07.2024, Time: 11:00 - 12:15, Lecture room: Alsh 2

Description

Chair TBA

Chair

TBA
TBA
TBA

ECSS Paris 2023: OP-BM05

Speaker A Francesco Salvaggio

Speaker A

Francesco Salvaggio
University of Turin, Department of medical sciences
Italy
"All sex differences in force-velocity relationship and rate of force development disappear after normalising for maximal force"

INTRODUCTION: Force-velocity (FV) relationship is characterised by dynamic force production capacities at high (V0) and low (F0) velocities, and then maximal power (Pmax). The rate of force development (RFD) represents a key measure determining the capacity to produce force quickly in isometric conditions starting from a relaxed state. When the FV relationship is obtained from average force and velocity over the movement, RFD could be an underlying factor of force production capacities at high velocity (V0, Boccia et al. 2024). This study aims to analyse comprehensively the variations in FV relationship and RFD parameters between sexes, notably to test the likely association between differences in both RFD and V0. METHODS: Single-leg knee extensors were tested under isometric and dynamic conditions in 53 healthy subjects (38% female). The protocol included two 5s maximal isometric voluntary contractions (MVC), 15 isometric burst-like contractions (at least 70% of MVC) and dynamic incremental-load knee extensions from the minimum load (i.e., 2.5 kg) up to the individual maximal load. Force and velocity were measured to calculate individual FV relationships using a hyperbolic model. Electromyographic signals were recorded using two 64-electrodes matrices placed on the vastus lateralis and the rectus femoris muscles. The root mean square (RMS) calculated during isometric and dynamic contractions was normalised to the RMS during the MVC. RESULTS: FV relationships were well fitted by hyperbolic regression (r² from 0.983 to 0.993). Independent samples t-test revealed significant sex differences (p<.001) in favour of males in isometric testing for RFD measured at 50 ms (effect size [ES]= 1.2), 100 ms (ES=2.1) and 150 ms (ES=2.4), peak RFD (ES=2.0), MVC (ES=2.1). However, after normalising all the parameters for MVC, the sex differences disappeared. Regarding FV parameters F0 (ES=0.9), Pmax (ES=1.0), and force at Pmax (ES=1.9) were greater in males (p<.001), while V0 was similar between sexes. Again, when normalising the force parameters (i.e. F0 and force at Pmax) for MVC, the sex differences disappeared. Similarly, the normalised EMG signals of both muscles showed no sex differences. CONCLUSION: Moderate to large sex differences were observed in favour of males in several parameters, including the RFD at 50ms, 100ms and 150ms, MVC, peak RFD, F0, maximal power and force at maximal power. However, after the normalisation of all the parameters mentioned above to MVC, those differences were no longer observed, suggesting that the maximal strength may explain most of the sex differences in rapid isometric and dynamic force production, without any differences in V0 between sexes. REFERENCES: Boccia et al., ECSS, 2024, The rate of force development as a determinant of maximal theoretical velocity in the force-velocity relationship.

Read CV Francesco Salvaggio

ECSS Paris 2023: OP-BM05

Speaker B Christopher D Connelly

Speaker B

Christopher D Connelly
Loughborough University, School of Sport, Exercise and Health Sciences
United Kingdom
"Age-related differences in motor unit discharge rate as a function of contraction intensity"

INTRODUCTION: Older adults undergo alterations in the neuromuscular system ultimately leading to reduced muscle force generating capacity. The force exerted by a muscle is controlled by motor unit (MU) recruitment and discharge rate, with the latter having been shown to decrease with advancing age. However, less is known about MU discharge rate modulation with increased contraction intensity in aged compared to younger individuals with alterations in motoneuron structural integrity, ionotropic synaptic input, and neuromodulation indicating potentially greater age-related effects at higher contraction intensities. Therefore, we aimed to examine tibialis anterior (TA) MU discharge rate modulation across a wide range of contraction intensities in older compared to young adults. METHODS: Fourteen young (6 female; 24±5 years) and 14 older adults (6 female; 71±4 years) matched for physical activity levels (3938 [2723, 6661] vs. 3431 [2068, 4795] MET.min/week, p=0.57) performed isometric dorsiflexion contractions both of a trapezoidal (10 s hold phase) and triangular shape (10 s ascending/descending phase) at 30%, 50% and 70% of maximum voluntary force (MVF). Multichannel electromyography signals were recorded from TA with a 64-channel array and were decomposed using Convolution Kernel Compensation algorithm into MU spike trains. From MU spike trains identified during trapezoidal contractions mean discharge rate during the hold phase was calculated. The MU spike trains during triangular contractions were smoothed with support vector regression, followed by the calculation of the onset-offset hysteresis of pairs of MUs (ΔF) to estimate the magnitude of persistent inward currents. Linear mixed effects models were used to determine if MU properties were predicted by age, contraction intensity and their interaction with MU recruitment threshold as a covariate. RESULTS: Dorsiflexion MVF was not significantly different between young and older adults (322 [278, 366] vs. 262 [222, 302] N; p=0.06). There was a significant interaction between age and contraction intensity for MU discharge rate (p=0.006). However, post hoc testing did not indicate any differences at individual contractions intensities (30% MVF: 14.5 [13.0, 16.0] vs. 12.7 [11.2, 14.1] pps, p=0.49; 50% MVF: 19.4 [17.9, 20.9] vs. 18.0 [16.5, 19.4] pps, p=0.71; 70% MVF: 26.0 [24.5, 27.5] vs. 25.3 [23.9, 26.8] pps; p=0.99). A significant age by contraction intensity interaction was also noted for ΔF (p<0.001), with greater ΔF noted for young compared to older adults at 70% (6.3 [5.7, 7.0] vs. 4.5 [3.8, 5.2] pps, p=0.006), but not 50% (5.8 [5.1, 6.4] vs. 4.4 [3.7, 5.1] pps, p=0.077) or 30% MVF (4.8 [4.1, 5.5] vs. 4.1 [3.4, 4.8] pps, p=0.6). CONCLUSION: These findings demonstrate that MU discharge properties are differentially modulated from low to high contraction intensities between young and older individuals in the TA, suggesting alterations in the gain modulation of aged motoneurons.

Read CV Christopher D Connelly

ECSS Paris 2023: OP-BM05

Speaker C Marco Gatti

Speaker C

Marco Gatti
Università degli studi di Pavia, Department of Public Health, Experimental and Forensic Medicine
Italy
"Effects of 14 days of steps reduction on neuromuscular function and fatigability in young adults"

INTRODUCTION: Physical inactivity (PI) is a critical issue for our society and is linked to increased risk of adverse health outcomes [1]. Disuse models used to simulate physiological consequences of PI, such as bedrest or lower limb suspension, showed muscle wasting and neuromuscular function (NMF) impairments in young adults [2,3]. After steps reduction (SR) interventions, a less invasive model of PI, mild muscle atrophy and controversial impairments in muscle force have been reported [4]. To date, no studies have fully explored NMF and fatigability changes after SR in young adults. This work aimed to assess in-vivo and ex-vivo muscle contractile proprieties and performance fatigability after 14 days of SR in healthy young adults. METHODS: We recruited 30 (n=16 women) young healthy, normally active, subjects that underwent to 14 days of SR to 1500 steps/day. They were tested before (T1) and after SR (T2). In each time point, participants performed 80 maximal-velocity isotonic concentric knee extension (KE) contractions (1 every 3s) with a load equivalent to 30% of the maximal voluntary isometric contraction (MVIC) torque. Mechanical power output (Pw) of each contraction was calculated. Before (PRE) and immediately after (POST) the fatiguing exercise, KE MVIC was performed. During MVIC interpolated twitch technique was used to investigate voluntary activation (VA). A set of 100Hz doublets (Db100) and single (St) transcutaneous electrical stimulations were also delivered through the femoral nerve on relaxed muscle. Finally, specific isometric force (P0/CSA) and maximal velocity (V0) were determined from skinned single muscle fibers samples from vastus lateralis. RESULTS: Overall daily steps were reduced by 82%. KE MVIC torque significantly decreased from T1 (149.750.0 Nm) to T2 (126.847.9 N m; p<.01). SR did not affected VA, St and Db100 torque. Initial Pw was lower T2 (2.970.83 W/Kg) compared to T1 (3.270.81 W/kg; p<.01). In POST, Pw and MVIC torque loss did not show any intervention effect. P0/CSA didn’t change in type 1 fibers, but a significant decrease in type 2 was found (10350 and 7736 kN*m2 in T1 and T2, respectively; p<.05). Moreover, V0 increased only for type 1 fibers (from 0.120.09 to 0.230.14 L/s; p<.05). CONCLUSION: In healthy young adults, mild reduction in physical activity like SR, caused a significant decrease in force production and mechanical Pw of KE, without affecting fatigability. Classical hallmarks of disuse [5] as reduction in type 2 fibers specific force and increase in type 1 fibers’ maximal velocity were found. This suggest that after SR, neuromuscular changes of knee extensors could be, at least in part, related to changes in contractile properties, although no changes were observed by electrical stimulated contractions. Thus, low mechanical load imposed for two weeks is sufficient to cause relevant functional impairments at skeletal muscle level.

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ECSS Paris 2023: OP-BM05