ECSS Paris 2023: OP-BM02
INTRODUCTION: Eccentric exercise training has been suggested to increase resting muscle fascicle length (FL) through sarcomerogenesis, especially in distal regions, reducing injury risk and promoting distal hypertrophy [1-3]. Animal data indicates that a greater fascicle elongation (fascicle strain; ƐPRE) during exercise promotes greater FL increases after training [4], although this has yet to be tested in humans. Simultaneously, detailed assessments of muscle function, including shifts in the muscle length of peak force production and shortening velocity [1,2], could help to identify the structural factors underlying any FL increases [1]. The present study investigated the hypothesis that increases in FL and muscle thickness (MT) would positively correlate with ƐPRE and changes in muscle function. METHODS: Distal and proximal tibialis anterior isokinetic eccentric ƐPRE, as well as resting architecture (via ultrasonography), and muscle function (via dynamometry), were assessed in 12 trained men (26 ± 4 y) before and after 18 unilateral isokinetic eccentric dorsiflexion training sessions (6 wk, 3/wk, 2-4 x 6-10 rep-max, 10°/s). Distal-proximal net architectural responses were computed as the distal minus the proximal relative changes. Dorsiflexion function was assessed through maximum voluntary isometric (5 angles) and dynamic (concentric, eccentric; 3 velocities) tests. RESULTS: Similar responses were found between distal and proximal regions for ƐPRE (59% vs. 60%; P=0.90), FL (7% vs. 6%; P=0.47), and MT (11% vs. 7%; P=0.58), although the values substantially varied between regions within individuals. A strong correlation was observed between distal-proximal net ƐPRE and distal-proximal net changes in FL (r=0.79, P<0.01) but not MT (r=0.17, P=0.59). However, no association was observed between region-averaged ƐPRE and region-averaged FL (r=0.04, P=0.91) or MT (r=0.03, P=0.92). Training significantly (P<0.05) improved isometric peak torque at shorter and longer muscle lengths (12%) and broadened the torque-angle relation (14%). Rate of torque development was increased at longer (0-75ms: 52%; 0-250ms: 23%), but not shorter or moderate lengths (P>0.05). Concentric peak torque (20%) and work (12%), and eccentric peak torque (14%) were increased in all tests. Isometric peak torque angle and estimated maximum shortening velocity were not changed significantly (P>0.05). CONCLUSION: Eccentric training increased FL and MT, which may influence both performance and injury risk [2], but changes were not specific to the distal region. Increases in FL were larger in regions with greater ƐPRE, whether proximal or distal, although individuals with greater ƐPRE did not exhibit greater increases in FL or MT. Serial sarcomere addition was likely not the primary driver of the FL increases, as some key functional adaptations were absent. References: 1 Hinks et al. 2022; J Appl Physiol. 2 Timmins et al. 2016; Br J Sports Med. 3 Franchi et al. 2017; Front Physiol. 4 Butterfield and Herzog 2006; Pflugers Arch.
Read CV Joao Pedro NunesECSS Paris 2023: OP-BM02
INTRODUCTION: Motor unit (MU) behaviour is the foundation of force output and is influenced by persistent inward currents (PICs), which amplify and prolong synaptic inputs and allow gain control of motor output. Despite known sex differences in MU discharge rate (DR) (1) and the magnitude of PICs (2), little is known about whether gain control differs between the sexes or across the female menstrual cycle (MC). Recent work suggests fluctuations in oestrogen (E) and progesterone (P) might contribute to modulations in DR at low force output (3), but as contraction intensity rises, estimates of PIC magnitude also increase (4). Examining MU behaviour during increasing intensity contractions might provide insight into the interactions between hormones and PICs. We hypothesised that DR and PICs would increase with contraction intensity, and intensity-specific increases will be enhanced when E concentration is elevated during the pre-ovulatory (OV) and mid-luteal (ML) phases; whereas elevated P concentration would blunt this increased gain in the ML phase. METHODS: Thirteen females (27 ± 6 years) participated across three institutes, as part of a wider study. Participants reported a regular MC, confirmed by cycle tracking and urine ovulation testing, retrospectively verified with serum hormone concentrations. After one tracked cycle, participants visited the lab in three phases of the subsequent MC(s): early follicular (EF), OV, and ML. During each visit, participants performed triangular dorsiflexion contractions at 30, 50 and 70% maximum, and high-density electromyographic signals were sampled and decomposed into individual MU spike trains. Smoothed DRs were analysed for peak DR, and DR hysteresis (ΔF) using the paired MU analysis technique. RESULTS: DR increased with contraction intensity, but was up to 10% higher during the ML phase compared to OV regardless of intensity (+0.74-1.99 pulses per second [pps]; p ≤ 0.043). Estimates of PIC magnitude were modulated by MC phase and intensity. At 30%, ΔF was up to 12% greater at ML (5.89 ± 0.29 pps) compared to EF (5.19 ± 0.28 pps) and OV (5.33 ± 0.28 pps; p ≤ 0.003) but not at 50 and 70% (p ≥ 0.139). There was a significant interaction between phase and intensity (p = 0.002) whereby ΔF did not increase with intensity during the ML phase as it did in the other phases. CONCLUSION: While DR was greater across all contraction intensities in the ML phase compared to OV, estimates of PICs across the MC were sensitive to contraction intensity, which presented as a saturation of PICs during the ML phase. This suggests that MC phase, and the associated fluctuations in E and P, do affect MU excitability, but these MC-related effects are diminished during higher intensity outputs. These data provide new mechanistic insight into motor output at a range of intensities across the MC. 1) Inglis et al., Appl Physiol Nutr Metab, 2020 2) Jenz et al., J Neurophysiol, 2023 3) Piasecki et al., Sports Med Open, 2023 4) Škarabot et al., bioRxiv, 2023
Read CV Padraig SpillaneECSS Paris 2023: OP-BM02
INTRODUCTION: Hamstring strain injuries are common in high-speed running-based sports, with up to 80% of cases affecting the biceps femoris long head (BFlh) [1]. Recently, high passive muscle stiffness has been identified as a risk factor for strain injuries [2], alongside low muscle strength and short fascicle lengths. Passive muscle stiffness has traditionally been recognized to chronically decrease with repeated passive lengthening (e.g., static stretching); however, its actual efficacy remains controversial [3]. Instead, during active lengthening (eccentric contraction), the combination of externally applied tensile and internally generated contractile forces theoretically imposes greater mechanical stress on muscle fibers and connective tissues [4], potentially leading to further decreases in passive muscle stiffness than static stretching. Here, we examined chronic adaptations of passive stiffness in the biarticular hamstring muscles after long-term eccentric-only knee flexion training (ECC) versus knee-extended static stretching (SS). METHODS: Eleven healthy young males participated in a 10-week intervention (three sessions per week) involving ECC for one leg and SS for the other. The ECC consisted of three sets of 10 repetitions performed at 50% of maximal eccentric torque, within 50 to 100% of the maximal range of motion, at an angular velocity of 5°/s. The SS involved three sets of stretching held for the same duration as ECC, at 100% of the maximal range of motion. Before and after the intervention, the shear moduli and architectures of the BFlh, semitendinosus, and semimembranosus (SM) were assessed using shear wave elastography and diffusion tensor MRI, respectively. Maximal isometric knee flexion torque was measured with a dynamometer. These variables were analyzed using the generalized mixed-effects models. RESULTS: The maximal isometric torque increased after ECC (57% [mean value], p < 0.01), but not after SS. The volumes of the three muscles substantially increased after ECC (22 to 42%, all p < 0.01), whereas BFlh volume did not increase after SS. The fascicle lengths of BFlh (34%) increased after ECC (p < 0.01), but not after SS. Notably, both shear moduli of BFlh (−11%) and SM (−25%) decreased after ECC (all p < 0.05), whereas only SM shear modulus (−18%) decreased after SS (p < 0.01). CONCLUSION: This study provides the first evidence that ECC has greater potential than SS for decreasing passive stiffness of a specific muscle, as well as increasing muscle strength and fascicle lengths within a matched intervention period. Our findings represent an important first step toward reconsidering the conventional approach to a chronic decrease in passive muscle stiffness and may offer practitioners a more comprehensive and time-efficient strategy aimed at preventing BFlh strain injury. REFERENCES: 1. Ekstrand et al. Sports Med (2012) 2. Miyamoto-Mikami et al. Med Sci Sports Exerc (2025) 3. Russell et al. Scand J Med Sci Sports (2024) 4. Crameri et al. J Physiol (2007)
Read CV Raki KawamaECSS Paris 2023: OP-BM02