ECSS Paris 2023: CP-BM02
INTRODUCTION: Resistance training is an effective strategy to counteract age-related neuromuscular deficits (1). In older adults, eccentric contractions are associated with lower perceived exertion and reduced metabolic demands, making them particularly suitable for resistance training interventions (2). However, eccentric exercise may also elicit greater neuromuscular fatigue and increase the risk of muscle damage (3). Muscle length has been identified as a key factor modulating these neuromuscular responses to eccentric exercise (4). However, the combined influence of eccentric contractions and muscle length on neuromuscular responses has not yet been investigated in older adults. Therefore, this study aimed to examine the acute and delayed neuromuscular responses to eccentric contractions performed at different muscle lengths in young and older adults. METHODS: Twenty-four young and twenty-six older adults participated in this study. Participants were assigned to either a short or long muscle length condition during maximal eccentric plantarflexor exercise, defined by the ankle joint angle (30° to 0° plantarflexion and 0° to −30° dorsiflexion, respectively). The exercise bout consisted of 10 sets of 10 maximal eccentric contractions. Maximal voluntary isometric contraction (MVIC) torque was assessed before, immediately after, and 1 h, 24 h, and 48 h post-exercise. Additional neuromuscular assessments, i.e., evoked responses and voluntary activation, were also performed. RESULTS: Despite a similar torque–time integral in both age groups, a muscle length condition effect was observed only in younger participants, with a greater immediate post-exercise reduction in MVIC torque at long compared with short muscle length condition (−28.1% vs. −8.9%, p = 0.031). In contrast, older adults showed similar MVIC torque losses across muscle length conditions (≈ −14–21%, p = 0.179). Evoked responses did not differ between age groups but were greater during the long muscle length condition, whereas voluntary activation differed significantly between age groups (p = 0.019). All variables returned to baseline within 24 h for all groups independently of muscle length condition, indicating no muscle damage CONCLUSION: Eccentric contractions performed at long muscle length induced greater immediate force loss in young adults, whereas muscle length did not modulate neuromuscular responses in older adults, despite a similar mechanical load during exercise. These findings indicate that, in older adults, eccentric exercise of the plantarflexors at long muscle length can be performed without exacerbating immediate and delayed neuromuscular impairments, supporting its safe use in resistance training interventions targeting this muscle group. (1) Fragala et al., 2019 (2) Cvečka et al., 2023 (3) Hayes et al., 2023 (4) Guilhem et al., 2016
Read CV Yohan BétusECSS Paris 2023: CP-BM02
INTRODUCTION: Associated Activity (AA) refers to unintended activity in non-target contralateral muscles during strong unilateral contractions. AA has been identified as a candidate mechanism for cross-education effects and tends to increase as task demand (e.g. fatigue) increases. There is limited research on AA in the lower limbs and during sustained maximal contractions. While females are generally less fatigable than males, there is limited research on sex differences in AA. The purpose of this study was to examine sex differences in fatigue and AA during a sustained unilateral knee extension maximum voluntary contraction (MVC) until time to task failure (TTF). METHODS: Thirty healthy participants (15 Females (F), 15 Males (M); Age:25±3yrs) with at least one year training experience completed brief 3-5s unilateral isometric knee extension (KE) and flexion (KF) MVCs on both limbs with simultaneous electromyographic (EMG) recordings (vastus lateralis and biceps femoris) to determine peak torque and maximal EMG signal. Participants then completed a sustained unilateral isometric KE MVC until failure, defined as torque below 40% MVC for five continuous seconds. Torque and EMG were analyzed as %MVC for the active (ACT) and EMG was analyzed as %MVC in the opposite contralateral (CONT) limb. TTF data were normalized to percentage of contraction duration (DUR) and analyzed across six time points: start-of-contraction (2-4s at start) followed by five 20% contraction-duration bins (0–20%, 20-40%, 40-60%, 60-80%, 80–100% of TTF duration). RESULTS: Seconds to failure was greater in females (F:98.4±26.7 vs M:81.4±12.3s; p=.038). For the ACT limb, there were significant decreases from start-of-contraction to 80-100% DUR for torque (F:90.6±5.9 vs 39.4±3.1; M:88.3±5.5 vs 40.6±3.5%MVC), KE EMG (F: 100.5±25.2 vs 67.6±30.0; M: 80.8±11.3 vs 58.2±20.7%MVC) and KF EMG (F:13.5±8.4 vs 8.4±5.0; M:11.0±8.5 vs 7.5±5.2%MVC; all p<.001), with no differences between sexes. For CONT limb, KE EMG was significantly greater in females (F:8.6±3.2 vs M:4.4 ±3.1%MVC; p=.002), pooled across all time points. CONT KF EMG increased over time (p=.028) pooled for both sexes from start-of-contraction to peak at 60-80% DUR (17.0±28.2 vs 25.2±32.6%MVC). CONCLUSION: When normalized based on percent contraction duration, torque and ACT EMG decreased as fatigue progressed for both sexes, with a similar fatigue profile. CONT limb muscle activity was evident in both the KF and KE, with higher activity in extensor muscles in females, despite greater resistance to fatigue. CONT antagonist (KF) muscle activity was relatively high, but quite variable. Higher CONT knee extensor activation in females may indicate sex-based differences in lower body AA.
Read CV Jaxon FunkECSS Paris 2023: CP-BM02
INTRODUCTION: The muscle’s in vivo force-length relationship dictates the magnitude of muscle activation and metabolic cost required to achieve a given force (Fletcher & MacIntosh, 2017) . Muscle contraction at longer or shorter lengths than optimal length results in less isometric force (Gordon et al. 1966). On the other hand, the necessary level of activation and muscle metabolic cost can be minimized if the muscle is operating near optimal length. However, force production efficiency relative to metabolic cost and muscle activation in vivo in humans remains poorly understood. The purpose of this study was to determine the influence of ankle joint angle (muscle length) on muscle O₂ extraction and neuromuscular efficiency (NME) during sustained submaximal plantar flexion. METHODS: Nine males and nine females participated in this study. Participants were seated upright in an isometric dynamometer with the knee fully extended. Baseline measurements were obtained at rest, with the ankle positioned at 20° plantar flexion. Participants performed maximal voluntary isometric contractions (MVC) with the ankle in neutral position. Subsequently, nine 2-min isometric plantar flexion contractions were performed at three joint angles: 10° dorsiflexion (DF10), neutral position (NP), and 10° plantar flexion (PF10), and at three contraction intensities (10, 20, and 30% MVC) in a randomized order with 3-min rest between trials. We measured ankle plantar flexion torque and muscle activity in medial and lateral gastrocnemius (MG and LG) and soleus (SOL) using electromyography (EMG) to calculate NME (ankle plantar flexion torque/EMG). Muscle O₂ extraction of MG and SOL was estimated from deoxygenation of hemoglobin and myoglobin (deoxy [Hb+Mb]) by time-resolved near-infrared spectroscopy. RESULTS: Ankle plantar flexion torque was greater in NP than in PF10 at 20% and 30% MVC (p<0.05). NME tended to be highest at DF10, followed by NP and PF10 for LG (p<0.01) and SOL (p<0.05) across all contraction intensities. However, MG showed greater NME at DF10 than at PF10 only at 10% MVC (p<0.05). The increase in deoxy [Hb+Mb] was significantly smaller at DF10 than at NP and PF10, only at 30% MVC for MG (p<0.05, respectively). CONCLUSION: Ankle joint angle influenced muscle O₂ extraction and neuromuscular efficiency during isometric plantar flexion torque production. Shorter muscle length (PF10) was associated with greater muscle O₂ extraction and muscle activation compared with longer muscle length (DF10). However, muscle O₂ extraction and neuromuscular responses differed depending on the muscle and contraction intensity. Overall, these findings suggest that longer muscle lengths of the triceps surae would allow more efficient torque production relative to muscle O₂ extraction and muscle activity.
Read CV Yoko KunimasaECSS Paris 2023: CP-BM02