ECSS Paris 2023: OP-BM03
INTRODUCTION: Tendons are mechanosensitive tissues that adapt in response to tensile strain, but the optimal stimulus for tendon adaptation is unknown. Isometric loading shows at least as much promise as eccentric loading but is understudied. We previously showed low-intensity isometric loading can improve tendon function and strength in a retrospective analysis of finger strength training data on rock climbers (PMID: 39560837), supported by our work on engineered ligaments, which indicates tenocytes are responsive to short (<10 min) load bouts irrespective of load magnitude (PMID: 28332110). Our hypothesis was that moderate- or long-duration isometric load would lead to improved tendon strength. METHODS: We performed a 3-week training study on adult rats using hindlimb electrical stimulation (stim) to induce controlled repeated isometric muscle contraction to determine how tendon adapts to different load magnitudes and durations with training. Each rat performed three 11-min training sessions per week (9 total). Protocols were matched for total stim time but varied in stim intensity: maximal or “high” load group vs. low intensity (50% of max) “low” load group and stim duration: (12 reps x 10s sets vs. 4 reps x 30s sets; 120s total). The three study groups (rep stim duration x intensity) were 10s x High, 10s x Low, and 30s x Low. Following training, we performed tendon tensile testing, then analyzed matrix stability using differential scanning calorimetry (DSC) and collagen content. The loading was optimized to the Achilles, a load bearing tendon, but we also analyzed the tibialis anterior (TA) tendon, which, due to stimulator placement in the posterior compartment of the distal hindlimb, received less muscle activation. RESULTS: Positive tendon adaptation was most evident in 30s x Low, with a trend toward higher maximal tensile load (MTL; p=0.0671) and an increased failure stress (p=0.0022) vs. control. By contrast, maximal loading was detrimental to the tendon: maximal tensile load (p=0.0106) and reduced failure stress (p=0.0005) in 10s x High vs. the contralateral control leg. Cross-sectional area (CSA) only increased in 10s x High (Achilles p=0.0753 (trend), TA p=0.0001) vs. control. Collagen (%) only changed in TA, indicating training can modestly impact CSA, but it is not the primary driver for the mechanical adaptations observed. The most apparent adaptations with DSC were on the proximal (muscle) end: proximal Achilles enthalpy (p=0.0165) and TA onset of melting temp (p=0.0105) increased in 30s x Low vs. control, and TA melt curve peak temp decreased in proximal TA (p=0.0309) indicating damage. CONCLUSION: Both load intensity and duration influenced the adaptive response of tendon to load, with the 30s x Low group exhibiting a superior adaptive response to training. The effect of training adaptation was greater in the TA, which as a positional tendon is subjected to much lower tensile loads in vivo than the Achilles.
Read CV Natalie GilmoreECSS Paris 2023: OP-BM03
INTRODUCTION: Explosive strength, quantified as the rate of torque development (RTD), can be viewed as a composite function resulting from the interaction between maximal strength and quickness in achieving a given level of torque. Therefore, RTD can improve in two ways: by increasing maximal voluntary torque (MVT) through maximal strength training or by increasing the rapidity of torque production through explosive strength training. This study aimed to compare the effects of explosive (EXP) versus sustained (STR) isometric training on RTD of the knee extensor muscles. METHODS: Twenty-two young adults underwent eight weeks of isometric training, two sessions per week. Participants trained one leg in EXP and the other in STR after randomized allocation counterbalanced by leg dominance. Both training interventions involved a linear increase in training volume. Force and High-Density surface EMG signals (HD-sEMG) from the three superficial knee extensor muscles were collected. MVT, time-locked absolute and normalized RTD, evoked RTD at 50 ms, and neural efficacy (voluntary/evoked RTD at 50 ms) were computed. Muscle excitation was calculated as the RMS of the HD-sEMG channel-wise normalized to the M-wave at each time window, and averaged across rectus femoris, vastus lateralis, and vastus medialis. M-wave peak-to-peak amplitude and conduction velocity were computed as measures of muscle excitability. RESULTS: MVT increased by 21.9% in STR and by 12.3% in EXP, with significantly larger gains in STR (time×training, p < .001). Absolute RTD increased by 11.9% to 24.0% in STR and by 16.5% to 24.6% in EXP, but no significant interaction between time × training, nor between time × training × epoch appeared. However, normalized RTD decreased by −1.7% to −8.9% in STR but increased +1.9% to +17.5% in EXP, showing a significant time × training interaction (p < .001). Evoked RTD at 50 ms significantly increased by 5.1% in both groups with no significant time × training interaction, whereas neural efficacy only improved in EXP with time × training interaction. Across time windows, muscle excitation increased by 13.0% to 19.6% in STR and by 16.2% to 19.8% in EXP without differences between trainings, while M-wave maximum amplitude and conduction velocity did not improve. CONCLUSION: These findings indicate that both maximal and explosive strength training enhance rapid torque production. Maximal strength training primarily increases the magnitude of torque that can be generated, thereby elevating the ceiling of torque available during contraction. In contrast, explosive strength training predominantly enhances the rate at which this available torque is developed.
Read CV Francesco SalvaggioECSS Paris 2023: OP-BM03
INTRODUCTION: Physical exercise (EX) improves cognitive function and mental health by promoting hippocampal neurogenesis through increased brain-derived neurotrophic factor (BDNF) signaling, a critical regulator of neuroplasticity (1). According to the molecular mechanisms underlying EX-induced brain BDNF overproduction, muscle myokines induced by contraction (2), such as FNDC5/irisin, can cross the blood-brain barrier and modulate brain BDNF production through muscle-brain crosstalk. Electromyostimulation (EMS), which electrically activates skeletal muscle via an external current, may therefore act as an alternative therapy to mimic some effects of EX by engaging this muscle-brain dialogue, particularly in patients with disabilities. Our team has previously demonstrated that acute EMS (two sessions, one week apart) significantly elevates hippocampal BDNF and neuroplasticity-related protein expression in rats, enhances executive function, and reduces anxiety in humans (3). To determine whether these benefits could be amplified, the same protocol was applied in rats under chronic conditions. METHODS: Chronic EMS was applied to the quadriceps of 8-week-old male Wistar rats for 7 days (30 min/day; 40 Hz, 400 μs pulse width, 7 s ON/14 s OFF; intensity 6–20 mA). Behavioral assessments for anxiety- and depression-like behaviors were performed 24 h after the last EMS session, followed by collection of blood, hippocampus, and quadriceps for ELISA, Western blot, and immunofluorescence analyses. RESULTS: Surprisingly, chronic EMS reduced hippocampal BDNF and neuroplasticity-related protein levels, while increasing pro-inflammatory cytokine expression in both the hippocampus and blood. These changes were associated with microglial and astrocytic activation in the hippocampus. EMS-treated rats also displayed anxiety and depressive-like behaviors. At the muscular level, an increase in FNDC5/irisin muscular expression was observed, but no change in irisin blood and brain levels. Furthermore, EMS caused tissue damage and elevated pro-inflammatory cytokines, along with increased serum corticosterone. CONCLUSION: Our data highlight that increased muscle FNDC5/irisin expression does not always reflect beneficial muscle-brain dialogue. In contrast, stimulation conditions that induce muscle damage trigger muscle-derived inflammatory signals, leading to maladaptive communication to the brain, hippocampal neuroinflammation, and altered BDNF. These findings highlight the importance of optimizing muscle stimulation protocols to minimize muscle damage, particularly to individuals suffering from muscle weakness.
Read CV Yuan WangECSS Paris 2023: OP-BM03