Do soleus responses to transcutaneous spinal cord stimulation show similar changes to H-reflex in response to Achilles tendon vibration?

INTRODUCTION/PURPOSE: Recently, the use of transcutaneous spinal cord stimulation (TSCS) has been proposed as a viable alternative to the H-reflex. The aim of the current study was to investigate to what extent the two modes of spinal cord excitability investigation would be similarly sensitive to the well-known vibration-induced depression. METHODS: Fourteen healthy participants (8 men and 6 women; age: 26.7 ± 4.8 years) were engaged in the study. The right soleus H-reflex and TSCS responses were recorded at baseline (PRE), during right Achilles tendon vibration (VIB) and following 20 min of vibration exposure (POST-VIB). Care was taken to match H-reflex and TSCS responses amplitude at PRE and to maintain effective stimulus intensities constant throughout time points. RESULTS: The statistical analysis showed a significant effect of time for the H-reflex, with VIB (13 ± 5% of maximal M-wave (Mmax) and POST-VIB (36 ± 4% of Mmax) values being lower than PRE-values (48 ± 6% of Mmax). Similarly, TSCS responses changed over time, VIB (9 ± 5% of Mmax) and POST-VIB (27 ± 5% of Mmax) values being lower than PRE-values (46 ± 6% of Mmax). Pearson correlation analyses revealed positive correlation between H-reflex and TSCS responses PRE-to-VIB changes, but not for PRE- to POST-VIB changes. CONCLUSION: While the sensitivity of TSCS seems to be similar to the gold standard H-reflex to highlight the vibratory paradox, both responses showed different sensitivity to the effects of prolonged vibration, suggesting slightly different pathways may actually contribute to evoked responses of both stimulation modalities.

Motor function and gait decline in individuals with cerebral palsy during adulthood: a narrative review of potential physiological determinants.

Cerebral palsy (CP) is the most common childhood-onset disability. The evolution of gait according to severity is well known amongst children and thought to peak between 8 and 12 years of age among those walking without assistive devices. However, among adults, clinical experience as well as scientific studies report, through clinical assessments, questionnaires and interviews, increasing walking difficulties leading to an increased dependency of assistive devices in everyday ambulation. For many individuals with CP, this change will occur around 30-40 years, with the risk of losing mobility increasing with age. This narrative review aims to first provide objective evidence of motor function and gait decline in adults with CP when ageing, and then to offer mechanistic hypotheses to explain those alterations. Many studies have compared individuals with CP to the typically developing population, yet the evolution with ageing has largely been understudied. Comorbid diagnoses comprise one of the potential determinants of motor function and gait decline with ageing in people with CP, with the first manifestations happening at an early age and worsening with ageing. Similarly, ageing appears to cause alterations to the neuromuscular and cardiovascular systems at an earlier age than their typically developing (TD) peers. Future studies should, however, try to better understand how the physiological particularities of CP change with ageing that could pave the way for better strategies for maintaining function and quality of life in people with CP.

Torque and Discomfort During Electrically Evoked Muscle Contractions in Healthy Young Adults: Influence of Stimulation Current and Pulse Frequency.

OBJECTIVE: To investigate (1) how current and pulse frequency of electrical stimulation (ES) as well as contraction mode (isometric, concentric, and eccentric) influence torque output and discomfort and (2) how familiarization by repeated ES sessions influences ratings of perceived discomfort. DESIGN: An experimental study, 3 sessions. SETTING: A university laboratory. PARTICIPANTS: Eight healthy participants (5 men, 3 women; mean age 25.2 years; N=8). INTERVENTIONS: Participants completed 3 trial days, each including 17 electrically evoked thigh muscle contractions. On each trial day, the first 6 contractions consisted of 2 isometric, 2 concentric, and 2 eccentric muscle contractions randomly ordered with a fixed stimulation current and pulse frequency (200 mA, 20 Hz), while the remaining 11 muscle contractions were all isometric with randomly ordered combinations of current (100-250 mA) and pulse frequency (20-100 Hz). MAIN OUTCOME MEASURES: Torque and perceived discomfort were measured for each ES-evoked contraction. RESULTS: Overall, the findings revealed that a higher stimulation frequency was associated with an increased torque without increased discomfort, while higher currents were associated with increases of both torque and discomfort. Contraction type did not influence level of discomfort, despite eccentric contractions eliciting higher torque compared with concentric and isometric contractions (P<.001). Finally, a significant familiarization to ES (P<.001) was observed after just 1 of 3 identical stimulation sessions. CONCLUSIONS: The outlined data suggest that to elicit high torque levels while minimizing levels of discomfort in young subjects, eccentric muscle contractions evoked with a low stimulation current, and a high pulse frequency are preferable. Furthermore, a single familiarization session significantly lowers rating of perceived discomfort during ES.

The Role of Plasma Extracellular Vesicles in Remote Ischemic Conditioning and Exercise-Induced Ischemic Tolerance.

Ischemic conditioning and exercise have been suggested for protecting against brain ischemia-reperfusion injury. However, the endogenous protective mechanisms stimulated by these interventions remain unclear. Here, in a comprehensive translational study, we investigated the protective role of extracellular vesicles (EVs) released after remote ischemic conditioning (RIC), blood flow restricted resistance exercise (BFRRE), or high-load resistance exercise (HLRE). Blood samples were collected from human participants before and at serial time points after intervention. RIC and BFRRE plasma EVs released early after stimulation improved viability of endothelial cells subjected to oxygen-glucose deprivation. Furthermore, post-RIC EVs accumulated in the ischemic area of a stroke mouse model, and a mean decrease in infarct volume was observed for post-RIC EVs, although not reaching statistical significance. Thus, circulating EVs induced by RIC and BFRRE can mediate protection, but the in vivo and translational effects of conditioned EVs require further experimental verification.

The effect of low-frequency fatigue on the torque-velocity relationship in human quadriceps.

Low-frequency fatigue (LFF) is usually defined as the decline in low:high-frequency force of electrically evoked isometric muscle contractions. The influence of LFF on dynamic muscle function is not well studied. Our aim was to assess the effect of LFF on the electrically evoked torque-velocity relationship in humans. Sixteen participants underwent a series of electrically evoked knee extensions in an isokinetic dynamometer to establish torque-velocity relationships at 15 and 50 Hz using isokinetic contractions. Hereafter, fatigue was induced by five sets of 10 repetitions of maximal voluntary dynamic knee extensions. After 30 min of rest, torque-velocity tests were repeated. Maximal torque (Fmax) was measured, whereas maximal contraction velocity (Vmax) and maximal power (Pmax) were estimated using Hill's force-velocity equation, 15:50 Hz ratios were calculated for Fmax, Vmax, and Pmax as markers of LFF. Fmax decreased by 40% at 15 Hz (P = 0.001) and by 15% at 50 Hz (P = 0.001) in the fatigued state. No significant change was detected for Vmax at 15 Hz [-2%, (P = 0.349)] or 50 Hz [+3% (P = 0.763)], whereas 15 and 50 Hz Pmax decreased by 30% (P = 0.004) and 10% (P = 0.008), respectively. Following the fatigue protocol, the 15:50 Hz Fmax ratio decreased by 31% (P < 0.001), indicating LFF. The 15:50 Hz Pmax ratio also decreased by 23% (P = 0.002), whereas the 15:50 Hz Vmax ratio was unchanged (P = 0.313). In conclusion, fatiguing contractions decreased Fmax and Pmax at both high and low stimulation frequencies, whereas Vmax appeared unaffected. Nevertheless, LFF influences power production during human dynamic contractions at a range of submaximal velocities.NEW & NOTEWORTHY Force-velocity relationships were established using either low- or high-frequency electrical stimulation before and after fatiguing voluntary eccentric/concentric contractions of the knee extensors. Low-frequency fatigue was assessed by the relative decrease in low- and high-frequency maximal torque, maximal shortening velocity, and maximal power estimated by the force-velocity relationship. Low-frequency fatigue manifests itself as a large decrease in low-frequency maximal force and power with a modest decrease in high-frequency maximal force and power. Contraction velocity does not seem to decrease in the same manner.

Six Weeks of Low-Load Blood Flow Restricted and High-Load Resistance Exercise Training Produce Similar Increases in Cumulative Myofibrillar Protein Synthesis and Ribosomal Biogenesis in Healthy Males.

Purpose: High-load resistance exercise contributes to maintenance of muscle mass, muscle protein quality, and contractile function by stimulation of muscle protein synthesis (MPS), hypertrophy, and strength gains. However, high loading may not be feasible in several clinical populations. Low-load blood flow restricted resistance exercise (BFRRE) may provide an alternative approach. However, the long-term protein synthetic response to BFRRE is unknown and the myocellular adaptations to prolonged BFRRE are not well described. Methods: To investigate this, 34 healthy young subjects were randomized to 6 weeks of low-load BFRRE, HLRE, or non-exercise control (CON). Deuterium oxide (D2O) was orally administered throughout the intervention period. Muscle biopsies from m. vastus lateralis were collected before and after the 6-week intervention period to assess long-term myofibrillar MPS and RNA synthesis as well as muscle fiber-type-specific cross-sectional area (CSA), satellite cell content, and myonuclei content. Muscle biopsies were also collected in the immediate hours following single-bout exercise to assess signaling for muscle protein degradation. Isometric and dynamic quadriceps muscle strength was evaluated before and after the intervention. Results: Myofibrillar MPS was higher in BFRRE (1.34%/day, p < 0.01) and HLRE (1.12%/day, p < 0.05) compared to CON (0.96%/day) with no significant differences between exercise groups. Muscle RNA synthesis was higher in BFRRE (0.65%/day, p < 0.001) and HLRE (0.55%/day, p < 0.01) compared to CON (0.38%/day) and both training groups increased RNA content, indicating ribosomal biogenesis in response to exercise. BFRRE and HLRE both activated muscle degradation signaling. Muscle strength increased 6-10% in BFRRE (p < 0.05) and 13-23% in HLRE (p < 0.01). Dynamic muscle strength increased to a greater extent in HLRE (p < 0.05). No changes in type I and type II muscle fiber-type-specific CSA, satellite cell content, or myonuclei content were observed. Conclusions: These results demonstrate that BFRRE increases long-term muscle protein turnover, ribosomal biogenesis, and muscle strength to a similar degree as HLRE. These findings emphasize the potential application of low-load BFRRE to stimulate muscle protein turnover and increase muscle function in clinical populations where high loading is untenable.