Meta-analysis prediction intervals are under reported in sport and exercise medicine.

AIM: Prediction intervals are a useful measure of uncertainty for meta-analyses that capture the likely effect size of a new (similar) study based on the included studies. In comparison, confidence intervals reflect the uncertainty around the point estimate but provide an incomplete summary of the underlying heterogeneity in the meta-analysis. This study aimed to estimate (i) the proportion of meta-analysis studies that report a prediction interval in sports medicine; and (ii) the proportion of studies with a discrepancy between the reported confidence interval and a calculated prediction interval. METHODS: We screened, at random, 1500 meta-analysis studies published between 2012 and 2022 in highly ranked sports medicine and medical journals. Articles that used a random effect meta-analysis model were included in the study. We randomly selected one meta-analysis from each article to extract data from, which included the number of estimates, the pooled effect, and the confidence and prediction interval. RESULTS: Of the 1500 articles screened, 866 (514 from sports medicine) used a random effect model. The probability of a prediction interval being reported in sports medicine was 1.7% (95% CI = 0.9%, 3.3%). In medicine the probability was 3.9% (95% CI = 2.4%, 6.6%). A prediction interval was able to be calculated for 220 sports medicine studies. For 60% of these studies, there was a discrepancy in study findings between the reported confidence interval and the calculated prediction interval. Prediction intervals were 3.4 times wider than confidence intervals. CONCLUSION: Very few meta-analyses report prediction intervals and hence are prone to missing the impact of between-study heterogeneity on the overall conclusions. The widespread misinterpretation of random effect meta-analyses could mean that potentially harmful treatments, or those lacking a sufficient evidence base, are being used in practice. Authors, reviewers, and editors should be aware of the importance of prediction intervals.

Ageing reduces persistent inward current contribution to motor neurone firing: Potential mechanisms and the role of exercise.

Nervous system deterioration is a primary driver of age-related motor impairment. The motor neurones, which act as the interface between the central nervous system and the muscles, play a crucial role in amplifying excitatory synaptic input to produce the desired motor neuronal firing output. For this, they utilise their ability to generate persistent (long-lasting) depolarising currents that increase cell excitability, and both amplify and prolong the output activity of motor neurones for a given synaptic input. Modulation of these persistent inward currents (PICs) contributes to the motor neurones' capacities to attain the required firing frequencies and rapidly modulate them to competently complete most tasks. Thus, PICs are crucial for adequate movement generation. Impairments in intrinsic motor neurone properties can impact motor unit firing capacity, with convincing evidence indicating that the PIC contribution to motor neurone firing is reduced in older adults. Indeed, this could be an important mechanism underpinning the age-related reductions in strength and physical function. Furthermore, resistance training has emerged as a promising intervention to counteract age-associated PIC impairments, with changes in PICs being correlated with improvements in muscular strength and physical function after training. In this review, we present the current knowledge of the PIC magnitude decline during ageing and discuss whether reduced serotonergic and noradrenergic input onto the motor neurones, voltage-gated calcium channel dysfunction or inhibitory input impairments are candidates that: (i) explain age-related reductions in the PIC contribution to motor neurone firing and (ii) underpin the enhanced PIC contribution to motor neurone firing following resistance training in older adults.

Longitudinal changes in intrinsic motoneuron excitability in amyotrophic lateral sclerosis are dependent on disease progression.

Increased amplitude of persistent inward currents (PICs) is observed in pre-symptomatic genetically modified SOD1 mice models of amyotrophic lateral sclerosis (ALS). However, at the symptomatic stage this reverses and there is a large reduction in PIC amplitude. It remains unclear whether these changes in PICs can be observed in humans, with cross-sectional studies in humans reporting contradictory findings. In people with ALS, we estimated the PIC contribution to self-sustained firing of motoneurons, using the paired-motor unit analysis to calculate the Δfrequency (ΔF), to compare the weaker and stronger muscles during the course of disease. We hypothesised that, with disease progression, ΔFs would relatively increase in the stronger muscles; and decline in the weaker muscles. Forty-three individuals with ALS were assessed in two occasions on average 17 weeks apart. Tibialis anterior high-density electromyograms were recorded during dorsiflexion (40% of maximal capacity) ramped contractions, followed by clinical tests. ∆F increased from 3.14 (2.57, 3.71) peaks per second (pps) to 3.55 (2.94, 4.17) pps on the stronger muscles (0.41 (0.041, 0.781) pps, standardised difference (d) = 0.287 (0.023, 0.552), P = 0.030). ∆F reduced from 3.38 (95% CI 2.92, 3.84) pps to 2.88 (2.40, 3.36) pps on the weaker muscles (-0.50 (-0.80, -0.21) pps, d = 0.353 (0.138, 0.567), P = 0.001). The ALSFRS-R score reduced 3.9 (2.3, 5.5) points. These data indicate that the contribution of PICs to motoneuron self-sustained firing increases over time in early stages of the disease when there is little weakness before decreasing as the disease progresses and muscle weakness exacerbates, in alignment with the findings from studies using SOD1 mice. KEY POINTS: Research on mouse model of amyotrophic lateral sclerosis (ALS) suggests that the amplitude of persistent inward currents (PICs) is increased in early stages before decreasing as the disease progresses. Cross-sectional studies in humans have reported contradictory findings with both higher and lower PIC contributions to motoneuron self-sustained firing. In this longitudinal (∼17 weeks) study we tracked changes in PIC contribution to motoneuron self-sustained firing, using the ΔF calculation (i.e. onset-offset hysteresis of motor unit pairs), in tibialis anterior muscles with normal strength and with clinical signs of weakness in people with ALS. ΔFs decreased over time in muscles with clinical signs of weakness. The PIC contribution to motoneuron self-sustained firing increases before the onset of muscle weakness, and subsequently decreases when muscle weakness progresses.

Caffeine does not influence persistent inward current contribution to motoneuron firing.

The purpose of this study was to investigate whether caffeine consumption would change persistent inward current (PIC) contribution to motoneuron firing at increased contraction intensities and after repetitive sustained maximal contractions. Before and after the consumption of 6 mg·kg-1 of caffeine or placebo, 16 individuals performed isometric triangular-shaped ramp dorsiflexion contractions (to 20% and 40% of peak torque), followed by four maximal contractions sustained until torque production dropped to 60% of maximum, and consecutive 20% triangular-shaped contractions. Tibialis anterior motor unit firing frequencies were analyzed from high-density surface electromyograms. PIC contribution to motor unit firing was estimated by calculating the delta frequency (ΔF) using the paired motor unit technique. Motoneuron peak firing frequencies at 20% and 40% contractions and total torque-time integral during the repetitive sustained maximal contractions were also assessed. ΔF increased 0.69 peaks per second (pps) (95% CI = -0.98, -0.405; d = -0.87) from 20% to 40% contraction intensities and reduced 0.85 pps (95% CI = 0.66, 1.05; d = 0.99) after the repetitive sustained maximal contractions, regardless of caffeine consumption. Participants produced 337 Nm·s (95% CI = 49.9, 624; d = 0.63) more torque integral during the repetitive sustained maximal contractions after caffeine consumption. A strong repeated-measures correlation (r = 0.61; 95% CI = 0.49, 0.69) was observed between reductions of ΔF and peak firing frequencies after the repetitive sustained maximal contractions. PIC contribution to motoneuron firing increases from 20% to 40% contraction intensities, with no effect of caffeine (on rested tibialis anterior). Repetitive sustained maximal contractions reduced PIC contribution to motoneuron firing, regardless of caffeine or placebo consumption, evidencing that changes in intrinsic motoneuron properties contributed to performance loss. Caffeine-attenuated reduction of torque production capacity was unlikely mediated by PICs.NEW & NOTEWORTHY Persistent inward current (PIC) contribution to motoneuron firing increases with contraction intensities and is reduced after repetitive sustained maximal contractions, regardless of caffeine consumption. Reductions of PIC contribution to motoneuron firing and peak firing frequencies were largely associated, evidencing a novel mechanism underpinning decrements in maximal torque production capacity following repetitive sustained maximal contractions. Caffeine consumption attenuated neuromuscular performance reductions-allowing higher time-torque integral production during repetitive sustained maximal contractions. This was unlikely mediated by PIC.

Intrinsic motor neuron excitability is increased after resistance training in older adults.

This study investigated the effects of high-intensity resistance training on estimates of the motor neuron persistent inward current (PIC) in older adults. Seventeen participants (68.5 ± 2.8 yr) completed a 2-wk nonexercise control period followed by 6 wk of resistance training. Surface electromyographic signals were collected with two 32-channel electrodes placed over soleus to investigate motor unit discharge rates. Paired motor unit analysis was used to calculate delta frequency (ΔF) as an estimate of PIC amplitudes during 1) triangular-shaped contractions to 20% of maximum torque capacity and 2) trapezoidal- and triangular-shaped contractions to 20% and 40% of maximum torque capacity, respectively, to understand their ability to modulate PICs as contraction intensity increases. Maximal strength and functional capacity tests were also assessed. For the 20% triangular-shaped contractions, ΔF [0.58-0.87 peaks per second (pps); P ≤ 0.015] and peak discharge rates (0.78-0.99 pps; P ≤ 0.005) increased after training, indicating increased PIC amplitude. PIC modulation also improved after training. During the control period, mean ΔF differences between 20% trapezoidal-shaped and 40% triangular-shaped contractions were 0.09-0.18 pps (P = 0.448 and 0.109, respectively), which increased to 0.44 pps (P < 0.001) after training. Also, changes in ΔF showed moderate to very large correlations (r = 0.39-0.82) with changes in peak discharge rates and broad measures of motor function. Our findings indicate that increased motor neuron excitability is a potential mechanism underpinning training-induced improvements in motor neuron discharge rate, strength, and motor function in older adults. This increased excitability is likely mediated by enhanced PIC amplitudes, which are larger at higher contraction intensities.NEW & NOTEWORTHY Resistance training elicited important alterations in soleus intrinsic motor neuronal excitability, likely mediated by enhanced persistent inward current (PIC) amplitude, in older adults. Estimates of PICs increased after the training period, accompanied by an enhanced ability to increase PIC amplitudes at higher contraction intensities. Our data also suggest that changes in PIC contribution to self-sustained discharging may contribute to increases in motor neuron discharge rates, maximal strength, and functional capacity in older adults after resistance training.

Increases in muscle temperature by hot water improve muscle contractile function and reduce motor unit discharge rates.

PURPOSE: Examine the effects of 42°C hot-water immersion on muscle contraction function and motor unit discharge rates. Voluntary and evoked contraction assessments were examined first with a concomitant increase in the core and muscle temperature, and thereafter with increased muscle temperature but cooled core temperature. METHODS: Fifteen participants (24.9 ± 5.6 years) performed neuromuscular assessments before, after, and ~15-min after either 90-min of 42°C (hot) or 36°C (control) water immersion. Maximal voluntary contraction (MVC) assessment of knee extension was performed along with surface electromyography (sEMG) (vastus lateralis and medialis [VL, VM]) and voluntary activation level (VAL). Resting evoked twitch was elicited for peak torque and time to peak torque analysis. In addition, the VL and VM motor unit discharge rates (MUDR) were measured. RESULTS: After hot-water immersion (core temperature ↑1°C; muscle temperature ↑2.4°C), MVC torque and VAL decreased (p < 0.05). The sEMG (VL and VM) and peak twitch torque did not change (p > 0.05), while time to peak torque decreased (p = 0.007). The VL and VM MUDR decreased, showing a time effect, after both water immersion conditions (36 and 42°C) (p > 0.001). Fifteen minutes after the hot-water immersion (core temperature at baseline; muscle temperature ↑1.4°C), MVC torque returned to baseline, but VAL remained lower. The sEMG (VL and VM) remained unchanged. Peak twitch torque increased (p < 0.002) and time to peak torque remained lower (p = 0.028). The MUDR remained lower after both water immersion conditions (p < 0.05). CONCLUSION: Increased core temperature evoked by 42°C hot-water immersion decreases MVC torque and VAL. However, a passive increase in muscle temperature improved evoked muscle contractile function (i.e., time to peak torque [after] and peak twitch torque [~15 min after]). Moreover, a passive increase in muscle temperature reduced the required MUDR to attain the same torque.

Lower motor unit discharge rates in gastrocnemius lateralis, but not in gastrocnemius medialis or soleus, in runners with Achilles tendinopathy: a pilot study.

OBJECTIVES: Deficits in muscle performance could be a consequence of a reduced ability of a motor neuron to increase the rate in which it discharges. This study aimed to investigate motor unit (MU) discharge properties of each triceps surae muscle (TS) and TS torque steadiness during submaximal intensities in runners with Achilles tendinopathy (AT). METHODS: We recruited runners with (n = 12) and without (n = 13) mid-portion AT. MU discharge rate was analysed for each of the TS muscles, using high-density surface electromyography during 10 and 20% isometric plantar flexor contractions. RESULTS: MU mean discharge rate was lower in the gastrocnemius lateralis (GL) in AT compared to controls. In AT, GL MU mean discharge rate did not increase as torque increased from 10% peak torque, 8.24 pps (95% CI 7.08 to 9.41) to 20%, 8.52 pps (7.41 to 9.63, p = 0.540); however, in controls, MU discharge rate increased as torque increased from 10%, 8.39 pps (7.25-9.53) to 20%, 10.07 pps (8.89-11.25, p < 0.001). There were no between-group difference in gastrocnemius medialis (GM) or soleus (SOL) MU discharge rates. We found no between-group differences in coefficient of variation of MU discharge rate in any of the TS muscles nor in TS torque steadiness. CONCLUSION: Our data demonstrate that runners with AT may have a lower neural drive to GL, failing to increase MU discharge rate to adjust for the increase in torque demand. Further research is needed to understand how interventions focussing on increasing neural drive to GL would affect muscle function in runners with AT.

Effects of α-lactalbumin on strength, fatigue and psychological parameters: a randomised double-blind cross-over study.

PURPOSE: The neurotransmitter serotonin has a strong effect on behaviour and motor control. Regarding motor control, serotonin contributes to the development of fatigue and is also involved in the ability of motor neurones to operate across a large range of forces (gain control). The consumption of tryptophan-rich supplements (such as α-lactalbumin) is of interest because this amino acid is the only precursor for brain serotonin synthesis. Therefore, the purpose of this study was to determine the effects of α-lactalbumin supplementation on neuromuscular performance. METHODS: Using a randomised double-blind cross-over design, 16 healthy participants performed plantar flexor and handgrip maximal voluntary contractions, a 30-s submaximal handgrip contraction, and a plantar flexor fatigue protocol before and 90 min after consuming either 40 g of α-lactalbumin, an isonitrogenous beverage (Zein) or an isocaloric beverage (corn-starch). Sleepiness, mood, and cognition were assessed to evaluate any psychological effects. RESULTS: α-Lactalbumin decreased force steadiness by 25% during the sustained submaximal handgrip contraction (p < 0.01) and induced greater fatigue (15% reduction in total torque-time integral, p = 0.01) during the fatigue protocol. These effects were not observed for the other control beverages. No effects were found for maximal or explosive strength, or psychological measurements. CONCLUSIONS: 40 g of α-lactalbumin increased handgrip force variability and reduced performance during fatiguing muscle contractions but did not influence brief maximal contractions or psychological parameters in healthy individuals. These findings support the hypothesis that the consumption of α-lactalbumin can increase motor neurone input-output gain and exacerbate central fatigue during sustained maximal exercise.

The effects of α-lactalbumin supplementation and handgrip contraction on soleus motoneuron excitability.

INTRODUCTION: We tested two strategies that hypothetically increase serotonin availability (α-lactalbumin consumption and a remote submaximal handgrip contraction) on estimates of persistent inward currents (PICs) amplitude of soleus muscle in healthy participants. METHODS: With a randomised, double-blind, and cross-over design, 13 healthy participants performed triangular-shaped ramp contractions with their plantar flexors (20% of maximal torque), followed by a 30-s handgrip sustained contraction (40% of maximal force) and consecutive repeated triangular-shaped contractions. This was performed before and after the consumption of either 40 g of α-lactalbumin, an isonitrogenous beverage (Zein) or an isocaloric beverage (Corn-starch). Soleus motor units discharge rates were analysed from high-density surface electromyography signals. PICs were estimated by calculating the delta frequency (ΔF) of motor unit train spikes using the paired motor unit technique. RESULTS: ΔF (0.19 pps; p = 0.001; d = 0.30) and peak discharge rate (0.20 pps; p < 0.001; d = 0.37) increased after the handgrip contraction, irrespective of the consumed supplement. No effects of α-lactalbumin were observed. CONCLUSIONS: Our results indicate that 40 g of α-lactalbumin was unable to modify intrinsic motoneuron excitability. However, performing a submaximal handgrip contraction before the plantar flexion triangular contraction was capable of increasing ΔF and discharge rates on soleus motor units. These findings highlight the diffused effects of serotonergic input, its effects on motoneuron discharge behaviour, and suggest a cross-effector effect within human motoneurons.

Facilitation-inhibition control of motor neuronal persistent inward currents in young and older adults.

A well-coordinated facilitation-inhibition control of motor neuronal persistent inward currents (PICs) via diffuse neuromodulation and local inhibition is essential to ensure motor units discharge at required times and frequencies. Present best estimates indicate that PICs are reduced in older adults; however, it is not yet known whether PIC facilitation-inhibition control is also altered with ageing. We investigated the responses of PICs to (i) a remote handgrip contraction, which is believed to diffusely increase serotonergic input onto motor neurones, and (ii) tendon vibration of the antagonist muscle, which elicits reciprocal inhibition, in young and older adults. High-density surface electromyograms were collected from soleus and tibialis anterior of 18 young and 26 older adults during triangular-shaped plantar and dorsiflexion contractions to 20% (handgrip experiments) and 30% (vibration experiments) of maximum torque (rise-decline rate of 2%/s). A paired-motor-unit analysis was used to calculate ∆F, which is assumed to be proportional to PIC strength. ΔF increased in both soleus (0.55 peaks per second (pps), 16.0%) and tibialis anterior (0.42 pps, 11.4%) after the handgrip contraction independent of age. Although antagonist tendon vibration reduced ΔF in soleus (0.28 pps, 12.6%) independent of age, less reduction was observed in older (0.42 pps, 10.7%) than young adults (0.72 pps, 17.8%) in tibialis anterior. Our data indicate a preserved ability of older adults to amplify PICs following a remote handgrip contraction, during which increased serotonergic input onto the motor neurones is expected, in both lower leg muscles. However, PIC deactivation in response to reciprocal inhibition was impaired with ageing in tibialis anterior despite being preserved in soleus. KEY POINTS: Motor neuronal persistent inward currents (PICs) are facilitated via diffuse neuromodulation and deactivated by local inhibition to ensure motor units discharge at required times and frequencies, allowing normal motor behaviour. PIC amplitudes appear to be reduced with ageing; however, it is not known whether PIC facilitation-inhibition control is also altered. Remote handgrip contraction, which should diffusely increase serotonergic input onto motor neurones, facilitated PICs similarly in both soleus and tibialis anterior of young and older adults. Antagonist tendon vibration, which induces reciprocal inhibition, reduced PICs in soleus in both young and older adults but had less effect in tibialis anterior in older adults. Data from lower-threshold motor units during low-force contractions suggest that PIC facilitation is preserved with ageing in soleus and tibialis anterior. However, the effect of reciprocal inhibition on the contribution of PICs to motor neurone discharge seems reduced in tibialis anterior but preserved in soleus.

Runners with mid-portion Achilles tendinopathy have greater triceps surae intracortical inhibition than healthy controls.

OBJECTIVES: This study aimed to investigate short-interval intracortical inhibition (SICI) and muscle function in the triceps surae of runners with mid-portion Achilles tendinopathy (AT). METHODS: Runners with (n = 11) and without (n = 13) AT were recruited. Plantar flexor isometric peak torque and rate of torque development (RTD) were measured using an isokinetic dynamometer. Triceps surae endurance was measured as single-leg heel raise (SLHR) to failure test. SICI was assessed using paired-pulse transcranial magnetic stimulation during a sustained contraction at 10% of plantar flexor isometric peak torque. RESULTS: Triceps surae SICI was 14.3% (95% CI: -2.1 to 26.4) higher in AT than in the control group (57.9%, 95% CI: 36.2 to 79.6; and 43.6% 95% CI: 16.2 to 71.1, p = 0.032) irrespective of the tested muscle. AT performed 16 (95% CI: 7.9 to 23.3, p < 0.001) fewer SLHR repetitions on the symptomatic side compared with controls, and 14 (95% CI: 5.8 to 22.0, p = 0.004), fewer SLHR repetitions on the non-symptomatic compared with controls. We found no between-groups differences in isometric peak torque (p = 0.971) or RTD (p = 0.815). PERSPECTIVE: Our data suggest greater intracortical inhibition for the triceps surae muscles for the AT group accompanied by reduced SLHR endurance, without deficits in isometric peak torque or RTD. The increased SICI observed in the AT group could be negatively influencing triceps surae endurance; thus, rehabilitation aiming to reduce intracortical inhibition should be considered to improve patient outcomes. Furthermore, SLHR is a useful clinical tool to assess plantar flexor function in AT patients.

Estimates of persistent inward currents increase with the level of voluntary drive in low-threshold motor units of plantar flexor muscles.

This study tested whether estimates of persistent inward currents (PICs) in the human plantar flexors would increase with the level of voluntary drive. High-density surface electromyograms were collected from soleus and gastrocnemius medialis of 21 participants (29.2 ± 2.6 yr) during ramp-shaped isometric contractions to 10%, 20%, and 30% (torque rise and decline of 2%/s and 30-s duration) of each participant's maximal torque. Motor units identified in all the contraction intensities were included in the paired-motor unit analysis to calculate delta frequency (ΔF) and estimate the PICs. ΔF is the difference in discharge rate of the control unit at the time of recruitment and derecruitment of the test unit. Increases in PICs were observed from 10% to 20% [Δ = 0.6 pulse per second (pps); P < 0.001] and from 20% to 30% (Δ = 0.5 pps; P < 0.001) in soleus and from 10% to 20% (Δ = 1.2 pps; P < 0.001) but not from 20% to 30% (Δ = 0.09 pps; P = 0.724) in gastrocnemius medialis. Maximal discharge rate increased for soleus and gastrocnemius medialis from 10% to 20% [Δ = 1.75 pps (P < 0.001) and Δ = 2.43 pps (P < 0.001), respectively] and from 20% to 30% [Δ = 0.80 pps (P < 0.017) and Δ = 0.92 pps (P = 0.002), respectively]. The repeated-measures correlation identified associations between ΔF and increases in maximal discharge rate for soleus (r = 0.64; P < 0.001) and gastrocnemius medialis (r = 0.77; P < 0.001). An increase in voluntary drive tends to increase PIC strength, which has key implications for the control of force but also for comparisons between muscles or studies when relative force levels might be different. Increases in voluntary descending drive amplify PICs in humans and provide an important spinal mechanism for motor unit discharging, and thus force output modulation.NEW & NOTEWORTHY Animal experiments and computational models have shown that motor neurons can amplify the synaptic input they receive via persistent inward currents. Here we show in humans that this amplification varies proportionally to the magnitude of the voluntary drive to the muscle.

Passive muscle stretching reduces estimates of persistent inward current strength in soleus motor units.

Prolonged (≥60 s) passive muscle stretching acutely reduces maximal force production at least partly through a suppression of efferent neural drive. The origin of this neural suppression has not been determined; however, some evidence suggests that reductions in the amplitude of persistent inward currents (PICs) in the motoneurons may be important. The aim of the present study was to determine whether acute passive (static) muscle stretching affects PIC strength in gastrocnemius medialis (GM) and soleus (SOL) motor units. We calculated the difference in instantaneous discharge rates at recruitment and de-recruitment (ΔF) for pairs of motor units in GM and SOL during triangular isometric plantar flexor contractions (20% maximum) both before and immediately after a 5 min control period and immediately after five 1 min passive plantar flexor stretches. After stretching, there was a significant reduction in SOL ΔF (-25.6%; 95% confidence interval, CI=-45.1% to -9.1%, P=0.002) but not GM ΔF These data suggest passive muscle stretching can reduce the intrinsic excitability, via PICs, of SOL motor units. These findings (1) suggest that PIC strength might be reduced after passive stretching, (2) are consistent with previously established post-stretch decreases in SOL but not GM EMG amplitude during contraction, and (3) indicate that reductions in PIC strength could underpin the stretch-induced force loss.

Neuromuscular determinants of explosive torque: Differences among strength-trained and untrained young and older men.

This study compared the differences in neural and muscular mechanisms related to explosive torque in chronically strength-trained young and older men (>5 years). Fifty-four participants were allocated into four groups according to age and strength training level: older untrained (n = 14; 65.6 ± 2.9 years), older trained (n = 12; 63.6 ± 3.8 years), young untrained (n = 14; 26.2 ± 3.7 years), and young trained (n = 14; 26.7 ± 3.4 years). Knee extension isometric voluntary explosive torque (absolute and normalized as a percentage of maximal voluntary torque) was assessed at the beginning of the contraction (ie, 50, 100, and 150 ms-T50, T100, and T150, respectively), and surface electromyogram (sEMG) amplitude (normalized as a percentage of sEMG recorded during maximal voluntary isometric contraction) at 0-50, 50-100, and 100-150 time windows. Supramaximal electrically evoked T50 was assessed with octet trains delivered to the femoral nerve (8 pulses at 300 Hz). Voluntary T50, T100, and T150 were higher for trained than untrained in absolute (P < 0.001) and normalized (P < 0.030) terms, accompanied by higher sEMG at 0-50, 50-100, and 100-150 ms (P < 0.001), and voluntary T50/octet T50 ratio for trained. Greater octet T50 was observed for the young trained (P < 0.001) but not for the older trained (P = 0.273) compared to their untrained counterparts. Age effect was observed for voluntary T50, T100, and T150 (P < 0.050), but normalization removed these differences (P > 0.417). Chronically strength-trained young and older men presented a greater explosive torque than their untrained pairs. In young trained, the greater explosive performance was attributed to enhanced muscular and neural mechanisms, while in older trained to neural mechanisms only.

Passive heat therapy: a promising preventive measure for people at risk of adverse health outcomes during heat extremes.

The world is experiencing increased frequency, duration, and severity of life-threatening heat extremes. Most hospitalizations and excess deaths during extreme heat events are associated with preexisting diseases in older adults. As climate change persists, the global population ages and the number of individuals with chronic diseases expands, more people are at risk of adverse health outcomes during extreme heat events. Therefore, proactive preventive measures are urgently needed to mitigate heat-related health risks within these populations. In this context, passive heat therapy (e.g., hot baths, saunas, and water-perfused suits) emerges as a promising countermeasure to improve physiological resilience to a warming planet. Passive heating improves cardiovascular function and overall health in older adults and individuals living with chronic diseases, offering the prospect of reducing cardiovascular strain during hotter days. Moreover, some studies suggest that passive heat therapy can be an effective strategy for heat acclimation (i.e., improved thermoregulation). This review describes the existing literature on the effects of passive heat therapy on cardiovascular and thermoregulatory responses in individuals with higher heat-related health risks and explores the use of passive heating as a strategy for heat acclimation to mitigate health risks during extreme heat events.NEW & NOTEWORTHY Passive heat therapy improves cardiovascular function and health in middle-aged and older adults living with or without chronic diseases. In addition, preliminary studies indicate that passive heat interventions can induce heat acclimation, improving thermoregulatory responses. Thus, passive heat therapy could serve as a preventive measure for people at risk of adverse health outcomes during extreme heat events, improving resilience to ongoing climate change.

Effects of Plyometric Training on Physical Performance: An Umbrella Review.

BACKGROUND: Plyometric training can be performed through many types of exercises involving the stretch-shortening cycle in lower limbs. In the last decades, a high number of studies have investigated the effects of plyometric training on several outcomes in different populations. OBJECTIVES: To systematically review, summarize the findings, and access the quality of published meta-analyses investigating the effects of plyometric training on physical performance. DESIGN: Systematic umbrella review of meta-analyses. DATA SOURCES: Meta-analyses were identified using a systematic literature search in the databases PubMed/MEDLINE, Scopus, SPORTDiscus, Web of Science, Cochrane Library and Scielo. ELIGIBILITY CRITERIA FOR SELECTING META-ANALYSES: Meta-analyses that examined the effects of plyometric training on physical fitness in different populations, age groups, and sex. RESULTS: Twenty-nine meta-analyses with moderate-to-high methodological quality were included in this umbrella review. We identified a relevant weakness in the current literature, in which five meta-analyses included control group comparisons, while 24 included pre-to-post-effect sizes. Trivial-to-large effects were found considering the effects of plyometric training on physical performance for healthy individuals, medium-trivial effects for the sports athletes' groups and medium effects for different sports athletes' groups, age groups, and physical performance. CONCLUSION: The available evidence indicates that plyometric training improves most related physical fitness parameters and sports performance. However, it is important to outline that most meta-analyses included papers lacking a control condition. As such, the results should be interpreted with caution. PROSPERO number: CRD42020217918.

Residual force enhancement in human skeletal muscles: A systematic review and meta-analysis.

OBJECTIVE: We reviewed and appraised the existing evidence of in vivo manifestations of residual force enhancement in human skeletal muscles and assessed, through a meta-analysis, the effect of an immediate history of eccentric contraction on the subsequent torque capacity of voluntary and electrically evoked muscle contractions. METHODS: Our search was conducted from database inception to May 2020. Descriptive information was extracted from, and quality was assessed for, 45 studies. Meta-analyses and metaregressions were used to analyze residual torque enhancement and its dependence on the angular amplitude of the preceding eccentric contraction. RESULTS: Procedures varied across studies with regards to muscle group tested, angular stretch amplitude, randomization of contractions, time window analyzed, and verbal command. Torque capacity in isometric (constant muscle tendon unit length and joint angle) contractions preceded by an eccentric contraction was typically greater compared to purely isometric contractions, and this effect was greater for electrically evoked muscle contractions than voluntary contractions. Residual torque enhancement differed across muscle groups for the voluntary contractions, with a significant enhancement in torque observed for the adductor pollicis, ankle dorsiflexors, ankle plantar flexors, and knee extensors, but not for the elbow and knee flexors. Meta-regressions revealed that the angular amplitude of the eccentric contraction (normalized to the respective joint's full range of motion) was not associated with the residual torque enhancement observed. CONCLUSION: There is evidence of residual torque enhancement for most, but not all, muscle groups, and residual torque enhancement is greater for electrically evoked than for voluntary contractions. Contrary to our hypothesis, and contrary to generally accepted findings on isolated muscle preparations, residual torque enhancement in voluntary and electrically evoked contractions does not seem to depend on the angular amplitude of the preceding eccentric contraction.

Pressure on the Electrode to Reduce Discomfort During Neuromuscular Electrical Stimulation in Individuals With Different Subcutaneous-Fat Thickness: Is the Procedure Effective and Reliable?

The addition of manual pressure on the electrode during neuromuscular electrical stimulation (NMES) has been used to reduce current intensity and perceived discomfort. In this study we aimed to test i) whether this approach affect the reliability of commonly made torque output measurements and ii) whether subcutaneous-fat thickness influence the efficacy of this approach in reducing current intensity and perceived discomfort. Twenty-one men (24 ± 3.1 years) performed knee extension maximal voluntary isometric contractions with and without manual pressure on the NMES femoral nerve electrode (superimposed and resting doublets, 2 pulses at 100 Hz) during two separate sessions. Torque output was measured in an isokinetic dynamometer and thigh subcutaneous-fat thickness assessed with ultrasonography. A scale of perceived discomfort was presented after contractions. Reductions in current intensity ( ) and discomfort during superimposed doublet ( p=0.002 ) and resting doublet ( p=0.002 ) were confirmed for the condition in which pressure was applied to the electrode. Fat thickness was correlated to changes in current intensity (r = 0.63; p = 0.002) and changes in discomfort (r = 0.45; p = 0.04) and no differences between pressure conditions and testing sessions were observed for torque output ( p > 0.05; ICC 0.95). Adding manual pressure during NMES on femoral nerve reduces discomfort and the maximal NMES intensity required to reach maximum torque without affecting torque output magnitude and reliability. Greater reduction in intensity and discomfort were observed in participants with higher subcutaneous-fat thickness levels after adding pressure on the electrode.