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Objectives To investigate the effect of progressive whole-body hyperthermia on maximal, and rapid voluntary torque production, and their neuromuscular determinants. Design Repeated measures, randomised. Methods Nine participants performed sets of neuromuscular assessments in HOT conditions (∼50 °C, ∼35% relative humidity) at rectal temperatures (Tre) of 37, 38.5 and 39.5 °C and in CON conditions (∼22 °C, ∼35% relative humidity) at a Tre of ∼37 °C and pre-determined comparative time-points. Electrically evoked twitch (single impulse) and octet (8 impulses at 300 Hz) responses were measured at rest. Maximum voluntary torque (MVT), surface electromyography (EMG) normalised to maximal M-wave, and voluntary activation (VA) were measured during 3−5 s isometric maximal voluntary contractions. Rate of torque development (RTD) and normalised EMG were measured during rapid voluntary isometric contractions from rest. Results All neuromuscular variables were unaffected by time in CON. In HOT, MVT, normalised EMG at MVT and VA were lower at 39.5 °C compared to 37 °C (p 0.05), despite lower normalised EMG at Tre 39.5 °C (p Conclusions Hyperthermia reduced late-phase voluntary RTD, likely due to reduced neural drive and the reduction in MVT. In contrast, early- and middle-phase voluntary RTD were unaffected by hyperthermia, likely due to the conflicting effects of reduced neural drive but faster intrinsic contractile properties.

Decreased mechanical work done by the trailing limb when descending a single-step could affect load development and increase injury risk on the leading limb. This study assessed the effect of trailing limb mechanics on the development of lead limb load during a step descent by examining individuals with unilateral transtibial amputations who are known to exhibit reduced work in the prosthetic limb.Eight amputees and 10 able-bodied controls walked 5 m along the length of a raised platform, descended a single-step of 14 cm height, and continued walking. The intact limb of amputees led during descent. Kinematic and kinetic data were recorded using integrated motion capture and force platform system. Lead limb loading was assessed through vertical ground reaction force, and knee moments and joint reaction forces. Sagittal-plane joint work was calculated for the ankle, knee, and hip in both limbs.No differences were found in lead limb loading despite differences in trail limb mechanics evidenced by amputees performing 58% less total work by the trailing (prosthetic) limb to lower the centre of mass (P = 0.004) and 111% less for propulsion (P 0.001). Amputees descended the step significantly slower (P = 0.003) and performed significantly greater lead limb ankle work (P = 0.017). After accounting for speed differences, initial loading at the knee was significantly higher in the lead limb of amputees versus controls.Increasing lead limb work and reducing forward velocity may be effective compensatory strategies to limit lead limb loading during a step descent, in response to reduced trailing limb work.

BACKGROUND: We have recently shown that a novel signalling kinase, inositol hexakisphosphate kinase 1 (IP6K1), is implicated in whole-body insulin resistance via its inhibitory action on Akt. Insulin and insulin like growth factor 1 (IGF-1) share many intracellular processes with both known to play a key role in glucose and protein metabolism in skeletal muscle. AIMS: We aimed to compare IGF/IP6K1/Akt signalling and the plasma proteomic signature in individuals with a range of BMIs after ingestion of lean meat. METHODS: Ten lean [Body mass index (BMI) (in kg/m(2)): 22.7 ± 0.4; Homeostatic model assessment of insulin resistance (HOMA(IR)): 1.36 ± 0.17], 10 overweight (BMI: 27.1 ± 0.5; HOMA(IR): 1.25 ± 0.11), and 10 obese (BMI: 35.9 ± 1.3; HOMA(IR): 5.82 ± 0.81) adults received primed continuous L-[ring-(13)C(6)]phenylalanine infusions. Blood and muscle biopsy samples were collected at 0 min (post-absorptive), 120 min and 300 min relative to the ingestion of 170 g pork loin (36 g protein and 5 g fat) to examine skeletal muscle protein signalling, plasma proteomic signatures, and whole-body phenylalanine disappearance rates (R(d)). RESULTS: Phenylalanine R(d) was not different in obese compared to lean individuals at all time points and was not responsive to a pork ingestion (basal, P = 0.056; 120 & 300min, P > 0.05). IP6K1 was elevated in obese individuals at 120 min post-prandial vs basal (P < 0.05). There were no acute differences plasma proteomic profiles between groups in the post-prandial state (P > 0.05). CONCLUSIONS: These data demonstrate, for the first time that muscle IP6K1 protein content is elevated after lean meat ingestion in obese adults, suggesting that IP6K1 may be contributing to the dysregulation of nutrient uptake in skeletal muscle. In addition, proteomic analysis showed no differences in proteomic signatures between obese, overweight or lean individuals.

Sarcopenia is defined as the combined loss of skeletal muscle strength, function, and/or mass with aging. This degenerative loss of muscle mass is associated with poor quality of life and early mortality humans. The loss of muscle mass occurs due to acute changes in daily muscle net protein balance (NPB). It is generally believed a poor NPB occurs due to reduced muscle protein synthetic responses to exercise, dietary amino acid availability, or an insensitivity of insulin to suppress breakdown. Hence, aging muscles appear to be resistant to the anabolic action of exercise and protein (amino acids or hormonal) when compared to their younger counterparts. The mechanisms that underpin anabolic resistance to anabolic stimuli (protein and resistance exercise) are multifactorial and may be partly driven by poor lifestyle choices (increased sedentary time and reduced dietary protein intake) as well as an inherent dysregulated mechanism in old muscles irrespective of the environmental stimuli. The insulin like growth factor 1 (IGF-1), Akt /Protein Kinase B and mechanistic target of rapamycin (mTOR) pathway is the primary driver between mechanical contraction and protein synthesis and may be a site of dysregulation between old and younger people. Therefore, our review aims to describe and summarize the differences seen in older muscle in this pathway in response to resistance exercise (RE) and describe approaches that researchers have sought out to maximize the response in muscle. Furthermore, this review will present the hypothesis that inositol hexakisphosphate kinase 1 (IP6K1) may be implicated in IGF-1 signaling and thus sarcopenia, based on recent evidence that IGF-1 and insulin share some intracellular bound signaling events and that IP6K1 has been implicated in skeletal muscle insulin resistance.

New findings What is the central question of this study? The effects of long-term muscle disuse on neuromuscular function are unclear because disuse studies are typically short term. In this study, we used a new model (unilateral transtibial amputees) to investigate the effects of long-term disuse on quadriceps neuromuscular function. What is the main finding and its importance? Kinetic analysis (knee-extension moments during gait) indicated habitual disuse of the amputated limb quadriceps, accompanied by lower quadriceps muscle strength (60-76%) and neural activation (32-44%), slower contractile properties and altered muscle architecture in the amputated limb, which could not be predicted from short-term disuse studies. Abstract The purpose of this study was to determine: (i) whether individuals with unilateral transtibial amputations (ITTAs), who habitually disuse the quadriceps muscles of their amputated limb, provide an effective model for assessing the effects of long-term muscle disuse; and (ii) the effects of such disuse on quadriceps muscle strength and neuromuscular function in this population. Nine ITTAs and nine control subjects performed isometric voluntary knee extensions of both limbs to assess maximal voluntary torque (MVT) and the rate of torque development (RTD). The interpolated twitch technique and EMG normalized to maximal M-wave were used to assess neural activation, involuntary (twitch and octet) contractions to assess intrinsic contractile properties, and ultrasound images of the vastus lateralis to assess muscle architecture. Clinical gait analysis was used to measure knee kinetic data during walking at an habitual speed. The ITTAs displayed 54-60% lower peak knee-extensor moments during walking in the amputated compared with intact/control limbs, but the intact and control limbs were comparable for loading during walking and muscle strength variables, suggesting that the intact limb provides a suitable internal control for comparison with the disused amputated limb. The MVT and RTD were ∼60 and ∼75% lower, respectively, in the amputated than intact/control limbs. The differences in MVT appeared to be associated with ∼40 and ∼43% lower muscle thickness and neural activation, respectively, and the differences in RTD appeared to be associated with the decline in MVT coupled with slowing of the intrinsic contractile properties. These results indicate considerable changes in strength and neuromuscular function with long-term disuse that could not be predicted from short-term disuse studies.

Background When stepping down from a raised surface, either a toe or heel contact strategy is performed. Increased vertical momentum is likely to be experienced during a step descent, yet the extent to which these descent strategies influence the development of load at the ground and knee has not been examined. Research Question Does descent strategy influence ground and knee joint loading? Does the contribution from leading and trailing limb joint mechanics differ between descent strategies? Methods Twenty-two healthy male participants (age: 34.0 ± 6.5 years, height: 179 ± 6.3 cm, mass: 83.5 ± 13 kg) walked along a raised platform, stepped down from a 14 cm height utilising either a toe (n = 10) or heel (n = 12) initial contact, and continued walking. Vertical ground reaction forces and knee external adduction and flexor moments were extracted for the duration of the braking phase. Joint work was calculated for the ankle, knee, and hip in both the leading and trailing limbs. Results Waveform analysis of the loading features indicated that a toe-contact strategy resulted in significantly reduced loading rates during early braking (1–32% of the braking phase) and significantly increased magnitude in late braking (55–96% of the braking phase). Individuals performing toe landings completed 33% greater overall work (p = 0.091) in the lead limb and utilised the lead limb ankle joint as the main shock absorber (79% of total lead limb work). Concurrently, the trailing limb performed 29% and 21% less work when lowering the centre of mass and propulsion, respectively, compared to a heel landing. Significance A toe-contact strategy results in reduced limb and knee joint loading rates through greater utilisation of the lead limb ankle joint. A heel-contact strategy, however, can reduce loading during late braking by utilising the functionality of the trailing limb.

Individuals with unilateral transtibial amputations experience greater work demand and loading on the intact limb compared with the prosthetic limb, placing this limb at a greater risk of knee joint degenerative conditions. It is possible that increased loading on the intact side may occur due to strength deficits and joint absorption mechanics. This study investigated the intact limb mechanics utilized to attenuate load, independent of prosthetic limb contributions and requirements for forward progression, which could provide an indication of deficiencies in the intact limb. Amputee and healthy control participants completed 3 unilateral drop landings from a 30-cm drop height. Joint angles at touchdown; range of motion; coupling angles; peak powers; and negative work of the ankle, knee, and hip were extracted together with isometric quadriceps strength measures. No significant differences were found in the load or movement mechanics (P ≥ .31, g ≤ 0.42), despite deficits in isometric maximum (20%) and explosive (25%) strength (P ≤ .13, g ≥ 0.61) in the intact limb. These results demonstrate that, when the influence from the prosthetic limb and task demand are absent, and despite deficits in strength, the intact limb adopts joint mechanics similar to able-bodied controls to attenuate limb loading.

Background\ud It has been found that alterations in passive muscle properties may be associated with low back pain, and these may be responsible for the altered gait parameters often observed in subjects with back pain. The purpose of the present study was to assess total hip and passive hip extensor moments in people with or without low back pain during the hip flexion component of walking.\ud Methods\ud 52 subjects volunteered for this study (low back pain group, n = 25 (male n = 13, female n = 12), control group, n = 27 (male n = 15, female n = 12)). Passive hip moments were calculated using an adapted force transducer during supine testing. A biomechanical model and predictive equation were used to calculate passive hip moments during walking. Total hip moments were calculated with the use of a 9 camera, 3-D motion-capture system.\ud Findings\ud Independent samples t-tests demonstrated no significant differences between groups for gait parameters or hip or knee angles. Results of the ANOVAs demonstrated significant differences in passive hip flexor moments during the second half of hip flexion ( P

The effect of different strength training regimes, and in particular training utilizing brief explosive contractions, on tendinous tissue properties is poorly understood. This study compared the efficacy of 12 weeks of knee extensor explosive-contraction (ECT; n = 14) vs. sustained-contraction (SCT; n = 15) strength training vs. a non-training control (n = 13) to induce changes in patellar tendon and knee extensor tendon-aponeurosis stiffness and size (patellar tendon, vastus-lateralis aponeurosis, quadriceps femoris muscle) in healthy young men. Training involved 40 isometric knee extension contractions (3 times/week): gradually increasing to 75% of maximum voluntary torque (MVT) before holding for 3 s (SCT), or briefly contracting as fast as possible to ~80% maximum voluntary torque (ECT). Changes in patellar tendon stiffness and Young’s modulus, tendon-aponeurosis complex stiffness, as well as quadriceps femoris muscle volume, vastus-lateralis aponeurosis area and patellar tendon cross-sectional area were quantified with ultrasonography, dynamometry, and magnetic resonance imaging. ECT and SCT similarly increased patellar tendon stiffness (20% vs. 16%, both p < 0.05 vs. control) and Young’s modulus (22% vs. 16%, both p < 0.05 vs. control). Tendon-aponeurosis complex high-force stiffness increased only after SCT (21%; p

Evidence suggests dietary nitrate supplementation enhances low-frequency (≤20 Hz) involuntary, but not voluntary, forces in unfatigued human muscle. We investigated the hypotheses that nitrate supplementation would also attenuate low-frequency fatigue and the loss of explosive voluntary forces in fatigued conditions.In a counterbalanced double-blinded order, 17 male participants completed two experimental trials after 7 d of dietary supplementation with either nitrate-rich (NIT) or nitrate-depleted (PLA) beetroot juice. Each trial consisted of measuring isometric knee extension forces during a series of explosive maximal voluntary contractions (MVC) and involuntary tetanic contractions (at 10, 20 50, and 100 Hz) in unfatigued conditions, followed by a fatigue protocol of 60 MVC and a repeat of the tetanic contractions immediately after the 60 MVC.In unfatigued conditions, there was no effect of NIT on any of the measured dependent variables, including maximal voluntary force, explosive impulse, and tetanic peak forces or peak rate of force developments at any frequency. In contrast, the percentage decline in explosive voluntary impulse from the first to the last 6 MVC in the fatigue protocol was lower in NIT (51.1% ± 13.9%) than in PLA (57.3% ± 12.4%; P = 0.039; d = 0.51). Furthermore, low-frequency fatigue determined via the percentage decline in the 20/50-Hz ratio was attenuated in NIT for tetanic peak force (NIT: 12.3% ± 12.0% vs PLA: 17.0% ± 10.1%; P = 0.110; d = 0.46) and tetanic peak rate force development (NIT: 12.3% ± 10.4% vs PLA: 20.3% ± 9.5%; P = 0.011; d = 0.83).Nitrate supplementation reduced the decline in explosive voluntary forces during a fatiguing protocol and attenuated low-frequency fatigue, likely due to reduced disruption of excitation-contraction coupling. However, contrary to previous findings, nitrate supplementation had no effect on contractile performance in unfatigued conditions.

This study investigated the neural and peripheral adaptations to short-term training for explosive force production. Ten men trained the knee extensors with unilateral explosive isometric contractions (1 s 'fast and hard') for 4 weeks. Before and after training, force was recorded at 50-ms intervals from force onset (F(50), F(100) and F(150)) during both voluntary and involuntary (supramaximal evoked octet; eight pulses at 300 Hz) explosive isometric contractions. Neural drive during the explosive voluntary contractions was measured with the ratio of voluntary/octet force, and average EMG normalized to the peak-to-peak M-wave of the three superficial quadriceps. Maximal voluntary force (MVF) was also measured, and ultrasonic images of the vastus lateralis were recorded during ramped contractions to assess muscle-tendon unit stiffness between 50 and 90% MVF. There was an increase in voluntary F(50) (+54%), F(100) (+15%) and F(150) (+14%) and in octet F(50) (+7%) and F(100) (+10%). Voluntary F(100) and F(150), and octet F(50) and F(100) increased proportionally with MVF (+11%). However, the increase in voluntary F(50) was +37% even after normalization to MVF, and coincided with a 42% increase in both voluntary/octet force and agonist-normalized EMG over the first 50 ms. Muscle-tendon unit stiffness between 50 and 90% MVF also increased. In conclusion, enhanced agonist neural drive and MVF accounted for improved explosive voluntary force production in the early and late phases of the contraction, respectively. The increases in explosive octet force and muscle-tendon unit stiffness provide novel evidence of peripheral adaptations within merely 4 weeks of training for explosive force production.

In this study we investigated the contribution of neural adaptations to strength changes after 4 weeks of unilateral isometric resistance training. Maximal and submaximal isometric knee extension contractions were assessed before and after training. Surface electromyography (EMG) data were collected from the agonist and antagonist muscles and normalized to evoked maximal M-wave and maximal knee flexor EMG, respectively. The interpolated twitch technique (ITT) was also used to determine activation at maximum voluntary force (MVF). MVF increased in the trained (+20%) and untrained (+8%) legs. Agonist EMG at MVF increased in the trained leg (+26%), although activation determined via the ITT was unchanged. In both legs the position of the force-agonist EMG relationship was unchanged, but antagonist coactivation was lower for all levels of agonist activation. Strength gains in the trained leg were due to enhanced agonist activation, whereas decreased coactivation may have affected strength changes in both legs.

Purpose: Electromechanical delay (EMD) and rate of force development (RFD) are determinants of explosive neuromuscular performance. We may expect a contrast in EMD and RFD between explosive power athletes, who have a demonstrable ability for explosive contractions, and untrained individuals. However, this comparison, and the neuromuscular mechanisms for any differences, has not been studied. Methods: The neuromuscular performance of explosive power athletes (n = 9) and untrained controls (n = 10) was assessed during a series of twitch, tetanic, explosive and maximum voluntary, isometric knee extensions. Knee extension force and EMG of the superficial quadriceps was measured in three 50 ms time windows from their onset, and normalised to strength and maximal M-wave (Mmax), respectively. Involuntary and voluntary EMD were determined from twitch and explosive voluntary contractions, respectively, and were similar for both groups. Results: The athletes were 28% stronger and their absolute RFD in the first 50 ms was 2-fold that of controls. Athletes had greater normalised RFD (4.86 ± 1.46 vs. 2.81 ± 1.20 MVC.s -1 ) and neural activation (mean quadriceps, 0.26 ± 0.07 vs. 0.15 ± 0.06 Mmax) during the first 50 ms of explosive voluntary contractions. Surprisingly the controls had a greater normalised RFD in the second 50 ms (6.68 ± 0.92 vs. 7.93 ± 1.11 MVC.s -1 ) and a greater change in EMG preceding this period. However, there were no differences in the twitch response or normalised tetanic RFD between groups. Conclusion: The differences in voluntary normalised RFD between athletes and controls were explained by agonist muscle neural activation, and not the similar intrinsic contractile properties of the groups.

Post-activation potentiation (PAP) is induced by a voluntary conditioning contraction (CC), performed typically at a maximal or near-maximal intensity, and has consistently been shown to increase both peak force and rate of force development during subsequent twitch contractions. The proposed mechanisms underlying PAP are associated with phosphorylation of myosin regulatory light chains, increased recruitment of higher order motor units, and a possible change in pennation angle. If PAP could be induced by a CC in humans, and utilized during a subsequent explosive activity (e.g. jump or sprint), it could potentially enhance mechanical power and thus performance and/or the training stimulus of that activity. However, the CC might also induce fatigue, and it is the balance between PAP and fatigue that will determine the net effect on performance of a subsequent explosive activity. The PAP-fatigue relationship is affected by several variables including CC volume and intensity, recovery period following the CC, type of CC, type of subsequent activity, and subject characteristics. These variables have not been standardized across past research, and as a result, evidence of the effects of CC on performance of subsequent explosive activities is equivocal. In order to better inform and direct future research on this topic, this article will highlight and discuss the key variables that may be responsible for the contrasting results observed in the current literature. Future research should aim to better understand the effect of different conditions on the interaction between PAP and fatigue, with an aim of establishing the specific application (if any) of PAP to sport.

Hyperthermia reduces volitional force production, voluntary muscle activation and agonist-electromyography (EMG) during a sustained maximal voluntary contraction (MVC) [1], [2]. This reduction in neuromuscular function may explain a reduced exercise capacity in the heat. Cooling of the neck has been shown to improve running capacity in the heat [3]; however the mechanism is unknown. The aim of the study was to investigate whether localised cooling of the head and neck during hyperthermia would affect neuromuscular function following 60 min of cycling in the heat.