David S. Kennedy

Firing of antagonist small-diameter muscle afferents reduces voluntary activation and torque of elbow flexors.

•  Maintained firing of fatigue‐sensitive small‐diameter muscle afferents is reported to reduce voluntary activation of the homonymous (fatigued) muscle. •  Our study determined if firing of fatigue‐sensitive afferents from elbow extensor muscles reduces voluntary activation and torque of the non‐fatigued elbow flexors. •  We examined voluntary activation of the elbow flexors by measuring changes in superimposed twitches evoked by magnetic cortical stimulation during maximal voluntary contractions. •  Following a fatiguing contraction of elbow extensors, the voluntary drive to unfatigued flexor muscles was reduced with continued activation of small‐diameter muscle afferents produced by a blood pressure cuff inflated to maintain muscle ischaemia. •  Continued discharge of small‐diameter muscle afferents from one muscle can decrease voluntary drive to another muscle in the same limb and can reduce its maximal voluntary torque.

Short Trips and Long Days: Safety and Health in Short-Haul Trucking:

This paper looks at the role and influence of contingent employment among short-haul truckers, an occupational group that has been little studied to date. A 2003 survey of Australian short-haul drivers examined the predictors of health and safety outcomes for all drivers and provided comparative information on the working hours, occupational safety and health, and work-life conflict of permanent employees, temporary (casual) employees, and owner-drivers. The main predictor of both illness and injury for all drivers was work-life conflict. The results show that contingent work is characteristic of short-haul trucking in Australia, especially among owner-drivers and casual employees. Contingent-work drivers differ from other drivers on a range of organizational characteristics, but not on safety and health outcomes. Contingent employment can take different forms, each of which is associated with a somewhat different set of effects on workers.

Fatigue-related firing of distal muscle nociceptors reduces voluntary activation of proximal muscles of the same limb.

With fatiguing exercise, firing of group III/IV muscle afferents reduces voluntary activation and force of the exercised muscles. These afferents can also act across agonist/antagonist pairs, reducing voluntary activation and force in nonfatigued muscles. We hypothesized that maintained firing of group III/IV muscle afferents after a fatiguing adductor pollicis (AP) contraction would decrease voluntary activation and force of AP and ipsilateral elbow flexors. In two experiments (n = 10) we examined voluntary activation of AP and elbow flexors by measuring changes in superimposed twitches evoked by ulnar nerve stimulation and transcranial magnetic stimulation of the motor cortex, respectively. Inflation of a sphygmomanometer cuff after a 2-min AP maximal voluntary contraction (MVC) blocked circulation of the hand for 2 min and maintained firing of group III/IV muscle afferents. After a 2-min AP MVC, maximal AP voluntary activation was lower with than without ischemia (56.2 ± 17.7% vs. 76.3 ± 14.6%; mean ± SD; P < 0.05) as was force (40.3 ± 12.8% vs. 57.1 ± 13.8% peak MVC; P < 0.05). Likewise, after a 2-min AP MVC, elbow flexion voluntary activation was lower with than without ischemia (88.3 ± 7.5% vs. 93.6 ± 3.9%; P < 0.05) as was torque (80.2 ± 4.6% vs. 86.6 ± 1.0% peak MVC; P < 0.05). Pain during ischemia was reported as Moderate to Very Strong. Postfatigue firing of group III/IV muscle afferents from the hand decreased voluntary drive and force of AP. Moreover, this effect decreased voluntary drive and torque of proximal unfatigued muscles, the elbow flexors. Fatigue-sensitive group III/IV muscle nociceptors act to limit voluntary drive not only to fatigued muscles but also to unfatigued muscles within the same limb.

Effects of fatigue on corticospinal excitability of the human knee extensors

What is the central question of this study? Do group III and IV muscle afferents act at the spinal or cortical level to affect the ability of the central nervous system to drive quadriceps muscles during fatiguing exercise? What is the main finding and its importance? The excitability of the motoneurone pool of vastus lateralis was unchanged by feedback from group III and IV muscle afferents. In contrast, feedback from these afferents may contribute to inhibition at the cortex. However, the excitability of the corticospinal pathway was not directly affected by feedback from these afferents. These findings are important for understanding neural processes during fatiguing exercise.

Motoneuron excitability of the quadriceps decreases during a fatiguing submaximal isometric contraction

During fatiguing voluntary contractions, the excitability of motoneurons innervating arm muscles decreases. However, the behavior of motoneurons innervating quadriceps muscles is unclear. Findings may be inconsistent because descending cortical input influences motoneuron excitability and confounds measures during exercise. To overcome this limitation, we examined effects of fatigue on quadriceps motoneuron excitability tested during brief pauses in descending cortical drive after transcranial magnetic stimulation (TMS). Participants ( n = 14) performed brief (~5-s) isometric knee extension contractions before and after a 10-min sustained contraction at ~25% maximal electromyogram (EMG) of vastus medialis (VM) on one ( n = 5) or two ( n = 9) days. Electrical stimulation over thoracic spine elicited thoracic motor evoked potentials (TMEP) in quadriceps muscles during ongoing voluntary drive and 100 ms into the silent period following TMS (TMS-TMEP). Femoral nerve stimulation elicited maximal M-waves (Mmax). On the 2 days, either large (~50% Mmax) or small (~15% Mmax) TMS-TMEPs were elicited. During the 10-min contraction, VM EMG was maintained ( P = 0.39), whereas force decreased by 52% (SD 13%) ( P < 0.001). TMEP area remained unchanged ( P = 0.9), whereas large TMS-TMEPs decreased by 49% (SD 28%) ( P = 0.001) and small TMS-TMEPs by 71% (SD 22%) ( P < 0.001). This decline was greater for small TMS-TMEPs ( P = 0.019; n = 9). Therefore, without the influence of descending drive, quadriceps TMS-TMEPs decreased during fatigue. The greater reduction for smaller responses, which tested motoneurons that were most active during the contraction, suggests a mechanism related to repetitive activity contributes to reduced quadriceps motoneuron excitability during fatigue. By contrast, the unchanged TMEP suggests that ongoing drive compensates for altered motoneuron excitability. NEW & NOTEWORTHY We provide evidence that the excitability of quadriceps motoneurons decreases with fatigue. Our results suggest that altered intrinsic properties brought about by repetitive activation of the motoneurons underlie their decreased excitability. Furthermore, we note that testing during voluntary contraction may not reflect the underlying depression of motoneuron excitability because of compensatory changes in ongoing voluntary drive. Thus, this study provides evidence that processes intrinsic to the motoneuron contribute to muscle fatigue of the knee extensors.

The effect of metabolic acidosis on maximal force production and muscle recruitment during repeated, submaximal calf contractions to task failure (705.11)

GABA and its synthesising enzyme, glutamate decarboxylase, have been detected in the rat kidney [1–2]. GABA has also been found in human plasma and urine [3–4] and most recently, a renoprotective role for GABA has been suggested [5]. We are systematically investigating functional roles for GABA and glutamate in the mammalian kidney. Contractile pericytes regulate vasa recta diameter in response to a number of endogenous vasoactive agents and in doing so regulate medullary blood flow (MBF) [6]. We have utilised the live kidney slice model [6] to demonstrate GABA-mediated constriction of vasa recta that was significantly greater at pericyte sites than at non-pericyte sites (p< 0.01). Conversely, the GABA substrate glutamate (100 ?M) caused a significantly greater vasodilation of vasa recta at pericyte sites compared to non-pericyte sites (p< 0.05). Data presented here identifies a novel role for GABA and glutamate in pericyte-mediated regulation of vasa recta diameter and thus MBF.Obesity frequently associates with chronic inflammatory diseases, including type 2 diabetes. In this study, a combination of a protein hydrolysate, LCPUFAs and a probiotic strain was investigated on the development of high fat diet -induced diabetic risk factors and complications in LDLr-/-.Leiden mice. Male LDLr-/-.Leiden mice at 12 wks of age received a high fat diet (HFD) for 21 wks with or without a combination of an extensive casein hydrolysate, docosahexaenoic acid (DHA), arachidonic acid (ARA), and Lactobacillus Rhamnosus GG (LGG). Both HFD and intervention diet were isocaloric and casein from HFD was replaced with casein hydrolysate in the test diets. The addition of DHA/ARA in the test diets was controlled for in the HFD. Moreover, a PBS gavage control group was included to control for potential effects of LGG gavage. There were significant beneficial effects of the hydrolysate/ARA/DHA/LGG composition versus the HFD control group including reduced body weight gain, lower plasma levels of insulin, cholesterol and triglycerides, lower systemic inflammation, improved adipose tissue quality and mass, and improved kidney and liver function. In a follow up study, evaluating the individual components of the test formulation, some of the outcomes were attributable to the hydrolysate or LGG. A combination of an extensive casein hydrolysate, ARA, DHA and LGG reduces the detrimental effects of HFD on the development of obesity and its metabolic complications. Main risk factors for the metabolic syndrome such as adipose tissue and chronic inflammation were markedly reduced which could provide a rationale for the beneficial effects observed.OBJECTIVETo evaluate the impact of a mobile phone SMS text message intervention on the exclusiveness of breastfeeding (EBF) in infants 0–6 months. METHODSA two-arm parallel randomized controlled tr...