Can muscle shortening alone, explain the energy cost of muscle contraction in vivo?

PURPOSE: Decreased whole-body energy cost of running has been associated with an increased Achilles tendon stiffness. It is usually assumed that this lower energy cost can be attributed to less muscle fascicle shortening with a stiffer tendon. Increased fiber shortening is an important determinant of muscle energetics in vitro. However, other factors, like increased muscle activation may be important when considering whole muscle energetics in vivo. METHODS: To determine the effects of a small additional muscle shortening on skeletal muscle energy requirement, 19 subjects performed 30 plantarflexions on two separate occasions: isometric (ISO) and isokinetic (KIN, 6.98 rad s(-1)), each with a target of 50 % of maximum isometric torque. Medial gastrocnemius muscle fascicle length (FL) was measured by ultrasound and rate of oxyhemoglobin (HbO2) desaturation was measured during blood flow occlusion using near-infrared spectroscopy. RESULTS: KIN resulted in significantly greater muscle shortening (23.8 ± 1.3 mm) than ISO (18.3 ± 1.0 mm, p < 0.001, mean ± SEM), and greater shortening velocity (KIN = 2.5 ± 0.3 FL s(-1), ISO = 1.1 ± 0.1 FL s(-1), p < 0.001). Rate of HbO2 desaturation was 19 ± 7 %, greater in KIN than ISO (p < 0.01), despite 19 ± 2 % lower mean torque (p < 0.001) and 9.8 ± 1.6 Nm s lower mean impulse per contraction (p < 0.001) in KIN compared to ISO. Root mean square for EMG was significantly greater (p < 0.05) during KIN (73 ± 3 %) than during ISO (63 ± 2 %). CONCLUSION: These results illustrate that muscle energy requirement is greater when muscle fascicle shortening and/or velocity of shortening is increased, and suggest that greater activation contributes to that increased energy requirement.

The associated impact of standardized admission screening on vancomycin-resistant Enterococci bloodstream infections.

OBJECTIVE: To determine whether discontinuing active screening for vancomycin-resistant Enterococcus (VRE) in Alberta, Canada, acute-care facilities had an associated impact on the rate of rise of hospital-acquired (HA) VRE bloodstream infection (VRE-BSI). SETTING: Acute-care facilities in Alberta, Canada. PATIENTS: All patients who were admitted to Alberta Health Services or Covenant Health acute-care facilities between January 1, 2013, and March 31, 2020, and who met the definition for hospital-acquired VRE-BSI were included in the analyses. METHODS: An intervention time-series Poisson regression was used to determine the slope change in VRE incidence between the pre- and postintervention (screening) periods. The patient population was separated into 3 cohorts: group 1 (low risk, VRE screening stopped), group 2 (high risk, VRE screening stopped), and group 3 (high risk, VRE screening continued). For all groups, a level- and slope-change model was used. RESULTS: We did not find a statistically significant difference in the slope change or rate of rise in VRE-BSI before and after the intervention, with incidence rate ratio (IRRs) of 1.015 (95% confidence interval [CI], 0.982-1.049), 1.025 (95% CI, 0.967-1.086), and 0.989 (95% CI, 0.924-1.059) for groups 1, 2 and 3, respectively. CONCLUSIONS: In Alberta, the rate of HA VRE-BSI has remained consistent, and our findings indicate that there has been no increase in the rate of rise of HA VRE-BSI in sites or units that discontinued screening for VRE, regardless of patient risk group.

Energy cost of running and Achilles tendon stiffness in man and woman trained runners.

The energy cost of running (E run), a key determinant of distance running performance, is influenced by several factors. Although it is important to express E run as energy cost, no study has used this approach to compare similarly trained men and women. Furthermore, the relationship between Achilles tendon (AT) stiffness and E run has not been compared between men and women. Therefore, our purpose was to determine if sex-specific differences in E run and/or AT stiffness existed. E run (kcal kg(-1) km(-1)) was determined by indirect calorimetry at 75%, 85%, and 95% of the speed at lactate threshold (sLT) on 11 man (mean ± SEM, 35 ± 1 years, 177 ± 1 cm, 78 ± 1 kg, [Formula: see text]1 = 56 ± 1 mL kg(-1) min(-1)) and 18 woman (33 ± 1 years, 165 ± 1 cm, 58 ± 1 kg, [Formula: see text]2 = 50 ± 0.3 mL kg(-1) min(-1)) runners. AT stiffness was measured using ultrasound with dynamometry. Man E run was 1.01 ± 0.06, 1.04 ± 0.07, and 1.07 ± 0.07 kcal kg(-1) km(-1). Woman E run was 1.05 ± 0.10, 1.07 ± 0.09, and 1.09 ± 0.10 kcal kg(-1) km(-1). There was no significant sex effect for E run or RER, but both increased with speed (P < 0.01) expressed relative to sLT. High-range AT stiffness was 191 ± 5.1 N mm(-1) for men and 125 ± 5.5 N mm(-1), for women (P < 0.001). The relationship between low-range AT stiffness and E run was significant at all measured speeds for women (r (2) = 0.198, P < 0.05), but not for the men. These results indicate that when E run is measured at the same relative intensity, there are no sex-specific differences in E run or substrate use. Furthermore, differences in E run cannot be explained solely by differences in AT stiffness.