Skeletal muscle mitochondrial correlates of critical power and W' in healthy active individuals.

The asymptote (critical power; CP) and curvature constant (W') of the hyperbolic power-duration relationship can predict performance within the severe-intensity exercise domain. However, the extent to which these parameters relate to skeletal muscle mitochondrial content and respiratory function is not known. Fifteen males (peak O2 uptake, 52.2 ± 8.7 mL kg-1 min-1; peak work rate, 366 ± 40 W; and gas exchange threshold, 162 ± 41 W) performed three to five constant-load tests to task failure for the determination of CP (246 ± 44 W) and W' (18.6 ± 4.1 kJ). Skeletal muscle biopsies were obtained from the vastus lateralis to determine citrate synthase (CS) activity, as a marker of mitochondrial content, and the ADP-stimulated respiration (P) and maximal electron transfer (E) through mitochondrial complexes (C) I-IV. The CP was positively correlated with CS activity (absolute CP, r = 0.881, P < 0.001; relative CP, r = 0.751, P = 0.001). The W' was not correlated with CS activity (P > 0.05). Relative CP was positively correlated with mass-corrected CI + IIE (r = 0.659, P = 0.038), with absolute CP being inversely correlated with CS activity-corrected CIVE (r = -0.701, P = 0.024). Relative W' was positively correlated with CS activity-corrected CI + IIP (r = 0.713, P = 0.021) and the phosphorylation control ratio (r = 0.661, P = 0.038). There were no further correlations between CP or W' and mitochondrial respiratory variables. These findings support the assertion that skeletal muscle mitochondrial oxidative capacity is positively associated with CP and that this relationship is strongly determined by mitochondrial content.

Intra-individual comparison of prostate-specific membrane antigen positron emission tomography/computed tomography versus bone scan in detecting skeletal metastasis at prostate cancer diagnosis.

OBJECTIVES: To compare the diagnostic performance and radiological staging impact of 68 Ga-prostate-specific membrane antigen positron emission tomography/computed tomography (PSMA PET/CT) compared to 99 Tc whole-body bone scan (WBBS) for the detection of skeletal metastasis in the primary staging of prostate cancer (PCa). PATIENTS AND METHODS: A prospective institutional database was retrospectively examined for patients who underwent both PSMA PET and WBBS within a 1 week interval for PCa primary staging. Lesions were categorised as 'negative', 'equivocal', or 'definite' based on nuclear medicine physician interpretation. Metastatic burden was characterised for each imaging modality according to three groups: (i) local disease (no skeletal metastases), (ii) oligometastatic disease (three or fewer skeletal metastases), or (iii) polymetastatic disease (more than three skeletal metastases). RESULTS: There were 667 patients included. The median (interquartile range) prostate-specific antigen level was 9.2 (6.2-16) ng/mL and 60% of patients were high risk according to a modified D'Amico risk classification. The overall distribution of skeletal metastasis detection changed across the two scans overall (P = 0.003), being maintained within high-risk (P = 0.030) and low-risk (P = 0.018) groups. PSMA PET/CT identified more definite skeletal metastases compared to WBBS overall (10.3% vs 7.3%), and according to risk grouping (high: 12% vs 9%, intermediate: 4% vs 1%). Upstaging was more common with PSMA PET/CT than WBBS (P = 0.001). The maximum standardised uptake value (SUVmax ) of the primary tumour was associated with upstaging of skeletal metastases on PSMA PET/CT (P = 0.025), while age was associated with upstaging on WBBS (P = 0.021). The SUVmax of the primary tumour and metastases were both higher according to extent of metastatic disease (P = 0.001 and P < 0.001, respectively). CONCLUSIONS: More skeletal metastases were detected with PSMA PET/CT than WBBS, resulting in a higher upstaging rate mostly in high-risk patients. The SUVmax of the primary tumour and metastases was associated with upstaging.

Modelling human endurance: power laws vs critical power.

The power-duration relationship describes the time to exhaustion for exercise at different intensities. It is believed to be a "fundamental bioenergetic property of living systems" that this relationship is hyperbolic. Indeed, the hyperbolic (a.k.a. critical-power) model which formalises this belief is the dominant tool for describing and predicting high-intensity exercise performance, e.g. in cycling, running, rowing or swimming. However, the hyperbolic model is now the focus of a heated debate in the literature because it unrealistically represents efforts that are short (< 2 min) or long (> 15 min). We contribute to this debate by demonstrating that the power-duration relationship is more adequately represented by an alternative, power-law model. In particular, we show that the often-observed good fit of the hyperbolic model between 2 and 15 min should not be taken as proof that the power-duration relationship is hyperbolic. Rather, in this range, a hyperbolic function just happens to approximate a power law fairly well. We also prove mathematical results which suggest that the power-law model is a safer tool for pace selection than the hyperbolic model and that the former more naturally models fatigue than the latter.

Co-occurrence of Proteus syndrome and ventricular tachycardia cardiac arrest in a teenager.

Proteus syndrome is an extremely rare overgrowth condition caused by a somatic variant of the AKT1 gene. It can involve multiple organ systems though rarely is there symptomatic cardiac involvement. Fatty infiltration of the myocardium has been described but has not been reported to cause functional or conduction abnormalities. We present an individual with Proteus syndrome who suffered a sudden cardiac arrest.

Calf blood flow is augmented in habitually aerobically trained versus untrained postmenopausal women in association with favorable modulation of vasculo-metabolic interactions.

INTRODUCTION: Menopause is associated with vascular dysfunction and increased risk of developing metabolic syndrome. Associations between vascular and metabolic health, and interactions with aerobic exercise training, are unknown in postmenopausal women (PMW). METHODS: In habitually aerobically trained PMW (PMWtr; n = 10; 57 ± 1 years; 40 ± 1 mL/kg/min), strain-gauge plethysmography was used to compare resting and peak calf blood flow (CBFr and CBFpk, respectively) and vascular resistance (CVRr; CVRpk) versus untrained PMW (PMWun; n = 13; 56 ± 1 years; 29 ± 1 mL/kg/min) and premenopausal women (PreM; n = 14; 26 ± 1 years; 40 ± 1 mL/kg/min). Vascular measures were taken before and 1 hour after 45 minutes of aerobic exercise (60% V̇ O2peak ), a known nitric oxide stimulus. Blood analyses included low- (LDLc) and high-density lipoprotein cholesterol (HDLc), insulin, and glucose. RESULTS: Pre-exercise, CBFr and CVRr did not differ (p > 0.05) between PMW groups, nor between PreM and PMWtr. CBFpk was highest (p < 0.05) and CVRpk was lowest (p < 0.05) in PMWtr. Blood markers were similar (p > 0.05) in PMW groups. However, in PMWtr, CBFpk was associated inversely (p < 0.05) with insulin (r = -0.725). Conversely, in PMWun, CBFpk correlated (p < 0.05) inversely with glucose (r = -0.717), positively with HDLc (r = 0.633), and CVRpk positively (p < 0.05) with LDLc (r = 0.568). Post-exercise, CBF increased and CVR decreased (p < 0.05) in all groups, yet CBFpk remained higher and CVRpk lower (p < 0.05) in PMWtr. CONCLUSION: In untrained PMW, peak CBF is associated inversely with circulating pro-atherogenic lipids and glucose. In contrast, peak CBF is associated inversely with insulin levels only in trained PMW. Habitual aerobic exercise may favorably modulate vasculo-metabolic interactions in PMW.

Exercise-induced skeletal muscle angiogenesis: impact of age, sex, angiocrines and cellular mediators.

Exercise-induced skeletal muscle angiogenesis is a well-known physiological adaptation that occurs in humans in response to exercise training and can lead to endurance performance benefits, as well as improvements in cardiovascular and skeletal tissue health. An increase in capillary density in skeletal muscle improves diffusive oxygen exchange and waste extraction, and thus greater fatigue resistance, which has application to athletes but also to the general population. Exercise-induced angiogenesis can significantly contribute to improvements in cardiovascular and metabolic health, such as the increase in muscle glucose uptake, important for the prevention of diabetes. Recently, our understanding of the mechanisms by which angiogenesis occurs with exercise has grown substantially. This review will detail the biochemical, cellular and biomechanical signals for exercise-induced skeletal muscle angiogenesis, including recent work on extracellular vesicles and circulating angiogenic cells. In addition, the influence of age, sex, exercise intensity/duration, as well as recent observations with the use of blood flow restricted exercise, will also be discussed in detail. This review will provide academics and practitioners with mechanistic and applied evidence for optimising training interventions to promote physical performance through manipulating capillarisation in skeletal muscle.

Flecainide administration in children: dosage, drug levels, and clinical effect.

Therapeutic drug monitoring of flecainide in children using plasma concentration measurements is undertaken by some clinicians. There is very little published evidence surrounding factors which influence plasma flecainide concentration, particularly in paediatric populations. We undertook a retrospective study of 45 children receiving flecainide to identify factors that influence its plasma concentration. Patients receiving a dose of 6 mg/kg/day had a higher mean plasma flecainide concentration than those receiving lower doses. Younger age and lighter weight were also associated with higher plasma flecainide concentrations. Children aged younger than 1 year receiving flecainide three times a day had a higher mean plasma flecainide concentration than older children who received flecainide twice a day. All supratherapeutic levels occurred in children aged less than 1 year who were receiving flecainide three times a day. Supratherapeutic levels were more common in those receiving 6 mg/kg/day while subtherapeutic levels were more common in those receiving 2 mg/kg/day. A supratherapeutic level did not correlate with adverse effects or clinical toxicity. Our results would suggest the need for a change of practice from prescribing flecainide at a frequency of three times a day in children aged younger than 1 year to twice a day in line with other ages.

Ischaemic preconditioning blunts exercise-induced mitochondrial dysfunction, speeds oxygen uptake kinetics but does not alter severe-intensity exercise capacity.

NEW FINDINGS: What is the central question of this study? Ischaemic preconditioning is a novel pre-exercise priming strategy. We asked whether ischaemic preconditioning would alter mitochondrial respiratory function and pulmonary oxygen uptake kinetics and improve severe-intensity exercise performance. What is the main finding and its importance? Ischaemic preconditioning expedited overall pulmonary oxygen uptake kinetics and appeared to prevent an increase in leak respiration, proportional to maximal electron transfer system and ADP-stimulated respiration, that was evoked by severe-intensity exercise in sham-control conditions. However, severe-intensity exercise performance was not improved. The results do not support ischaemic preconditioning as a pre-exercise strategy to improve exercise performance in recreationally active participants. ABSTRACT: We examined the effect of ischaemic preconditioning (IPC) on severe-intensity exercise performance, pulmonary oxygen uptake ( V ̇ O 2 ${\dot V_{{{\rm{O}}_{\rm{2}}}}}$ ) kinetics, skeletal muscle oxygenation (muscle tissue O2 saturation index) and mitochondrial respiration. Eight men underwent contralateral IPC (4 × 5 min at 220 mmHg) or sham-control (SHAM; 20 mmHg) before performing a cycling time-to-exhaustion test (92% maximum aerobic power). Muscle (vastus lateralis) biopsies were obtained before IPC or SHAM and ∼1.5 min postexercise. The time to exhaustion did not differ between SHAM and IPC (249 ± 37 vs. 240 ± 32 s; P = 0.62). Pre- and postexercise ADP-stimulated (P) and maximal (E) mitochondrial respiration through protein complexes (C) I, II and IV did not differ (P > 0.05). Complex I leak respiration was greater postexercise compared with baseline in SHAM, but not in IPC, when normalized to wet mass (P = 0.01 vs. P = 0.19), mitochondrial content (citrate synthase activity, P = 0.003 vs. P = 0.16; CI+IIP, P = 0.03 vs. P = 0.23) and expressed relative to P (P = 0.006 vs. P = 0.30) and E (P = 0.004 vs. P = 0.26). The V ̇ O 2 ${\dot V_{{{\rm{O}}_{\rm{2}}}}}$ mean response time was faster (51.3 ± 15.5 vs. 63.7 ± 14.5 s; P = 0.003), with a smaller slow component (270 ± 105 vs. 377 ± 188 ml min-1 ; P = 0.03), in IPC compared with SHAM. The muscle tissue O2 saturation index did not differ between trials (P > 0.05). Ischaemic preconditioning expedited V ̇ O 2 ${\dot V_{{{\rm{O}}_{\rm{2}}}}}$ kinetics and appeared to prevent an increase in leak respiration through CI, when expressed proportional to E and P evoked by severe-intensity exercise, but did not improve exercise performance.

Disturbance of desire-goal motivational dynamics during different exercise intensity domains.

PURPOSE: The desire-goal motivational conflict helps explain endurance performance; however, the physiological concomitants are unknown. The present study examined disturbances in desire to reduce effort and performance goal value across moderate, heavy, and severe exercise intensity domains, demarcated by the first (LT1) and second (LT2) lactate thresholds. In addition, the within-person relationships among blood lactate concentration, heart rate, and desire-goal conflict were examined. METHODS: Thirty participants (53% female, Mage  = 21.03 years; SD = 2.06 years) completed an incremental cycling exercise test, in which work rate was increased by 25 watts every four minutes, until voluntary exhaustion or sufficient data from the severe intensity domain had been collected. Desire to reduce effort, performance goal value, blood lactate concentration (for determination of LT1 and LT2), and heart rate were measured at the end of each stage and analyzed using multilevel models. RESULTS: The desire to reduce effort increased over the exercise test with additional shifts and accelerations after each lactate threshold. The performance goal did not show general declines, nor did it shift at LT1. However, the performance goal value shifted at LT2, and the rate of change increased at both thresholds. Within-person variation in blood lactate concentration positively correlated with the desire to reduce effort and negatively correlated with the performance goal. Within-person variation in heart rate correlated with desire to reduce effort but not the performance goal. CONCLUSION: Transitioning through both lactate thresholds is important phases for motivation during progressive exercise, particularly for the desire to reduce effort. Within-person variation in blood lactate concentration is more influential for motivation, compared with heart rate.

Publisher Correction: Image Data Resource: a bioimage data integration and publication platform.

This paper was originally published under standard Nature America Inc. copyright. As of the date of this correction, the Resource is available online as an open-access paper with a CC-BY license. No other part of the paper has been changed.

Blood-flow-restricted exercise: Strategies for enhancing muscle adaptation and performance in the endurance-trained athlete.

NEW FINDINGS: What is the topic of this review? Blood-flow-restricted (BFR) exercise represents a potential approach to augment the adaptive response to training and improve performance in endurance-trained individuals. What advances does it highlight? When combined with low-load resistance exercise, low- and moderate-intensity endurance exercise and sprint interval exercise, BFR can provide an augmented acute stimulus for angiogenesis and mitochondrial biogenesis. These augmented acute responses can translate into enhanced capillary supply and mitochondrial function, and subsequent endurance-type performance, although this might depend on the nature of the exercise stimulus. There is a requirement to clarify whether BFR training interventions can be used by high-performance endurance athletes within their structured training programme. ABSTRACT: A key objective of the training programme for an endurance athlete is to optimize the underlying physiological determinants of performance. Training-induced adaptations are governed by physiological and metabolic stressors, which initiate transcriptional and translational signalling cascades to increase the abundance and/or function of proteins to improve physiological function. One important consideration is that training adaptations are reduced as training status increases, which is reflected at the molecular level as a blunting of the acute signalling response to exercise. This review examines blood-flow-restricted (BFR) exercise as a strategy for augmenting exercise-induced stressors and subsequent molecular signalling responses to enhance the physiological characteristics of the endurance athlete. Focus is placed on the processes of capillary growth and mitochondrial biogenesis. Recent evidence supports that BFR exercise presents an intensified training stimulus beyond that of performing the same exercise alone. We suggest that this has the potential to induce enhanced physiological adaptations, including increases in capillary supply and mitochondrial function, which can contribute to an improvement in performance of endurance exercise. There is, however, a lack of consensus regarding the potency of BFR training, which is invariably attributable to the different modes, intensities and durations of exercise and BFR methods. Further studies are needed to confirm its potential in the endurance-trained athlete.

Bladder infusion versus standard catheter removal for trial of void: a systematic review and meta-analysis.

PURPOSE: To compare the efficacy and time-to-discharge of two methods of trial of void (TOV): bladder infusion versus standard catheter removal. METHODS: Electronic searches for randomized controlled trials (RCTs) comparing bladder infusion versus standard catheter removal were performed using multiple electronic databases from dates of inception to June 2020. Participants underwent TOV after acute urinary retention or postoperatively after intraoperative indwelling catheter (IDC) placement. Quality assessment and meta-analyses were performed, with odds ratio and mean time difference used as the outcome measures. RESULTS: Eight studies, comprising 977 patients, were included in the final analysis. Pooled meta-analysis demonstrated that successful TOV was significantly higher in the bladder infusion group compared to standard TOV (OR 2.41, 95% CI 1.53-3.8, p = 0.0005), without significant heterogeneity (I2=19%). The bladder infusion group had a significantly shorter time-to-decision in comparison to standard TOV (weighted mean difference (WMD)-148.96 min, 95% CI - 242.29, - 55.63, p = 0.002) and shorter time-to-discharge (WMD - 89.68 min, 95% CI - 160.55, - 18.88, p = 0.01). There was no significant difference in complication rates between the two groups. CONCLUSION: The bladder infusion technique of TOV may be associated with a significantly increased likelihood of successful TOV and reduced time to discharge compared to standard TOV practices.

The Image Data Resource: A Bioimage Data Integration and Publication Platform.

Access to primary research data is vital for the advancement of science. To extend the data types supported by community repositories, we built a prototype Image Data Resource (IDR) that collects and integrates imaging data acquired across many different imaging modalities. IDR links data from several imaging modalities, including high-content screening, super-resolution and time-lapse microscopy, digital pathology, public genetic or chemical databases, and cell and tissue phenotypes expressed using controlled ontologies. Using this integration, IDR facilitates the analysis of gene networks and reveals functional interactions that are inaccessible to individual studies. To enable re-analysis, we also established a computational resource based on Jupyter notebooks that allows remote access to the entire IDR. IDR is also an open source platform that others can use to publish their own image data. Thus IDR provides both a novel on-line resource and a software infrastructure that promotes and extends publication and re-analysis of scientific image data.

A prospective, matched comparison of ultra-low and standard-dose computed tomography for assessment of renal colic.

OBJECTIVE: To determine the diagnostic accuracy of ultra-low-dose computed tomography (ULDCT) compared with standard-dose CT (SDCT) in the evaluation of patients with clinically suspected renal colic, in addition to secondary features (hydroureteronephrosis, perinephric stranding) and additional pathological entities (renal masses). PATIENTS AND METHODS: A prospective, comparative cohort study was conducted amongst patients presenting to the emergency department with signs and symptoms suggestive of renal or ureteric colic. Patients underwent both SDCT and ULDCT. Single-blinded review of the image sets was performed independently by three board-certified radiologists. RESULTS: Among 21 patients, the effective radiation dose was lower for ULDCT [mean (SD) 1.02 (0.16) mSv] than SDCT [mean (SD) 4.97 (2.02) mSv]. Renal and/or ureteric calculi were detected in 57.1% (12/21) of patients. There were no significant differences in calculus detection and size estimation between ULDCT and SDCT. A higher concordance was observed for ureteric calculi (75%) than renal calculi (38%), mostly due to greater detection of calculi of <3 mm by SDCT. Clinically significant calculi (≥3 mm) were detected by ULDCT with high specificity (97.6%) and sensitivity (100%) compared to overall detection (specificity 91.2%, sensitivity 58.8%). ULDCT and SDCT were highly concordant for detection of secondary features, while ULDCT detected less renal cysts of <2 cm. Inter-observer agreement for the ureteric calculi detection was 93.9% for SDCT and 87.8% for ULDCT. CONCLUSION: ULDCT performed similarly to SDCT for calculus detection and size estimation with reduced radiation exposure. Based on this and other studies, ULDCT should be considered as the first-line modality for evaluation of renal colic in routine practice.

Single use versus reusable digital flexible ureteroscopes: A prospective comparative study.

OBJECTIVES: To compare the performance and surgical outcomes of two different single-use digital flexible ureteroscopes with a reusable video flexible ureteroscope. METHODS: Patients undergoing retrograde flexible ureteroscopy at Nepean Hospital, Sydney, Australia, were included in this study. Three different flexible ureteroscopes were used in this study: (i) single-use digital LithoVue (Boston Scientific, Marlborough, MA, USA); (ii) single-use digital PU3022A (Pusen, Zhuhai, China); and (iii) reusable digital URF-V2 (Olympus, Tokyo, Japan). Visibility and maneuverability was rated on a 5-point Likert scale by the operating surgeon. Operative outcomes and complications were collected and analyzed. RESULTS: A total of 150 patients were included in the present study. Of these, 141 patients had ureteroscopy for stone treatment, four for endoscopic combined intrarenal surgery and five for diagnostic/tumor treatment. There were 55 patients in the LithoVue group, 31 in the PU3022A group and 64 patients in the Olympus URF-V2 group. The URF-V2 group had higher visibility scores than both the single-use scopes and higher maneuverability scores when compared with the PU3022A. The LithoVue had higher visibility and maneuverability scores when compared with the PU3022A. There were no differences in operative time, rates of relook flexible ureteroscopes, scope failure or complication rates observed. CONCLUSIONS: Single-use digital flexible ureteroscopes have visibility and maneuverability profiles approaching that of a reusable digital flexible ureteroscope. Single-use flexible ureteroscopes achieve similar clinical outcomes to the more expensive reusable versions.

Impacts of Early- and Late-Terminated Cover Crops on Gas Fluxes.

Cover crops (CCs) could alter soil processes, but the effects of early versus late termination of CCs on gas fluxes are not well known. We evaluated temporal changes in fluxes of CO, NO, and CH and related soil properties in no-till corn ( L.) with and without winter rye ( L.) CCs that were terminated early (30 d before planting) or late (at planting) in a rainfed silty clay loam and an irrigated silt loam in Nebraska from April 2016 to June 2017. Gas fluxes, soil temperature, and soil water content were measured biweekly to monthly, and wet aggregate stability and particulate organic matter concentrations were measured seasonally. We also compared our results with a global literature review. Late-terminated CCs did not affect CH fluxes but increased daily CO fluxes by 59% compared with no CC at both sites and NO fluxes by 92% at the rainfed site only. Early termination did not affect gas fluxes. Termination date did not affect cumulative fluxes and had minimal effects on soil properties. The literature review supports our study results, which indicate that CC effects on (i) CO fluxes are driven by plant respiration during the CC growing period, and (ii) NO and CH fluxes are minimal under grass CCs. Overall, under no-till, CC termination date has small effects on NO and CH fluxes, but late CC termination can increase CO fluxes in spring due to greater biomass yield compared with early termination.

Ischemic preconditioning enhances critical power during a 3 minute all-out cycling test.

This study tested the hypothesis that ischemic preconditioning (IPC) would increase critical power (CP) during a 3 minute all-out cycling test. Twelve males completed two 3 minute all-out cycling tests, in a crossover design, separated by 7 days. These tests were preceded by IPC (4 x 5 minute intervals at 220 mmHg bilateral leg occlusion) or SHAM treatment (4 x 5 minute intervals at 20 mmHg bilateral leg occlusion). CP was calculated as the mean power output during the final 30 s of the 3 minute test with W' taken as the total work done above CP. Muscle oxygenation was measured throughout the exercise period. There was a 15.3 ± 0.3% decrease in muscle oxygenation (TSI; [Tissue saturation index]) during the IPC stimulus, relative to SHAM. CP was significantly increased (241 ± 65 W vs. 234 ± 67 W), whereas W' (18.4 ± 3.8 vs 17.9 ± 3.7 kJ) and total work done (TWD) were not different (61.1 ± 12.7 vs 60.8 ± 12.7 kJ), between the IPC and SHAM trials. IPC enhanced CP during a 3 minute all-out cycling test without impacting W' or TWD. The improved CP after IPC might contribute towards the effect of IPC on endurance performance.

Metadata management for high content screening in OMERO.

High content screening (HCS) experiments create a classic data management challenge-multiple, large sets of heterogeneous structured and unstructured data, that must be integrated and linked to produce a set of "final" results. These different data include images, reagents, protocols, analytic output, and phenotypes, all of which must be stored, linked and made accessible for users, scientists, collaborators and where appropriate the wider community. The OME Consortium has built several open source tools for managing, linking and sharing these different types of data. The OME Data Model is a metadata specification that supports the image data and metadata recorded in HCS experiments. Bio-Formats is a Java library that reads recorded image data and metadata and includes support for several HCS screening systems. OMERO is an enterprise data management application that integrates image data, experimental and analytic metadata and makes them accessible for visualization, mining, sharing and downstream analysis. We discuss how Bio-Formats and OMERO handle these different data types, and how they can be used to integrate, link and share HCS experiments in facilities and public data repositories. OME specifications and software are open source and are available at https://www.openmicroscopy.org.

The functional significance of hamstrings composition: is it really a "fast" muscle group?

Hamstrings muscle fiber composition may be predominantly fast-twitch and could explain the high incidence of hamstrings strain injuries. However, hamstrings muscle composition in vivo, and its influence on knee flexor muscle function, remains unknown. We investigated biceps femoris long head (BFlh) myosin heavy chain (MHC) composition from biopsy samples, and the association of hamstrings composition and hamstrings muscle volume (using MRI) with knee flexor maximal and explosive strength. Thirty-one young men performed maximal (concentric, eccentric, isometric) and explosive (isometric) contractions. BFlh exhibited a balanced MHC distribution [mean ± SD (min-max); 47.1 ± 9.1% (32.6-71.0%) MHC-I, 35.5 ± 8.5% (21.5-60.0%) MHC-IIA, 17.4 ± 9.1% (0.0-30.9%) MHC-IIX]. Muscle volume was correlated with knee flexor maximal strength at all velocities and contraction modes (r = 0.62-0.76, P < 0.01), but only associated with late phase explosive strength (time to 90 Nm; r = -0.53, P < 0.05). In contrast, BFlh muscle composition was not related to any maximal or explosive strength measure. BFlh MHC composition was not found to be "fast", and therefore composition does not appear to explain the high incidence of hamstrings strain injury. Hamstrings muscle volume explained 38-58% of the inter-individual differences in knee flexor maximum strength at a range of velocities and contraction modes, while BFlh muscle composition was not associated with maximal or explosive strength.