Need to revise classification of physical activity intensity in older adults? The use of estimated METs, measured METs and V̇O2Reserve.

BACKGROUND: Multiples of resting metabolic rate (RMR) are often used to classify physical activity intensity, a concept known as the Metabolic Equivalent of Task (MET). However, the METs metrics may misclassify physical activity intensity in older adults because of age related changes in RMR and maximal aerobic capacity (V̇O2max). This study aimed to 1) compare classifications of activity intensity by estimated (METsestimated) and measured (METsmeasured) METs and 2) compare physical activity classified by absolute (METsmeasured) versus relative intensity (%V̇O2Reserve) in older adults. METHODS: Ninety-eight adults aged 75-90 years participated in the study. RMR and V̇O2 during sitting, standing, daily activities and 6-minute-walking-test were measured. V̇O2Reserve was defined as the difference between V̇O2max and RMR. Moderate and vigorous intensity was classified as 3 and 6 METs and 40% and 60% of V̇O2Reserve, respectively. Paired t-tests and a confusion matrix were used to investigate aim 1 and 2, respectively. RESULTS: METsmeasured was 24% lower than the standard 1 MET of 3.5 ml O2·min-1·kg-1. METsestimated underestimated the intensity during daily and walking activities when compared to METsmeasured. Nevertheless, when comparing METsmeasured to percentages of V̇O2Reserve, a mismatch was shown for moderate intensity in 47-67% of the participants during daily activities, and 21% of the participants during self-selected gait speed. CONCLUSION: Applying METsestimated for older adults leads to potential underestimation of physical activity intensity, suggesting that current classification metrics should be revised for older adults. V̇O2Reserve is a candidate metric for establishing precise physical activity intensity cut-points for older adults.

Differential utilisation of subcellular skeletal muscle glycogen pools: a comparative analysis between 1 and 15 min of maximal exercise.

In skeletal muscle, glycogen particles are distributed both within and between myofibrils, as well as just beneath the sarcolemma. Their precise localisation may influence their degradation rate. Here, we investigated how exercise at different intensities and durations (1- and 15-min maximal exercise) with known variations in glycogenolytic rate and contribution from anaerobic metabolism affects utilisation of the distinct pools. Furthermore, we investigated how decreased glycogen availability achieved through lowering carbohydrate and energy intake after glycogen-depleting exercise affect the storage of glycogen particles (size, numerical density, localisation). Twenty participants were divided into two groups performing either a 1-min (n = 10) or a 15-min (n = 10) maximal cycling exercise test. In a randomised, counterbalanced, cross-over design, the exercise tests were performed following short-term consumption of two distinct diets with either high or moderate carbohydrate content (10 vs. 4 g kg-1 body mass (BM) day-1) mediating a difference in total energy consumption (240 vs. 138 g kg-1 BM day-1). Muscle biopsies from m. vastus lateralis were obtained before and after the exercise tests. Intermyofibrillar glycogen was preferentially utilised during the 1-min test, whereas intramyofibrillar glycogen was preferentially utilised during the 15-min test. Lowering carbohydrate and energy intake after glycogen-depleting exercise reduced glycogen availability by decreasing particle size across all pools and diminishing numerical density in the intramyofibrillar and subsarcolemmal pools. In conclusion, distinct subcellular glycogen pools were differentially utilised during 1-min and 15-min maximal cycling exercise. Additionally, lowered carbohydrate and energy consumption after glycogen-depleting exercise altered glycogen storage by reducing particle size and numerical density, depending on subcellular localisation. KEY POINTS: In human skeletal muscle, glycogen particles are localised in distinct subcellular compartments, referred to as intermyofibrillar, intramyofibrillar and subsarcolemmal pools. The intermyofibrillar and subsarcolemmal pools are close to mitochondria, while the intramyofibrillar pool is at a distance from mitochondria. We show that 1 min of maximal exercise is associated with a preferential utilisation of intermyofibrillar glycogen, and, on the other hand, that 15 min of maximal exercise is associated with a preferential utilisation of intramyofibrillar glycogen. Furthermore, we demonstrate that reduced glycogen availability achieved through lowering carbohydrate and energy intake after glycogen-depleting exercise is characterised by a decreased glycogen particle size across all compartments, with the numerical density only diminished in the intramyofibrillar and subsarcolemmal compartments. These results suggest that exercise intensity influences the subcellular pools of glycogen differently and that the dietary content of carbohydrates and energy is linked to the size and subcellular distribution of glycogen particles.

Substrate utilization and durability during prolonged intermittent exercise in elite road cyclists.

PURPOSE: This study investigated the effects of prolonged intermittent cycling exercise on peak power output (PPO) and 6-min time-trial (6 min-TT) performance in elite and professional road cyclists. Moreover, the study aimed to determine whether changes in performance in the fatigued state could be predicted from substrate utilization during exercise and laboratory measures obtained in a fresh state. METHODS: Twelve cyclists (age: 23 years [21;25]; body mass: 71.5 kg [66.7;76.8]; height: 181 cm [178;185]; V ˙ O2peak: 73.6 ml kg-1 min-1 [71.2;76.0]) completed a graded submaximal cycling test to determine lactate threshold (LT1), gross efficiency (GE), and maximal fat oxidation (MFO) as well as power output during a maximal 6 min-TT (MPO6 min) in a fresh condition. On a separate day, the cyclists completed a 4-h intermittent cycling protocol with a high CHO intake (100 g h-1). Substrate utilization and PPO was measured hourly during the protocol, which was followed by another 6 min-TT. RESULTS: MPO6 min and PPO was reduced by 10% [4;15] and 6% [0;6], respectively, after the cycling protocol. These reductions were accompanied by reductions in the anaerobic energy contribution and V ˙ O2peak, whereas the average V ˙ O2 during the 6 min-TT was unchanged. Correlation analyses showed no strong associations between reductions in MPO6 min and PPO and laboratory measures (i.e., LT1, GE, MFO, V ˙ O2peak) obtained in the fresh condition. Additionally, fat oxidation rates during the cycling protocol were not related to changes in neither PPO nor MPO6 min. CONCLUSION: PPO and MPO6 min were reduced following prolonged intermittent cycling, but the magnitude of these reductions could not be predicted from laboratory measures obtained in the fresh condition.

Eight weeks of heavy strength training increases hemoglobin mass and V̇o2peak in well-trained to elite female and male rowers.

O2-transport and endurance exercise performance are greatly influenced by hemoglobin mass (Hbmass), which largely depends on lean body mass (LBM). This study investigated the effects of 8 wk with three weekly sessions of conventional (3-SET: 3 × 10 reps) or high-volume strength training (10-SET: 5-10 × 10 reps) on LBM, Hbmass, muscle strength, and exercise performance in female and male rowers. Hematological parameters were obtained through CO rebreathing and body composition by dual-energy X-ray absorptiometry (DEXA) scans before and after the training period. Concomitantly, V̇o2peak was determined during 2-km ergometer rowing and muscle strength by isometric midthigh pull. There were no differences in training responses between groups for any of the parameters. Pooled data revealed overall increments for Hbmass (10-SET: 882 ± 199 g to 897 ± 213 g; 3-SET: 936 ± 245 g to 962 ± 247 g, P = 0.02) and V̇o2peak (10-SET: 4.3 ± 1.0 to 4.4 ± 0.9 L·min-1; 3-SET: 4.5 ± 0.9 to 4.6 ± 0.9 L·min-1, P = 0.03), whereas LBM remained unchanged (10-SET: 58.7 ± 10.5 to 58.7 ± 10.1 kg; 3-SET: 64.1 ± 10.8 to 64.5 ± 10.6 kg, P = 0.42). Maximal isometric midthigh pull strength increased (10-SET: 224 ± 47 kg to 237 ± 55 kg; 3-SET: 256 ± 77 kg to 281 ± 83 kg, P = 0.001). Strong associations were observed between LBM and Hbmass and V̇o2peak (r2 = 0.88-0.90), entailing sex differences in Hbmass and V̇o2peak. Normalizing V̇o2peak to LBM reduced the sex difference to ∼10%, aligning with the sex difference in Hbmass·LBM-1. Strength training successfully increased Hbmass and V̇o2peak in elite female and male rowers, without an additional effect from increased training volume. Moreover, sex differences in V̇o2peak were mainly explained by differences in LBM, but likely also by differences in Hbmass·LBM-1.NEW & NOTEWORTHY This study in female and male rowers demonstrates that hemoglobin mass (Hbmass), V̇o2peak, and muscle strength increases with 8 wk of heavy strength training and that this response is not different between conventional (3 × 10 repetitions) and high-volume strength training (10 × 10 repetitions). Moreover, female rowers exhibited less hemoglobin per kilogram of lean body mass compared with their male counterparts, which likely contributes to sex differences in V̇o2peak and rowing performance.

Lowered muscle glycogen reduces body mass with no effect on short-term exercise performance in men.

Performance in short-duration sports is highly dependent on muscle glycogen, but the total degradation is only moderate and considering the water-binding property of glycogen, unnecessary storing of glycogen may cause an unfavorable increase in body mass. To investigate this, we determined the effect of manipulating dietary carbohydrates (CHO) on muscle glycogen content, body mass, and short-term exercise performance. In a randomized and counterbalanced cross-over design, twenty-two men completed two maximal cycle tests of either 1-min (n = 10) or 15-min (n = 12) duration with different pre-exercise muscle glycogen levels. Glycogen manipulation was initiated three days prior to the tests by exercise-induced glycogen depletion followed by ingestion of a moderate (M-CHO) or high (H-CHO) CHO-diet. Subjects were weighed before each test, and muscle glycogen content was determined in biopsies from m. vastus lateralis before and after each test. Pre-exercise muscle glycogen content was lower following M-CHO than H-CHO (367 mmol · kg-1 DW vs. 525 mmol · kg-1 DW, p < 0.00001), accompanied by a 0.7 kg lower body mass (p < 0.00001). No differences were observed in performance between diets in neither the 1-min (p = 0.33) nor the 15-min (p = 0.99) test. In conclusion, pre-exercise muscle glycogen content and body mass were lower after ingesting moderate compared with high amounts of CHO, while short-term exercise performance was unaffected. This demonstrates that adjusting pre-exercise glycogen levels to the requirements of competition may provide an attractive weight management strategy in weight-bearing sports, particularly in athletes with high resting glycogen levels.

Contractile Properties of MHC I and II Fibers From Highly Trained Arm and Leg Muscles of Cross-Country Skiers.

INTRODUCTION: Little is known about potential differences in contractile properties of muscle fibers of the same type in arms and legs. Accordingly, the present study was designed to compare the force-generating capacity and Ca2+ sensitivity of fibers from arm and leg muscles of highly trained cross-country skiers. METHOD: Single muscle fibers of m. vastus lateralis and m. triceps brachii of eight highly trained cross-country skiers were analyzed with respect to maximal Ca2+-activated force, specific force and Ca2+ sensitivity. RESULT: The maximal Ca2+-activated force was greater for myosin heavy chain (MHC) II than MHC I fibers in both the arm (+62%, P < 0.001) and leg muscle (+77%, P < 0.001), with no differences between limbs for each MHC isoform. In addition, the specific force of MHC II fibers was higher than that of MHC I fibers in both arms (+41%, P = 0.002) and legs (+95%, P < 0.001). The specific force of MHC II fibers was the same in both limbs, whereas MHC I fibers from the m. triceps brachii were, on average, 39% stronger than fibers of the same type from the m. vastus lateralis (P = 0.003). pCa50 was not different between MHC I and II fibers in neither arms nor legs, but the MHC I fibers of m. triceps brachii demonstrated higher Ca2+ sensitivity than fibers of the same type from m. vastus lateralis (P = 0.007). CONCLUSION: Comparison of muscles in limbs equally well trained revealed that MHC I fibers in the arm muscle exhibited a higher specific force-generating capacity and greater Ca2+ sensitivity than the same type of fiber in the leg, with no such difference in the case of MHC II fibers. These distinct differences in the properties of fibers of the same type in equally well-trained muscles open new perspectives in muscle physiology.

Performance effects of periodized carbohydrate restriction in endurance trained athletes - a systematic review and meta-analysis.

Endurance athletes typically consume carbohydrate-rich diets to allow for optimal performance during competitions and intense training. However, acute exercise studies have revealed that training or recovery with low muscle glycogen stimulates factors of importance for mitochondrial biogenesis in addition to favourable metabolic adaptations in trained athletes. Compromised training quality and particularly lower intensities in peak intervals seem to be a major drawback from dietary interventions with chronic carbohydrate (CHO) restriction. Therefore, the concept of undertaking only selected training sessions with restricted CHO availability (periodized CHO restriction) has been proposed for endurance athletes. However, the overall performance effect of this concept has not been systematically reviewed in highly adapted endurance-trained athletes. We therefore conducted a meta-analysis of training studies that fulfilled the following criteria: a) inclusion of females and males demonstrating a VO2max ≥ 55 and 60 ml · kg- 1 · min- 1, respectively; b) total intervention and training periods ≥ 1 week, c) use of interventions including training and/or recovery with periodized carbohydrate restriction at least three times per week, and d) measurements of endurance performance before and after the training period. The literature search resulted in 407 papers of which nine studies fulfilled the inclusion criteria. The subsequent meta-analysis demonstrated no overall effect of CHO periodization on endurance performance compared to control endurance training with normal (high) CHO availability (standardized mean difference = 0.17 [- 0.15, 0.49]; P = 0.29). Based on the available literature, we therefore conclude that periodized CHO restriction does not per se enhance performance in endurance-trained athletes. The review discusses different approaches to CHO periodization across studies with a focus on identifying potential physiological benefits.

Effects of Acute Exercise and Training on the Sarcoplasmic Reticulum Ca2+ Release and Uptake Rates in Highly Trained Endurance Athletes.

Little is presently known about the effects of acute high-intensity exercise or training on release and uptake of Ca2+ by the sarcoplasmic reticulum (SR). The aims here were to characterize this regulation in highly trained athletes following (1) repeated bouts of high-intensity exercise and (2) a period of endurance training including high-intensity sessions. Eleven cross-country skiers (25 ± 4 years, 65 ± 4 mL O2⋅kg-1⋅min-1) performed four self-paced sprint time-trials (STT 1-4) lasting ≈ 4 min each (STT 1-4) and separated by 45 min of recovery; while 19 triathletes and road cyclists (25 ± 4 years, 65 ± 5 mL O2⋅kg-1⋅min-1) completed 4 weeks of endurance training in combination with three sessions of high-intensity interval cycling per week. Release (µmol⋅g-1 prot⋅min-1) and uptake [tau (s)] of Ca2+ by SR vesicles isolated from m. triceps brachii and m. vastus lateralis were determined before and after STT 1 and 4 in the skiers and in m. vastus lateralis before and after the 4 weeks of training in the endurance athletes. The Ca2+ release rate was reduced by 17-18% in both limbs already after STT 1 (arms: 2.52 ± 0.74 to 2.08 ± 0.60; legs: 2.41 ± 0.45 to 1.98 ± 0.51, P < 0.0001) and attenuated further following STT 4 (arms: 2.24 ± 0.67 to 1.95 ± 0.45; legs: 2.13 ± 0.51 to 1.83 ± 0.36, P < 0.0001). Also, there was a tendency toward an impairment in the SR Ca2+ uptake from pre STT1 to post STT4 in both arms and legs (arms: from 22.0 ± 3.7 s to 25.3 ± 6.0 s; legs: from 22.5 ± 4.7 s to 25.5 ± 7.7 s, P = 0.05). Endurance training combined with high-intensity exercise increased the Ca2+ release rate by 9% (1.76 ± 0.38 to 1.91 ± 0.44, P = 0.009), without altering the Ca2+ uptake (29.6 ± 7.0 to 29.1 ± 8.7 s; P = 0.98). In conclusion, the Ca2+ release and uptake rates by SR in exercising limbs of highly trained athletes declines gradually by repetitive bouts of high-intensity exercise. We also demonstrate, for the first time, that the SR Ca2+ release rate can be enhanced by a specific program of training in highly trained athletes, which may have important implications for performance parameters.

No Superior Adaptations to Carbohydrate Periodization in Elite Endurance Athletes.

PURPOSE: The present study investigated the effects of periodic carbohydrate (CHO) restriction on endurance performance and metabolic markers in elite endurance athletes. METHODS: Twenty-six male elite endurance athletes (maximal oxygen consumption (V˙O2max), 65.0 mL O2·kg·min) completed 4 wk of regular endurance training while being matched and randomized into two groups training with (low) or without (high) CHO manipulation 3 d·wk. The CHO manipulation days consisted of a 1-h high-intensity bike session in the morning, recovery for 7 h while consuming isocaloric diets containing either high CHO (414 ± 2.4 g) or low CHO (79.5 ± 1.0 g), and a 2-h moderate bike session in the afternoon with or without CHO. V˙O2max, maximal fat oxidation, and power output during a 30-min time trial (TT) were determined before and after the training period. The TT was undertaken after 90 min of intermittent exercise with CHO provision before the training period and both CHO and placebo after the training period. Muscle biopsies were analyzed for glycogen, citrate synthase (CS) and ß-hydroxyacyl-coenzyme A dehydrogenase (HAD) activity, carnitine palmitoyltransferase (CPT1b), and phosphorylated acetyl-CoA carboxylase (pACC). RESULTS: The training effects were similar in both groups for all parameters. On average, V˙O2max and power output during the 30-min TT increased by 5% ± 1% (P < 0.05) and TT performance was similar after CHO and placebo during the preload phase. Training promoted overall increases in glycogen content (18% ± 5%), CS activity (11% ± 5%), and pACC (38% ± 19%; P < 0.05) with no differences between groups. HAD activity and CPT1b protein content remained unchanged. CONCLUSIONS: Superimposing periodic CHO restriction to 4 wk of regular endurance training had no superior effects on performance and muscle adaptations in elite endurance athletes.

Muscle glycogen content modifies SR Ca2+ release rate in elite endurance athletes.

PURPOSE: The aim of the present study was to investigate the influence of muscle glycogen content on sarcoplasmic reticulum (SR) function and peak power output (Wpeak) in elite endurance athletes. METHODS: Fourteen highly trained male triathletes (VO2max = 66.5 ± 1.3 mL O2·kg·min), performed 4 h of glycogen-depleting cycling exercise (HRmean = 73% ± 1% of maximum). During the first 4 h of recovery, athletes received either water (H2O) or carbohydrate (CHO), separating alterations in muscle glycogen content from acute changes affecting SR function and performance. Thereafter, all subjects received CHO-enriched food for the remaining 20-h recovery period. RESULTS: Immediately after exercise, muscle glycogen content and SR Ca release rate was reduced to 32% ± 4% (225 ± 28 mmol·kg dw) and 86% ± 2% of initial levels, respectively (P < 0.01). Glycogen markedly recovered after 4 h of recovery with CHO (61% ± 2% of preexercise) and SR Ca release rate returned to preexercise level. However, in the absence of CHO during the first 4 h of recovery, glycogen and SR Ca release rate remained depressed, with the normalization of both parameters at the end of the 24 h of recovery after receiving a CHO-enriched diet. Linear regression demonstrated a significant correlation between SR Ca release rate and muscle glycogen content (P < 0.01, r = 0.30). The 4 h of cycling exercise reduced Wpeak by 5.5%-8.9% at different cadences (P < 0.05), and Wpeak was normalized after 4 h of recovery with CHO, whereas Wpeak remained depressed (P < 0.05) after water provision. Wpeak was fully recovered after 24 h in both the H2O and the CHO group. CONCLUSION: In conclusion, the present results suggest that low muscle glycogen depresses muscle SR Ca release rate, which may contribute to fatigue and delayed recovery of Wpeak 4 h postexercise.