Coactivation pattern in human quadriceps during isokinetic knee-extension by muscle functional MRI

We tested activation patterns of individual muscles and neuromuscular compartments (NMCs) of the quadriceps femoris (QF) during knee-extension exercises (KEE). Eight healthy men performed repetitive isokinetic KEE at 120° s^–1. Before and after KEE, muscle functional magnetic resonance images of the thigh were taken to calculate transverse relaxation times (T2) for the individual muscles of the QF. In terms of exercised T2, a moderate correlation was observed between the rectus femoris (RF) and vastus medialis (VM) (r=0.493, P<0.001), and between the vastus lateralis (VL) and VM (r=0.347, P<0.01). Furthermore, a strong correlation was also observed between the vastus intermedius (VI) and VM (r=0.719, P<0.0001), and between the VL and VI (r=0.783, P<0.0001). There was a significant difference in T2 between the proximal and distal NMCs of the RF. We conclude that at least two strategies are applied for force production in the human QF: (1) coactivation of specific pairs of individual muscles and (2) regional-specific activation in NMCs in the RF.     

Effects of calcium channel blockers on the contractility of the filariidAcanthocheilonema viteae

The role of calcium in muscle contractility was explored in the filarial nematodeAcanthocheilonema viteae (Dipetalonema viteae). The parasite was slit open longitudinally and mounted in a smooth-muscle chamber that had been filled with aerated (95% N₂/5% CO₂) physiological solution at 37°C. Nifedipine (10^–6 m) and cadmium (3×10^–5 m) reduced the spontaneous isotonic contractions ofA. viteae, whereas verapamil (10^–5 m) and diltiazem (10^–5 m) enhanced them. The effects of nifedipine and verapamil did not appear to be due to the solvent ethanol. All of the drugs reduced the maximal contraction induced by acetylcholine (ACh, 10^–5 m), although nifedipine was the most potent. After the exposure of worm preparations to a calcium-free medium containing ethyleneglycol-bis-(-aminoethyl ether)-N,N,N,N-tetraacetic acid (EGTA, 10^–4 m) for 1 h, application of ACh (10^–5 m) induced a small, transient contraction. Subsequent applications of ACh in this medium had no effect. Thus, the nematode muscle contraction appears to depend on extracellular calcium. Nifedipine, diltiazem, and verapamil could act by reducing the calcium influx across the muscle membrane.     

The effects of indomethacin and prostaglandins E2 and F2α on canine tracheal mucus generation

The effects of prostaglandins on respiratory smooth muscle are well known; however, their role in mucus generation has received little attention. This investigation was undertaken to study the actions of prostaglandin E₂ (PGE₂) and prostaglandin F₂ (PGF₂) on secretion and synthesis of respiratory mucus glycoproteins. The method employed canine tracheal explants; radiochemical precursors were used in the culture media for labelling and quantitation of the secreted glycoproteins.Glycoprotein secretion was not significantly influenced by PGE₂ (1×10^–10 to 1×10^–3 M) or PGF₂ (1×10^–8 to 1×10^–4 M). However, PGF₂ (1×10^–3 M and 1×10^–2 M), significantly stimulated glycoprotein secretion. The mechanism may have involved a contractile effect on myoepithelial cells of the submucosal glands. Synthesis of glycoproteins was significantly inhibited by PGF₂ (1×10^–5 M), indomethacin (1×10^–6 M), and the combination PGF₂ (1×10^–5 M)/indomethacin (1×10^–6 M). PGE₂ (1×10^–5 M) and arachidonic acid (1×10^–5 M) were without effect. These data suggested that indomethacin inhibited glycoprotein synthesis by a mechanism other than an action on prostaglandin synthetase. In addition, because these agents inhibited synthesis of glycoproteins which were labeled with³H-glucosamine,³⁵S-sulfate, and¹⁴C-serine, it appears that their action was primarily upon synthesis of the protein core.     

Biarticular and monoarticular muscle activation and injury in human quadriceps muscle

We hypothesized that activation of the quadriceps femoris muscle group during eccentric exercise is related to the increase in magnitude of several markers of muscle injury that developed during the next week. Fourteen male subjects performed six to eight sets of five to ten repetitions of single-leg eccentric-only seated knee extension exercise. Magnetic resonance (MR) images were collected before and immediately after exercise and on days 2–4 and 6 after eccentric exercise. Changes in maximal voluntary contraction (MVC), perceived soreness, muscle volume and muscle transverse relaxation of water protons (T₂) were determined for the quadriceps femoris muscle group each day. Changes in muscle volume and T₂ were determined every day for each muscle [vastus lateralis (VL), vastus medialis (VM), vastus intermedius (VI), rectus femoris (RF)] of the quadriceps femoris group. Post-exercise T₂ was greater than pre-exercise T₂ (P<0.05) for all muscles. The acute ΔT₂ (Post-Pre) was similar (P>0.05) among VL, VM, VI, and RF [5.5 (0.3) ms], suggesting that the four muscles were equally activated during eccentric exercise. In the week after eccentric exercise, subjects experienced delayed-onset muscle soreness (DOMS) and all muscles demonstrated a delayed increase in T₂ above pre-exercise values (P<0.05), suggesting that muscle injury had occurred. For the quadriceps femoris muscle group, there was no correlation between acute ΔT₂ and delayed (peak T₂ during days 2, 3, 4, 6 minus pre-exercise T₂) ΔT₂ (r=0.04, P>0.05). Similar results were obtained when VL, VM, VI and RF were examined separately. Of the four muscles in quadriceps femoris, the biarticular RF experienced greater muscle injury [delayed ΔT₂=15.2 (2.0) ms] compared to the three monoarticular vasti muscles [delayed ΔT₂=7.7 (1.3) ms; P<0.05]. We propose that the disproportionate muscle injury to RF resulted from an ineffective transfer of torque from the knee to hip joint during seated eccentric knee extension exercise, thus causing RF to dissipate greater energy than normal. We conclude that in humans, muscle activation is not a unique determinant of muscle injury. Electronic Publication     

Basolateral membrane chloride permeability of A6 cells: implication in cell volume regulation

The permeability to Cl^– of the basolateral membrane (blm) was investigated in renal (A6) epithelial cells, assessing their role in transepithelial ion transport under steady-state conditions (isoosmotic) and following a hypoosmotic shock (i.e. in a regulatory volume decrease, RVD). Three different complementary studies were made by measuring: (1) the Cl^– transport rates (F/F _o · s^–1 (× 10^–3)), where F is the fluorescence of N-(6-methoxyquinoyl) acetoethyl ester, MQAE, and F _o the maximal fluorescence (×10^–3) of both membranes by following the intracellular Cl^–3 activities (a _iCl^–, measured with MQAE) after extracellular Cl^– substitution (2) the blm ⁸⁶Rb and ³⁶Cl uptakes and (3) the cellular potential and Cl^– current using the wholecell patch-clamp technique to differentiate between the different Cl^– transport mechanisms. The permeability of the blm to Cl^– was found to be much greater than that of the apical membranes under resting conditions: a _iCl^– changes were 5.3±0.7 mM and 25.5±1.05 mM (n=79) when Cl^– was substituted by NO₃ ^– in the media bathing apical and basolateral membranes. The Cl^– transport rate of the blm was blocked by bumetanide (100 M) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 50 M) but not by N-phenylanthranilic acid (DPC, 100 M). ⁸⁶Rb and ³⁶C1 uptake experiments confirmed the presence of a bumetanide- and a NPPB-sensitive Cl^– pathway, the latter being approximately three times more important than the former (Na/K/2Cl cotransporter). Application of a hypoosmotic medium to the serosal side of the cell increased F/F _o · s^–1 (×10^–3) after extracellular Cl^–3 substitution (1.03±0.10 and 2.45±0.17 arbitrary fluorescent units·s^–1 for isoosmotic and hypoosmotic conditions respectively, n=11); this F/F _o·s^–1 (×10^–3) increase was totally blocked by serosal NPPB application; on the other hand, cotransporter activity was decreased by the hypoosmotic shock. Cellular Ca²⁺ depletion had no effect on F/F _o·s^–1 (×10^–3) under isoosmotic conditions, but blocked the F/F _o·s^–1 (×10^–3) increase induced by a hypoosmotic stress. Under isotonic conditions the measured cellular potential at rest was –37.2±4.0 mV but reached a maximal and transient depolarization of –25.1±3.7 mV (n=9) under hypoosmotic conditions. The cellular current at a patch-clamping cellular potential of –85 mV (close to the Nernst equilibrium potential for K⁺) was blocked by NPPB and transiently increased by hypoosmotic shock ( 50% maximum increase). This study demonstrates that the major component of Cl^– transport through the blm of the A6 monolayer is a conductive pathway (NPPB-sensitive Cl^– channels) and not a Na/K/2Cl cotransporter. These channels could play a role in transepithelial Cl^– absorption and cell volume regulation. The increase in the blm Cl^– conductance, inducing a depolarization of these membranes, is proposed as one of the early events responsible for the stimulation of the ⁸⁶Rb efflux involved in cell volume regulation.     

Recruitment of fibre types and quadriceps muscle portions during repeated, intense knee-extensor exercise in humans

To investigate recruitment of slow-twitch (ST) and fast-twitch (FT) muscle fibres, as well as the involvement of the various quadriceps femoris muscle portions during repeated, intense, one-legged knee-extensor exercise, 12 healthy male subjects performed two 3-min exercise bouts at ~110% maximum thigh O₂ consumption (EX1 and EX2) separated by 6 min rest. Single-fibre metabolites were determined in successive muscle biopsies obtained from the vastus lateralis muscle (n=6) and intra-muscular temperatures were continuously measured at six quadriceps muscle sites (n=6). Creatine phosphate (CP) had decreased (P<0.05) by 27, 73 and 88% in ST fibres and 25, 71 and 89% in FT fibres after 15 and 180 s of EX1 and after 180 s of EX2, respectively. CP was below resting mean–1 SD in 15, 46, 84 and 100% of the ST fibres and 9, 48, 85 and 100% of the FT fibres at rest, after 15 and 180 s of EX1 and after 180 s of EX2, respectively. A significant muscle temperature increase (T_m) occurred within 2–4 s at all quadriceps muscle sites. T_m varied less than 10% between sites during EX1, but was 23% higher (P<0.05) in the vastus lateralis than in the rectus femoris muscle during EX2. T_m in the vastus lateralis was 101 and 109% of the mean quadriceps value during EX1 and EX2, respectively. We conclude that both fibre types and all quadriceps muscle portions are recruited at the onset of intense knee-extensor exercise, that essentially all quadriceps muscle fibres are activated during repeated intense exercise and that metabolic measurements in the vastus lateralis muscle provide a good indication of the whole-quadriceps muscle metabolism during repeated, intense, one-legged knee-extensor exercise.     

The effects of voluntary contraction effort on quadriceps femoris electromyogram median frequency in humans: a muscle and sex comparison

The purpose of this study was to examine the effect of voluntary contraction efforts on the median frequency (f _med) of the electromyogram (EMG) recorded from the quadriceps femoris muscle in healthy men and women. A group of 30 healthy volunteers (15 men, 15 women) were assessed for EMG activity of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles during isometric contractions with the knee at 60° flexion. Subjects performed a series of 5 s maximal voluntary isometric contractions that anchored the perceptual range with a "10" on a 10-point scale. Sub-maximal isometric contractions were then separately performed at the following perceived effort levels on the 10-point scale: 1, 2, 3, 4, 5, 6, 7, 8 and 9, in a random order. Subjects were instructed to maintain the contraction at each perceived level of effort for 5 s. The f _med of the three muscles was assessed using a power spectrum analysis performed over 11 consecutive, 512 ms, epochs overlapping each other by half their length during the middle 3 s of each contraction. The f _med for each of the 11 epochs was then determined for each muscle, followed by calculation of the means and normalized coefficients of variation [(standard deviation/mean)×100%] for each contraction. The results demonstrated that the mean f _med of VL was significantly greater than those of the other two muscles, and that f _med of RF was significantly greater than that of VM. The VL muscle demonstrated a significant increase in mean f _med across the contraction efforts, compared to the VM and RF muscles that displayed a significant decrease. The men displayed significantly higher f _med values for the VM muscle than did the women, as well as showing a significantly greater increase across the contraction efforts for the VL muscle. The variability of f _med was shown to be significantly higher for the VM muscle, compared to the VL and RF muscles. The findings of this study suggest that the f _med statistic is most sensitive to contraction intensity efforts for the VL muscle, and that men display significantly higher values for the VL and VM muscles, compared to women. Electronic Publication     

Cardiac electrophysiological effects of histamine,H1 and H2 agonists

The effects of histamine, H₁ and H₂ agonists on the transmembrane action potentials (APs) of electrically driven left auricle of guinea-pig were studied in Tyrode and in 25 mM K⁺ Tyrode solution. Under physiological conditions, histamine (10^–7–10^–5 M) increased the amplitude and the plateau phase of AP. The H₁ agonist pyridylethylamine (PEA; 10^–5–10^–4 M) exerted similar effects as histamine while the H₂-agonist impromidine (10^–8–10^–7 M) decreased the duration of AP. PEA restored the abolished electromechanical activity in high K⁺ (25 mM)-depolarized atria. These slow APs were unaffected by the -adrenergic blocker pindolol (5×10^–7 M) but were abolished by the H₁ antagonist mepyramine (10^–5 M). Neither dimaprit nor impromidine was able to induce slow AP. Our results suggest that in guinea-pig left atria histamine enhances the slow inward Ca²⁺ current mediated by H₁ receptors.     

Membrane electrical properties of vascular smooth muscle from the guinea pig superior mesenteric artery

Some electrical membrane properties of an isolated small artery, namely, the superior mesenteric artery of the guinea pig, were studied by intracellular microelectrodes. The mean resting membrane potential (E _m) was –54 mV. The average slope of theE _m vs. log [K]_o curve (between 10 and 100 mM [K]_o) was 32 mV/decade, and the curve extrapolated to a [K]_i of 160 mM. The ratio of Na⁺ permeability to K⁺ permeability (P _Na/P _K) at 4.0 mM [K]_o calculated from the Goldman constant-field equation (assuming Cl^– to be passively distributed) was 0.18 (E _m=–46 mV after a 5 min exposure to ouabain to suppress any electrogenic pump potential). The normal input resistance (R _in) averaged 8.5 m. Choline substitution for Na⁺ or amiloride, an agent known to depressP _Na, hyperpolarized the muscle to about –63 mV without a significant change inR _in. Ba²⁺ (0.5 mM) depolarized the muscle to –37 mV, increasedR _in to 15 m, and produced spontaneous action potentials in this normally quiescent artery; tetraethylammonium (TEA, 5 mM) enabled large overshooting action potentials to be produced upon stimulation. Glutamate of NO ₃ ^– substitution for Cl^– produced an initial depolarization followed by a return to the original resting potential within 10 min; readdition of 25 mM Cl^– transiently hyperpolarized the muscle markedly, followed by a return to the originalE _m. These data indicate that Cl^– is passively distributed and does not contribute to the steady-state resting potential in this vascular muscle. The data also suggest that the relatively lowE _m in this arterial muscle is not due to a low [K]_i, but is due to a highP _Na/P _K ratio, presumably related to a low K⁺ conductance (g _K). Since Ba²⁺ and TEA⁺ are known to decrease restingg _K and K⁺ activation, the data also suggest that K⁺ activation could inhibit action potential generation.     

Effects of prostaglandins on Na transport in isolated collecting tubules

Direct tubular effects of prostaglandins (PG's) on Na transport were examined in isolated cortical and medullary collecting tubules of rabbits perfused in vitro. The animals were treated with deoxycorticosterone acetate (DOCA, 1 mg kg^–1 day^–1, i.m.) for 3–6 days before experiments. In the cortical collecting tubules PGE₂ (1.2×10^–7–2.5×10^–5 M), E₁ (1.2×10^–5 M) and F₂(1.2×10^–5 M) added to the bath caused reversible decreases in transtubular potential difference (PD_t). But neither PGE₂ (1.2 ×10^–5 M) added to the perfusate nor PGA₂ (1.2 ×10^–5 M) added to the bath had an effect on PD_t. The net Na absorption was decreased with PGE₂ (1.2×10^–5 M) added to the bath from 8.6±1.36 to 1.5±1.04 pEq cm^–1 s^–1 (P<0.02). In rabbits not pretreated with DOCA, the net Na absorption was reduced from 2.73±0.74 to 1.02±0.74 pEq cm^–1 s^–1 (P<0.01). In the outer medullary collecting tubules PGE₂ (1.2×10^–5 M) added to the bath also caused a reversible decrease in PD_t. It is concluded that PGE₂, F₂ and E₁ inhibit Na absorption in the collecting tubules by acting on the peritubular membrane.     

Dihydropyridine receptors in transverse tubules from normal and dystrophic chicken skeletal muscle

Summary Calcium overload is a fundamental pathogenic event associated with chronic muscle degeneration in muscular dystrophies. The possibility that l-type voltage-dependent calcium channels were involved in the etiology of chicken muscular dystrophy was investigated by studying the dihydropyridine receptors in transverse tubule membranes isolated from skeletal muscle of normal (line 412) and dystrophic (line 413) chickens. The yield of T-tubular protein from dystrophic muscle was considerably increased compared with that from normal muscle (2.51±0.18 vs 1.04±0.31 mg protein × 100 g muscle^–1). The binding of the calcium channel antagonist (+) [³H]PN200-110 to the dihydropyridine receptor in transverse tubule preparations was relatively slow, markedly affected by temperature and required divalent cations. (+) [³H]PN200-110 equilibrium binding assays revealed a single class of high-affinity sites and showed that maximum binding capacity (B_max) (3.17±0.47 for normal and 3.51±0.52 pmol × mg protein^–1 for dystrophic transverse tubules) and dissociation constant (K_d) (0.32±0.07 and 0.26±0.09 nm, respectively) were not significantly different in normal and dystrophic membranes. Kinetic studies indicated that normal and dystrophic transverse tubules did not differ significantly in association (2.54×10⁶ and 2.27×10⁶ m ^–1 s^–1, respectively) and dissociation (8.5×10^–4 and 9.3×10^–4 s^–1, respectively) rate constants. Since dissociation kinetics for both preparations were monoexponential under all the experimental conditions employed, no low-affinity binding sites for (+) [³H]PN200-110 could be detected in chicken transverse tubules membranes. However, immunoblot assay, using a monoclonal antibody, revealed that dystrophic transverse tubules as compared with normal membranes were enriched twofold with the ₁-subunit of the dihydropyridine receptor. Therefore, although dihydropyridine-binding sites were not altered in transverse tubule membranes from dystrophic chicken skeletal muscle, both the increased yield in T-tubule vesicles and the enhanced immunodetection of the ₁-subunit of the dihydropyridine receptor, suggest that total content in dihydropyridine receptor is higher in dystrophic than in normal muscle.     

Predicting sweat loss response to exercise,environment and clothing

Summary Metabolic heat production (M), clothing heat transfer characteristics, and the environment dictate a required evaporative cooling (E_req) from the body to maintain thermal balance. However, the maximal evaporative capacity (E_max) is dictated by vapor transfer properties of the clothing and environment. Relationships between metabolic load, environmental conditions, clothing and sweat loss were studied in 34 heat-acclimatized males categorized into four groups (eight, eight, eight, and ten subjects) and exposed to various environmental conditions (ambient temperature, 20–54 C, and relative humidity, 10–90%), three levels of metabolic rate (resting; walking 1.34 m·s^–1, level; or walking 1.34 m·s^–1, 5% grade) while wearing various clothing ensembles (shorts and T-shirts, fatigues, fatigues plus overgarment, or sweat suit). Individual groups were not exposed to all combinations. Exposures lasted 120 min: either 10 min rest — 50 min exercise — 10 min rest — 50 min exercise, or 120 min at rest. Physiological measurements included heart rate, rectal temperature, mean skin temperature, energy expenditure and sweat loss (m_sw). E_max and E_req were calculated from environmental conditions, metabolism, clothing insulation and permeability. The ratio E_req/m_sw was found to correlate with E_max and not with M. The predictive equation for sweat loss was: m_sw=18.7×E_req×(E_max)^–0.455 within the limits 50req<360; W·m^–2 and 20max<525; W·m^–2. This formula predicts sweat loss for specific work loads, climates and clothing ensembles.     

Electromyographic fatigue thresholds of the superficial muscles of the quadriceps femoris

The purpose of this investigation was to compare the thresholds of neuromuscular fatigue determined simultaneously from the vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) muscles using the electromyographic fatigue threshold (EMG_FT) test, Eight adult volunteers [mean (SD) age, 33 (10) years] served as subjects for this investigation. The results of a one-way repeated measured ANOVA indicated that there was a significant (P < 0.05) difference among the mean EMG_FT values for the VL [248(31)W], VM [223(43)W] and RF [220(30)W] muscles. Tukey post-hoc comparisons indicated that the EMG_FT for the RF was significantly (P < 0.05) lower than that of the VL. These findings suggested that during cycle ergometry there is a dissociation in neuromuscular fatigue characteristics of the superficial muscles of the quadriceps femoris group.     

Contractile properties and ultrastructure of three types of muscle fibre in the dogfish myotome

Summary Three main types of fibre can be differentiated in the adult dogfish myotome at the immediate post-anal level. An outer band of muscle consists of 80–90 pale multiply innervated fibres (superficial fibres). These fibres are 80–90 m in diameter, lack M-lines and have a low Ca²⁺-activated myosin ATPase activity. Volume densities of myofibrils (V_v(my,f)) and mitochondria (V_v(mt,f)) are respectively 76 and 9.5%. Beneath this layer are around 8000 red multiply innervated fibres. These have an average diameter of 25–40 m. V_v(my,f) and V_v(mt,f) are 62 and 21% respectively, and M-lines are present. Around 11000 white focally innervated twitch fibres lie beneath the red fibre zone. White fibres with an average diameter of 80–120 m have a high Ca²⁺-activated myosin ATPase activity and V_v(my,f) and V_v(mt,f) are 78 and 5% respectively.Contractile properties of single skinned fibres were determined at 12° C. Maximum Ca²⁺ activated tensions (kN m^–2) and unloaded contraction speeds (muscle lengths s^–1) were 49 and 0.5 for superficial, 70 and 1.4 for red and 180 and 4.4 for white muscle fibres.Superficial fibres have not been reported in other elasmobranchs with the exception of the closely related nursehound (Scyliorhinus stellaris L.) It is suggested that they are specialized for sustained force generation, having a tonic (postural) rather than a locomotor role.     

Muscle buffer capacity and aerobic fitness are associated with repeated-sprint ability in women

In addition to a high aerobic fitness, the ability to buffer hydrogen ions (H⁺) may also be important for repeated-sprint ability (RSA). We therefore investigated the relationship between muscle buffer capacity (m_{in vivo} and m_{in vitro}) and RSA. Thirty-four untrained females [mean (SD): age 19 (1) years, maximum oxygen uptake (O_2peak) 42.3 (7.1) ml·kg^–1·min^–1] completed a graded exercise test (GXT), followed by a RSA cycle test (five 6-s sprints, every 30 s). Capillary blood was sampled during the GXT and before and after the RSA test to determine blood pH (pH_b) and lactate concentration ([La^–]_b). Muscle biopsies were taken before (n=34) and after (n=23) the RSA test to determine muscle lactate concentration ([La^–]_i), hydrogen ion concentration ([H⁺]_i) pH_i, m_{in vivo} and m_{in vitro}. There were significant correlations between work decrement (%) and m_{in vivo} (r=–0.72, P<0.05), O_2peak (r=–0.62, P<0.05), lactate threshold (LT) (r=–0.56, P<0.05) and changes in [H⁺]_i (r=0.41, P<0.05). There were however, no significant correlations between work decrement and m_{in vitro}, or changes in [La^–]_i, or [La^–]_b. There were also no significant correlations between total work (J·kg^–1) during the RSA test and m_{in vitro}, m_{in vivo}, or changes in [La^–]_i, pH_i, [La^–]_b, or pH_b. There were significant correlations between total work (J·kg^–1) and both O_2peak (r=0.60, P<0.05) and LT(r=0.54, P<0.05). These results support previous research, identifying a relationship between RSA and aerobic fitness. This study is the first to identify a relationship between m_{in vivo} and RSA. This suggests that the ability to buffer H⁺ may be important for maintaining performance during brief, repeated sprints.     

Regional differences in blood flow, glucose uptake and fatty acid uptake within quadriceps femoris muscle during dynamic knee-extension exercise

The purpose of the present study was to investigate the regional differences in glucose and fatty acid uptake within skeletal muscle during exercise. Blood flow (BF), glucose uptake (GU) and free fatty acid uptake (FFAU) were measured in four different regions (vastus lateralis, VL; rectus femoris, RF; vastus intermedius, VI; and vastus medialis, VM) of the quadriceps femoris (QF) muscle during low-intensity, knee-extension exercise using positron emission tomography. BF was higher in VI than in VL, RF and VM (P < 0.05). FFAU was higher in VI (P < 0.001) but also in VM (P < 0.05) compared with VL and RF. In contrast, GU was higher in RF compared with VL (P < 0.05) but was not significantly different to VM or VI (both P = NS). FFAU within these four muscle regions correlated significantly with BF (r = 0.951, P < 0.05), whereas no significant relationship was observed between GU and BF (r = 0.352, P = NS). Therefore, skeletal muscle FFAU, but not GU, appears to be associated with BF during low-intensity exercise. The present results also indicate considerable regional differences in substrate use within working QF muscle. As such, an important methodological outcome from these results is that one sample from a specific part of the QF muscle does not represent the response in the entire QF muscle group.     

Co-activation and tension-regulating phenomena during isokinetic knee extension in sedentary and highly skilled humans

The aim of this study was to examine isokinetic torque produced by highly skilled (HS) and sedentary (S) human subjects, during knee extension, during maximal voluntary and superimposed electrical activation. To verify the level of activation of agonist (vastus lateralis, VL, and vastus medialis, VM) and antagonist muscles (semi-tendineous, ST), during maximal voluntary activation, their myo-electrical activities were detected and quantified as root mean square (rms) amplitude. Ten HS and ten S subjects performed voluntary and superimposed isometric actions and isokinetic knee extensions at 14 angular velocities (from –120 to 300°·s^–1). The rms amplitude of each muscle was normalized with respect to its rms amplitude when acting as agonist at 15°·s^–1. Whatever the angular velocity considered, peals torque and constant angular torque at 65° HS were significantly higher (P < 0.05) than those of S. Eccentric superimposed torque of S, but not HS, was significantly higher (P < 0.05) than voluntary torque at –120, –90, –60 and –30°·s^–1 angular velocities. For a given velocity, the rms amplitude of VL and VM were significantly lower (P < 0.05), during eccentric than during concentric actions, in S, but not in HS. However, whatever the angular velocity, ST co-activation in HS was significantly lower (P < 0.05) than in S. We concluded that co-activation phenomenon could partly explain differences in isokinetic performances. Differences between voluntary and superimposed eccentric torques as well as lower agonist rms amplitude during eccentric action in S, support the possibility of the presence of a tension-regulating mechanism in sedentary subjects.     

Relationships between postcompetition blood lactate concentration and average running velocity over 100-m and 200-m races

The relationships between anaerobic glycolysis and average velocity () sustained during sprint running were studied in 12 national level male sprinters. A blood sample was obtained within 3 min of the completion of semi-finals and finals in the 100-m and 200-m Cameroon national championships and blood lactate concentration ([la^–]_b) was measured. The 35-m times were video-recorded. The 100-m and 200-m [la^–]_b were 8.5 (SD 0.8) and 10.3 (SD 0.8) mmol·l^–1, respectively. These were not correlated with the performances. Over 200 m [la^–]_b was correlated with the sustained over the last 165 m (r=0.65,P<0.05). In the 9 athletes who participated in both the 100-m and 200-m races, the difference between the [la^–]_b measured at the end of the two races was negatively correlated to the difference in v sustained over the two races (r=0.76,P>0.02). Energy expenditure during sprint running was estimated from the [la^–]_b values. This estimate was mainly based on the assumption that a 1 mmol·l^–1 increase in [la^–]_b corresponds to the energy produced by the utilization of 3.30 ml O₂·kg^–1. The energy cost of running was estimated at 0.275 (SD 0.02) ml O₂·kg^–1·m^–1 over 200-m and 0.433 (SD 0.03) ml O₂·kg^–1·m^–1 over 100-m races. These results would suggest that at the velocities studied anaerobic glycolysis contributes to at least 55% of the energy expenditure related to sprint running. However, the influence of both mechanical factors and the contribution of other energy processes obscure the relationship between [la^–]_b and performance.     

Skeletal muscle buffering capacity is higher in the superficial vastus than in the soleus of spontaneously running rats

Skeletal muscle buffering capacity (βm_titr) was determined in soleus (type I) and superficial vastus (type II) muscles of 16 Long–Evans rats with differing levels of spontaneous activity and in 11 sedentary control rats. βm_titr was 24% higher (P<0.001) in superficial vastus muscle than in soleus muscle (268±50 vs. 216±30 μmol H⁺ g muscle dry wt^-1 pH unit^-1) (mean±SD). There was no relationship between βm_titr and mean weekly running distance amongst spontaneously running rats, nor was βm_titr any greater in these rats than in a group of sedentary control rats. Protein to wet wt ratio was 31% higher (P<0.0001) in the superficial vastus muscle when compared with soleus muscle (22.04±3.74 vs. 16.77±3.00 mg protein, 100 mg wet wt muscle^-1), but there was no relationship between protein to wet wt ratio and running distance. Initial muscle homogenate pH (pH_i) was lower in superficial vastus muscle compared with soleus muscle (6.36±0.25 vs. 6.63±0.16). Running rats had a significantly lower pH_i in both soleus and superficial vastus than sedentary controls. There was an exponential relationship between weekly running distance and pH_i in both the superficial vastus muscle (r=-0.86, P<0.001) and the soleus muscle (r=-0.73, P<0.01). Citrate synthase activity correlated with weekly running distance in superficial vastus muscle (r=0.66, P<0.01) but not in soleus muscle. The results confirm a higher βm_titr in the type II superficial vastus muscle when compared with the predominantly type I soleus muscle. We suggest that this may be partly the result of a higher protein concentration in type II muscle. Future studies measuring βm_titr in mixed muscle (e.g. human vastus lateralis) should report fibre type composition.     

Cardiac output during exercise in paraplegic subjects

Summary The purpose of this study was to measure the cardiac output using the CO₂ rebreathing method during submaximal and maximal arm cranking exercise in six male paraplegic subjects with a high level of spinal cord injury (HP). They were compared with eight able bodied subjects (AB) who were not trained in arm exercise. Maximal O₂ consumption ( O_2max) was lower in HP (1.1 1·min^–1, SD 0.1; 17.5 ml·min^–·kg, SD 4) than in AB (2.5 1·min^–1, SD 0.6; 36.7 ml·min^–1·kg, SD 10.7). Maximal cardiac output was similar in the groups (HP, 141·min^–1 SD 2.6; AB, 16.81·min^–1 SD 4). The same result was obtained for maximal heart rate (f _c,max (HP, 175 beats·min^–1, SD 18; AB, 187 beats·min^–, SD 16) and the maximal stroke volume (HP, 82 ml, SD 13; AB, 91 ml, SD 27). The slopes of the relationshipf _c/ O₂ were higher in HP than AB (P<0.025) but when expressed as a % O_2max there were no differences. The results suggests a major alteration of oxygen transport capacity to active muscle mass in paraplegics due to changes in vasomotor regulation below the level of the lesion.