Does autonomic blockade reveal a potent contribution of nitric oxide to locomotion-induced vasodilation?

We sought to test the role of nitric oxide (NO) in governing skeletal muscle (iliac) vascular conductance during treadmill locomotion in dogs (n = 6; 3.2 and 6.4 km/h at 0% grade, and 6.4 km/h at 10% grade). As seen previously, the increase in muscle vascular conductance accompanying treadmill locomotion was little influenced by NO synthase inhibition alone with N(omega)-nitro-L-arginine methyl ester (L-NAME, 10 mg/kg iv), but the absolute value of conductance achieved during locomotion was reduced. Such ambiguous results provide an unclear picture regarding the importance of NO during locomotion. However, muscle vasodilation is normally restrained by the sympathetic system during locomotion. Thus a significant contribution by NO to the increase in vascular conductance that accompanies locomotion could be masked by partial withdrawal of the competing influence of sympathetic vasoconstrictor nerve activity secondary to the rise in arterial pressure following systemic L-NAME administration. To test this possibility, we compared the rise in muscle vascular conductance before and after L-NAME treatment while ganglionic transmission was blocked by hexamethonium. Under these conditions, L-NAME significantly reduced both the rise in vascular conductance (by 32%, P < 0.001) and the absolute level of vascular conductance (by 30%, P < 0.001) achieved during locomotion with no effect on blood flow. Thus augmented NO production normally provides a significant drive to relax vascular smooth muscle in active skeletal muscle during locomotion. Potential deficits stemming from the absence of NO following L-NAME treatment are masked by less intense sympathetic restraint when autonomic function is intact.     

Muscle pump function during locomotion: mechanical coupling of stride frequency and muscle blood flow

We imposed opposing oscillations in treadmill speed and grade on nine rats to test for direct mechanical coupling between stride frequency and hindlimb blood flow. Resting hindlimb blood flow was 15.5 +/- 1.7 ml/min. For 90 s at 7.5 m/min, rats alternated walking at -10 degrees for 10 s and +10 degrees for 10 s. This elicited oscillations in hindlimb blood flow having an amplitude of 4.1 +/- 0.5 ml/min (18% of mean flow) with a delay presumably due to metabolic vasodilation. Similar oscillations in speed (5.5-9.5 m/min) elicited oscillations in hindlimb blood flow (amplitude 3.4 +/- 0.5 ml/min, 15% of mean flow) with less of a delay, possibly due to changes in vasodilation and muscle pump function. We then simultaneously imposed these speed and grade oscillations out of phase (slow uphill, fast downhill). The rationale was that the oscillations in vasodilation evoked by the opposing oscillations in speed and grade would cancel each other, thereby testing the degree to which stride frequency affects hindlimb blood flow directly (i.e., muscle pumping). Opposing oscillations in speed and grade evoked oscillations in hindlimb blood flow having an amplitude of 3.3 +/- 0.6 ml/min (16% of mean flow) with no delay and directly in phase with the changes in speed and stride frequency. The finding that hindlimb blood flow changes directly with speed (when vasodilation caused by changes in speed and grade oppose each other) indicates that there is a direct coupling of stride frequency and hindlimb blood flow (i.e., muscle pumping).     

Delay of muscle vasodilation to changes in work rate (treadmill grade) during locomotion.

Previous studies examining the delay to the onset of vasodilation have primarily focused on the onset of exercise, a setting complicated by the fact that the muscle pump and the vasodilator systems are both activated, making it difficult to attribute changes in blood flow to one or both. The goal here was to determine the delay to the onset of vasodilation after changes in work rate imposed by changes in treadmill grade (work intensity) during locomotion at a steady speed. The rationale was that constant speed would help ensure constant muscle pump activity (contraction frequency) such that vasodilator responses could be examined in isolation. Seven Sprague-Dawley rats underwent three trials each in which treadmill incline was suddenly ( approximately 1 s) elevated from -10 degrees to +10 degrees. The delay to the onset of vasodilation averaged 5.0 +/- 1.8 s, and this delay was not altered by inhibition of nitric oxide synthase. Similar or longer delays were seen during sinusoidal exercise. Thus there is a significant delay before the onset of vasodilation after an increase in work intensity (muscle force) during locomotory exercise at constant speed.     

Neck muscle fatigue and spatial orientation during stepping in place in humans.

Neck proprioceptive input, as elicited by muscle vibration, can produce destabilizing effects on stance and locomotion. Neck muscle fatigue produces destabilizing effects on stance, too. Our aim was to assess whether neck muscle fatigue can also perturb the orientation in space during a walking task. Direction and amplitude of the path covered during stepping in place were measured in 10 blindfolded subjects, who performed five 30-s stepping trials before and after a 5-min period of isometric dorsal neck muscle contraction against a load. Neck muscle electromyogram amplitude and median frequency during the head extensor effort were used to compute a fatigue index. Head and body kinematics were recorded by an optoelectronic system, and stepping cadence was measured by sensorized insoles. Before the contraction period, subjects normally stepped on the spot or drifted forward. After contraction, some subjects reproduced the same behavior, whereas others reduced their forward progression or even stepped backward. The former subjects showed minimal signs of fatigue and the latter ones marked signs of fatigue, as quantified by the dorsal neck electromyogram index. Head position and cadence were unaffected in either group of subjects. We argue that the abnormal fatigue-induced afferent input originating in the receptors transducing the neck muscle metabolic state can modulate the egocentric spatial reference frame. Notably, the effects of neck muscle fatigue on orientation are opposite to those produced by neck proprioception. The neck represents a complex source of inputs capable of modifying our orientation in space during a locomotor task.     

Coupling of cardiac and locomotor rhythms

The pressure within exercising skeletal muscle rises and falls rhythmically during normal human locomotion, the peak pressure reaching levels that intermittently impede blood flow to the exercising muscle. Speculating that a reciprocal relationship between the timing of peak intramuscular and pulsatile arterial pressures should optimize blood flow through muscle and minimize cardiac load, we tested the hypothesis that heart rate becomes entrained with walking and running cadence at some locomotion speeds, by means of electrocardiography and an accelerometer to provide signals reflecting heart rate and cadence, respectively. In 18 of 25 subjects, 1:1 coupling of heart and step rates was present at one or more speeds on a motorized treadmill, generally at moderate to high exercise intensities. To determine how exercise specific this phenomenon is, and to refute the competing hypothesis that coupling is due to vertical accelerations of the heart during locomotion, we had 12 other subjects cycle on an electronically braked bicycle ergometer. Coupling was found between heart rate and pedaling frequency in 10 of them. Cardiac-locomotor coupling appears to be a normal physiological phenomenon, and its identification provides a fresh perspective from which to study endurance.     

Tension relaxation after stretch in resting mammalian muscle fibers: stretch activation at physiological temperatures.

Tension responses to ramp stretches of 1-3% Lo (fiber length) in amplitude were examined in resting muscle fibers of the rat at temperatures ranging from 10 degrees C to 36 degrees C. Experiments were done using bundles of approximately 10 intact fibers isolated from the extensor digitorum longus (a fast muscle) and the soleus (a slow muscle). At low temperatures (below approximately 20 degrees C), the tension response consisted of an initial rise to a peak during the ramp followed by a complex tension decay to a plateau level; the tension decay occurred at approximately constant sarcomere length. The tension decay after a standard stretch at approximately 3-4.Lo/s contained a fast, an intermediate, and a (small amplitude) slow component, which at 10 degrees C (sarcomere length approximately 2.5 microns) were approximately 2000.s-1, approximately 150.s-1, and approximately 25.s-1 for fast fibers and approximately 2000.s-1, approximately 70.s-1 and approximately 8.s-1 for slow fibers, respectively. The fast component may represent the decay of interfilamentary viscous resistance, and the intermediate component may be due to viscoelasticity in the gap (titin, connectin) filament. The two- to threefold fast-slow muscle difference in the rate of passive tension relaxation (in the intermediate and the slow components) compares with previously reported differences in the speed of their active contractions; this suggests that "passive viscoelasticity" is appropriately matched to contraction speed in different muscle fiber types. At approximately 35 degrees C, the fast and intermediate components of tension relaxation were followed by a delayed tension rise at approximately 10.s-1 (fast fibers) and 2.5.s-1 (slow fibers); the delayed tension rise was accompanied by sarcomere shortening. BDM (5-10 mM) reduced the active twitch and tetanic tension responses and the delayed tension rise at 35 degrees C; the results indicate stretch sensitive activation in mammalian sarcomeres at physiological temperatures.     

Frequency-dependent power output and skeletal muscle design

Cyclically contracting muscles provide power for a variety of processes including locomotion, pumping blood, respiration, and sound production. In the current study, we apply a computational model derived from force–velocity relationships to explore how sustained power output is systematically affected by shortening velocity, operational frequency, and strain amplitude. Our results demonstrate that patterns of frequency dependent power output are based on a precise balance between a muscle's intrinsic shortening velocity and strain amplitude. We discuss the implications of this constraint for skeletal muscle design, and then explore implications for physiological processes based on cyclical muscle contraction. One such process is animal locomotion, where musculoskeletal systems make use of resonant properties to reduce the amount of metabolic energy used for running, swimming, or flying. We propose that skeletal muscle phenotype is tuned to this operational frequency, since each muscle has a limited range of frequencies at which power can be produced efficiently. This principle also has important implications for our understanding muscle plasticity, because skeletal muscles are capable of altering their active contractile properties in response to a number of different stimuli. We discuss the possibility that muscles are dynamically tuned to match the resonant properties of the entire musculoskeletal system.     

The physiological basis of slow locomotion in chamaeleons

The African chamaeleon, Chamaeleo senegalensis, will not move faster than approximately 0.1 m/second at 23 degrees C, whereas the lizard Agama agama, like most lizards its size, runs at speeds more than 10X as fast. To account for this difference, we measured various physiological parameters of the iliofibularis muscle of both lizards. The maximum speed of tetanic contraction of unloaded Chamaeleo muscle was half as fast as that of Agama muscle (2.5 vs. 5.8 resting lengths per second). Heavily loaded Chamaeleo iliofibularis contracted at nearly 1/4 the speed of Agama muscle. Time to peak isometric twitch tension and time to half relaxation were twice as long in Chamaeleo as in Agama (122 vs. 58 msec, and 168 vs. 81 msec). Much more of the Chamaeleo muscle consisted of tonic muscle fibers, and the Chamaeleo muscle, compared to Agama muscle, showed physiological evidence of having a significant amount of tonic fibers (potassium contracture and high tetanus to twitch ratios). Finally, the myofibrillar ATPase activity of the Chamaeleo muscle was 1/3 that of Agama muscle. Thus, these results show that the slow locomotion of old world chamaeleons can, in part, be explained by the physiology, biochemistry, and fiber-type distribution of their muscles.     

The muscle twitch and the maximum swimming speed of giant bluefin tuna, Thunnus thynnus L.

Sustained swimming of bluefin tuna was analysed from video recordings made of a captive patrolling fish school [lengths (L) 1.7–3.3 m, body mass (M) 54–433 kg]. Speeds ranged from 0.6 to 1.2 L s⁻¹ (86–260 km day⁻¹) while stride length during steady speed swimming varied between 0.54 and 0.93 L. Maximum swimming speed was estimated by measuring twitch contraction of the anaerobic swimming muscle in pithed fish 5 min after death. Muscle contraction time increased from the shortest just behind the head (30–50 ms at 20% L) to the longest at the tail peduncle (80–90 ms at 80% L) (all at 28°C). A fish (L = 2.26 m) with a muscle contraction time of 50 ms at 25% L can have a maximum tail beat frequency of 10 Hz and maximum swimming speed of 15m s⁻¹ (54km h⁻¹) with a stride length of 0.65L. With a stride length of 1 L a speed of 22.6 m s⁻¹ (81.4 km h⁻¹) is possible. Power used at maximum speed was estimated for this fish at between 10 and 40 kW, with corresponding values for the drag coefficient at a Reynolds number of 4.43 × 10⁷ of 0.0007 and 0.0027.     

Changes in surface EMG parameters during static and dynamic fatiguing contractions.

The effect of contraction types on muscle fiber conduction velocity (MFCV), median frequency (MDF) and mean amplitude (AMP) of surface electromyography was examined in the vastus lateralis of 19 healthy male adults. The subjects performed knee extension both statically and dynamically until they were exhausted. The static contraction was a sustained isometric extension of the knee at a joint angle of 90 degrees with 50% of the maximum voluntary contraction (MVC) load. The dynamic contraction was a repetitive isotonic extension of the knee between the angles of 90 degrees and 180 degrees with the same 50% MVC load at a frequency of 10 times per minute. MFVC during the static contraction significantly decreased during the exercise (p < 0.01). On the other hand, MFVC during the dynamic contraction did not significantly change throughout the exercise. MDF decreased and AMP increased during both types of contractions (p < 0.01). Because the blood flow within the muscle is maintained during the dynamic contraction by enhanced venous return from the contracting muscle, these results suggested that MFVC is affected by the metabolic state in the muscle and the changes in MDF cannot be explained only by that of MFVC.     

Force-velocity relations and fiber composition in human knee extensor muscles.

Standardized measurements of dynamic strength of the kneee extensor muscles were performed in 25 healthy male subjects (17-37 yr) by means of isokinetic contractions, i.e., knee extensions with constant angular velocities. Overall variation between double determinations of maximal torque throughout the 90 degrees arc of motion (0 degrees = fully extended leg) averaged 10% for the different constant velocities chosen. At any given angle of the knee the torque produced was higher for isometric than for dynamic contractions. Dynamic torque decreased gradually with increased speed of shortening. Peak dynamic torque was reached at knee angles in the range: 55-66 degrees, with a displacement toward smaller knee angles with higher angular velocities. Correlations were demonstrated between peak torque produced at the highest speed of muscle shortening and percent as well as relative area of fast twitch fibers in the contracting muscle. In addition muscles with a high percentage of fast twitch fibers had the highest maximal contraction speeds. These observations on intact human skeletal muscle are consistent with earlier findings in animal skeletal muscle preparations.     

Twitch potentiation is greater after a fatiguing submaximal isometric contraction performed at short vs. long quadriceps muscle length.

Endurance time of a submaximal sustained contraction is longer when the muscle is fatigued in a shortened position. The aim of the present study was to compare central and peripheral mechanisms of fatigue after an isometric contraction of the knee extensor muscles performed at 20% maximal voluntary contraction (MVC) at two knee angles (35 degrees , short length vs. 75 degrees , long length; 0 degrees = full extension) until exhaustion. Eleven men (24 +/- 4 yr) attended two experimental randomized sessions. Endurance time was greater at 35 degrees compared with 75 degrees (974 +/- 457 vs. 398 +/- 144 s; P < 0.001) despite a similar reduction in knee extensor MVC (-28.4 +/- 16.0%, P < 0.001 vs. -27.6 +/- 18.8%, P < 0.001, respectively). Voluntary activation level was similarly depressed after the fatiguing contraction performed at the two muscle lengths (-19 +/- 16.7% at 35 degrees , P < 0.01 vs. -13.7 +/- 14.5% at 75 degrees , P < 0.01). After the fatiguing contraction, peak twitch potentiation was observed only at the short length (+31.8 +/- 17.6% at 35 degrees , P < 0.01 vs. +6.4 +/- 21.3% at 75 degrees , P > 0.05), whereas M-wave properties were similarly altered for the two angles. These results suggest that 1) central fatigue at task failure for a sustained isometric contraction was not dependent on the muscle length, and 2) the longer endurance time of a sustained isometric contraction performed at a shortened length is related to potentiation. It is suggested that the greater endurance time of a sustained isometric contraction observed at 35 degrees is related to the occurrence of potentiation at this short length, because central fatigue is similar at task failure for both tasks.     

Timbal muscle physiology in the endothermic cicada Tibicen winnemanna (Homoptera: Cicadidae).

The operative muscle temperature and contraction kinetics are described for the endothermic cicada Tibicen winnemanna (Davis). Measurements of timbal muscle temperature in the field demonstrate that timbal muscle temperature is elevated above ambient temperature during activity. Timbal muscle temperature increases as the acoustic output progresses from a 'warm-up' buzz (27.9-29.7 degrees C) to full song production (36.3-39.5 degrees C). Mean muscle-ambient temperature difference increases from 5.8 degrees C in buzzing animals to 13.1 degrees C in animals producing a full calling song. Twitch rise time and onset to 50% relaxation time decrease while tension production increases with increasing temperature to 40 degrees C. Mean force production at 20 degrees C was determined to be 0.22+/-0.06 N/cm(2). The characteristics of T. winnemanna muscle are similar to those reported in other cicada species. Analysis of the songs produced by animals with one timbal destroyed and the sounds produced by mechanical manipulation suggest that the timbals alternately contract and relax in the sequence IN(1)-IN(2)-OUT(1)-OUT(2). Measurements of the inferred period of muscle contraction from song recordings and the contraction kinetics of isolated timbal muscles measured in the laboratory show that the temperature of the timbal muscle must be elevated in order for the cicada to be able to produce the calling song.     

Increased rate of force development and neural drive of human skeletal muscle following resistance training.

The maximal rate of rise in muscle force [rate of force development (RFD)] has important functional consequences as it determines the force that can be generated in the early phase of muscle contraction (0-200 ms). The present study examined the effect of resistance training on contractile RFD and efferent motor outflow ("neural drive") during maximal muscle contraction. Contractile RFD (slope of force-time curve), impulse (time-integrated force), electromyography (EMG) signal amplitude (mean average voltage), and rate of EMG rise (slope of EMG-time curve) were determined (1-kHz sampling rate) during maximal isometric muscle contraction (quadriceps femoris) in 15 male subjects before and after 14 wk of heavy-resistance strength training (38 sessions). Maximal isometric muscle strength [maximal voluntary contraction (MVC)] increased from 291.1 +/- 9.8 to 339.0 +/- 10.2 N. m after training. Contractile RFD determined within time intervals of 30, 50, 100, and 200 ms relative to onset of contraction increased from 1,601 +/- 117 to 2,020 +/- 119 (P < 0.05), 1,802 +/- 121 to 2,201 +/- 106 (P < 0.01), 1,543 +/- 83 to 1,806 +/- 69 (P < 0.01), and 1,141 +/- 45 to 1,363 +/- 44 N. m. s(-1) (P < 0.01), respectively. Corresponding increases were observed in contractile impulse (P < 0.01-0.05). When normalized relative to MVC, contractile RFD increased 15% after training (at zero to one-sixth MVC; P < 0.05). Furthermore, muscle EMG increased (P < 0.01-0.05) 22-143% (mean average voltage) and 41-106% (rate of EMG rise) in the early contraction phase (0-200 ms). In conclusion, increases in explosive muscle strength (contractile RFD and impulse) were observed after heavy-resistance strength training. These findings could be explained by an enhanced neural drive, as evidenced by marked increases in EMG signal amplitude and rate of EMG rise in the early phase of muscle contraction.     

Marching to the beat of the same drummer: the spontaneous tempo of human locomotion.

Laboratory studies have suggested that the preferred cadence of walking is approximately 120 steps/min, and the vertical acceleration of the head exhibits a dominant peak at this step frequency (2 Hz). These studies have been limited to short periods of walking along a predetermined path or on a treadmill, and whether such a highly tuned frequency of movement can be generalized to all forms of locomotion in a natural setting is unknown. The aim of this study was to determine whether humans exhibit a preferred cadence during extended periods of uninhibited locomotor activity and whether this step frequency is consistent with that observed in laboratory studies. Head linear acceleration was measured over a 10-h period in 20 subjects during the course of a day, which encompassed a broad range of locomotor (walking, running, cycling) and nonlocomotor (working at a desk, driving a car, riding a bus or subway) activities. Here we show a highly tuned resonant frequency of human locomotion at 2 Hz (SD 0.13) with no evidence of correlation with gender, age, height, weight, or body mass index. This frequency did not differ significantly from the preferred step frequency observed in the seminal laboratory study of Murray et al. (Murray MP, Drought AB, and Kory RC. J Bone Joint Surg 46A: 335-360, 1964). [1.95 Hz (SD 0.19)]. On the basis of the frequency characteristics of otolith-spinal reflexes, which drive lower body movement via the lateral vestibulospinal tract, and otolith-mediated collic and ocular reflexes that maintain gaze when walking, we speculate that this spontaneous tempo of locomotion represents some form of central "resonant frequency" of human movement.     

Effect of contraction frequency on leg blood flow during knee extension exercise in humans.

Previous studies in isolated muscle preparations have shown that muscle blood flow becomes compromised at higher contraction frequencies. The purpose of this study was to examine the effect of increases in contraction frequency and muscle tension on mean blood flow (MBF) during voluntary exercise in humans. Nine male subjects [23.6 +/- 3.7 (SD) yr] performed incremental knee extension exercise to exhaustion in the supine position at three contraction frequencies [40, 60, and 80 contractions/min (cpm)]. Mean blood velocity of the femoral artery was determined beat by beat using Doppler ultrasound. MBF was calculated by using the diameter of the femoral artery determined at rest using echo Doppler ultrasound. The work rate (WR) achieved at exhaustion was decreased (P < 0.05) as contraction frequency increased (40 cpm, 16.2 +/- 1.4 W; 60 cpm, 14.8 +/- 1.4 W; 80 cpm, 13.2 +/- 1.3 W). MBF was similar across the contraction frequencies at rest and during the first WR stage but was higher (P < 0.05) at 40 than 80 cpm at exercise intensities >5 W. MBF was similar among contraction frequencies at exhaustion. In humans performing knee extension exercise in the supine position, muscle contraction frequency and/or muscle tension development may appreciably affect both the MBF and the amplitude of the contraction-to-contraction oscillations in muscle blood flow.     

Muscle mechanical advantage of human walking and running: implications for energy cost.

Muscular forces generated during locomotion depend on an animal's speed, gait, and size and underlie the energy demand to power locomotion. Changes in limb posture affect muscle forces by altering the mechanical advantage of the ground reaction force (R) and therefore the effective mechanical advantage (EMA = r/R, where r is the muscle mechanical advantage) for muscle force production. We used inverse dynamics based on force plate and kinematic recordings of humans as they walked and ran at steady speeds to examine how changes in muscle EMA affect muscle force-generating requirements at these gaits. We found a 68% decrease in knee extensor EMA when humans changed gait from a walk to a run compared with an 18% increase in hip extensor EMA and a 23% increase in ankle extensor EMA. Whereas the knee joint was extended (154-176 degrees) during much of the support phase of walking, its flexed position (134-164 degrees) during running resulted in a 5.2-fold increase in quadriceps impulse (time-integrated force during stance) needed to support body weight on the ground. This increase was associated with a 4.9-fold increase in the ground reaction force moment about the knee. In contrast, extensor impulse decreased 37% (P < 0.05) at the hip and did not change at the ankle when subjects switched from a walk to a run. We conclude that the decrease in limb mechanical advantage (mean limb extensor EMA) and increase in knee extensor impulse during running likely contribute to the higher metabolic cost of transport in running than in walking. The low mechanical advantage in running humans may also explain previous observations of a greater metabolic cost of transport for running humans compared with trotting and galloping quadrupeds of similar size.     

Thermal dependence of isotonic contractile properties of skeletal muscle and sprint performance of the lizardDipsosaurus dorsalis

Contractile properties of the fast-twitch glycolytic (FG) portion of the iliofibularis muscle and sprint running performance were studied at approximately 5 degrees C intervals from 15-44 degrees C in the lizard Dipsosaurus dorsalis. Maximal running velocity (VR) and stride frequency (f) were both greatest when body temperature (Tb) was 40 degrees C, the field-active Tb in Dipsosaurus. At 40 degrees C VR was 4.3 +/- 0.2 m/s and f was 13.5 +/- 0.5 s-1. Between 25 and 40 degrees C, the thermal dependencies of VR and f were approximately constant (Q10's of 1.31 and 1.36 got VR and f, respectively). Below 25 degrees C performance declined more markedly with decreasing temperature. At 20 degrees C strides were qualitatively normal, but VR was only half of the value at 25 degrees C. At 15 degrees C the lizards were substantially incapacitated, and VR was 10% of the value at 20 degrees C. Stride length was approximately 0.33 m and changed very little with Tb from 20-44 degrees C. The time dependent contractile properties of FG muscle were affected more by temperature than was sprint performance. The maximal velocity of shortening at zero load (VO) was 18.7 0/s at 40 degrees C and had a Q10 of 1.7 from 25-40 degrees C. Maximal power output (Wmax) determined from the force-velocity curve was 464 W/kg at 40 degrees C. Below 40 degrees C max varied with temperature with a Q10 of 2-3. The shape of the force-velocity curve changed little with temperature (Wmax/POVO = 0.11). Between 25 and 40 degrees C a relatively temperature-independent process must modulate the effects of temperature on the contractile properties of the muscles that supply the power for burst locomotion. Storage and recovery of elastic energy appears to be a likely candidate for such a process. Below 25 degrees C, however, the contraction time is prolonged to such an extent that the f attainable is limited by the minimum time taken to contract and relax the muscles.     

Low-intensity training produces muscle adaptations in rats with femoral artery stenosis.

The effectiveness of a mild-intensity exercise program to induce adaptations within skeletal muscle of animals with peripheral arterial insufficiency was evaluated using an isolated perfused hindlimb preparation at a muscle blood flow similar to the peak found in vivo. Adult rats were subjected to bilateral femoral artery stenosis sufficient to limit peak blood flow during exercise but not alter resting blood flow. Stenosed-trained (Sten-Trained) rats walked on a treadmill at an easily achieved speed (20 m/min with a 15% grade) 5 days wk. Exercise tolerance improved from 10 min initially to 2 h/day. Non-stenosed-sedentary (Non-Sten-Sed) and stenosed-sedentary (Sten-Sed) animals were limited to cage activity. Oxygen delivery to the contracting muscles was similar among groups (7.0 +/- 0.4, 7.3 +/- 0.6, and 6.6 +/- 0.6 mumol.min-1.g-1 in Non-Sten-Sed, Sten-Sed, and Sten-Trained, respectively; n = 13 each). Force development was better maintained by Sten-Trained muscle (P less than 0.001) during a sequence of tetanic contraction conditions. Peak oxygen consumption was greater (P less than 0.05) in the Sten-Trained (5.23 +/- 0.34 mumol.min-1.g-1) than in Non-Sten-Sed (4.08 +/- 0.35) and Sten-Sed (4.34 +/- 0.37) rats. The increased peak oxygen extraction (P less than 0.05) by the muscle of the Sten-Trained rats (82.5 +/- 7.1% of oxygen inflow vs. 58.7 +/- 4.7 and 57.4 +/- 5.0%, respectively) was probably related to the increased muscle capillarity and mitochondrial enzyme activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

MMG and EMG responses of the superficial quadriceps femoris muscles.

Eighteen adults performed isometric muscle actions of the leg extensors at 25, 50, 75, and 100% maximal voluntary contraction (%MVC) at leg flexion angles of 25, 50, and 75 degrees. The results indicated that isometric torque production increased as leg flexion angle increased (75 degrees > 50 degrees > 25 degrees). For each muscle tested (rectus femoris, vastus lateralis, and vastus medialis), the EMG amplitude increased up to 100%MVC at each leg flexion angle (25, 50, and 75 degrees). The MMG amplitude for each muscle, however, increased up to 100%MVC at 25 and 50 degrees of leg flexion, but plateaued from 75 to 100%MVC at 75 degrees of leg flexion. We hypothesize that the varied patterns for the MMG amplitude-isometric torque relationships were due to leg flexion angle differences in: (1) muscle stiffness, (2) intramuscular fluid pressure, or (3) motor unit firing frequency.