Impaired muscle metaboreflex-induced increases in ventricular function in heart failure

We investigated to what extent heart failure alters the ability of the muscle metaboreflex to improve ventricular function. Dogs were chronically instrumented to monitor mean arterial pressure (MAP), cardiac output (CO), heart rate (HR), stroke volume (SV), and central venous pressure (CVP) at rest and during mild treadmill exercise (3.2 km/h) before and during reductions in hindlimb blood flow imposed to activate the muscle metaboreflex. These control experiments were repeated at constant heart rate (ventricular pacing 225 beats/min) and at constant heart rate coupled with a beta-adrenergic blockade (atenolol, 2 mg/kg iv) in normal animals and in the same animals after the induction of heart failure (HF, induced via rapid ventricular pacing). In control experiments in normal animals, metaboreflex activation caused tachycardia with no change in SV, resulting in large increases in CO and MAP. At constant HR, large increases in CO still occurred via significant increases in SV. Inasmuch as CVP did not change in this setting and that beta-adrenergic blockade abolished the reflex increase in SV at constant HR, this increase in SV likely reflects increased ventricular contractility. In contrast, after the induction of HF, much smaller increases in CO occurred with metaboreflex activation because, although increases in HR still occurred, SV decreased thereby limiting any increase in CO. At constant HR, no increase in CO occurred with metaboreflex activation even though CVP increased significantly. After beta-adrenergic blockade, CO and SV decreased with metaboreflex activation. We conclude that in HF, the ability of the muscle metaboreflex to increase ventricular function via both increases in contractility as well as increases in filling pressure are markedly impaired.     

Muscle mechanosensitive reflex is suppressed in the conscious condition: effect of anesthesia.

To test the hypothesis that a muscle mechanosensitive reflex is suppressed in the conscious condition, we examined the effect of anesthesia on the cardiovascular responses to passive mechanical stretch of the hindlimb triceps surae muscle in six conscious cats. The triceps surae muscle was manually stretched for 30 s by extending the hip and knee joints and subsequently by dorsiflexing the ankle joint; the lateral gastrocnemius muscle was lengthened by 19 +/- 2.6 mm. Heart rate (HR) and mean arterial blood pressure (MAP) did not change significantly during passive stretch of the muscle in the conscious condition. At 10-40 min after intravenously administering pentobarbital sodium (20-25 mg/kg), the identical passive stretch of the triceps surae muscle was able to induce the cardiovascular responses; HR and MAP were increased by 14 +/- 1.3 beats/min and 14 +/- 1.4 mmHg, respectively, and the cardiovascular responses were sustained throughout the passive stretch. In contrast, stretching skin on the triceps surae muscle evoked no significant changes in HR and MAP in the anesthetized condition. When anesthesia became light 40-90 min after injection of pentobarbital and the animals started to show spontaneous body movement, the cardiovascular response to passive muscle stretch tended to be blunted again. It is therefore concluded that passive mechanical stretch of skeletal muscle is capable of evoking the reflex cardiovascular response, which is suppressed in the conscious condition but exaggerated by anesthesia.     

Myofascial force transmission between the human soleus and gastrocnemius muscles during passive knee motion

The plantarflexors of the lower limb are often assumed to act as independent actuators, but the validity of this assumption is the subject of considerable debate. This study aims to determine the degree to which passive changes in gastrocnemius muscle length, induced by knee motion, affect the tension in the adjacent soleus muscle. A second aim is to quantify the magnitude of myofascial passive force transmission between gastrocnemius and adjacent soleus. Fifteen healthy volunteers participated. Simultaneous ultrasound images of the gastrocnemius and soleus muscles were obtained during passive knee flexion (0-90°), while keeping the ankle angle fixed at either 70° or 115°. Image correlation analysis was used to quantify muscle fascicle lengths in both muscles. The data show that the soleus muscle fascicles elongate significantly during gastrocnemius shortening. The approximate change in passive soleus force as a result of the observed change in fascicle length was estimated and appears to be <5 N, but this estimate is sensitive to the assumed slack length of soleus.     

Passive resistance of the human knee: The effect of immobilization

The effects of immobilization on both the circumferential dimensions of the lower limb, and the passive resistance of the knee to sinusoidal motion in the flexion-extension plane, were studied in patients immobilized after tibial fractures or ligamentous injuries of the knee. Immobilization resulted in a decrease of circumferential dimensions of the lower limb, indicating atrophy of thigh and calf musculature. The equilibrium angle of the knee, i.e. the angle at which the net passive moment equals zero, approached the angle of the knee during immobilization. At knee angles of 60° (the maximal angle imposed on the immobilized knee) and 45°, the resistance was increased, and at angles of 30° and 15°, the resistance was decreased. The passive resistances at ± 22.5, 15 and 7.5° relative to the equilibrium angle were all increased. A simple model, taking into account changes in the length of muscles at the anterior and posterior side of the knee, explains changes in passive resistance. The duration of immobilization for patients with ligamentous injuries of the knee was less than 50% of that for patients with tibial fractures, whose ankle joint was not immobilized, both resulted in a smaller muscular atrophy for patients with ligamentous injuries. Differences in passive resistance between unaffected and immobilized legs were the same in both groups of patients. Changes in passive resistance in the immobilized leg for those with ligamentous injury are probably not the effect of immobilization alone but the combined effect of immobilization and ligamentous lesions.     

In vivo changes in the human patellar tendon moment arm length with different modes and intensities of muscle contraction

The purpose of this study was to examine the effect of different muscle contraction modes and intensities on patellar tendon moment arm length (d(PT)). Five men performed isokinetic concentric, eccentric and passive knee extensions at an angular velocity of 60 deg/s and six men performed gradually increasing to maximum effort isometric muscle contractions at 90( composite function) and 20( composite function) of knee flexion. During the tests, lateral X-ray fluoroscopy imaging was used to scan the knee joint. The d(PT) differences between the passive state and the isokinetic concentric and extension were quantified at 15( composite function) intervals of knee joint flexion angle. Furthermore, the changes of the d(PT) as a function of the isometric muscle contraction intensities were determined during the isometric knee extension at 90( composite function) and 20( composite function) of knee joint flexion. Muscle contraction-induced changes in knee joint flexion angle during the isometric muscle contraction were also taken into account for the d(PT) measurements. During the two isometric knee extensions, d(PT) increased from rest to maximum voluntary muscle contraction (MVC) by 14-15%. However, when changes in knee joint flexion angle induced by the muscle contraction were taken into account, d(PT) during MVC increased by 6-26% compared with rest. Moreover, d(PT) increased during concentric and eccentric knee extension by 3-15%, depending on knee flexion angle, compared with passive knee extension. These findings have important implications for estimating musculoskeletal loads using modelling under static and dynamic conditions.     

Control of cutaneous vascular conductance and sweating during recovery from dynamic exercise in humans.

The purpose of the study was to examine the effect of 1) passive (assisted pedaling), 2) active (loadless pedaling), and 3) inactive (motionless) recovery modes on mean arterial pressure (MAP), skin blood flow (SkBF), and sweating during recovery after 15 min of dynamic exercise. It was hypothesized that an active recovery mode would be most effective in attenuating the fall in MAP, SkBF, and sweating during exercise recovery. Six male subjects performed 15 min of cycle ergometer exercise at 70% of their predetermined peak oxygen consumption followed by 15 min of 1) active, 2) passive, or 3) inactive recovery. Mean skin temperature (T(sk)), esophageal temperature (T(es)), SkBF, sweating, cardiac output (CO), stroke volume (SV), heart rate (HR), total peripheral resistance (TPR), and MAP were recorded at baseline, end exercise, and 2, 5, 8, 12, and 15 min postexercise. Cutaneous vascular conductance (CVC) was calculated as the ratio of laser-Doppler blood flow to MAP. In the active and passive recovery modes, CVC, sweat rate, MAP, CO, and SV remained elevated over inactive values (P < 0.05). The passive mode was equally as effective as the active mode in maintaining CO, SV, MAP, CVC, and sweat rate above inactive recovery. Sweat rate was different among all modes after 8 min of recovery (P < 0.05). TPR during active recovery remained significantly lower than during recovery in the passive and inactive modes (P < 0.05). No differences in either T(es) or T(sk) were observed among conditions. Given that MAP was higher during passive and active recovery modes than during inactive recovery suggests differences in CVC may be due to differences in baroreceptor unloading and not factors attributed to central command. However, differences in sweat rate may be influenced by factors such as central command and mechanoreceptor stimulation.     

Relationships between the extent of apnea-induced bradycardia and the vascular response in the arm and leg during dynamic two-legged knee extension exercise

Our aim was to test the hypothesis that apnea-induced hemodynamic responses during dynamic exercise in humans differ between those who show strong bradycardia and those who show only mild bradycardia. After apnea-induced changes in heart rate (HR) were evaluated during dynamic exercise, 23 healthy subjects were selected and divided into a large response group (L group; n = 11) and a small response group (S group; n = 12). While subjects performed a two-legged dynamic knee extension exercise at a work load that increased HR by 30 beats/min, apnea-induced changes in HR, cardiac output (CO), mean arterial pressure (MAP), arterial O(2) saturation (Sa(O(2))), forearm blood flow (FBF), and leg blood flow (LBF) were measured. During apnea, HR in the L group (54 ± 2 beats/min) was lower than in the S group (92 ± 3 beats/min, P < 0.05). CO, Sa(O(2)), FBF, LBF, forearm vascular conductance (FVC), leg vascular conductance (LVC), and total vascular conductance (TVC) were all reduced, and MAP was increased in both groups, although the changes in CO, TVC, LBF, LVC, and MAP were larger in the L group than in the S group (P < 0.05). Moreover, there were significant positive linear relationships between the reduction in HR and the reductions in TVC, LVC, and FVC. We conclude that individuals who show greater apnea-induced bradycardia during exercise also show greater vasoconstriction in both active and inactive muscle regions.     

Muscle-damaging exercise affects isokinetic torque more at short muscle length

The aim of this study was to investigate the differences in the length-dependent changes in quadriceps muscle torque during voluntary isometric and isokinetic contractions performed after severe muscle-damaging exercise. Thirteen physically active men (age = 23.8 ± 3.2 years, body weight = 77.2 ± 4.5 kg) performed stretch-shortening cycle (SSC) exercise comprising 100 drop jumps with 30-second intervals between each jump. Changes in the voluntary and electrically evoked torque in concentric and isometric conditions at different muscle lengths, muscle soreness, and plasma creatine kinase (CK) activity were assessed within 72 hours after SSC exercise. Isokinetic knee extension torque decreased significantly (p < 0.05) at all joint angles after SSC exercise. At 2 minutes and at 72 hours after SSC exercise, the changes in knee torque were significantly smaller at 80° (where 180° = full knee extension) than at 110-130°. At 2 minutes after SSC exercise, the optimal angle for isokinetic knee extension torque shifted by 9.5 ± 8.9° to a longer muscle length (p < 0.05). Electrically induced torque at low-frequency (20-Hz) stimulation decreased significantly more at a knee joint angle of 130° than at 90°. The subjects felt acute muscle pain and CK activity in the blood increased to 1,593.9 ± 536.2 IU·L?1 within 72 hours after SSC exercise (p < 0.05). This study demonstrates that the effect of muscle-damaging exercise on isokinetic torque is greatest for contractions at short muscle lengths. These findings have practical importance because the movements in most physical activities are dynamic in nature, and the decrease in torque at various points in the range of motion during exercise might affect overall performance.     

A model of semitendinosus muscle sarcomere length, knee and hip joint interaction in the frog hindlimb

The interaction between the semitendinosus muscle and both hip and knee joint angles was examined in the frog (Rana pipiens) hindlimb. Sarcomere length was measured by laser diffraction in passive muscle during hip and knee rotation. A model was then developed to predict semitendinosus sarcomere length as a function of both hip and knee flexion angle. Based on published frog muscle fiber length-tension [Gordon, A. M. et al., J. Physiol. 184, 170-192 (1966)] and force-velocity [Edman, K. A. P., J. Physiol. 291, 143-159 (1979)] properties, and published joint angles during hopping [Calow, L. J. and Alexander, R. McN., J. Zool. (Lond.) 171, 293-321 (1973)], muscle sarcomere length, force and hip and knee torque during a hop were predicted. The semitendinosus muscle generally operated on the descending limb of the length-tension curve at normal joint angle combinations. The model predicted that, during a single coordinated movement, a period of sarcomere shortening (concentric) was followed by a period of sarcomere lengthening (eccentric). Based on calculated torque profiles at the hip and knee joints, this study suggested that the semitendinosus muscle probably functions more as a hip extensor than a knee flexor. In addition, based on the nature of the shortening-lengthening cycle, the semitendinosus may act to mechanically link the force of knee extension to hip extension.     

Knee and ankle joint torque-angle relationships of multi-joint leg extension

The force-length-relation (F-l-r) is an important property of skeletal muscle to characterise its function, whereas for in vivo human muscles, torque-angle relationships (T-a-r) represent the maximum muscular capacity as a function of joint angle. However, since in vivo force/torque-length data is only available for rotational single-joint movements the purpose of the present study was to identify torque-angle-relationships for multi-joint leg extension. Therefore, inverse dynamics served for calculation of ankle and knee joint torques of 18 male subjects when performing maximum voluntary isometric contractions in a seated leg press. Measurements in increments of 10° knee angle from 30° to 100° knee flexion resulted in eight discrete angle configurations of hip, knee and ankle joints. For the knee joint we found an ascending-descending T-a-r with a maximum torque of 289.5° ± 43.3 Nm, which closely matches literature data from rotational knee extension. In comparison to literature we observed a shift of optimum knee angle towards knee extension. In contrast, the T-a-r of the ankle joint vastly differed from relationships obtained for isolated plantar flexion. For the ankle T-a-r derived from multi-joint leg extension subjects operated over different sections of the force-length curve, but the ankle T-a-r derived from isolated joint efforts was over the ascending limb for all subjects. Moreover, mean maximum torque of 234.7 ± 56.6 Nm exceeded maximal strength of isolated plantar flexion (185.7 ± 27.8 Nm). From these findings we conclude that muscle function between isolated and more physiological multi-joint tasks differs. This should be considered for ergonomic and sports optimisation as well as for modelling and simulation of human movement.     

Nonthermoregulatory control of cutaneous vascular conductance and sweating during recovery from dynamic exercise in women.

The purpose of the study was to examine the effect of 1) active (loadless pedaling), 2) passive (assisted pedaling), and 3) inactive (motionless) recovery modes on mean arterial pressure (MAP), cutaneous vascular conductance (CVC), and sweat rate during recovery after 15 min of dynamic exercise in women. It was hypothesized that an active recovery mode would be most effective in attenuating the fall in MAP, CVC, and sweating during exercise recovery. Ten female subjects performed 15 min of cycle ergometer exercise at 70% of their predetermined peak oxygen consumption followed by 20 min of 1) active, 2) passive, or 3) inactive recovery. Mean skin temperature (Tsk), esophageal temperature (Tes), skin blood flow, sweating, cardiac output (CO), stroke volume (SV), heart rate (HR), total peripheral resistance (TPR), and MAP were recorded at baseline, end exercise, and 2, 5, 8, 12, 15, and 20 min postexercise. Cutaneous vascular conductance (CVC) was calculated as the ratio of laser-Doppler blood flow to MAP. In the active recovery mode, CVC, sweat rate, MAP, CO, and SV remained elevated over inactive values (P < 0.05). The passive mode was equally as effective as the active mode in maintaining MAP. Sweat rate was different among all modes after 12 min of recovery (P < 0.05). TPR during active recovery remained significantly lower than during recovery in the inactive mode (P < 0.05). No differences in either Tes or Tsk were observed among conditions. The results indicate that CVC can be modulated by central command and possibly cardiopulmonary baroreceptors in women. However, differences in sweat rate may be influenced by factors such as central command, mechanoreceptor stimulation, or cardiopulmonary baroreceptors.     

Triceps surae muscle-tendon unit length changes as a function of ankle joint angles and contraction levels: the effect of foot arch deformation

The purpose of this study was to clarify how foot deformation affects the relationship between triceps surae muscle-tendon unit (MTU) length and ankle joint angle. For six women and six men a series of sagittal magnetic resonance (MR) images of the right foot were taken, and changes in MTU length (the displacement of the calcaneal tuberosity), foot arch angle, and ankle joint angle were measured. In the passive session, each subject's ankle joint was secured at 10° dorsiflexed position, neutral position (NP), and 10° and 20° plantar flexed positions while MR images were acquired. In the active session, each subject was requested to perform submaximal isometric plantar flexions (30%, 60%, and 80% of voluntary maximum) at NP. The changes in MTU length in each trial were estimated by two different formulae reported previously. The changes of the measured MTU length as a function of ankle joint angles observed in all trials of the active session were significantly (p<0.05) larger than corresponding values in the passive session and by the estimation formulae. In the passive session, MTU length changes were significantly smaller than the estimated values when the ankle was plantar flexed. The foot arch angle increased as the contraction level increased from rest (117 ± 4°) to 80% (125 ± 3°), and decreased as the ankle was positioned further into plantar flexion in the passive session (115 ± 3°). These results indicate that foot deformation profoundly affects the triceps surae MTU length-ankle joint angle relationship during plantar flexion.     

Effects of step length and step frequency on lower-limb muscle function in human gait

The aim of this study was to quantify the effects of step length and step frequency on lower-limb muscle function in walking. Three-dimensional gait data were used in conjunction with musculoskeletal modeling techniques to evaluate muscle function over a range of walking speeds using prescribed combinations of step length and step frequency. The body was modeled as a 10-segment, 21-degree-of-freedom skeleton actuated by 54 muscle-tendon units. Lower-limb muscle forces were calculated using inverse dynamics and static optimization. We found that five muscles – GMAX, GMED, VAS, GAS, and SOL – dominated vertical support and forward progression independent of changes made to either step length or step frequency, and that, overall, changes in step length had a greater influence on lower-limb joint motion, net joint moments and muscle function than step frequency. Peak forces developed by the uniarticular hip and knee extensors, as well as the normalized fiber lengths at which these muscles developed their peak forces, correlated more closely with changes in step length than step frequency. Increasing step length resulted in larger contributions from the hip and knee extensors and smaller contributions from gravitational forces (limb posture) to vertical support. These results provide insight into why older people with weak hip and knee extensors walk more slowly by reducing step length rather than step frequency and also help to identify the key muscle groups that ought to be targeted in exercise programs designed to improve gait biomechanics in older adults.     

In vivo motion transmission in the inactive gastrocnemius medialis muscle-tendon unit during ankle and knee joint rotation.

The purposes of this study were: (a) to quantify the influence of passive ankle and knee joint angular displacement on the estimated mechanical and architectural properties of the gastrocnemius medialis (GM) muscle-tendon unit, and (b) to determine the strain distribution of separate structures (tendon, aponeurosis and fascicle) during passive lengthening of the GM muscle-tendon unit at rest. Ten male subjects participated in the study. The passive ankle and knee joint movements were performed on an isokinetic dynamometer. The kinematics of the left leg were recorded using the Vicon 624 system with 8 cameras. Two ultrasound probes were used to examine the elongation of the tendon, the aponeurosis, the fascicles and the angle of pennation of the GM. To calculate the elongation of the GM muscle-tendon unit the Achilles tendon path was reconstructed using a series of small reflective markers. The results show that the passive ankle joint angular displacement has a considerable influence on the elongation of the tendinous and architectural structures of the GM muscle-tendon unit. In contrast, the influence of knee joint angular displacement on the GM fascicle length and pennation angle becomes relevant only at knee angles greater than 144 degrees . The contribution of the tendon to the elongation of the GM muscle-tendon unit at rest is relevant because of its greater resting length in comparison to the resting length of the GM fascicles. The results indicate the existence of slackness in the inactive GM muscle-tendon unit between 121 degrees and 107 degrees ankle angle and between 65 degrees and 144 degrees knee angle.     

Dynamic time course of hemodynamic responses after passive head-up tilt and tilt back to supine position.

Mechanisms involved in the control of arterial pressure during postural changes were studied by analysis of the dynamic time course of cardiovascular changes during head-up tilt (HUT) and tilt back to supine position (TB). Beat-to-beat values of cardiovascular variables were recorded continuously before, during, and after passive HUT to 30 degrees in seven healthy humans. Left cardiac stroke volume (SV, Doppler ultrasound), mean arterial blood pressure (MAP), heart rate (HR), cardiac output (CO), and total peripheral conductance (TPC) were recorded. During HUT, MAP at the level of the carotid baroreceptors decreased by approximately 5 mmHg. There was a striking asymmetry between the time courses of cardiovascular changes on HUT and on TB. Adjustments generally took up to 30 s after HUT, whereas most changes were completed during the first 10 s after TB. Cardiovascular reflex adjustments of HR and TPC were more symmetrical. After HUT, SV was maintained during the first 4-6 s and then decreased steadily during the next 30 s to a stable level approximately 25% below its pretilt value. However, after TB, SV increased rapidly to its pretilt value in <10 s. This asymmetry in SV dynamics may be explained in part by a more rapid change in left cardiac filling after TB than after HUT. On TB, there must be a rapid inflow of stagnant blood from the legs, whereas venous valves will impede backward filling of veins in the lower body on HUT. In conclusion, we have revealed a characteristic asymmetry in cardiovascular responses to inverse variations in gravity forces in humans. This asymmetry can be explained in part by nonlinear, hydrodynamic factors, such as the one-way effect of venous valves in the lower part of the body.     

Residual force enhancement during voluntary contractions of knee extensors and flexors at short and long muscle lengths

Residual force enhancement (RFE) is a term describing the observation that muscle tension during a contraction that includes a stretch and hold remains above that during an isometric contraction at the hold length. RFE has been observed during in vitro and in vivo experiments, but results involving voluntary contractions are mixed, particularly with respect to large muscles. The purpose of this study was to determine if RFE can be observed in large muscles such as knee extensors and flexors at joint configurations corresponding to the ascending and descending limbs of the muscle force-length curve. Two groups of twenty participants (ten males and ten females per group) performed maximum voluntary contractions on a Biodex machine in purely isometric conditions and in isometric conditions immediately following eccentric stretch. Knee extension trials were performed at 40° (short muscles) and 100° (long muscles) flexion from full extension (0°), and knee flexion trials were performed at 70° (short muscles) and 10° (long muscles) flexion. Stretch-isometric trials terminated at these angles following 30° of eccentric motion at 30°/s. Statistically-significant RFE was observed for both tasks at long-muscle joint configurations, but was not observed for either task at short-muscle joint configurations. Passive torque enhancement was also observed following muscle relaxation at long-muscle joint configurations for both tasks, but for only knee flexion at short-muscle joint configurations. These results reinforce for voluntary contractions of large muscles the RFE behavior observed in smaller muscles, and provide further evidence that RFE occurs primarily on the descending limb of the muscle force-length curve.     

Cardiovascular, metabolic and plasma catecholamine responses to passive and active exercises.

Eight healthy male volunteers (aged 19.6+/-3.0 years) were submitted to the unloaded active (AE) and passive (PE) cycling exercise-tests performed on an adapted cycle ergometer at a pedalling rate of 50 rpm. Intensity of active exercise was about 10% of VO2 max. In the PE exercise test the ergometer was moved electrically. During both tests the systolic time intervals (STI), stroke volume (SV), heart rate (HR), blood pressure (BP), oxygen uptake (VO2), rating of perceived exertion (RPE), electrical muscle activity (EMG), plasma adrenaline (A), noradrenaline (NE) and blood lactate (LA) concentrations were measured. Exercise induced changes in VO2, RPE and EMG were significantly higher during AE than PE. Shortening of the pre-ejection period (PEP) and diminishing of the PEP to ejection time (ET) ratio were similar in both types of exercise, whereas HR increased only during AE. A significant increase in cardiac output (p<0.01) resulted from increased SV (p<0.01) during PE and from increased HR (p <0.01) during AE. MAP increased only during PE and it was higher than at rest and during AE (p<0.01). Absence of changes in SV and MAP during AE may be considered as a secondary effect of the decrease in TPR. Plasma catecholamines did not increase above resting values in either type of exercise. Blood LA concentration increased during both PE and AE but it reached higher values (p<0.01) after the latter test. The present data suggest that the inotropic state depends on the mechanoreflexes originated in skeletal muscles. However, contribution of changes in preload to shortening of PEP can not be excluded.     

In vivo passive mechanical properties of the human gastrocnemius muscle belly

The purpose of the present study was to determine the in vivo passive mechanical properties, including the length below the slack length, of the gastrocnemius muscle (GAS) belly in humans. Transverse ultrasound images of the medial head of the GAS were taken in 11 subjects during passive knee extension from 80 degrees to 5 degrees with a constant ankle joint angle of 10 degrees (0 degrees is the neutral ankle position: positive values for dorsiflexion). The change in passive ankle joint moment (Mp), which is produced only by the GAS length change, was also measured during passive knee extension. The onset of Mp during passive knee extension was found to be 43+/-8 degrees (mean+/-SD) when the baseline of the Mp was set at the average Mp in the range of 55-60 degrees where the Mp was almost constant (SD<0.03 Nm). At this onset, the muscle fascicle length of the GAS (Lf) was 46+/-7 mm (slack length; Lfs). Lf at 80 degrees was 6+/-4 mm (13+/-6%) less than the Lfs, and Lf at 5 degrees was 12+/-5 mm (27+/-11%) greater than the Lfs. The passive force-resisting compression of the GAS did not produce a dorsiflexion moment in the joint angle range adopted. The passive ankle joint moment increased linearly with Lf (coefficient of determination (R2)=0.85-0.96), and the slopes of the relationships between Lf and Mp, and between the relative Lf to Lfs and Mp were 0.093+/-0.038 Nm/mm and 0.043+/-0.021 Nm/%Lfs. The findings of the present study can be implemented in musculoskeletal modeling, which would provide a more accurate evaluation of the passive mechanical properties of muscle during movement.     

A new method for measuring passive length-tension properties of human gastrocnemius muscle in vivo

The study of muscle growth and muscle length adaptations requires measurement of passive length-tension properties of individual muscles, but until now such measurements have only been made in animal muscles. We describe a new method for measuring passive length-tension properties of human gastrocnemius muscles in vivo. Passive ankle torque and ankle angle data were obtained as the ankle was rotated through its full range with the knee in a range of positions. To extract gastrocnemius passive length-tension curves from passive torque-angle data it was assumed that passive ankle torque was the sum of torque due to structures which crossed only the ankle joint (this torque was a 6-parameter function of ankle joint angle) and a torque due to the gastrocnemius muscle (a 3-parameter function of knee and ankle angle). Parameter values were estimated with non-linear regression and used to reconstruct passive length-tension curves of the gastrocnemius. The reliability of the method was examined in 11 subjects by comparing three sets of measurements: two on the same day and the other at least a week later. Length-tension curves were reproducible: the average root mean square error was 5.1+/-1.1 N for pairs of measurements taken within a day and 7.3+/-1.2 N for pairs of measurements taken at least a week apart (about 3% and 6% of maximal passive tension, respectively). Length-tension curves were sensitive to mis-specification of moment arms, but changes in length-tension curves were not. The new method enables reliable measurement of passive length-tension properties of human gastrocnemius in vivo, and is likely to be useful for investigation of changes in length-tension curves over time.     

Peripheral chemoreceptor contributions to sympathetic and cardiovascular responses during hypercapnia

We tested the hypothesis that integrated sympathetic and cardiovascular reflexes are modulated by systemic CO2 differently in hypoxia than in hyperoxia (n = 7). Subjects performed a CO2 rebreathe protocol that equilibrates CO2 partial pressures between arterial and venous blood and that elevates end tidal CO2 (PET(CO2)) from approximately 40 to approximately 58 mmHg. This test was repeated under conditions where end tidal oxygen levels were clamped at 50 (hypoxia) or 200 (hyperoxia) mmHg. Heart rate (HR; EKG), stroke volume (SV; Doppler ultrasound), blood pressure (MAP; finger plethysmograph), and muscle sympathetic nerve activity (MSNA) were measured continuously during the two protocols. MAP at 40 mmHg PET(CO2) (i.e., the first minute of the rebreathe) was greater during hypoxia versus hyperoxia (P < 0.05). However, the increase in MAP during the rebreathe (P < 0.05) was similar in hypoxia (16 +/- 3 mmHg) and hyperoxia (17 +/- 2 mmHg PET(CO2)). The increase in cardiac output (Q) at 55 mmHg PET(CO2) was greater in hypoxia (2.61 +/- 0.7 L/min) versus hyperoxia (1.09 +/- 0.44 L/min) (P < 0.05). In both conditions the increase in Q was due to elevations in both HR and SV (P < 0.05). Systemic vascular conductance (SVC) increased to similar absolute levels in both conditions but rose earlier during hypoxia (> 50 mmHg PET(CO2)) than hyperoxia (> 55 mmHg). MSNA increased earlier during hypoxic hypercapnia (> 45 mmHg) compared with hyperoxic hypercapnia (> 55 mmHg). Thus, in these conscious humans, the dose-response effect of PET(CO2) on the integrated cardiovascular responses was shifted to the left during hypoxic hypercapnia. The combined data indicate that peripheral chemoreceptors exert important influence over cardiovascular reflex responses to hypercapnia.