Cardiorespiratory responses to fatiguing dynamic and isometric hand-grip exercise

In occupational work, continuous repetitive and isometric actions performed with the upper extremity primarily cause local muscle strain and musculoskeletal disorders. They may also have some adverse effects on the cardiorespiratory system, particularly, through the elevation of blood pressure. The aim of the present study was to compare peak cardiorespiratory responses to fatiguing dynamic and isometric hand-grip exercise. The subjects were 21 untrained healthy men aged 24–45 years. The dynamic hand-grip exercise (DHGE) was performed using the left hand-grip muscles at the 57 (SD 4)% level of each individual's maximal voluntary contraction (MVC) with a frequency of 51 (SD 4) grips · min^−l. The isometric hand-grip exercise (IHGE) was done using the right hand at 46 (SD 3)% of the MVC. The endurance time, ventilatory gas exchange, heart rate (HR) and blood pressure were mea- sured during both kinds of exercise. The mean endurance times for DHGE and IHGE were different, 170 (SD 62) and 99 (SD 27) s, respectively (P < 0.001). During DHGE the mean peak values of the breathing frequency [20 (SD 6) breaths · min⁻¹] and tidal volume [0.89 (SD 0.34) l] differed significantly (P < 0.01) from peak values obtained during IHGE [15 (SD 5) breaths · min⁻¹, and 1.14 (SD 0.32) l, respectively]. The corresponding peak oxygen consumptions, pulmonary ventilations, HR and systolic blood pressures did not differ, and were 0.51 (SD 0.06) and 0.46 (SD 0.11) l · min⁻¹, 17.1 (SD 3.0) and 16.7 (SD 4.7) l · min⁻¹, 103 (SD 18) and 102 (SD 17) beats · min⁻¹, and 156 (SD 17) and 161 (SD 17) mmHg, respectively. The endurance times of both DHGE and IHGE were short (<240 s). The results indicate that the peak responses for the ventilatory gas exchange, HR and blood pressure were similar during fatiguing DHGE and IHGE, whereas the breathing patterns differed significantly between the two types of exercise. The present findings emphasize the importance of following ergonomic design principles in occupational settings which aim to reduce the output of force, particularly in tasks requiring isometric and/or one-sided repetitive muscle actions. Accepted: 16 February 2000     

Effect of co-contractions on the cardiovascular response to submaximal static handgrip

The aim of this study was to investigate the influence of concomitant involuntary contractions of different muscles on heart rate (HR) and blood pressure (BP) during a sustained, submaximal handgrip. Nine male subjects [23.6 (0.4) years, 177.0 (1.5) cm, and 73.0 (2.7) kg, means (SE)] participated in the experiment. The maximal integrated electromyographic activity (IEMG_max) of four ipsilateral muscles, flexor digitorum (FD), biceps brachii (BB), rectus abdominalis (RA) and vastus lateralis (VA), was recorded. Then, after 30 min of rest, the subjects maintained a submaximal isometric handgrip for 2 min. Heart rate (HR), systolic (SBP) and diastolic (DBP) blood pressure and surface electromyography (EMG) of the four muscles were recorded. The amplitude and power spectrum of the EMG were analysed. During the handgrip the force was kept constant at 43 (1)% of the maximum voluntary contraction (MVC) only for 90 (12) s. After that time, the subjects were unable to maintain the target force which decreased continuously up to the end of the contraction (P < 0.01) with a residual force of 27 (3)% MVC at t=120 s. HR increased from 75 (3) beats · min⁻¹ at rest to 109 (6) beats · min⁻¹ at t=120 s (P < 0.01). SBP and DBP also increased from 112 (5) and 81 (2) mmHg to 176 (5) and 133 (7) mmHg, respectively (P < 0.01). The EMG activity rose significantly for both FD and BB with a moderate increase for RA and VL. In fact, the individual contributions of FD and BB to the EMG activity of the four muscles were 52 (2)% and 37 (2)%, respectively, whereas the RA and VL contributed only 9 (1)% and 1.4 (0.1)%. The amplitude ratio of FD's EMG to the total EMG activity of the four muscles from which recordings were made decreased with time from 72% to 33% (P < 0.01). The central command's level of activation, as reflected by the increased EMG activity of the four recorded muscles, was probably high enough to stimulate the cardiovascular centres through cortical spread (sometimes known as cortical irradiation). On the other hand, maintaining an isometric handgrip at 43% MVC reduced local muscle blood flow and metabolites known to stimulate type III and IV afferents then accumulated, which in turn induced a reflex-mediated elevation of blood pressure. However, the relative forces developed by the co-contracting muscles were of low intensity (less than 20% MVC) and short duration compared to those of the muscle group under study. These results suggest that the mass of the muscle groups recruited during a fatiguing submaximal handgrip contributes little to the cardiovascular response. Accepted: 20 July 2000     

Noninvasive assessment of cardiac contractility by using (dP/dt)/P of carotid artery pulses during exercise

In earlier studies we have shown that both the pressure (P) of the carotid artery pulse (CAP) and its first derivative (CAP dP/dt) could be recorded during moderate exercise. To establish that the CAP (dP/dt)/P is a noninvasive substitute for the left ventricular (LV) value, LV (dP/dt)/P, an index of cardiac contractility, we studied CAP (dP/dt)/P under various states of activity in the autonomic nervous system in 12 healthy male subjects. Increased sympathetic nerve activities yielded by passive tilting, emotional load, or cold stress increased CAP (dP/dt)/P significantly (P< 0.05). Increased parasympathetic nerve activity by ocular compression, however, did not significantly affect the value. Moderate exercise at a heart rate of approximately 150 beats·min^–1 increased it significantly from 16.7 to 25.2·s^–1 in a supine position (P<0.001) and from 16.6 to 24.8·s^–1 in an upright position (P<0.001). It increased monotonically as heart rate increased, but the slope was steeper when the heart rate was greater than approximately 100 beats·min^–1 than it was when the rate was less than 100 beats·min^–1. In conclusion, the present study indicated that CAP (dP/dt)/P can be used as a noninvasive index of cardiac contractility even in moderate exercise.     

Continuous heart rate monitoring over 1 week in teenagers aged 11–16 years

Heart rate (HR) was monitored in 66 French pubertal boys (B, n=28) and girls (G, n=38) aged 11–16 years to evaluate habitual physical activity (HPA) over a 1-week period in the winter. The HR and the percentage of heart rate reserve (%HRR) were taken to be indexes of the metabolic activity for the whole day and for the different parts of the day. The HPA was evaluated from the time spent each day below 50%HRR, between 50%–70%HRR and above 70%HRR, which related to the time spent in no or low physical activity (NLPA), moderate physical activity (MPA) and vigorous physical activity (VPA), respectively. No sex differences were observed in the average %HRR each day {%HRR_mean, [B 30 (SD 4)%; G 32 (SD 4)%]} or in NLPA [B 715 (SD 61) min, G 711 (SD 81) min] and VPA [B 19 (SD 16) min, G 21 (SD 21) min] throughout the week. During school days, daily %HRR_mean was 7% smaller in 14–16 year olds compared to 11–13 year olds. This was linked to a decrease in MPA and a concomitant increase in NLPA (P<0.05). Daily %HRR_mean varied significantly during the week (range: 28–34% HRR). There were significant differences among the periods of the day (P<0.05). The HR was the greatest during physical education lessons [128 (SD 11) beats · min⁻¹], recreation [113 (SD 15) beats · min⁻¹] and lunch break [108 (SD 12) beats · min⁻¹] and the lowest during the evening [94 (SD 10) beats · min⁻¹]. It was only during the lunch breaks that %HRR_mean was greater (P<0.05) on school days than on free days. Of all the teenagers studied 32% were considered active during the week. Accepted: 9 June 1997     

Blood flow in the triceps brachii muscle in humans during sustained submaximal isometric contractions

The main purpose of this study was to determine the extent to which blood flow through the profunda artery within the triceps brachii muscle may be compromised during maintained low-force isometric fatiguing contractions. Doppler ultrasound techniques were used to record mean blood velocity and arterial diameter of the profunda brachii artery during sustained isometric contractions of 20% maximal voluntary contraction. The arterial diameter did not change throughout the contraction. Thus, blood velocity was considered to be an indicator of blood flow. The mean blood velocity increased initially and then remained constant during the contraction period. When compared to rest [0.06 (SD 0.03) m s^–1] mean blood velocity was significantly larger at the start of the contraction [0.13 (SD 0.07) m s^–1] and larger yet during recovery following the contraction [0.30 (SD 0.14) m s^–1]. Although blood flow through the conduit artery did not drop during the contraction, the post-contraction hyperaemia suggested that circulatory compromise might have occurred at the level of the capillary beds. Electronic Publication     

Changes in muscle sympathetic nerve activity and calf blood flow during combined leg and forearm exercise

In order to examine efferent sympathetic nerve control of the peripheral circulation during exercise, muscle sympathetic nerve activity (MSNA), calf blood flow (CBF), heart rate (HR), blood pressure (BP) and oxygen uptake were measured during combined foot and forearm exercise. An initial period of rhythmic foot exercise (RFE) (60 min^-1 at 10% of maximal voluntary contraction (MVC) was followed by the addition of rhythmic handgrip exercise (RFE+OCCL) (60 min at 30% of MVC) and by forearm ischaemia after handgrip exercise while continuing RFE (RFE + OCCL). During RFE, CBF in the working leg, HR and oxygen increased respectively by 560%, 121% and 144% when compared with the control rest period, but MSNA (burst rate) was reduced by 13% (P > 0.05) and BP was unchanged. During RFE+RHG, HR, BP and oxygen uptake were greater than during RFE alone. There was no change in CBF, but a significant increase occurred in calf vascular resistance (CVR) and MSNA increased to 121% of the control level. During RFE + OCCL, MSNA, CVR and BP were all higher than during RFE alone, whereas HR and oxygen uptake decreased slightly, although they remained higher than the control values. The increase in CVR in the working leg and the rise in BP during RFE+RHG or RFE+OCCL might be linked to enhancement of MSNA, which may have been reflexly evoked by input from muscle metabolic receptors in the working forearm.     

Sodium citrate ingestion and muscle performance in acute hypobaric hypoxia

Eight subjects were studied on four occasions following ingestion of a 300-ml solution containing either sodium citrate (C, 0.4g · kg^–1 body mass) or placebo (P, sodium chloride 0.045 g · kg^–1 body mass), at local barometric pressure (N, P _B approximately 740 mmHg, 98.7 kPa) or hypobaric hypoxia (HH, P _B = 463 mmHg, 61.7 kPa). At 2 h after ingestion of the solution, the subjects performed prolonged isometric knee-extension at 35% of the maximal voluntary contraction (MVC) measured either in N or HH. Results showed that ingestion of C led to an improvement in muscle endurance (P < 0.01). However, this increase in endurance time for knee extensor muscles was only significant in N ( +22%, P < 0.05, compared to + 15%, NS, at N and HH, respectively). Following ingestion of sodium citrate, pre-exercise bicarbonate concentrations and pH levels were significantly higher than those measured after P ingestion. A significant treatment effect was observed for blood lactate concentrations with values higher for C than for P after 4, 6 and 10 min of recovery (P < 0.05). Electromyographic signals (EMG) were obtained from the vastus lateralis muscle during the prolonged isometric contraction at 35% MVC. The mean power frequency (MPF) significantly decreased in time under both N-P and N-C conditions. In HH, no significant decrease in MPF was observed with time. The results suggest that C ingestion was an ergogenic aid enhancing endurance during a sustained isometric contraction. In addition, it is suggested that fatigue during prolonged isometric contraction in HH was not directly related to factors determining the EMG signs of fatigue.     

Muscle performance following fatigue induced by isotonic and quasi‐isometric contractions of rat extensor digitorum longus and soleus muscles in vitro

Aim: To study the effect of contraction mode on fatigue development. Methods: Muscle fatigue was induced by isotonic and quasi‐isometric contractions in rat soleus (SOL) and extensor digitorum longus (EDL) muscles, using identical stimulation protocol (60 Hz, 400 ms s^?1) for 100 s in SOL and 60 s in EDL. Fatigue was quantified as the decline in peak values of shortening, shortening velocity, relaxation and work during the isotonic contractions, and, correspondingly, of force, rate of force development, relaxation and work during the quasi‐isometric contractions. Maximal test contractions (60 Hz, 1.5 s) performed before and after fatigue were analysed for decline in force development (F_max), rate of force development (dF/dt_max) and relaxation (?dF/dt_max). Results: F _max declined to significantly lower values after isotonic than after quasi‐isometric fatiguing contractions (fatigued in percentage of unfatigued): 58.5 ± 6.4% vs. 64.4 ± 7.0% in SOL, and 30.4 ± 4.1% vs. 33.3 ± 3.6% in EDL, respectively. The same pattern was seen for dF/dt_max which decreased to: 46.3 ± 9.9% vs. 52.3 ± 8.5% in SOL, and 19.1 ± 4.3% vs. 22.3 ± 3.2% in EDL after isotonic and quasi‐isometric contractions, respectively. Similarly, when comparing fatigue development during the two contraction modes, the respective fatigue variables decreased more rapidly and to lower levels during isotonic vs. quasi‐isometric contractions. During maximal test contractions, the dynamic fatigue variables (±dF/dt_max) declined to significantly lower levels than F_max. Conclusions: Fatigue development was significantly larger during isotonic vs. quasi‐isometric contractions. The use of force as the only experimental fatigue variable may underestimate the functional impairment of fatigued muscle, neglecting the fatigue effect on time and length dimensions.     

Cardiovascular response to static handgrip in trained and untrained men

Summary The influence of aerobic capacity on the cardiovascular response to handgrip exercise, in relation to the muscle mass involved in the effort, was tested in 8 trained men (T) and 17 untrained men (U). The subjects performed handgrip exercises with the right-hand (RH), left-hand (LH) and both hands simultaneously (RLH) at an intensity of 25% of maximal voluntary contraction force. Maximal aerobic capacity was 4.3 l·min^–1 in T and 3.21·min^–1 in U (P<0.01). The endurance time for handgrip was longer in T than in U by 29% (P<0.05) for RH, 38% (P<0.001) for LH and 24% (P<0.001) for RLH. Heart rate (f _c) was significantly lower in T than in U before handgrip exercise, and showed smaller increases (P<0.01) at the point of exhaustion: 89 vs 106 beats·min^–1 for RH, 93 vs 100 beats·min^–1 for LH and 92 vs 108 beats·min^–1 for RLH. Stroke volume (SV) at rest was greater in T than in U and decreased significantly (P<0.05) during handgrip exercise in both groups of subjects. At the point of exhaustion SV was still greater in T than in U: 75 vs 57 ml for RH, 76 vs 54 ml for LH and 76 vs 56 ml for RLH. During the last seconds of handgrip exercise, the left ventricular ejection time was longer in T than in U. Increases in cardiac output (Q _c) and systolic blood pressure did not differ substantially between T and U, nor between the handgrip exercise tests. It was concluded that handgrip exercise caused similar increases inQ _c in both T and U but in T the increased level ofQ _c was an effect of greater SV and lowerf _c than in U. Doubling the muscle mass did not alter the cardiovascular response to handgrip exercise in either T or U.     

Force-frequency relationship,contraction duration and recirculating fraction of calcium in postnatally developing rat heart ventricles: correlation with heart rate

Heart rates (HR) of awake unrestrained animals, isometric contraction duration and force-frequency relationship of ventricular tissue were determined in adult and postnatally developing rats. Resting HR was lowest in newborns (256 beats min^-1), reached maximum at the age of 2.5 weeks (506 beats min^-1) and then declined to the level of adult rats (381 beats min^-1). Duration of isometric contraction correlated negatively with HR. Time to peak tension (TPT) was 185 ms in newborns but fell rapidly during the first days of post-natal life. Minimum was attained at the age of 2.5 weeks (TPT = 98 ms), followed by a slight prolongation towards adulthood. Recirculating fraction of activator Ca²⁺increased parallel with HR, being 6% in newborns, 33% in 11-day-old pre-weanlings, and 87% in adult rats. Similar developmental pattern of the parameters suggests that a post-natal increase in HR and a shortening of contraction duration are closely associated with a shift from extracellular to intracellular source of activator Ca²⁺. Force–frequency curves were similar at different developmental stages and consisted of three phases; a negative staircase between 0.05 and 1.0 Hz, a positive staircase between 1.0 and 4.0 Hz, and a secondary decline above 4.0 Hz. In adult rats the positive force staircase was weak or absent. Furthermore, our results show that negative staircase is not only a property of adult rat heart but is present, and even more pronounced, in preweanling and weanling rat heart. Therefore negative staircase is not solely explained by quantitative changes in the contribution of sarcoplasmic reticulum (SR) to contractile activation, but rather by the mechanisms which regulate loading and/or release of sarcoplasmic reticular Ca²⁺.     

Endurance training slows down the kinetics of heart rate increase in the transition from moderate to heavier submaximal exercise intensities

Summary To find out whether endurance training influences the kinetics of the increases in heart rate (f _c) during exercise driven by the sympathetic nervous system, the changes in the rate off _c adjustment to step increments in exercise intensities from 100 to 150 W were followed in seven healthy, previously sedentary men, subjected to 10-week training. The training programme consisted of 30-min cycle exercise at 50%–70% of maximal oxygen uptake ( O_2max) three times a week. Every week during the first 5 weeks of training, and then after the 10th week the subjects underwent the submaximal three-stage exercise test (50, 100 and 150 W) with continuousf _c recording. At the completion of the training programme, the subjects' O_2max had increased significantly(39.2 ml·min^–1·kg^–1, SD 4.7 vs 46 ml·min^–1·kg^–1, SD 5.6) and the steady-statef _c at rest and at all submaximal intensities were significantly reduced. The greatest decrease in steady-statef _c was found at 150 W (146 beats·min^–1, SD 10 vs 169 beats·min^–1, SD 9) but the difference between the steady-statef _c at 150 W and that at 100 W (f _c) did not decrease significantly (26 beats·min^–1, SD 7 vs 32 beats·min^–1, SD 6). The time constant () of thef _c increase from the steady-state at 100 W to steady-state at 150 W increased during training from 99.4 s, SD 6.6 to 123.7 s, SD 22.7 (P<0.01) and the acceleration index (A=0.63·f _c·^–1) decreased from 0.20 beats·min^–1·s^–1, SD 0.05 to 0.14 beats·min^–1·s^–1, SD 0.04 (P<0.02). The major part of the changes in and A occurred during the first 4 weeks of training. It was concluded that heart acceleration following incremental exercise intensities slowed down in the early phase of endurance training, most probably due to diminished sympathetic activation.     

Left ventricular performance during prolonged exercise and early recovery in healthy subjects

The effect of semi-supine long lasting exercise to exhaustion [61 (SD 10) min] on left ventricular systolic performance was studied by echocardiography in 16 young healthy volunteers. During the incremental phase of exercise, the ejection fraction increased from 65.2 (SD 4.1)% to 80.1 (SD 4.8)% (P<0.0001), then it levelled off up to the end of exercise [81.7 (SD 4.4)%,P<0.0001 vs rest]. During recovery, the ejection fraction rapidly and steadily decreased to a value similar to that at rest [66.1 (SD 5.0)%, n.s.). A similar pattern was shown by the systolic blood pressure/end-systolic volume coefficient, which rose from 3.2 (SD 0.8) mmHg · ml^–1 to 7.5 (SD 2.7) mmHg · ml^–1 (P < 0.0001) in the initial phase and subsequently did not change until the end of exercise [7.0 (SD 2.2) mmHg · ml^–1,P<0.0001 vs rest], to fall sharply after the cessation of exercise [2.9 (SD 1.1) mmHg · ml^–1 at the 10th min, n.s. vs rest]. Exercise and recovery indices of left ventricular performance were not correlated with exercise duration, maximal heart rate and increase in free fatty acids. The present results indicated that, after the initial increase, left ventricular performance remained elevated during prolonged high intensity exercise and that conclusions on exercise cardiac performance drawn from postexercise data can be misleading.     

Heart rate overshoot at the beginning of muscle exercise

Summary A characteristic notch in the heart rate (f _c) on-response at the beginning of square-wave exercise is described in 7 very fit marathon runners and 12 sedentary young men, during cycle tests at 30% and 60% of maximal oxygen consumption (VO_2max). The (f _c) notch revealed af _c overshoot with respect to the (f _c) values predicted from exponential beat-by-beat fitted models. While at 30% of (VO_2max). all subjects showed af _c over-shoot, at 60% of (VO_2max). it occurred in the marathon runners but not in the sedentary subjects. The mean time of occurrence of thef _c overshoot from the onset of the exercise was 16.7 (SD 4.7) s and 12.2 (SD 3.2) s at 30% of (VO_2max). in the runners and the sedentary subjects respectively, and 23.8 (SD 8.8) s at 60% of (VO_2max). in the runners. The amplitude of the overshoot, with respect to rest, was 41 (SD 12) beats·min^–1and 31 (SD 4) beats·min^–1 at 30% of (VO_2max). in the runners and the sedentary subjects respectively, and 46 (SD 19) beats·min^–1 at 60% of (VO_2max). in the runners. The existence and the amplitude of thef _c overshoot may have been related to central command and muscle heart reflex mechanisms and thus may have been indicators of changes in the balance between sympathetic and parasympathetic activity occurring in fit and unfit subjects.     

Short-term cardiovascular responses to rapid whole-body tilting during exercise

Our objective was to characterize the responses of heart rate (HR) and arterial blood pressure (BP) to changes in posture during concomitant dynamic leg exercise. Ten men performed dynamic leg exercise at 50, 100, and 150 W and were rapidly and repeatedly tilted between supine (0°) and upright (80°) positions at 2-min intervals. Continuous recordings of BP and HR were made, and changes in central blood volume were estimated from transthoracic impedance. Short-lasting increases in BP were observed immediately upon tilting from the upright to the supine position (down-tilt), averaging +18 mmHg (50 W) to +31 mmHg (150 W), and there were equally short-lasting decreases in BP, ranging from −26 to −38 mmHg upon tilting from supine to upright (up-tilt). These components occurred for all pressure parameters (systolic, mean, diastolic, and pulse pressures). We propose that these transients reflect mainly tilt-induced changes in total peripheral resistance resulting from decreases and increases of the efficiency of the venous muscle pump. After 3–4 s (down-tilt) and 7–11 s (up-tilt) there were large HR transients in a direction opposite to the pressure transients. These HR transients were larger during the down-tilt (−15 to −26 beats · min⁻¹) than during the up-tilt (+13 to +17 beats · min⁻¹), and increased in amplitude with work intensity during the down-tilt. The tilt-induced HR fluctuations could be modelled as a basically linear function of an arterial baroreflex input from a site half-way between the heart and the carotid sinus, and with varying contributions of fast vagal and slow sympathetic HR responses resulting in attenuated tachycardic responses to hypotensive stimuli during exercise. Accepted: 24 August 1999     

Intramuscular pressures during exercise: an evaluation of a fiber optic transducer-tipped catheter system

Summary The efficacy of a modified fibre optic transducer-tipped catheter system for measuring intramuscular pressures during exercise was determined. A microcapillary infusion technique using a catheter was employed as the standard of comparison due to its established dynamic properties. Pressures were measured in the tibialis anterior muscle of six healthy adults at rest before exercise, during isometric and concentric exercise, and at rest after exercise. The fibre optic system measured contraction pressures equal to the microcapillary infusion technique during all phases of the exercise protocols but recorded a lower relaxation pressure during isometric exercise and a lower rest pressure following 20 min of concentric exercise. Negative relaxation pressures were recorded by the fibre optic system for two subjects during continuous concentric exercise. It is hypothesized that a piston effect, due to the sliding of muscle fibres at the catheter tip following a contraction, rendered falsely low pressures during relaxation and that this artefact was reflected in the subsequent rest pressure following exercise. The larger volume (157 mm³) and area (3.49 mm²) of the fibre optic catheter in the muscle made it more prone to this effect than the conventional catheter (39 mm³ and 0.87 mm², respectively). The fibre optic system may be preferred when recording the musclecontraction pressures during complex limb movements but should not be used when assessing the relaxation pressures or the pressure at rest following exercise.This project was performed at the Division of Orthopaedics and Rehabilitation, Veterans Administration Medical Center and the University of California San Diego     

Cardiovascular responses to head-out water immersion in Korean women breath-hold divers

Head-out water immersion (HOI) increases cardiac output (CO) for a given oxygen consumption. To investigate whether professional breath-hold divers show a similar response, cardiovascular responses to HOI were compared between six Korean women breath-hold divers, six non-diving housewives and six non-diving young women at rest and while performing leg cycle exercise of moderate intensity (metabolic rate = ~100 W m^–2) in water at a thermoneutral temperature (34.5°C). In all three groups, HOI increased CO markedly due to a rise in stroke volume, with no significant change in heart rate (HR) and arterial blood pressure (BP). Thus, total peripheral resistance (TPR) and arterio–venous oxygen content difference fell significantly. During dynamic exercise in water CO increased mainly due to a rise in HR. The arterial systolic BP rose slightly with no significant change in diastolic BP, and the TPR fell 20–40% with similar responses among the three groups of subjects. This study showed that both at rest and during exercise, cardiovascular responses to immersion do not vary significantly with age and water immersion experience.     

Buffer capacity and lactate accumulation in skeletal muscle of trained and untrained men

Buffer capacity (β) of skeletal muscle has been determined in trained (n=7) and in sedentary subjects (n=8). The trained subjects were active in ball games where a high degree of anaerobic energy utilization is required. Percentage fibre type occurrence in the thigh muscle was not significantly different in the two groups. However, there was a tendency towards a higher proportion of type I (slow-twitch) fibres (61.5±11.6% vs. 50.2±12.5%) and a lower proportion of type IIB fibres (2.1±3.5% vs 14.1±16.3%) in the trained subjects. The proportion of the cross-sectional area of the muscle biopsies that was made up of type I or type II fibres was not different in the two groups. All subjects performed an isometric contraction of the knee extensors to fatigue at 61% of their maximal voluntary contraction force. Muscle biopsies were taken from the quadriceps femoris muscle at rest and immediately after contraction. The buffer capacity of muscle was calculated from: β= (Muscle lactate (work)-Muscle lactate (rest))/(Muscle pH (rest) -Muscle pH (work)). A higher buffer capacity (p<0.05) was observed in the trained subjects (β=194±30 mmolxpH^-1xkg^-1 dry wt.) compared to the sedentary group (β=164±20) (mean±SD). An unexpected finding was that muscle lactate after contraction to fatigue was lower (30%, p<0.01) and muscle pH was higher (6.80±0.06 vs. 6.61±0.12, p<0.01) in the trained subjects than in the sedentary controls. Creatine phosphate stores were almost completely depleted in both groups. Post-exercise lactate values were related to the proportion of type II fibres in the muscle (p<0.01). There was, however, no statistical correlation betwe β and fibre type occurrence (p>0.05). In summary, the present results indicate that skeletal muscle buffer capacity can be changed by training in man. Furthermore, it is concluded that the lower lactate accumulation and pH decline after an isometric contraction to fatigue that was observed in the trained compared to the sedentary subjects is related to the training per se. However, the tendency towards a lower type I (slowtwitch) fibre percentage in the trained subjects is likely to have contributed to the observed differences.     

Heart rate at the onset of muscle contraction and during passive muscle stretch in humans: a role for mechanoreceptors

Previous evidence suggests that the heart rate (HR) increase observed with isometric exercise is dependent on different afferent mechanisms to those eliciting the increase in blood pressure (BP). Central command and muscle metaboreceptors have been shown to contribute to this differential effect. However, in experimental animals passive stretch of the hindlimb increases HR suggesting that small fibre mechanoreceptors could also have a role. This has not been previously shown in humans and was investigated in this study. Healthy human volunteers were instrumented to record BP, ECG, respiration, EMG of rectus femoris and gastrocnemius and contraction force of triceps surae. Voluntary isometric contraction of triceps surae elicited a significant HR change in the first three respiratory cycles at 40 % of maximum voluntary contraction whereas BP did not change significantly until after 30 s. This suggests that different mechanisms are involved in the initiation of the cardiovascular changes. Sustained passive stretch of triceps surae for 1 min, by dorsiflexion of the foot, caused a significant (   P < 0.05  ) increase in HR (5 ± 2.6 beats min⁻¹) with no significant change in BP. A time domain measure of cardiac vagal activity was reduced significantly during passive stretch from 69.7 ± 12.9 to 49.6 ± 8.9 ms. Rapid rhythmic passive stretch (0.5 Hz for 1 min) was without significant effect suggesting that large muscle proprioreceptors are not involved. We conclude that in man small fibre muscle mechanoreceptors responding to stretch, inhibit cardiac vagal activity and thus increase HR. These afferents could contribute to the initial cardiac acceleration in response to muscle contraction.     

Development,validity and reliability of a new pressure air biofeedback device (PAB) for measuring isometric extension strength of the lumbar spine

This study describes the development of a new portable muscle testing device, using air pressure as a biofeedback and strength testing tool. For this purpose, a pressure air biofeedback device (PAB^®) was developed to measure and record the isometric extension strength of the lumbar multifidus muscle in asymptomatic and low back pain (LBP) persons. A total of 42 subjects (age 47.58 years, ±18.58) participated in this study. The validity of PAB^® was assessed by comparing a selected measure, air pressure force in millibar (mb), to a standard criterion; calibrated weights in kilograms (kg) during day-to-day tests. Furthermore, clinical trial-to-trial and day-to-day tests of maximum voluntary isometric contraction (MVIC) of L5 lumbar multifidus were done to compare air pressure force (mb) to electromyography (EMG) in microvolt (μV) and to measure the reliability of PAB^®. A highly significant relationship were found between air pressure output (mb) and calibrated weights (kg). In addition, Pearson correlation calculations showed a significant relationship between PAB^® force (mb) and EMG activity (μV) for all subjects (n?=?42) examined, as well as for the asymptomatic group (n?=?24). No relationship was detected for the LBP group (n?=?18). In terms of lumbar extension strength, we found that asymptomatic subjects were significantly stronger than LBP subjects. The results of the PAB^® test differentiated between LBP and asymptomatic subject’s lumbar isometric extension strength without any risk to the subjects and also indicate that the lumbar isometric extension test with the new PAB^® device is reliable and valid.     

Metabolic recovery of mouse extensor digitorum longus and soleus muscle

Heat produced by a 1-s isometric tetanus of mouse extensor digitorum longus muscle (EDL; n=6) and a 1.5-s isometric tetanus of soleus muscle (n=7) was measured with thermopiles at 20 °C, and separated into initial heat (I) and recovery heat (R). In EDL the initial heat was 190±40 (SD) mJ g^–1 and in soleus 52±9 (SD) mJ g^–1. The recovery heat production rate immediately following the tetanus was almost zero in both muscles. It rose in 12±6 s (EDL) and in 30±3 s (soleus) to a maximum, to decrease thereafter monoexponentially with a time constant of 30.7±5.7 s (EDL) and 41.7±7.2 s (soleus). The measured recovery ratio (R/I) differed between EDL (0.95±0.14) and soleus (1.54±0.22). The value for soleus muscles was significantly different from the theoretical value of 1.13. EDL muscles were freeze-clamped at rest (n=10) and during the recovery phase, 1 min after the onset of the tetanus (n=10), to determine lactate and creatine phosphate. It was found that no significant amount of net lactate was produced. The amount of creatine phosphate reformed corresponded to the recovery heat produced. The results suggest that metabolic recovery after short tetani of EDL and soleus muscles occurs predominantly through oxidative phosphorylation, but knowledge of respiratory control in the living cell is insufficient to explain its slow onset immediately following contraction and the finding that EDL recovers faster than soleus.