Force–velocity and unloaded shortening velocity during graded potassium contractures in frog skeletal muscle fibres

Steady-state conditions of contraction, at maximal and submaximal forces, were produced in intact single muscle fibres, from Rana esculenta, using full tetani and graded K⁺-contractures. The uniformity in radial direction, of spreading of activation produced in K⁺-contractures, was checked in relation to the fibre diameters. The absolute isometric force was similar in tetani and maximal contractures, for fibres with diameters between 40 and 60 m, but not for fibres with diameters greater than about 70 m in which contracture force never reached tetanic force. The force–[K⁺]_o relation was similar for fibres with diameters between 40 and 60 m, but it was right shifted and it had a minor slope for fibres with diameters greater than 65–70 m. This suggests that only in the small diameter fibres (40–60 m) the activation does not fail to penetrate uniformly from the surface towards the fibre core. For fibres selected in the diameter range between 40 and 60 m, force–velocity relations and unloaded shortening velocities were determined in tetani and maximal and submaximal contractures. Data were obtained across a force range of 0.3 to 1 P ₀ (tetanic plateau force). Controlled velocity method was used to obtain force–velocity relations, and slack test to determine the unloaded shortening velocity (V _U). The values of the parameters characterising the force–velocity relation (V ₀ and a/P ₀) and V _U as determined by the slack test did not differ significantly in tetani and contractures, independent of the activation level or absolute force developed by the fibre. These results show that, at least within the range of forces tested, crossbridge kinetics is independent of the number of cycling crossbridges, in agreement with the prediction of the recruitment model of myofilament activation.     

Influence of muscle architecture on the torque and power–velocity characteristics of young and elderly men

This study investigated the contribution of muscle architecture to the differences in the torque–velocity and power–velocity relationships between older (OM n = 9, aged 69–82 years) and younger men (YM n = 15, aged 19–35 years). Plantarflexors’ (PF) maximal isometric and concentric torques were recorded at 0.87, 1.75, 2.62, 3.49 and 4.36 rad s⁻¹. Physiological cross-sectional area (PCSA) was calculated as the ratio of muscle volume (determined by magnetic resonance imaging) to muscle fascicle length (L _f, measured by ultrasonography). GM PCSA and L _f of the OM were, respectively, 14.3% (P < 0.05) and 19.3% (P < 0.05) smaller than of the YM. In the OM, GM maximum isometric torque and maximum contraction velocity (V _max), estimated from Hill’s equation were, respectively, 48.5 and 38.2% lower (P < 0.001) than in the YM. At all contraction velocities, the OM produced less torque than the YM (46.3% of YM at 0.87 rad s⁻¹ to 14.7% at 4.36 rad s⁻¹, P < 0.001). Peak power (PP) of the OM was 80% lower than that of the YM and normalisation of PP to muscle volume only reduced this difference by 10%. Normalisation of torque to PCSA reduced, but did not eliminate, differences in torque between YM and OM (9.6%) and differences in torque/PCSA increased with contraction velocity (P < 0.05). After normalisation of velocity to L _f, the difference in V _max between the OM and the YM was reduced to 15.9%. Thus, although muscle architecture contributes significantly to the differences in the torque– and power–velocity properties of OM and YM, other contractile factors, intrinsic to the muscle, seem to play a role. It is noteworthy that the deficit in PP between OM and YM is far greater than that of muscle torque, even after normalisation of PP to muscle volume. This finding likely plays an important role in the loss of mobility in old age.     

Force–velocity relationship of leg extensors obtained from loaded and unloaded vertical jumps

Purpose

Resent research has suggested that loaded multi-joint movements could reveal a linear force–velocity (F–V) relationship. The aim of the present study was to evaluate the F–V relationship both across different types of vertical jumps and across different F and V variables.

Methods

Ten healthy subjects performed maximum various vertical jumps that were either loaded or unloaded by constant external forces of up to 30 % of their body weight. Both the maximum and averaged F and V data were recorded.

Results

The observed F–V relationships proved to be strong (median correlation coefficients ranged 0.78–0.93) and quasi-linear. Their F- and V-intercepts and the calculated maximum power (P) were highly reliable (0.85 < ICC < 0.98), while their concurrent validity with respect to their directly measured values was on average moderate-to-large. The obtained F–V relationships also revealed that (1) the assessment of maximum F and P could be somewhat more reliable and valid than the assessment of maximum V, (2) natural countermovement jumps should be employed rather than the jumps performed from a fixed squat position, while (3) both maximum and averaged F and V variables could be used despite revealing markedly different regression parameters.

Conclusions

The data generally reveal a reliable, valid, strong and quasi-linear F–V relationship across variety of vertical jumps and the recorded F and V variables. Therefore, we conclude that the loaded vertical jumps could be developed into a routine method for testing the force, velocity, and power generating capacity of leg extensors.     

Energy cost and body centre of mass’ 3D intracycle velocity variation in swimming

The purpose of this study was to examine the relationship between the energy cost (C) and the 3D intracycle velocity variation (IVV; swimming direction--x, vertical--y and lateral--z axes) throughout the 200 m front crawl event. Ten international level swimmers performed a maximal 200 m front crawl swim followed by 50, 100 and 150 m bouts at the same pace as in the 200 m splits. Oxygen consumption was measured during the bouts and blood samples were collected before and after each one. The C was calculated for each 50 m lap as the ratio of the total energy expenditure (three energy pathways) to the distance. A respiratory snorkel and valve system with low hydrodynamic resistance was used to measure pulmonary ventilation and to collect breathing air samples. Two above water and four underwater cameras videotaped the swim bouts and thereafter APAS was used to assess the centre of mass IVV (x, y and z components). The increase in the C was significantly associated with the increase in the IVV in x for the first 50 m lap (R = -0.83, P < 0.01). It is concluded that the IVV relationship with C in a competitive event does not present the direct relationship found in the literature, revealing a great specificity, which suggests that the relation between these two parameters could not be used as a performance predictor in competitive events.     

Quadriceps maximal power and optimal shortening velocity in 335 men aged 23–88 years

The ability to develop adequate quadriceps muscle power may be highly predictive of subsequent disability among older persons. Rate as well as quantitative (sarcopenia) and qualitative (among other slowing of muscles) contributors to that age-related power decline are poorly known. The relationship of quadriceps maximal short-term power (P_max) and corresponding optimal shortening velocity (_opt) with age was assessed in 335 healthy men aged 23–88 years. The P_max and _opt were measured on a friction loaded non-isokinetic cycle ergometer. Anthropometric dimensions were used to estimate lean thigh volume (LTVest) and quadriceps mass. The decline in P_max across the adult life span (10.7% per decade) was greater than the usually reported decrease in maximal muscle strength. Power decreased already after the fourth decade. Both muscle mass (4.1% decline for LTVest or 3.4% for quadriceps mass per decade) and _opt (6.6% decline per decade) contributed to the decrease in power. Age contributed to the variability in P_max independently to the LTVest/quadriceps mass and _opt. The age-related decrease pattern of P_max reflects both stabilization (or even increase) of anthropometric measures (LTVest or quadriceps mass) from youth to middleage and systematic decline of _opt already from the thirties. This implicates more focus on velocity-orientated training as a means of enhancing leg power and improving functional status.     

Simultaneous determination of wave speed and arrival time of reflected waves using the pressure–velocity loop

In a previous paper we demonstrated that the linear portion of the pressure–velocity loop (PU-loop) corresponding to early systole could be used to calculate the local wave speed. In this paper we extend this work to show that determination of the time at which the PU-loop first deviates from linearity provides a convenient way to determine the arrival time of reflected waves (Tr). We also present a new technique using the PU-loop that allows for the determination of wave speed and Tr simultaneously. We measured pressure and flow in elastic tubes of different diameters, where a strong reflection site existed at known distances away form the measurement site. We also measured pressure and flow in the ascending aorta of 11 anaesthetised dogs where a strong reflection site was produced through total arterial occlusion at four different sites. Wave speed was determined from the initial slope of the PU-loop and Tr was determined using a new algorithm that detects the sampling point at which the initial linear part of the PU-loop deviates from linearity. The results of the new technique for detecting Tr were comparable to those determined using the foot-to-foot and wave intensity analysis methods. In elastic tubes Tr detected using the new algorithm was almost identical to that detected using wave intensity analysis and foot-to-foot methods with a maximum difference of 2%. Tr detected using the PU-loop in vivo highly correlated with that detected using wave intensity analysis (r ² = 0.83, P < 0.001). We conclude that the new technique described in this paper offers a convenient and objective method for detecting Tr, and allows for the dynamic determination of wave speed and Tr, simultaneously.     

V max estimate from three-parameter critical velocity models: validity and impact on 800 m running performance prediction

The purpose of this study was to evaluate the validity of maximal velocity (V _max) estimated from three-parameter systems models, and to compare the predictive value of two- and three-parameter models for the 800 m. Seventeen trained male subjects performed five randomly ordered constant velocity tests (CVT), a maximal velocity test (mean velocity over the last 10 m portion of a 40 m sprint) and a 800 m time trial (V _{800 m}). Five systems models (two three-parameter and three two-parameter) were used to compute V _max (three-parameter models), critical velocity (CV), anaerobic running capacity (ARC) and V _{800 m} from times to exhaustion during CVT. V _max estimates were significantly lower than (0.19<Bias<0.24 m s⁻¹) and poorly associated (0.44<r<0.49) with actual V _max (8.43±0.33 m s⁻¹). Critical velocity (CV) alone explained 40–62% of the variance in V _{800 m}. Combining CV with other parameters of each model to produce a calculated V _{800 m} resulted in a clear improvement of this relationship (0.83<r<0.94). Three-parameter models had a better association (0.93<r<0.94) and a lower bias (0.00<Bias<0.04 m s⁻¹) with actual V _{800 m} (5.87±0.49 m s⁻¹) than two-parameter models (0.83<r<0.91, 0.06<Bias<0.20). If three-parameter models appear to have a better predictive value for short duration events such as the 800 m, the fact the V _max is not associated with the ability it is supposed to reflect suggests that they are more empirical than systems models.     

Serum hormone and myocellular protein recovery after intermittent runs at the velocity associated with V˙O2max

The responses of serum myocellular proteins and hormones to exercise were studied in ten well-trained middle-distance runners [maximal oxygen consumption (V˙O_2max)?=?69.4?(5.1)?ml?·?kg^?1?·?min^?1] during 3 recovery days and compared to various measures of physical performance. The purpose was to establish the duration of recovery from typical intermittent middle-distance running exercises. The subjects performed, in random, order two 28-min treadmill running exercises at a velocity associated with V˙O_2max: 14 bouts of 60-s runs with 60?s of rest between each run (IR₆₀) and 7 bouts of 120-s runs with 120?s of rest between each run (IR₁₂₀). Before the exercises (pre- exercise), 2?h after, and 1, 2 and 3 days after the exercises, the same series of measurements were performed, including those for serum levels of the myocellular proteins creatine kinase, myoglobin and carbonic anhydrase III (S-CK, S-Mb and S-CA III, respectively), serum hormones testosterone, Luteinizing hormone, follicle-stimulating hormone and cortisol (S-testosterone, S-LH, S-FSH and S-cortisol, respectively) and various performance parameters: maximal vertical jump height (CMJ) and stride length, heart rate and ratings of perceived exertion during an 8-min run at 15?km?·?h^?1 (SL_15?km·h?1, HR_15?km?·?h?1 and RPE_15?km?·?h?1, respectively). Two hours after the end of both exercise bouts the concentration of each measured serum protein had increased significantly (P?15?km?·?h?1 or CMJ. During the recovery days only S-CK was significantly raised (P?P?15?km?·?h?1 (P?120 the post-exercise responses returned to their pre-exercise levels within the 3 days of recovery. The present findings suggest that a single 28-min intermittent middle-distance running exercise does not induce changes in serum hormones of well-trained runners during recovery over 3 days, while changes in S-CK, CMJ and RPE_15?km?·?h?1 indicate that 2–3 days of light training may be needed before the recovery at muscle level is complete.     

Effect of countermovement on power–force–velocity profile

Purpose

To study the effect of a countermovement on the lower limb force–velocity (F–v) mechanical profile and to experimentally test the influence of F–v mechanical profile on countermovement jump (CMJ) performance, independently from the effect of maximal power output (P _max).

Methods

Fifty-four high-level sprinters and jumpers performed vertical maximal CMJ and squat jump (SJ) against five to eight additional loads ranging from 17 to 87 kg. Vertical ground reaction force data were recorded (1,000 Hz) and used to compute center of mass vertical displacement. For each condition, mean force, velocity, and power output were determined over the entire push-off phase of the best trial, and used to determine individual linear F–v relationships and P _max. From a previously validated biomechanical model, the optimal F–v profile maximizing jumping performance was determined for each subject and used to compute the individual mechanical F–v imbalance (Fv _IMB) as the difference between actual and optimal F–v profiles.

Results

A multiple regression analysis clearly showed (r ² = 0.952, P < 0.001, SEE 0.011 m) that P _max, Fv _IMB and lower limb extension range (h _PO) explained a significant part of the interindividual differences in CMJ performance (P < 0.001) with positive regression coefficients for P _max and h _PO and a negative one for Fv _IMB.

Conclusion

Compared to SJ, F–v relationships were shifted to the right in CMJ, with higher P _max, maximal theoretical force and velocity (+35.8, 20.6 and 13.3 %, respectively). As in SJ, CMJ performance depends on Fv _IMB, independently from the effect of P _max, with the existence of an individual optimal F–v profile (Fv _IMB having an even larger influence in CMJ).     

Does the conduction velocity distribution change along the nerve?

Nerve conduction velocity distribution (CVD) is a very useful tool to examine the state and function of nerves. Only one record of compound action potential (CAP) may be sufficient to determine the CVD if the shape functions of the single fiber action potentials (SFAP) of fibers are known. Otherwise, CAP recordings from different locations are necessary to determine CVD. In this case, we confront the problem of whether the shape of the CVD changes along the nerve, because many methods that attempt to determine the CVD are based on the assumption that the CVD is invariant along the nerve. There is not a complete solution to this problem, but there are many suggestions allied with the recording conditions to minimise this effect. The other effect that may influence both shapes of CAP and CVD along the nerve is the volume conductor effect. If a suitable model could isolate and eliminate the volume conductor effect, then the spatial variation of CVD may be attributed to the natural conditions of the nerve. In this study, we followed a procedure to eliminate volume conductor effect and then applied our previously published model to examine the spatial variations in CVD. The results show that CVDs estimated at discrete points along the nerve trunk have significantly different patterns. Consequently, it may be concluded that CVD is not uniform along an isolated nerve trunk contrary to the assumptions of the most CVD estimation methods.     

Velocity at V̇O2 max and peak treadmill velocity are not influenced within or across the phases of the menstrual cycle

Velocity at VO_{2 max} (vO_{2 max}) and peak treadmill velocity (PTV) are variables highly predictive of endurance performance. However, how these variables are affected by the menstrual cycle is unknown. The aim of this study was to assess the effect of the menstrual cycle on vO_{2 max} and PTV. Ten, female runners were studied across three menstrual cycles. Training, menstrual history and mood states were assessed for 2 months, with daily salivary samples taken to detect menstrual phases. During the third menstrual cycle, participants completed a maximal test to determine O_{2 max}, vO_{2 max} and PTV in the early follicular phase, late follicular phase, early luteal phase, late luteal phase and menses. Progesterone increased at the onset of the luteal phase [mean (SEM); 490 (73.6) pmol l^–1] compared to the follicular phase [344.6 (59.7) pmol l^–1). No significant differences in the psychological mood states between the phases of the menstrual cycle were found (P>0.05). No significant differences in vO_{2 max} (P=0.611), or PTV (P=0.472) were found between the phases of the menstrual cycle. Thus, vO_{2 max} and PTV are not affected by the monthly menstrual cycle in female endurance runners.     

Can socioeconomic factors account for “atypical” correlations between timing,peak velocity,and intensity of adolescent growth in Taiwanese girls?

This study uses longitudinal height records of girls in two urban and one rural area in Taiwan. Individual height records were modeled with the Preece-Baines Model 1 (PB1) function to test two related hypotheses: 1) Taiwanese students who experienced a relatively stable, affluent growth environment from an early age, as judged from parental education and stability of residence type, will have a pattern of correlations for the timing and intensity of the growth spurt similar to those of European and American females; and 2) those students whose parents gained the wherewithal to move from single-story to multi-story dwellings while they were in primary school will have the most atypical patterns. The extent to which these and other sociodemographic factors influence pubertal spurt velocities and increments of adolescent growth were explored as well using multiple regression analyses. Results support the view that rapid socioeconomic change in Taiwan influenced the relationship between the timing and intensity of adolescent growth in stature. Children in the more stable environments in both urban areas had patterns of correlations typical of population samples from developed countries. The most atypical correlations in both areas were found among those who likely experienced the greatest improvement in socioeconomic status during primary school. These represent positive values previously unreported in the literature. Differences in amounts of growth, though in accord with these patterns, were quite small. Am. J. Hum. Biol. 12:102–117, 2000. © 2000 Wiley-Liss, Inc.     

Left ventricular flow propagation velocity measurement: Is it cast in stone?

This study aims to investigate the measurement of left ventricular flow propagation velocity, V _p, using phase contrast magnetic resonance imaging and to assess the discrepancies resulting from inflow jet direction and individual left ventricular size. Three V _p measuring techniques, namely non-adaptive (NA), adaptive positions (AP) and adaptive vectors (AV) method, were suggested and compared. We performed the comparison on nine healthy volunteers and nine post-infarct patients at four measurement positions, respectively, at one-third, one-half, two-thirds and the conventional 4 cm distances from the mitral valve leaflet into the left ventricle. We found that the V _p measurement was affected by both the inflow jet direction and measurement positions. Both NA and AP methods overestimated V _p, especially in dilated left ventricles, while the AV method showed the strongest correlation with the isovolumic relaxation myocardial strain rate (r = 0.53, p < 0.05). Using the AV method, notable difference in mean V _p was also observed between healthy volunteers and post-infarct patients at positions of: one-half (81 ± 31 vs. 58 ± 25 cm/s), two-thirds (89 ± 32 vs. 45 ± 15 cm/s) and 4 cm (98 ± 23 vs. 47 ± 13 cm/s) distances. The use of AV method and measurement position at one-half distance was found to be the most suitable method for assessing diastolic dysfunction given varying left ventricular sizes and inflow jet directions.     

Does critical swimming velocity represent exercise intensity at maximal lactate steady state?

Summary The purpose of this investigation was to determine whether the critical swimming velocity ( _crit), which is employed in competitive swimming, corresponds to the exercise intensity at maximal lactate steady state. _crit is defined as the swimming velocity which could theoretically be maintained forever without exhaustion and expression as the slope of a regression line between swimming distances covered and the corresponding times. A total of eight swimmers were instructed to swim two different distances (200 m and 400 m) at maximal effort and the time taken to swim each distance was measured. In the present study, _crit is calculated as the slope of the line connecting the two times required to swim 200 m and 400 m. vcrit determined by this new simple method was correlated significantly with swimming velocity at 4 mmol · 1^–1 of blood lactate concentration (r = 0.914,P < 0.01) and mean velocity in the 400m freestyle (r = 0.977,P < 0.01). In the maximal lactate steady-state test, the subjects were instructed to swim 1600 m (4 x 400 m) freestyle at three constant velocities (98010, 100% and 102070 of _crit). At 100% _crit blood lactate concentration showed a steady-state level of approximately 3.2 mmol · 1^– from the first to the third stage and at 98% of _crit lactate concentration had a tendency to decrease significantly at the fourth stage. On the other hand, at 102% of _crit, blood lactate concentration increased progressively and those of the third and fourth stages were significantly higher than those at 100% of _crit (P<0.05). These data suggest that _crit, which can be calculated by performing two timed, maximal effort swimming tests, may correspond to the exercise intensity at maximal lactate steady state.     

Force–velocity profiling of sprinting athletes: single-run vs. multiple-run methods

European Journal of Applied Physiology - This study explored the agreement between a single-run and a multiple-run method for force–velocity (Fv) profiling of sprinting athletes; we evaluated...     

Blood pressure response to force–velocity properties of the knee-hip extension movement

This study aimed to examine the effects of maximum static and dynamic forces during and after knee-hip extension movement on blood pressure. Blood pressure was measured with a combination of oscillometric and tonometry methods before, during, immediately after and 30 s after knee-hip extension movements performed under maximum isometric and various isotonic force conditions on the servo-controlled dynamometer. The force–velocity relation of knee-hip extension movement was linear (r ² = 0.9989), so that maximum isometric force (F _max) and unloaded velocity (V _max) were obtained by extrapolation. F _max coincided with measured maximum isometric force (F ₀) (F ₀/F _max = 1.03 ± 0.25). During isometric contraction, mean arterial pressure (MAP) increased to a larger extent and the increase was significantly higher than those during all controlled-load range of isotonic force measurements. The magnitude of MAP response during maximum isometric exercise was positively correlated with both F ₀ (r = 0.687, P < 0.01) and V _max (r = 0.586, P < 0.05). On the other hand, there was no significant correlation between F ₀ and V _max (r = 0.451, P > 0.05). It is suggested that measurements of muscular function with isotonic trials cause smaller increase in blood pressure than isometric trials do. Also, it was indicated that individuals with greater muscular strength and speed might respond with larger changes in blood pressure to strenuous muscular exercises.