Preferred and optimal stride frequency,stiffness and economy: changes with fatigue during a 1-h high-intensity run

Metabolic cost of submaximal running at constant speed is influenced by various factors including fatigue and kinematic characteristics. Metabolic costs typically drift upwards during extended running while stride characteristics often shift away from initial. When non-fatigued, experienced runners naturally optimize stride frequency in a manner that minimizes oxygen uptake. An initial objective was to determine whether runners demonstrate a similar self-optimizing capability when fatigued where stride characteristics have perhaps shifted away from the initial state. A secondary objective involved measurement of vertical and leg stiffness characteristics as a potential explanation for frequency changes with fatigue. We hypothesized that runners decrease stride frequency and stiffness with fatigue while optimizing these characteristics to minimize metabolic cost. Sixteen experienced runners completed a near maximal effort 1-h treadmill run at a constant speed. Preferred and optimal stride frequencies (PSF and OSF) were measured near the beginning and end of the hour run using frequencies ±4 and ±8% around PSF. From vertical force data recorded throughout the run, leg and vertical stiffness were determined. As expected, oxygen uptake significantly increased during the run from 45.9 to 47.4 ml kg⁻¹ min⁻¹ (P = 0.004). There was no difference between preferred and optimal stride frequencies at the beginning or the end of running (P = 0.978), however both PSF and OSF significantly decreased from 1.45 to 1.43 Hz (P = 0.026). All runners self-optimized stride frequency at the beginning and end of one-hour of running despite changes of optimal stride frequency. Stiffness and stride frequency changes were subject specific with some runners exhibiting little to no change. No clear relationship of frequency or stiffness changes to economy was found.     

The effects of a single bout of downhill running and ensuing delayed onset of muscle soreness on running economy performed 48 h later

Delayed onset of muscle soreness (DOMS) is a common response to exercise involving significant eccentric loading. Symptoms of DOMS vary widely and may include reduced force generating capacity, significant alterations in biochemical indices of muscle and connective tissue health, alteration of neuromuscular function, and changes in mechanical performance. The purpose of the investigation was to examine the effects of downhill running and ensuing DOMS on running economy and stride mechanics. Nine, well-trained distance runners and triathletes participated in the study. Running economy was measured at three relative intensities [65, 75, and 85% of maximal aerobic capacity (V̇O_2peak)] before (RE1) and 48 h after (RE2) a 30-min downhill run (−10%) at 70% V̇O_2peak. Dependent variables included leg muscle soreness, rate of oxygen consumption (V̇O₂), minute ventilation, respiratory exchange ratio, lactate, heart rate, and stride length. These measurements were entered into a two-factor multivariate analysis of variance (MANOVA). The analysis revealed a significant time effect for all variables and a significant interaction (time × intensity) for lactate. The energy cost of locomotion was elevated at RE2 by an average of 3.2%. This was coupled with a significant reduction in stride length. The change in V̇O₂ was inversely correlated with the change in stride length (r= −0.535). Lactate was significantly elevated at RE2 for each run intensity, with a mean increase of 0.61 mmol l⁻¹. Based on these findings, it is suggested that muscle damage led to changes in stride mechanics and a greater reliance on anaerobic methods of energy production, contributing to the change in running economy during DOMS.     

Adaptability in frequency and amplitude of leg movements during human locomotion at different speeds

In this study of human locomotion we investigate to what extent the normal frequency and amplitude of leg movements can be modified voluntarily at different constant velocities, and how these modifications are accomplished in terms of changes in duration and length of the support and swing phases of the stride cycle. Eight healthy male subjects performed walking and running on a motor-driven treadmill at speeds ranging from 1.0 to 3.0 m s-1 (walking) and 1.5 to 8.0 m s-1 (running), respectively. At each speed the subjects walked and ran with: normal stride frequency; the highest possible stride frequency, and the lowest possible stride frequency. Time for foot contact was measured with a special pressure transducer system under the sole of each shoe. At all speeds of walking and running it was possible to either increase or decrease the frequency of leg movements; that is, to decrease or increase stride cycle duration. The range of variation decreased with increasing speed. The mean overall stride frequency range was 0.41 (low frequency walk 1.0 m s-1)-3.57 Hz (high-frequency run 1.5 m s-1). Stride length ranged 0.40 (high frequency walk 1.0 m s-1)-5.00 m (low frequency run 6.0 m s-1). At normal frequency the overall ranges of stride frequency and length were 0.83-1.95 Hz and 1.16-4.10 m, respectively. The stride frequency increased with speed in low frequency walking and running (as in normal frequency) and decreased in high frequency, despite the effort to maintain extreme frequencies. Only in high frequency walking could the stride frequency be kept approximately constant.(ABSTRACT TRUNCATED AT 250 WORDS)     

Force-, EMG-, and elasticity-velocity relationships at submaximal,maximal and supramaximal running speeds in sprinters

Summary The relationships between ground reaction forces, electromyographic activity (EMG), elasticity and running velocity were investigated at five speeds from submaximal to supramaximal levels in 11 male and 8 female sprinters. Supramaximal running was performed by a towing system. Reaction forces were measured on a force platform. EMGs were recorded telemetrically with surface electrodes from the vastus lateralis and gastrocnemius muscles, and elasticity of the contact leg was evaluated with spring constant values measured by film analysis. Data showed increases in most of the parameters studied with increasing running speed. At supramaximal velocity (10.36±0.31 m×s⁻¹; 108.4±3.8%) the relative increase in running velocity correlated significantly (P<0.01) with the relative increase in stride rate of all subjects. In male subjects the relative change in stride rate correlated with the relative change of IEMG in the eccentric phase (P<0.05) between maximal and supramaximal runs. Running with the towing system caused a decrease in elasticity during the impact phase but this was significant (P<0.05) only in the female sprinters. The average net resultant force in the eccentric and concentric phases correlated significantly (P<0.05−0.001) with running velocity and stride length in the maximal run. It is concluded that (1) increased neural activation in supramaximal effort positively affects stride rate and that (2) average net resultant force as a specific force indicator is primarily related to stride length and that (3) the values in this indicator may explain the difference in running velocity between men and women.     

Relationship between the efficiency of muscular work during jumping and the energetics of running

Summary The running economy of seventeen athletes was studied during running at a low speed (3.3 m · s^–1) on a motor-driven treadmill. The net energetic cost during running expressed in kJ·kg^–1·km^–1 was on average 4.06. As expected, a positive relationship was found between the energetic cost and the percentage of fast twitch fibres (r=0.60,n=17,p<0.01). In addition, the mechanical efficiency during two different series of jumps performed with and without prestretch was measured in thirteen subjects. The effect of prestretch on muscle economy was represented by the ratio between the efficiency of muscular work performed during prestretch jumps and the corresponding value calculated in no prestretch conditions. This ratio demonstrated a statistically significant relationship with energy expenditure during running (r=–0.66,n=13,P<0.01), suggesting that the elastic behaviour of leg extensor muscles is similar in running and jumping if the speeds of muscular contraction during eccentric and concentric work are of similar magnitudes.     

Neuromuscular factors determining 5 km running performance and running economy in well-trained athletes

This study investigated the effects of the neuromuscular and force–velocity characteristics in distance running performance and running economy. Eighteen well-trained male distance runners performed five different tests: 20 m maximal sprint, running economy at the velocity of 4.28 m s⁻¹, 5 km time trial, maximal anaerobic running test (MART), and a treadmill test to determine VO_2max. The AEMG ratio was calculated by the sum average EMG (AEMG) of the five lower extremity muscles during the 5 km divided by the sum AEMG of the same muscles during the maximal 20 m sprinting. The runners’ capacity to produce power above VO_2max (MART VO_2gain) was calculated by subtracting VO_2max from the oxygen demand of the maximal velocity in the MART (V _MART). Velocity of 5 km (V _5K) correlated with V _MART (r=0.77, p<0.001) and VO_2max (r=0.49, p<0.05). Multiple linear regression analysis showed that MART VO_2gain and VO_2max explained 73% of the variation in V _5K. A significant relationship also existed between running economy and MART VO_2gain (r=0.73, p<0.01). A significant correlation existed between V _5K and AEMG ratio during the ground contact phase at the 3 km (r=0.60, p<0.05) suggesting that neural input may affect distance running performance. The results of the present study support the idea that distance running performance and running economy are related to neuromuscular capacity to produce force and that the V _MART can be used as a determinant of distance-running performance.     

Metabolic cost of exercise and physical performance in children with some observations on external loading

Summary The metabolic cost (VO₂) of running was studied on a motor-driven treadmill in nine athletic boys, five athletic girls, and nine active boys aged 11–13 years and the results compared with their performance times during racing out of doors. On 15 of the children, additional observations of the effects of external loading on aerobic power output were made. The results showed that VO₂ was proportional to body weight in children but when expressed in ml·kg^–1·min^–1, VO₂ for a given speed of running was significantly higher in children than expected from previously collected data on adults. There were no significant differences between aerobic cost of running of the athletic boys, girls, or the active boys. The increased VO₂ ml·kg^–1·min^–1 in children appeared to be independent of stride length and frequency but external loading equivalent to 5% of body weight reduced VO₂ (ml·kg^–1·min^–1), particularly at the higher speeds. It was suggested in young active and athletic children due to their relatively light body weights and highly developed aerobic power outputs, that the required frequency of leg movement was not optimally matched to the force necessary to produce the most economic conversion of aerobic energy into mechanical work. Thus, in competitive events their performance times were related to their maximal aerobic power output (r=–0.75) but their times were always inferior to those which one might have expected from previous aerobic power weight data collected on adult male and female athletes.     

The influence of prior cycling on biomechanical and cardiorespiratory response profiles during running in triathletes

The aim of the present study was to determine the effects of 40 km of cycling on the biomechanical and cardiorespiratory responses measured during the running segment of a classic triathlon, with particular emphasis on the time course of these responses. Seven male triathletes underwent four successive laboratory trials: (1) 40 km of cycling followed by a 10-km triathlon run (TR), (2) a 10-km control run (CR) at the same speed as TR, (3) an incremental treadmill test, and (4) an incremental cycle test. The following ventilatory data were collected every minute using an automated breath-by-breath system: pulmonary ventilation (V˙ _E, l · min⁻¹), oxygen uptake (V˙O₂, ml · min⁻¹ · kg⁻¹), carbon dioxide output (ml · min⁻¹), respiratory equivalents for oxygen (V˙ _E/V˙O₂) and carbon dioxide (V˙ _E/V˙CO₂), respiratory exchange ratio (R) respiratory frequency (f, breaths · min⁻¹), and tidal volume (ml). Heart rate (HR, beats · min⁻¹) was monitored using a telemetric system. Biomechanical variables included stride length (SL) and stride frequency (SF) recorded on a video tape. The results showed that the following variables were significantly higher (analysis of variance, P < 0.05) for TR than for CR: V˙O₂ [51.7 (3.4) vs 48.3 (3.9) ml · kg⁻¹ · min⁻¹, respectively], V˙ _E [100.4 (1.4) l · min⁻¹ vs 84.4 (7.0) l · min⁻¹], V˙ _E/V˙O₂ [24.2 (2.6) vs 21.5 (2.7)] V˙ _E/V˙CO₂ [25.2 (2.6) vs 22.4 (2.6)], f [55.8 (11.6) vs 49.0 (12.4) breaths · min⁻¹] and HR [175 (7) vs 168 (9) beats · min⁻¹]. Moreover, the time needed to reach steady-state was shorter for HR and V˙O₂ (1 min and 2 min, respectively) and longer for V˙ _E (7 min). In contrast, the biomechanical parameters, i.e. SL and SF, remained unchanged throughout TR versus CR. We conclude that the first minutes of the run segment after cycling in an experimental triathlon were specific in terms of V˙O₂ and cardiorespiratory variables, and nonspecific in terms of biomechanical variables. Accepted: 7 July 1997     

Factors limiting force during slow,shortening actions of the quadriceps femoris muscle group in vivo

Speed-torque relations of the quadriceps femors muscle group were determined using eight healthy subjects. Isometric or isovelocity, shortening muscle actions were performed at 15 speeds (0–6.28 rad s^-1) and torque was measured 0.78 and 0.52 rad below horizontal. Unilateral actions were evoked by surface electrical stimulation or by maximal voluntary effort. Stimulation current evoked a torque equal to approx. 70% of maximal voluntary isometric and was held constant across speeds. For the voluntary or stimulation tests, torque decreased (P < 0.05) with increasing speed. This response in relative terms was greater (P < 0.05) for the stimulation than for the voluntary tests. The difference in the decline in relative torque for the simulation vs. voluntary tests was not influenced by the angle at which torque was measured, and thereby muscle length. Speed-torque data for the two stimulation tests fit a linearized plot of a hyperbolic relation for the higher tests speeds. When torque was measured at the greater joint angle (0.78 rad), and thereby longer muscle length, the equation was Po – P/V = Po 0.724-1.078 (p<0.05, r²= 0.95) for speeds greater than 0.70 rad s^-1. For the shorter muscle length, the equation for data collected at speeds greater than 1.13 rad s“¹ was Po-P/V = Po 0.467 + 4.61 (P < 0.05, r²= 0.91). Inclusion of data for the next slower speed markedly compromised the ‘fit’ for either linear relation. The results suggest that the extent of force limitation by ‘some neural inhibitory mechanism’ during maximal voluntary effort has previously been over-estimated because the speed-torque relation for the quadriceps femoris muscle group was not hyperbolic in nature at ‘slow’ speeds when involuntary, shortening actions were studied. The magnitude of force limitation does not, however, appear to depend on muscle length.     

Locomotor response to levodopa in fluctuating Parkinson’s disease

The aim of this study was to quantify the dynamic response of locomotion to the first oral levodopa administration of the day in patients with fluctuating Parkinson’s disease (PD). Stride length, walking speed, cadence and gait variability were measured with an ambulatory gait monitor in 13 PD patients (8 males) with a clinical history of motor fluctuations. The Unified Parkinson’s Disease Rating Scale (UPDRS) gait score (part 29) was also determined by a movement disorders specialist from video recordings. Subjects arrived in the morning in an ‘off’ state (no PD medication) and walked for a maximum length of 100 m. They then took their usual morning dose of oral levodopa and repeated the walking task at 13 min intervals (on average) over a 90 min period. Changes in stride length over time were fit with a Hill (Emax) function. Latency (time until stride length increased 15% of the difference between baseline and maximum response) and the Hill coefficient (shape of the ‘off–on’ transition) were determined from the fitted curve. Latency varied from 4.7 to 53.3 min post-administration [23.31 min (SD 14.9)], and was inversely correlated with age at onset of PD (R = −0.83; P = 0.0004). The Hill coefficient (H) ranged from a smooth hyperbolic curve (0.9) to an abrupt ‘off–on’ transition (16.9), with a mean of 8.1 (SD 4.9). H correlated with disease duration (R = 0.67; P = 0.01) and latency (R = 0.67; P = 0.01), and increased with Hoehn & Yahr stage in the ‘off’ state (P = 0.02) from 5.7 (SD 3.5) (H&Y III) to 11.9 (SD 4.7) (H&Y IV). Walking speed correlated with changes in mean stride length, whereas cadence and gait variability did not. UPDRS gait score also reflected improving gait in the majority of subjects (8), providing clinical confirmation of the objective measures of the locomotor response to levodopa. Increasing abruptness (H) of the ‘off–on’ transition with disease duration is consistent with results from finger-tapping studies, and may reflect reduced buffering capacity of pre-synaptic nigrostriatal dopaminergic neurons. Ambulatory monitoring of gait objectively measures the dynamic locomotor response to levodopa, and this information could be used to improve daily management of motor fluctuations. Dr. Steven Moore was supported in part by NASA grant NNJ04HF51G.     

Changes in spring-mass model characteristics during repeated running sprints

This study investigated fatigue-induced changes in spring-mass model characteristics during repeated running sprints. Sixteen active subjects performed 12?×?40?m sprints interspersed with 30?s of passive recovery. Vertical and anterior?Cposterior ground reaction forces were measured at 5?C10?m and 30?C35?m and used to determine spring-mass model characteristics. Contact (P?<?0.001), flight (P?<?0.05) and swing times (P?<?0.001) together with braking, push-off and total stride durations (P?<?0.001) lengthened across repetitions. Stride frequency (P?<?0.001) and push-off forces (P?<?0.05) decreased with fatigue, whereas stride length (P?=?0.06), braking (P?=?0.08) and peak vertical forces (P?=?0.17) changes approached significance. Center of mass vertical displacement (P?<?0.001) but not leg compression (P?>?0.05) increased with time. As a result, vertical stiffness decreased (P?<?0.001) from the first to the last repetition, whereas leg stiffness changes across sprint trials were not significant (P?>?0.05). Changes in vertical stiffness were correlated (r?>?0.7; P?<?0.001) with changes in stride frequency. When compared to 5?C10?m, most of ground reaction force-related parameters were higher (P?<?0.05) at 30?C35?m, whereas contact time, stride frequency, vertical and leg stiffness were lower (P?<?0.05). Vertical stiffness deteriorates when 40?m run-based sprints are repeated, which alters impact parameters. Maintaining faster stride frequencies through retaining higher vertical stiffness is a prerequisite to improve performance during repeated sprinting.     

Determination of muscular fatigue in elite runners

This study analyses the changes in the electromyographic activity (EMG) of six major muscles of the leg during an incremental running test carried out on a treadmill. These muscles, the gluteus maximus (GM), biceps femoris (BF), vastus lateralis (VL), rectus femoris (RF), tibialis anterior (TA) and gastrocnemius (Ga) are known to have quite different functions during running. The aim of this study was to develop a methodology adapted to the analysis of integrated EMG (iEMG) running results, and to test the chronology of the onset of fatigue of the major muscles involved in running. Nine well-trained subjects [ O_2max 76 (2.9) ml.min^–1.kg^–1] took part in this study. They completed a running protocol consisting of 4 min stages, incrementally increasing in speed until exhaustion. The EMG signal was recorded during ten bursts of activation analysed separately at 45 s and 3 min 40 s of each stage. During running, consideration of the alteration in stride frequency with either an increase in speed or the onset of fatigue appears to be an indispensable part of the assessment of muscular fatigue. This allows the comparison of muscular activation between the various stage speeds by the use of common working units. Distance seems to be the only working unit that allows this comparison and thus the determination of the appearance of fatigue during running. The biarticular hip-mobilising muscles (RF and BF), which present two different bursts of activation during one running cycle, are the muscles that show the earliest signs of fatigue.     

The transition between walking and running in humans: metabolic and mechanical aspects at different gradients

Five subjects walked and ran at overlapping speeds and different gradients on a motorized treadmill. At each gradient the speed was obtained at which walking and running have the same metabolic cost (Sm) and the speed of spontaneous (Ss) transition between the two gaits was measured. Ss was found to be statistically lower than Sm at all gradients, the difference being in the range of 0.5–0.9 km h^-1. The motion analysis of walking reveals that at all gradients and at increasing speed: (1) the percentage of recovery, an index of mechanical energy saving related to the pendulum–like characteristic of walking, decreases; (2) the lower limb spread reaches a limit in walking; and consequently (3) both the stride frequency and the internal mechanical work, due to limb acceleration in relation to the body centre of mass, increase much more in walking than in running. Switching to a run, although implying a higher frequency, makes the internal work decrease as a result of the lower limb spread. In this paper several influences, such as the ‘ratings of perceived exertion’ (RPE), on the choice of beginning to run when it is more economical to walk, are discussed. A tentative hypothesis on the determinants of Ss, which is emphasized to be a speed which has to be studied in detail but is generally avoided in locomotion, is based on a comfort criterion from peripheric afferences and is reflected by the fact that at Ss a running stride costs as much as a walking stride. A preliminary measure of the subjects' behaviour during spontaneous overground locomotion, where the progression speed can be changed freely, reveals that the running speed immediately following gait transition is approximately 2 km h^-1 higher than the ‘last’ walking speed, supporting the hypothesis of metabolic energy minimization.     

Running training and adaptive strategies of locomotor-respiratory coordination

It has been suggested that stronger coupling between locomotory and breathing rhythms may occur as a result of training in the particular movement pattern and also may reduce the perceived workload or metabolic cost of the movement. Research findings on human locomotor–respiratory coordination are equivocal, due in part to the fact that assessment techniques range in sensitivity to important aspects of coordination (e.g. temporal ordering of patterns, half-integer couplings and changes in frequency and phase coupling). An additional aspect that has not received much attention is the adaptability of this coordination to changes in task constraints. The current study investigated the effect of running training on the locomotor-respiratory coordination and the adaptive strategies observed across a wide range of walking and running speeds. Locomotor-respiratory coordination was evaluated by the strength and variability of both frequency and phase coupling patterns that subjects displayed within and across the speed conditions. Male subjects (five runners, five non-runners) locomoted at seven different treadmill speeds. Group results indicated no differences between runners and non-runners with respect to breathing parameters, stride parameters, as well as the strength and variability of the coupling at each speed. Individual results, however, showed that grouping subjects masks large individual differences and strategies across speeds. Coupling strategies indicated that runners show more stable dominant couplings across locomotory speeds than non-runners do. These findings suggest that running training does not change the strength of locomotor–respiratory coupling but rather how these systems adapt to changing speeds. Electronic Publication     

Oxygenation in vastus lateralis and lateral head of gastrocnemius during treadmill walking and running in humans

The purposes of this study were to compare the deoxygenation patterns of the vastus lateralis (VL) and the lateral head of gastrocnemius (GL) and examine the relationship between the muscle oxygenation level and pulmonary oxygen uptake (VO₂) during graded treadmill exercise. Changes in oxygenation in each muscle were measured using near infrared spectroscopy (NIRS). Eight healthy male subjects participated in this study. Two NIRS probes were placed on VL and GL, and thereafter the leg arteries were occluded in all subjects to enable normalization of the NIR signals. The subjects then walked at 4 km·h^–1 and 6 km·h^–1, and then ran continuously at speeds ranging from 8 km·h^–1 to 16 km·h^–1. The muscle oxygenation level was defined as being 100% at rest and 0% at its lowest value during occlusion. Pulmonary VO₂ was measured using indirect calorimetry. After the subjects had started walking, the muscle oxygenation in VL increased and exceeded the level at rest. Thereafter, the muscle oxygenation in both muscles decreased in relation to the increase in speed (P<0.001). A significant difference in the level of muscle oxygenation between VL and GL was found at speeds of 10 km·h^–1 and 12 km·h^–1 (P<0.05). The muscle oxygenation level at 16 km·h^–1 was [mean (SEM)] 51.9 (4.6)% in VL and 52.8 (3.6)% in GL. There was a negative relationship between pulmonary VO₂ and the muscle oxygenation level (VL: r=–0.803 to –0.986; GL: r=–0.848 to –0.963, P<0.05). We concluded that the pattern of deoxygenation between VL and GL was somewhat different and that the muscle oxygenation level was associated with pulmonary VO₂. Electronic Publication     

Quasi-stiffness of the knee joint is influenced by walking on a destabilising terrain

BackgroundPredictive models have been devised to estimate the necessary quasi-stiffness that a transfemoral prosthesis should be set to aligning the body and gait parameters of the user. Current recommendations exist only for walking over level ground. This study aimed to ascertain whether walking across destabilising terrain influences the quasi-stiffness of the knee joint thus influencing prosthetic engineering.MethodsTen healthy males (age: 25.1 ± 2.5 years; mean ± sd, height: 1.78 ± 0.05 m, weight: 84.40 ± 11.02 kg) performed 14 gait trials. Seven trials were conducted over even ground and seven over 20 mm ballast. Three-dimensional motion capture and ground reaction force were collected. Paired samples t-tests and Wilcoxon signed ranked test compared variables including; quasi-stiffness, gait speed, stride length and stride width.ResultsQuasi-stiffness (d = 0.562, P = 0.001) and stride width (d = 0.909, P < 0.001) were significantly greater in the destabilising terrain condition. Gait speed (r = −0.731, P = 0.001) was significantly greater in the control condition. No significant difference was seen in stride length (d = 0.583, P = 0.016).ConclusionsAn increase in quasi-stiffness when walking across destabilising terrain was attributed to a magnified shock absorption mechanism, facilitating an increased flexion angle during the stance phase. This causes a lower centre of mass resulting in the musculoskeletal system having to produce a greater knee extensor moment to prevent the knee collapsing. Therefore, transfemoral prostheses should be tuned to apply increased extension moments if ambulation is to occur on a destabilising terrain.     

Changes in muscle fascicles of tibialis anterior during anisometric contractions are not associated with motor-output variability of the ankle dorsiflexors in young and old adults

This study examined the associations between the fluctuations of foot acceleration during shortening and lengthening contractions with the electromyographic (EMG) activity of lower leg muscles and ultrasound measures of tibialis anterior fascicle length and pennation angle. Young (24.9 ± 4.17 years) and old (74.8 ± 3.31 years) adults lifted and lowered a submaximal load with the foot at different speeds (3°/s–50°/s). The standard deviation (SD) of foot acceleration normalized to the load lifted was similar for young (12.2 ± 7.22 cm s⁻²/kg) and old adults (14.3 ± 8.03 cm s⁻²/kg; P = 0.093). The changes in tibialis anterior muscle fascicle length and pennation angle were similar for young and old adults (P ≥ 0.233), but greater for shortening (fascicle length: 0.937 ± 0.633 cm, pennation angle: 1.61 ± 0.918o) than for lengthening contractions (fascicle length: 0.806 ± 0.521 cm, pennation angle: 0.966 ± 0.632o; P ≤ 0.014). The changes in fascicle length and pennation angle were not associated with the SD of foot acceleration (r ² ≤ 0.031; P ≥ 0.092). The surface EMG of tibialis anterior was greater for the shortening contractions than for the lengthening contractions (P < 0.001), but triceps surae EMG was similar for the two types of contractions (P = 0.304). The results suggested that the influence of movement speed on variability in performance was similar for shortening and lengthening contractions with the dorsiflexor muscles; furthermore, old adults were able to match the performance of young adults.     

Pygmy locomotion

The hypothesis that Pygmies may differ from Caucasians in some aspects of the mechanics of locomotion was tested. A total of 13 Pygmies and 7 Caucasians were asked to walk and run on a treadmill at 4–12 km · h^–1. Simultaneous metabolic measurements and three-dimensional motion analysis were performed allowing the energy expenditure and the mechanical external and internal work to be calculated. In Pygmies the metabolic energy cost was higher during walking at all speeds (P < 0.05), but tended to be lower during running (NS). The stride frequency and the internal mechanical work were higher for Pygmies at all walking (P < 0.05) and running (NS) speeds although the external mechanical work was similar. The total mechanical work for Pygmies was higher during walking (P < 0.05), but not during running and the efficiency of locomotion was similar in all subjects and speeds. The higher cost of walking in Pygmies is consistent with the allometric prediction for smaller subjects. The major determinants of the higher cost of walking was the difference in stride frequency (+9.45, SD 0.44% for Pygmies), which affected the mechanical internal work. This explains the observed higher total mechanical work of walking in Pygmies, even when the external component was the same. Most of the differences between Pygmies and Caucasians, observed during walking, tended to disappear when the speed was normalized as the Fronde number. However, this was not the case for running. Thus, whereas the tested hypothesis must be rejected for walking, the data from running, do indeed suggest that Pygmies may differ in some aspects of the mechanics of locomotion.     

Relationship between in vivo muscle force at different speeds of isokinetic movements and myosin isoform expression in men and women

In an attempt to explore the relationship between force production during voluntary contractions at different speeds of isokinetic movement and the myofibrillar protein isoform expression in humans, an improved isokinetic dynamometer that corrects for gravitation, controls for acceleration and deceleration, and identifies a maximum voluntary activation was used. Muscle torque recordings were compared at the same muscle length (knee angle) and the torque was calculated as the average torque at each angle over a large knee angle range (75°–25°) to reduce the influence of small torque oscillation on the calculated torque. Muscle torque at fast (240° s⁻¹) versus slow (30° s⁻¹) speeds of movement, torque normalized to muscle cross-sectional area (specific tension), and absolute torque at fast speeds of movement were measured in 34 young healthy male and female short-, middle-, and long-distance runners. The relationship between the different measures of muscle function and the expression of myosin heavy chain (MyHC) isoforms using enzyme–histochemical and electrophoretic protein separation techniques were investigated. A significant correlation between the 240° s⁻¹ vs 30° s⁻¹ torque ratio and the relative area of the type II fibers and type II MyHC isoforms were observed in both the men (r=0.74;P<0.001) and the women (r=0.81; P<0.05). Thus, the present results confirm a significant relationship between in vivo human muscle function and the MyHC isoform expression in the contracting muscle. Electronic Publication     

Ergometry for estimation of mechanical power output in sprinting in humans using a newly developed self-driven treadmill

An evaluation of mechanical power during walking and running in humans was undertaken after developing a specially designed running ergometer (RE) in which the subjects gripped the handlebar in front of them keeping both arms straight and in a horizontal position. Ten subjects participated in comparisons of the mean horizontal pushing force (MF _am) on the handlebar with the mean horizontal ground reaction force (MF _fp) recorded by force platform under the RE during five different constant speeds of walking or running and sprint running with maximal effort. Mechanical power developed during sprint running on the RE was compared with a 50 m sprint. Mean linear velocity (Mv) of the RE belt was recorded by the rotary encoder attached to the axis of the belt. Mean mechanical power calculated from the handlebar setting (MP _am=MF _am × Mv) was compared to that calculated from force platform recordings (MP _fp=MF _fp × Mv). A high test-retest reproducibility was observed for both MF _fp (r=0.889) and MF _am (r=0.783). Larger values for the coefficient of variation for MF _am (11.3%–15.8%) were observed than for MF _fp (3.3%–8.2%). The MP _am, which were obtained from five different constant speeds of walking, running and sprint running were closely correlated to those of MP _fp (y=0.98x − 19.10,r=0.982, P < 0.001). In sprint running, MP _am was 521.7 W (7.67 W · kg⁻¹) and was correlated to the 50 m sprint time (r=−0.683, P < 0.01). It is concluded that the newly developed RE was useful in the estimation of mechanical power output during human locomotion such as when walking, jogging and sprinting. Accepted: 10 October 2000