Sprint performance-duration relationships are set by the fractional duration of external force application

We hypothesized that the maximum mechanical power outputs that can be maintained during all-out sprint cycling efforts lasting from a few seconds to several minutes can be accurately estimated from a single exponential time constant (k(cycle)) and two measurements on individual cyclists: the peak 3-s power output (P(mech max)) and the maximum mechanical power output that can be supported aerobically (P(aer)). Tests were conducted on seven subjects, four males and three females, on a stationary cycle ergometer at a pedal frequency of 100 rpm. Peak mechanical power output (P(mech max)) was the highest mean power output attained during a 3-s burst; the maximum power output supported aerobically (P(aer)) was determined from rates of oxygen uptake measured during a progressive, discontinuous cycling test to failure. Individual power output-duration relationships were determined from 13 to 16 all-out constant load sprints lasting from 5 to 350 s. In accordance with the above hypothesis, the power outputs measured during all-out sprinting efforts were estimated to within an average of 34 W or 6.6% from P(mech max), P(aer), and a single exponential constant (k(cycle) = 0.026 s(-1)) across a sixfold range of power outputs and a 70-fold range of sprint trial durations (R2 = 0.96 vs. identity, n = 105; range: 180 to 1,136 W). Duration-dependent decrements in sprint cycling power outputs were two times greater than those previously identified for sprint running speed (k(run) = 0.013 s(-1)). When related to the respective times of pedal and ground force application rather than total sprint time, decrements in sprint cycling and running performance followed the same time course (k = 0.054 s(-1)). We conclude that the duration-dependent decrements in sprinting performance are set by the fractional duration of the relevant muscular contractions.     

High-speed running performance is largely unaffected by hypoxic reductions in aerobic power.

We tested the importance of aerobic metabolism to human running speed directly by altering inspired oxygen concentrations and comparing the maximal speeds attained at different rates of oxygen uptake. Under both normoxic (20.93% O2) and hypoxic (13.00% O2) conditions, four fit adult men completed 15 all-out sprints lasting from 15 to 180 s as well as progressive, discontinuous treadmill tests to determine maximal oxygen uptake and the metabolic cost of steady-state running. Maximal aerobic power was lower by 30% (1.00 +/- 0.15 vs. 0.77 +/- 0.12 ml O2. kg-1. s-1) and sprinting rates of oxygen uptake by 12-25% under hypoxic vs. normoxic conditions while the metabolic cost of submaximal running was the same. Despite reductions in the aerobic energy available for sprinting under hypoxic conditions, our subjects were able to run just as fast for sprints of up to 60 s and nearly as fast for sprints of up to 120 s. This was possible because rates of anaerobic energy release, estimated from oxygen deficits, increased by as much as 18%, and thus compensated for the reductions in aerobic power. We conclude that maximal metabolic power outputs during sprinting are not limited by rates of anaerobic metabolism and that human speed is largely independent of aerobic power during all-out runs of 60 s or less.     

Energetics of high-speed running: integrating classical theory and contemporary observations

We hypothesized that the anaerobic power and aerobic power outputs during all-out runs of any common duration between 10 and 150 s would be proportional to the maximum anaerobic (E(an-max)) and aerobic powers (E(aer-max)) available to the individual runner. Seventeen runners who differed in E(an-max) and E(aer-max) (5 sprinters, 5 middle-distance runners, and 7 long distance runners) were tested during treadmill running on a 4.6 degrees incline. E(an-max) was estimated from the fastest treadmill speed subjects could attain for eight steps. E(aer-max) was determined from a progressive, discontinuous, treadmill test to failure. Oxygen deficits and rates of uptake were measured to assess the respective anaerobic and aerobic power outputs during 11-16 all-out treadmill runs that elicited failure between 10 and 220 s. We found that, during all-out runs of any common duration, the relative anaerobic and aerobic powers utilized were largely the same for sprint, middle-distance, and long-distance subjects. The similar fractional utilization of the E(an-max) and E(aer-max) available during high-speed running 1) provides empirical values that modify and advance classic theory, 2) allows rates of anaerobic and aerobic energy release to be quantified from individual maxima and run durations, and 3) explains why the high-speed running performances of different event specialists can be accurately predicted (R(2) = 0.97; n = 254) from two direct measurements and the same exponential time constant.     

A simple method for computing sprint acceleration kinetics from running velocity data: Replication study with improved design

Measuring the ground reaction forces (GRF) underlying sprint acceleration is important to understanding the performance of such a common task. Until recently direct measurements of GRF during sprinting were limited to a few steps per trial, but a simple method (SM) was developed to estimate GRF across an entire acceleration. The SM utilizes displacement- or velocity-time data and basic computations applied to the runner’s center of mass and was validated against compiled force plate (FP) measurements; however, this validation used multiple-trials to generate a single acceleration profile, and consequently fatigue and error may have introduced noise into the analyses. In this study, we replicated the original validation by comparing the main sprint kinetics and force-velocity-power variables (e.g. GRF and its horizontal and vertical components, mechanical power output, ratio of horizontal component to resultant GRF) between synchronized FP data from a single sprinting acceleration and SM data derived from running velocity measured with a 100 Hz laser. These analyses were made possible thanks to a newly developed 50-m FP system providing seamless GRF data during a single sprint acceleration. Sixteen trained male sprinters performed two all-out 60-m sprints. We observed good agreement between the two methods for kinetic variables (e.g. grand average bias of 4.71%, range 0.696 ± 0.540–8.26 ± 5.51%), and high inter-trial reliability (grand average standard error of measurement of 2.50% for FP and 2.36% for the SM). This replication study clearly shows that when implemented correctly, this method accurately estimates sprint acceleration kinetics.     

Intermuscular coordination in a sprint push-off.

This study was designed to investigate the patterns of intermuscular coordination during a sprinting event. In previous research it was found that despite the indeterminacy problem of movement control, movements like vertical jumping, speed skating and cycling are performed in a stereotyped manner. It was hypothesized that this might be due to constraints associated with the transformation of joint rotations into the desired translation. The objective of the present study was to determine the extent to which the intermuscular coordination patterns during other movements also are performed in a stereotyped manner and, if that is true, whether this can be understood on the basis of such constraints. Seven elite sprint runners were instructed to execute an explosive sprinting dash. Ground reaction forces and cinematographic data were recorded for the second stance phase of the sprint. Simultaneously, electromyographic activity of nine leg muscles was recorded telemetrically. Linked-segment modeling was used to obtain net joint moments and net joint powers. Different athletes appeared to perform the sprint in a stereotyped manner. The muscle coordination pattern is characterized by a proximo to distal sequence in timing of the monoarticular muscles. When compared to the sequential pattern found in jumping, the biarticular hamstrings and rectus femoris muscles behave differently; in the sprint a more pronounced reciprocal activity between these muscles exists. The resulting movement pattern is characterized by a sequence of upper leg extension and plantar flexion. The observed sequence in timing of muscle activation patterns is aimed at solving the problems associated with the earlier identified geometrical and anatomical constraint. However, the coordination pattern cannot be fully understood on the basis of these constraints. A specific constraint is identified with respect to the direction of the ground reaction force, which explains the pronounced reciprocal activity of the biarticular hamstring and rectus femoris muscles. The intermuscular coordination pattern in the sprint can be seen as a compromise between the specific requirement of the sprint and the advantageous effect of a proximo to distal sequence as found previously for jumping.     

A power equation for the sprint in speed skating.

An analysis of the start of the 500 m speed skating races during the 1988 Olympic Winter Games showed a remarkably high correlation between the acceleration of the skater in the first second of the sprint and the final time (r = -0.75). In this study a power equation is used to explain this high coefficient of correlation. The performance in speed skating is determined by the capability of external power production by the speed skater. This power is necessary to overcome the air and ice friction and to increase the kinetic energy of the skater. Numerical values of the power dissipated to air and ice friction, both dependent on speed, are obtained from ice friction and wind tunnel experiments. Using aerobic and anaerobic power production as measured during supra maximal bicycle tests of international-level speed skaters, a model of the kinetics of power production is obtained. Simulation of power production and power dissipation yields values of speed and acceleration and, finally, the performance time of the sprint during speed skating. The mean split time at 100 m and the final time at 500 m in these races, derived from simulation, were 10.57 s (+/- 0.31) and 37.82 s (+/- 0.96), respectively. The coefficient of correlation between the simulated 500 m times and the actual 500 m times was 0.90. From the results of this study it can be concluded that the distribution of the available anaerobic energy is an important factor in the short lasting events. For the same amount of anaerobic energy the better sprinters appear to be able to liberate considerably more energy at the onset of the race than skaters of lower performance level.     

Six sessions of sprint interval training increases muscle oxidative potential and cycle endurance capacity in humans.

Parra et al. (Acta Physiol. Scand 169: 157-165, 2000) showed that 2 wk of daily sprint interval training (SIT) increased citrate synthase (CS) maximal activity but did not change "anaerobic" work capacity, possibly because of chronic fatigue induced by daily training. The effect of fewer SIT sessions on muscle oxidative potential is unknown, and aside from changes in peak oxygen uptake (Vo(2 peak)), no study has examined the effect of SIT on "aerobic" exercise capacity. We tested the hypothesis that six sessions of SIT, performed over 2 wk with 1-2 days rest between sessions to promote recovery, would increase CS maximal activity and endurance capacity during cycling at approximately 80% Vo(2 peak). Eight recreationally active subjects [age = 22 +/- 1 yr; Vo(2 peak) = 45 +/- 3 ml.kg(-1).min(-1) (mean +/- SE)] were studied before and 3 days after SIT. Each training session consisted of four to seven "all-out" 30-s Wingate tests with 4 min of recovery. After SIT, CS maximal activity increased by 38% (5.5 +/- 1.0 vs. 4.0 +/- 0.7 mmol.kg protein(-1).h(-1)) and resting muscle glycogen content increased by 26% (614 +/- 39 vs. 489 +/- 57 mmol/kg dry wt) (both P < 0.05). Most strikingly, cycle endurance capacity increased by 100% after SIT (51 +/- 11 vs. 26 +/- 5 min; P < 0.05), despite no change in Vo(2 peak). The coefficient of variation for the cycle test was 12.0%, and a control group (n = 8) showed no change in performance when tested approximately 2 wk apart without SIT. We conclude that short sprint interval training (approximately 15 min of intense exercise over 2 wk) increased muscle oxidative potential and doubled endurance capacity during intense aerobic cycling in recreationally active individuals.     

Effect of power, pedal rate, and force on average muscle fiber conduction velocity during cycling.

Muscle fiber conduction velocity (MFCV) provides indications on motor unit recruitment strategies due to the relation between conduction velocity and fiber diameter. The aim of this study was to investigate MFCV of thigh muscles during cycling at varying power outputs, pedal rates, and external forces. Twelve healthy male participants aged between 19 and 30 yr cycled on an electronically braked ergometer at 45, 60, 90, and 120 rpm. For each pedal rate, subjects performed two exercise intensities, one at an external power output corresponding to the previously determined lactate threshold (100% LT) and the other at half of this power output (50% LT). Surface electromyogram signals were detected during cycling from vastus lateralis and medialis muscles with linear adhesive arrays of eight electrodes. In both muscles, MFCV was higher at 100% LT compared with 50% LT for all average pedal rates except 120 rpm (mean +/- SE, 4.98 +/- 0.19 vs. 4.49 +/- 0.18 m/s; P < 0.001). In all conditions, MFVC increased with increasing instantaneous knee angular speed (from 4.14 +/- 0.16 to 5.08 +/- 0.13 m/s in the range of instantaneous angular speeds investigated; P < 0.001). When MFCV was compared at the same external force production (i.e., 90 rpm/100% LT vs. 45 rpm/50% LT, and 120 rpm/100% LT vs. 60 rpm/50% LT), MFCV was higher at the faster pedal rate (5.02 +/- 0.17 vs. 4.64 +/- 0.12 m/s, and 4.92 +/- 0.19 vs. 4.49 +/- 0.11 m/s, respectively; P < 0.05) due to the increase in inertial power required to accelerate the limbs. It was concluded that, during repetitive dynamic movements, MFCV increases with the external force developed, instantaneous knee angular speed, and average pedal rate, indicating progressive recruitment of large, high conduction velocity motor units with increasing muscle force.     

Skeletal muscle metabolic and ionic adaptations during intense exercise following sprint training in humans.

The effects of sprint training on muscle metabolism and ion regulation during intense exercise remain controversial. We employed a rigorous methodological approach, contrasting these responses during exercise to exhaustion and during identical work before and after training. Seven untrained men undertook 7 wk of sprint training. Subjects cycled to exhaustion at 130% pretraining peak oxygen uptake before (PreExh) and after training (PostExh), as well as performing another posttraining test identical to PreExh (PostMatch). Biopsies were taken at rest and immediately postexercise. After training in PostMatch, muscle and plasma lactate (Lac(-)) and H(+) concentrations, anaerobic ATP production rate, glycogen and ATP degradation, IMP accumulation, and peak plasma K(+) and norepinephrine concentrations were reduced (P<0.05). In PostExh, time to exhaustion was 21% greater than PreExh (P<0.001); however, muscle Lac(-) accumulation was unchanged; muscle H(+) concentration, ATP degradation, IMP accumulation, and anaerobic ATP production rate were reduced; and plasma Lac(-), norepinephrine, and H(+) concentrations were higher (P<0.05). Sprint training resulted in reduced anaerobic ATP generation during intense exercise, suggesting that aerobic metabolism was enhanced, which may allow increased time to fatigue.     

Decreased PDH activation and glycogenolysis during exercise following fat adaptation with carbohydrate restoration

Five days of a high-fat diet while training, followed by 1 day of carbohydrate (CHO) restoration, increases rates of whole body fat oxidation and decreases CHO oxidation during aerobic cycling. The mechanisms responsible for these shifts in fuel oxidation are unknown but involve up- and downregulation of key regulatory enzymes in the pathways of skeletal muscle fat and CHO metabolism, respectively. This study measured muscle PDH and HSL activities before and after 20 min of cycling at 70% VO2peak and 1 min of sprinting at 150% peak power output (PPO). Estimations of muscle glycogenolysis were made during the initial minute of exercise at 70% VO2peak and during the 1-min sprint. Seven male cyclists undertook this exercise protocol on two occasions. For 5 days, subjects consumed in random order either a high-CHO (HCHO) diet (10.3 g x kg(-1) x day(-1) CHO, or approximately 70% of total energy intake) or an isoenergetic high-fat (FAT-adapt) diet (4.6 g x kg(-1) x day(-1) FAT, or 67% of total energy) while undertaking supervised aerobic endurance training. On day 6 for both treatments, subjects ingested an HCHO diet and rested before their experimental trials on day 7. This CHO restoration resulted in similar resting glycogen contents (FAT-adapt 873 +/- 121 vs. HCHO 868 +/- 120 micromol glucosyl units/g dry wt). However, the respiratory exchange ratio was lower during cycling at 70% VO2peak in the FAT-adapt trial, which resulted in an approximately 45% increase and an approximately 30% decrease in fat and CHO oxidation, respectively. PDH activity was lower at rest and throughout exercise at 70% VO2peak (1.69 +/- 0.25 vs. 2.39 +/- 0.19 mmol x kg wet wt(-1) x min(-1)) and the 1-min sprint in the FAT-adapt vs. the HCHO trial. Estimates of glycogenolysis during the 1st min of exercise at 70% VO2peak and the 1-min sprint were also lower after FAT-adapt (9.1 +/- 1.1 vs. 13.4 +/- 2.1 and 37.3 +/- 5.1 vs. 50.5 +/- 2.7 glucosyl units x kg dry wt(-1) x min(-1)). HSL activity was approximately 20% higher (P = 0.12) during exercise at 70% VO2peak after FAT-adapt. Results indicate that previously reported decreases in whole body CHO oxidation and increases in fat oxidation after the FAT-adapt protocol are a function of metabolic changes within skeletal muscle. The metabolic signals responsible for the shift in muscle substrate use during cycling at 70% VO2peak remain unclear, but lower accumulation of free ADP and AMP after the FAT-adapt trial may be responsible for the decreased glycogenolysis and PDH activation during sprinting.     

Cardiorespiratory and neuromuscular responses to motocross riding

The aim of the present study was to examine physiological and neuromuscular responses during motocross riding at individual maximal speed together with the riding-induced changes in maximal isometric force production. Seven A-level (group A) and 5 hobby-class (group H) motocross-riders performed a 30-minute riding test on a motocross track and maximal muscle strength and oxygen uptake (VO2max) tests in a laboratory. During the riding the mean (+/-SD) VO2 reduced in group A from 86 +/- 10% to 69 +/- 6% of the maximum (P < 0.001), whereas in group H the corresponding reduction was from 94 +/- 25% to 82 +/- 20% (P < 0.05). This relative VO2 during the riding correlated with riding speed (r = 0.70, P < 0.01). Heart rate (HR) was maintained at the level of 97 +/- 7% of its maximum in group A and at 98 +/- 3% in group H. Mean muscle activation of the lower body during riding varied between 24% and 38% of its maximum in group A and between 40% and 45% in group H. In conclusion, motocross is a sport that causes great physical stress and demands on both skill and physical capacity of the rider. Physical stress occurs as the result of handling of the bike when receiving continuous impacts in the situation requiring both aerobic and anaerobic metabolism. Our data suggest that both maximal capacity and strain during the ride should be measured to analyze the true physiological and neuromuscular demands of motocross ride. For the practice, this study strongly suggests to train not only aerobic and anaerobic capacity but also to use strength and power training for successful motocross riding.     

The development of contact force construction in the dynamic-contact task of cycling [corrected

Purposeful movement requires that an individual produce appropriate joint torques to accelerate segments, and when environmental contact is involved, to develop task-appropriate contact forces. Developmental research has been confined largely to the mastery of unconstrained movement skills (pointing, kicking). The purpose of this study was to study the developmental progression that characterizes the interaction of muscular and non-muscular forces in tasks constrained by contact with the environment. Seven younger children (YC, 6-8 years), 7 older children (OC, 9-11 years) and 7 adults (AD) pedaled an ergometer (80 rpm) at an anthropometrically scaled cycling power. Resultant forces measured at the pedal's surface were decomposed into muscle, inertia and gravity components. Muscle pedal forces were further examined in terms of the underlying lower extremity joint torques and kinematic weights that constitute the muscular component of the pedal force. Data showed children applied muscle forces to the pedal in a significantly different manner compared to adults, and that this was due to the children's lower segmental mass and inertia. The children adjusted the contribution of the proximal joint muscle torques to compensate for reduced contributions to the resultant pedal force by gravitational and inertial components. These data show that smaller segmental mass and inertia limit younger children's ability to construct the dynamic-contact task of cycling in an adult-like form. On the basis of these results, however, the children's response was not "immature". Rather, the results show a task-appropriate adaptation to lower segmental mass and inertia.     

Foot and ankle forces during an automobile collision: the influence of muscles

Muscles have a potentially important effect on lower extremity injuries during an automobile collision. Computational modeling can be a powerful tool to predict these effects and develop protective interventions. Our purpose was to determine how muscles influence peak foot and ankle forces during an automobile collision. A 2-D bilateral musculoskeletal model was constructed with seven segments. Six muscle groups were included in the right lower extremity, each represented by a Hill muscle model. Vehicle deceleration data were applied as input and the resulting movements were simulated. Three models were evaluated: no muscles (NM), minimal muscle activation at a brake pedal force of 400 N (MN), and maximal muscle activation to simulate panic braking (MX). Muscle activation always resulted in large increases in peak joint force. Peak ankle joint force was greatest for MX (10120 N), yet this model also had the lowest peak rearfoot force (629 N). Peak force on the Achilles tendon was 4.5 times greater, during MX (6446 N) compared to MN (1430 N). We conclude that (1). external and internal forces are dependent on muscles, (2). muscle activation level could exacerbate axial loading injuries, (3). external and internal forces can be inversely related once muscle properties are included.     

Influence of training status on maximal accumulated oxygen deficit during all-out cycle exercise

The influence of training status on the maximal accumulated oxygen deficit (MAOD) was used to assess the validity of the MAOD method during supramaximal all-out cycle exercise. Sprint trained (ST; n = 6), endurance trained (ET; n = 8), and active untrained controls (UT; n = 8) completed a 90 s all-out variable resistance test on a modified Monark cycle ergometer. Pretests included the determination of peak oxygen uptake ( O_2peak) and a series (5–8) of 5-min discontinuous rides at submaximal exercise intensities. The regression of steady-state oxygen uptake on power output to establish individual efficiency relationships was extrapolated to determine the theoretical oxygen cost of the supramaximal power output achieved in the 90 s all-out test. Total work output in 90 s was significantly greater in the trained groups (P<0.05), although no differences existed between ET and ST. Anaerobic capacity, as assessed by MAOD, was larger in ST compared to ET and UT. While the relative contributions of the aerobic and anaerobic energy systems were not significantly different among the groups, ET were able to achieve significantly more aerobic work than the other two groups, while ST were able to achieve significantly more anaerobic work. Peak power and peak pedalling rate were significantly higher in ST. The results suggested that MAOD determined during all-out exercise was sensitive to training status and provided a useful assessment of anaerobic capacity. In our study sprint training, compared with endurance training, appeared to enhance significantly power output and high intensity performance over brief periods (up to 60 s), yet few overall differences in performance (i.e. total work) existed during 90 s of all-out exercise.     

A cycle ergometer mounted on a standard force platform for three-dimensional pedal forces measurement during cycling

This report describes a new method allowing to measure the three-dimensional forces applied on right and left pedals during cycling. This method is based on a cycle ergometer mounted on a force platform. By recording the forces applied on the force platform and applying the fundamental mechanical equations, it was possible to calculate the instantaneous three-dimensional forces applied on pedals. It was validated by static and dynamic tests. The accuracy of the present system was -7.61 N, -3.37 N and -2.81 N, respectively, for the vertical, the horizontal and the lateral direction when applying a mono-directional force and -4.52 N when applying combined forces. In pedaling condition, the orientation and magnitude of the pedal forces were comparable to the literature. Moreover, this method did not modify the mechanical properties of the pedals and offered the possibility for pedal force measurement with materials often accessible in laboratories. Measurements obtained showed that this method has an interesting potential for biomechanical analyses in cycling.     

Inhibition by nifedipine of the indirectly induced contractile response of the rat diaphragm

The effects of nifedipine (30 micrograms/ml) on isometric force production of in-vitro rat diaphragm were studied during direct and indirect modes of muscle activation. During direct muscle stimulation, nifedipine potentiated isometric force during twitch and unfused tetanic stimulation. Indirectly elicited responses, evoked by stimulation of the phrenic nerve, were uniformly depressed following nifedipine. Inhibition of indirect force production increased with time, while force potentiation with direct activation remained constant. We conclude that inhibitory effects of nifedipine on the phrenic nerve-diaphragm preparation are specific for nerve or neuromuscular junction.     

Influence of short vs. long repetition sprint training on selected fitness components in young soccer players

The aim of this study was to compare the effect of short-sprint repetition and long-sprint repetition training (SST, LST), matched for total distance, on selected fitness components in young soccer players. Thirty young (14-15 years) soccer players were randomly assigned to either the short-sprint training group or long-sprint training group and completed 2 similar sets of fitness tests before and after 7 weeks of training. The 2 training programs consisted of SST (4-6 sets of 4 × 50-m all-out sprint) and LST (4-6 sets of 200-m run at 85% of maximum speed), each performed 3 times a week. Before training, there were no baseline between-group differences in predicted VO2max, standing long jump, 30-m sprint time, 4 × 10-m shuttle running time, and 250-m running time. Both training programs led to a significant improvement in VO2max (predicted from the 20-m shuttle run, p < 0.01), with no between-group difference (p = 0.14). Both training programs also led to a significant improvement in the anaerobic fitness variables of 30-m sprint time (p < 0.01), 4 × 10-m shuttle running time (p < 0.01), and 250-m running time (p < 0.01), with no between-group differences. Neither of the training programs had a significant effect on standing long jump (p = 0.21). The study showed that long, near-maximal sprints, and short, all-out sprint training, matched for total distance, are equally effective in enhancing both the aerobic and anaerobic fitness of young soccer players. Therefore, to maintain a player's training interest and enthusiasm, coaches may alternate between these methods during the busy soccer season.     

The effect of pedal rate and time of day on the time to exhaustion from high-intensity exercise

The aim of this study was to examine the supposed influence of pedal rate on the diurnal fluctuation of the time to exhaustion from high-intensity exercise. Eleven male cyclists performed three tests at 06:00 h and three at 18:00 h at a free pedal rate (FPR) and two imposed pedal rates (80% and 120% of the FPR). They performed the tests until exhaustion using a power output corresponding to 95% maximal power (Pmax). Time to exhaustion, rectal temperature, oxygen consumption (.VO2), M. quadriceps, vastus medialis, M. biceps femoris electromyographic Root Mean Square activity rise (RMS slope), and blood lactate concentration were measured. The mean time to exhaustion recorded at 18:00 h (270.6+/-104.8 sec) was greater than at 06:00 h (233.9+/-84.9 sec). The time to exhaustion was significantly greater when the pedal rate was imposed at 80% versus 120% FPR. The blood lactate concentration and absolute core temperature at the point of exhaustion were significantly higher during tests done at 18:00 h. There was no diurnal variation in core temperature increase, .VO2, and RMS slope. The time-of-day effect for every variable did not depend on pedal rate. Diurnal variations in maximal aerobic endurance cannot be explained by a change in aerobic metabolism or in muscular fatigue. The origin of the diurnal variation in the time to exhaustion is likely to lie in greater participation in anaerobic metabolism. Also, the influence of temperature on neuromuscular functioning as an explanation for the diurnal variation in performance cannot be excluded in this study. The hypothesis on the basis of which pedal rate would influence diurnal variations in time to exhaustion in cycling was not validated by this research.     

Prediction of pedal forces in bicycling using optimization methods

The bicycle-rider system is modeled as a planar five-bar linkage with pedal forces and pedal dynamics as input. The pedal force profile input is varied, maintaining constant average bicycle power, in order to obtain the optimal pedal force profile that minimizes two cost functions. One cost function is based on joint moments and the other is based on muscle stresses. Predicted (optimal) pedal profiles as well as joint moment time histories are compared to representative real data to examine cost function appropriateness. Both cost functions offer reasonable predictions of pedal forces. The muscle stress cost function, however, better predicts joint moments. Predicted muscle activity also correlates well with myoelectric data. The factors that lead to effective (i.e. low cost) pedalling are examined. Pedalling effectiveness is found to be a complex function of pedal force vector orientation and muscle mechanics.     

Pre-impact lower extremity posture and brake pedal force predict foot and ankle forces during an automobile collision

BACKGROUND: The purpose of this study was to determine how a driver's foot and ankle forces during a frontal vehicle collision depend on initial lower extremity posture and brake pedal force. METHOD OF APPROACH: A 2D musculoskeletal model with seven segments and six right-side muscle groups was used. A simulation of a three-second braking task found 3647 sets of muscle activation levels that resulted in stable braking postures with realistic pedal force. These activation patterns were then used in impact simulations where vehicle deceleration was applied and driver movements and foot and ankle forces were simulated. Peak rearfoot ground reaction force (F(RF)), peak Achilles tendon force (FAT), peak calcaneal force (F(CF)) and peak ankle joint force (F(AJ)) were calculated. RESULTS: Peak forces during the impact simulation were 476 +/- 687 N (F(RF)), 2934 +/- 944 N (F(CF)) and 2449 +/- 918 N (F(AJ)). Many simulations resulted in force levels that could cause fractures. Multivariate quadratic regression determined that the pre-impact brake pedal force (PF), knee angle (KA) and heel distance (HD) explained 72% of the variance in peak FRF, 62% in peak F(CF) and 73% in peak F(AJ). CONCLUSIONS: Foot and ankle forces during a collision depend on initial posture and pedal force. Braking postures with increased knee flexion, while keeping the seat position fixed, are associated with higher foot and ankle forces during a collision.