Horizontal Propulsive Force Research Articles

Ground Reaction Forces and Energy Exchange During Underwater Walking.

Underwater walking was a crucial step in the evolutionary transition from water to land. Underwater walkers use fins and/or limbs to interact with the benthic substrate and produce propulsive forces. The dynamics of underwater walking remain poorly understood due to the lack of a sufficiently sensitive and waterproof system to measure substrate reaction forces (SRFs). Using an underwater force plate (described in our companion paper), we quantify SRFs during underwater walking in axolotls (Ambystoma mexicanum) and Spot prawn (Pandalus platyceros), synchronized with videography. The horizontal propulsive forces were greater than the braking forces in both species to overcome hydrodynamic drag. In axolotls, potential energy (PE) fluctuations were far smaller than kinetic energy (KE) fluctuations due to high buoyant support (97%), whereas the magnitudes were similar in the prawn due to lower buoyant support (93%). However, both species show minimal evidence of exchange between KE and PE, which, along with the effects of hydrodynamic drag, is incompatible with inverted pendulum dynamics. Our results show that, despite their evolutionary links, underwater walking has fundamentally different dynamics compared with terrestrial walking and emphasize the substantial consequences of differences in body plan in underwater walking.

Change of Direction Speed and Technique Modification Training Improves 180° Turning Performance, Kinetics, and Kinematics.

This study aimed to examine the effects of change of direction (COD) speed and technique modification training on 180° turning performance (completion time, ground contact time [GCT], and exit velocity), kinetics, and kinematics. A non-randomised 6 week intervention study was administered. Thirteen male multidirectional sport athletes formed the intervention group (IG), participating in two COD speed and technique modification sessions per week. A total of 12 male multidirectional sport athletes formed the control group (CG). All subjects performed six modified 505 trials, whereby pre-to-post-intervention biomechanical changes were evaluated using three-dimensional motion analysis. Two-way mixed analysis of variances revealed significant interaction effects (group × time) for completion time, mean horizontal propulsive force (HPF), horizontal to vertical mean braking and propulsive force ratios for the penultimate (PFC) and final foot contact (FFC), FFC peak knee flexion and PFC hip flexion angle (p ≤ 0.040, η2 = 0.170–0.417). The IG displayed small to large improvements post-intervention in these aforementioned variables (p ≤ 0.058, g = 0.49–1.21). Turning performance improvements were largely to very largely (p ≤ 0.062, r or ρ = 0.527–0.851) associated with increased mean HPF, more horizontally orientated FFC propulsive force and PFC braking force, and greater pelvic rotation, PFC hip flexion, and PFC velocity reductions. COD speed and technique modification is a simple, effective training strategy that enhances turning performance.

The Stair Climb Power Test

The Stair Climb Power Test

Biomechanical Determinants of the Modified and Traditional 505 Change of Direction Speed Test.

Dos'Santos, T, McBurnie, A, Thomas, C, Comfort, P, and Jones, PA. Biomechanical determinants of the modified and traditional 505 change of direction speed test. J Strength Cond Res 34(5): 1285-1296, 2020-The aim of this study was to investigate the whole-body biomechanical determinants of 180° change of direction (COD) performance. Sixty-one male athletes (age: 20.7 ± 3.8 years, height: 1.77 ± 0.06 m, mass: 74.7 ± 10.0 kg) from multiple sports (soccer, rugby, and cricket) completed 6 trials of the modified and traditional 505 on their right leg, whereby 3D motion and ground reaction force data were collected during the COD. Pearson's and Spearman's correlations were used to explore the relationships between biomechanical variables and COD completion time. Independent t-tests and Hedges' g effect sizes were conducted between faster (top 20) and slower (bottom 20) performers to explore differences in biomechanical variables. Key kinetic and kinematic differences were demonstrated between faster and slower performers with statistically significant (p ≤ 0.05) and meaningful differences (g = 0.56-2.70) observed. Faster COD performers displayed greater peak and mean horizontal propulsive forces (PF) in shorter ground contact times, more horizontally orientated peak resultant braking and PFs, greater horizontal to vertical mean and peak braking and PF ratios, greater approach velocities, and displayed greater reductions in velocity over key instances of the COD. In addition, faster performers displayed greater penultimate foot contact (PFC) hip, knee, and ankle dorsi-flexion angles, greater medial trunk lean, and greater internal pelvic and foot rotation. These aforementioned variables were also moderately to very largely (r or ρ = 0.317-0.795, p ≤ 0.013) associated with faster COD performance. Consequently, practitioners should focus not only on developing their athletes' ability to express force rapidly, but also develop their technical ability to apply force horizontally. In addition, practitioners should consider coaching a 180° turning strategy that emphasizes high PFC triple flexion for center of mass lowering while also encouraging whole-body rotation to effectively align the body toward the exit for faster performance.

Mechanical Determinants of Faster Change of Direction Speed Performance in Male Athletes.

Dos'Santos, T, Thomas, C, Jones, PA, and Comfort, P. Mechanical determinants of faster change of direction speed performance in male athletes. J Strength Cond Res 31(3): 696-705, 2017-Mechanical variables during change of directions, for example, braking and propulsive forces, impulses, and ground contact times (GCT) have been identified as determinants of faster change of direction speed (CODS) performance. The purpose of this study was to investigate the mechanical determinants of 180° CODS performance with mechanical characteristic comparisons between faster and slower performers; while exploring the role of the penultimate foot contact (PEN) during the change of direction. Forty multidirectional male athletes performed 6 modified 505 (mod505) trials (3 left and right), and ground reaction forces were collected across the PEN and final foot contact (FINAL) during the change of direction. Pearson's correlation coefficients and coefficients of determination were used to explore the relationship between mechanical variables and mod505 completion time. Independent T-tests and Cohen's d effect sizes (ES) were conducted between faster (n = 10) and slower (n = 10) mod505 performers to explore differences in mechanical variables. Faster CODS performance was associated (p ≤ 0.05) with shorter GCTs (r = 0.701-0.757), greater horizontal propulsive forces (HPF) (r = -0.572 to -0.611), greater horizontal braking forces (HBF) in the PEN (r = -0.337), lower HBF ratios (r = -0.429), and lower FINAL vertical impact forces (VIF) (r = 0.449-0.559). Faster athletes demonstrated significantly (p ≤ 0.05, ES = 1.08-2.54) shorter FINAL GCTs, produced lower VIF, lower HBF ratios, and greater HPF in comparison to slower athletes. These findings suggest that different mechanical properties are required to produce faster CODS performance, with differences in mechanical properties observed between fast and slower performers. Furthermore, applying a greater proportion of braking force during the PEN relative to the FINAL may be advantageous for turning performance.

Biomechanical Insights Into Differences Between the Mid-Acceleration and Maximum Velocity Phases of Sprinting.

Yu, J, Sun, Y, Yang, C, Wang, D, Yin, K, Herzog, W, and Liu, Y. Biomechanical insights into differences between the mid-acceleration and maximum velocity phases of sprinting. J Strength Cond Res 30(7): 1906-1916, 2016-Investigating the differences between distinct phases of sprint running may increase the knowledge about the specific physical abilities needed for different phases of sprinting. Differences between the mid-acceleration and maximum velocity phases of sprint running have not yet been adequately investigated. Twenty male sprinters performed maximum-effort sprint runs, and measurements were made at 12 m from start for the mid-acceleration phase and at 40 m from the start for the maximum velocity phase. Kinematic data and ground reaction forces (GRFs) were collected at a rate of 200 and 1000 Hz, respectively. Intersegmental dynamics analysis was performed to investigate the interaction of muscle torque (MUS) with other passive torques. The peak horizontal braking force was significantly lower for the acceleration compared with that for the maximal velocity phase, whereas the peak horizontal propulsive force was similar for both phases. The peak MUS at the hip and knee joints for the braking phase was significantly smaller in the acceleration phase than in the maximum velocity phase. In conclusion, compared with the maximum velocity phase, the lower horizontal braking force was the primary cause for the increase in running velocity during the mid-acceleration phase. The force produced by lower limb muscles required to counteract external torques caused by the horizontal braking force in the braking phase was smaller during the acceleration phase than the maximum velocity phase. Therefore, training aimed at reducing the horizontal braking force might be more important than increasing the force produced by the lower limb muscles for success of the mid-acceleration phase.

The effects of Task-Oriented Motor Training on gait characteristics of patients with type 2 diabetes neuropathy.

BackgroundIt is known that general gait training improves lower extremity muscle strength and endurance in Diabetes Neuropathy (DN). But, it is still unknown whether Task-Oriented (TO) gait training would change gait biomechanics and the risk of falling in DN. TO gait training focuses on promoting timing and coordination of lower extremity movements through goal-directed practices with sufficient repetition.MethodsA group of 14 patients with DN participated in a time-series study. All subjects participated in four sessions of assessments (Initial, Pre, Post and Follow-Up). Training was twice a week for 12 weeks. Vertical and horizontal Ground Reaction Forces (GRF), Time Get up and Go (TGUG) and Fall Efficacy Scale-International (FES-I) were evaluated. Gait training started with stepping patterns that progressively changed to complicated patterns of walking. Then, training continued combining walking patterns with upper extremity activities and then ended with treadmill-paced practice.ResultsDN patients significantly increased Second Vertical Peak Force and Horizontal Propulsive Force in addition decrease in Minimum Vertical Force. TGUG significantly decreased while FES-I reflected significant increase after gait training.DiscussionConclusively, training not only improved gait performance, confidence in daily activities and attenuated risk of falling, but also helped DN patients to improve feet biomechanics, muscles timing and coordination.ConclusionsGait training with respect to principles of motor learning allowed patients to effectively improve through sessions.

Free Flight Testing and Performance Evaluation of a Passively Morphing Ornithopter

Unmanned Aerial Vehicles (UAVs) are proliferating in both the civil and military markets. Flapping wing UAVs, or ornithopters, have the potential to combine the agility and maneuverability of rotary wing aircraft with excellent performance in the low Reynolds number flight regimes. The purpose of this paper is to present new free flight experimental results for an ornithopter equipped with single degree of freedom compliant spines. The compliant spines are designed and optimized in terms of mass, maximum von-Mises stress, and desired wing bending deflections. The spines are inserted in an experimental ornithopter wing leading edge spar, in order to achieve a set of desired kinematics during the up and down strokes of a flapping cycle. The ornithopter is flown at Wright Patterson Air Force Base in the Air Force Research Laboratory Small Unmanned Air Systems (SUAS) indoor flight facility. Vicon® motion tracking cameras are used to track the motion of the vehicle for four different wing configurations. The effect of the presence of the compliant spine on the wings and body kinematics, as well as the leading edge spar deflection during free flight is presented in this paper. Several metrics were used to evaluate the vehicle performance with various compliant spine designs inserted in the leading edge spar of the wings. Results show that passively morphing the wings, via adding compliance in the leading edge spar, does not require additional power expenditure and is beneficial to the overall vertical and horizontal propulsive force production.

The mechanical cost of transport of fast running animals

Regarding running animals, algebraic expressions for the horizontal (ωx) and vertical (ωy) components of the mechanical cost of transport are deduced for a ground force pattern based on the Spring–mass model. Defining μ˜ as the maximum ground forces ratio μ˜=max(Fx)/max(Fy), the analysis shows that the mechanical cost of transport ωx+ωy for fast running animals is approximately proportional to μ˜, and to the relative contact length, and positively correlated to the limb take-off angle and the collision angle. The vertical cost ωy is shown to approximate to zero for fast running animals. Sustained top running speeds are predicted to require the largest possible values of μ˜ and therefore relatively large horizontal propulsive forces, as well as a minimum possible ground contact time. The equations also show that animals running relatively slow would tend to prefer certain interval of values for parameter μ˜, which would minimize both their mechanical cost of transport and their metabolic cost of transport. Very large animals are suspected to be less capable of developing large values of μ˜, which possibly renders them incapable of developing great speeds.

A comparison of lower limb EMG and ground reaction forces between barefoot and shod gait in participants with diabetic neuropathic and healthy controls

BackgroundIt is known that when barefoot, gait biomechanics of diabetic neuropathic patients differ from non-diabetic individuals. However, it is still unknown whether these biomechanical changes are also present during shod gait which is clinically advised for these patients. This study investigated the effect of the participants own shoes on gait biomechanics in diabetic neuropathic individuals compared to barefoot gait patterns and healthy controls.MethodsGround reaction forces and lower limb EMG activities were analyzed in 21 non-diabetic adults (50.9 ± 7.3 yr, 24.3 ± 2.6 kg/m2) and 24 diabetic neuropathic participants (55.2 ± 7.9 yr, 27.0 ± 4.4 kg/m2). EMG patterns of vastus lateralis, lateral gastrocnemius and tibialis anterior, along with the vertical and antero-posterior ground reaction forces were studied during shod and barefoot gait.ResultsRegardless of the disease, walking with shoes promoted an increase in the first peak vertical force and the peak horizontal propulsive force. Diabetic individuals had a delay in the lateral gastrocnemius EMG activity with no delay in the vastus lateralis. They also demonstrated a higher peak horizontal braking force walking with shoes compared to barefoot. Diabetic participants also had a smaller second peak vertical force in shod gait and a delay in the vastus lateralis EMG activity in barefoot gait compared to controls.ConclusionsThe change in plantar sensory information that occurs when wearing shoes revealed a different motor strategy in diabetic individuals. Walking with shoes did not attenuate vertical forces in either group. Though changes in motor strategy were apparent, the biomechanical did not support the argument that the use of shoes contributes to altered motor responses during gait.

Do horizontal propulsive forces influence the nonlinear structure of locomotion?

BackgroundSeveral investigations have suggested that changes in the nonlinear gait dynamics are related to the neural control of locomotion. However, no investigations have provided insight on how neural control of the locomotive pattern may be directly reflected in changes in the nonlinear gait dynamics. Our simulations with a passive dynamic walking model predicted that toe-off impulses that assist the forward motion of the center of mass influence the nonlinear gait dynamics. Here we tested this prediction in humans as they walked on the treadmill while the forward progression of the center of mass was assisted by a custom built mechanical horizontal actuator.MethodsNineteen participants walked for two minutes on a motorized treadmill as a horizontal actuator assisted the forward translation of the center of mass during the stance phase. All subjects walked at a self-select speed that had a medium-high velocity. The actuator provided assistive forces equal to 0, 3, 6 and 9 percent of the participant's body weight. The largest Lyapunov exponent, which measures the nonlinear structure, was calculated for the hip, knee and ankle joint time series. A repeated measures one-way analysis of variance with a t-test post hoc was used to determine significant differences in the nonlinear gait dynamics.ResultsThe magnitude of the largest Lyapunov exponent systematically increased as the percent assistance provided by the mechanical actuator was increased.ConclusionThese results support our model's prediction that control of the forward progression of the center of mass influences the nonlinear gait dynamics. The inability to control the forward progression of the center of mass during the stance phase may be the reason the nonlinear gait dynamics are altered in pathological populations. However, these conclusions need to be further explored at a range of walking speeds.

Local Anesthesia of the Sole of the Foot Alters Lower Limb Kinematics and Kinetics During Running

1649 PURPOSE: The purpose of this study was to investigate whether local anesthesia of the sole of the foot would alter lower limb kinematics, kinetics and perceptions of comfort during running. METHODS: The study was conducted as a randomised double blind placebo controlled crossover clinical trial (n = 10). Either a topical anaesthetic cream (EMLA™, lignocaine and prilocaine 25 mg/g) or placebo (CON) was applied after which subjects rested in sitting for 60 minutes and then performed 10 running trials in the Motion Analysis Laboratory to determine their kinetics and lower limb kinematics during running. Thereafter, they were required to run 840 metres on an indoor tartan track and rate their perceived comfort (Scale 0–10) during the run. RESULTS: The step length (m) (EMLA: 0.68 ± 0.065, CON: 0.718 ± 0.081: p = 0.0036) and stride length (m) (EMLA: 1.375 ± 0.146, CON: 1.43 ± 0.18: p = 0.026) in the laboratory and mean velocity (m/sec) (EMLA: 3.87 ± 0.358, CON: 4.07 ± 0.317: p = 0.0036) on the indoor track decreased significantly following EMLA™. Peak horizontal propulsive force (BW) was significantly reduced during the EMLA trial compared with the CON trial (EMLA: 0.162 ± 0.044 BW, CON: 0.177 ± 0.049 BW: p = 0.043). There was a significant increase (p = 0.0053) in dorsiflexion of the ankle at heelstrike (Ahs) during the EMLA trial (degrees) (EMLA: −3.23 ± 4.19, CON: 0.233 ± 4.9) compared with CON. There was also a significant increase of dorsiflexion of the ankle at the time of peak knee extension (Asw) (degrees) (EMLA: −4.113 ± 4.55, CON: −1.59 ± 4.55: p = 0.0078) and at the time of peak knee flexion in the stance phase (Ast) (degrees) (EMLA: −9.69 ± 5.43, CON: −7.2 ± 6.3: p = 0.028) during the EMLA trial compared with CON. CONCLUSION: The application of a topical local anesthetic cream to the sole of the foot reduced self-selected running speed and altered selected kinetic and kinematic variables during running. Sensory input from cutaneous afferents in the foot alters gait mainly by changing running speed.

Local Anesthesia of the Sole of the Foot Alters Lower Limb Kinematics and Kinetics During Running

1649 PURPOSE: The purpose of this study was to investigate whether local anesthesia of the sole of the foot would alter lower limb kinematics, kinetics and perceptions of comfort during running. METHODS: The study was conducted as a randomised double blind placebo controlled crossover clinical trial (n = 10). Either a topical anaesthetic cream (EMLA™, lignocaine and prilocaine 25 mg/g) or placebo (CON) was applied after which subjects rested in sitting for 60 minutes and then performed 10 running trials in the Motion Analysis Laboratory to determine their kinetics and lower limb kinematics during running. Thereafter, they were required to run 840 metres on an indoor tartan track and rate their perceived comfort (Scale 0–10) during the run. RESULTS: The step length (m) (EMLA: 0.68 ± 0.065, CON: 0.718 ± 0.081: p = 0.0036) and stride length (m) (EMLA: 1.375 ± 0.146, CON: 1.43 ± 0.18: p = 0.026) in the laboratory and mean velocity (m/sec) (EMLA: 3.87 ± 0.358, CON: 4.07 ± 0.317: p = 0.0036) on the indoor track decreased significantly following EMLA™. Peak horizontal propulsive force (BW) was significantly reduced during the EMLA trial compared with the CON trial (EMLA: 0.162 ± 0.044 BW, CON: 0.177 ± 0.049 BW: p = 0.043). There was a significant increase (p = 0.0053) in dorsiflexion of the ankle at heelstrike (Ahs) during the EMLA trial (degrees) (EMLA: −3.23 ± 4.19, CON: 0.233 ± 4.9) compared with CON. There was also a significant increase of dorsiflexion of the ankle at the time of peak knee extension (Asw) (degrees) (EMLA: −4.113 ± 4.55, CON: −1.59 ± 4.55: p = 0.0078) and at the time of peak knee flexion in the stance phase (Ast) (degrees) (EMLA: −9.69 ± 5.43, CON: −7.2 ± 6.3: p = 0.028) during the EMLA trial compared with CON. CONCLUSION: The application of a topical local anesthetic cream to the sole of the foot reduced self-selected running speed and altered selected kinetic and kinematic variables during running. Sensory input from cutaneous afferents in the foot alters gait mainly by changing running speed.

THE RELATIONSHIP BETWEEN RUNNING STYLE, BIOMECHANICAL VARIABLES AND COMMON MUSCULOSKELETAL RUNNING INJURIES

Traditional biomechanical variables such as ground reaction force parameters, and kinematic variables such as abnormal movements of segments of the feet and lower limbs, are not strongly associated with common running injuries. The aim of this study was to compare traditional (kinetic and kinematic) and novel (mechanical work and power at a joint) biomechanical variables between injured runners (IJ, n = 26, 32.2±7.1 yrs, 1.76±0.08m. 74.613.7kg, anterior knee pain syndrome, n = 17;Iliotibial band friction syndrome, n = 4; medial tibial stress syndrome, n = 3; Achilles tendinosis, n = 2) and matched injury-free runners (IF, n = 43, 33.4±11.8 yrs. 1.73±0.09m, 71.4±13.2kg). Data were collected with a Vicon 3D Motion Analysis System and processed in GaitLab®. Running speeds and kinematic variables were similar, but horizontal propulsive force (BW)(IF, 0.18±0.04; IJ, 0.16±0.03)(p= 0.03), and loading rate of the vertical impact force at the peak magnitude (VLR, BW/s) (IF, 44.5±9.5; IJ, 39.8±8.6) was greater in the IF group. The peak negative knee power (PnKp in Watts/kg: IJ, 7.6±1.8; IF, 6.6±0.1) and the eccentric knee work (EKw in J/kg: IJ, 0.47±0.13; IF, 0.40±0.16) were greater in the IJ group (p < 0.05). Biomechanical variables that are associated with common running injuries are kinetic rather than kinematic, with increased PnKP and EKw being novel biomechanical descriptors in injured runners.

Test-Retest Reliability of Selected Ground Reaction Force Parameters and Their Symmetry during Running

The aim of our study was to assess the interday test-retest reliability (focussing on the separate contribution of systematic and random error) of selected 10-trial mean ground reaction force (GRF) parameters and GRF symmetry indices measured during running. Ten competitive male heel-strike runners (aged, 26.2 ± 5.7 years) performed 10 successful running trials across the force platform at a constant velocity of 4.0 m · s-1 ±10% wearing their customary running footwear. The testing procedure was repeated under similar conditions 1 week later. The results showed no statistically significant differences between the means of Test 1 and Test 2 for most GRF parameters and symmetry indices, indicating non-significant systematic error. Correlation coefficients ranged from 0.73 to 0.99 for GRF parameters. Random error was small with SEmeas less than 10% of the Test 1 mean value for almost all GRF parameters. Symmetry indices displayed correlation coefficients ranging from −0.44 to 0.91. Based on these and the size of the SEmeas, the symmetry indices displayed variable reliability, with the most reliable being those associated with peak vertical active force and peak horizontal propulsive force.

Locomotion on the water surface: propulsive mechanisms of the fisher spider

Using kinematic and mechanical experiments, we have shown how fisher spiders, Dolomedes triton (Araneae, Pisauridae), can generate horizontal propulsive forces using their legs. This horizontal thrust is provided primarily by the drag of the leg and its associated dimple as both move across the water surface. Less important sources of resistance are surface tension and bow waves. The relative contributions of drag, surface tension and bow waves were examined in several different ways. In one experiment, we measured the forces acting on a leg segment as water flowed past it in non-turbulent flow; the bow wave was not present at leg relative velocities below 0.2ms-1 and thus cannot play a role in thrust production at low leg speeds. In a second experiment, we varied the surface tension by altering the concentration of ethanol from 0% to 9% in the experimental water tank. At a constant dimple depth, force varied little with changes in surface tension, a result consistent with the hypothesis that drag is the primary source of resistance. In addition, however, as surface tension decreased from 0.072 to 0.064Nm-1, the power exponent of the relationship between force and velocity (as measured by the exponent of the power function relating the two variables) increased; at lower surface tensions, down to 0.054Nm-1, the power exponent of the relationship between force and velocity decreased. These results suggest an influence of surface tension (albeit still secondary to drag) in generating horizontal resistance to leg movement. We also measured flow disturbance in the water downstream from a leg segment and confirmed that, even at velocities well below 0.2ms-1, the leg-cum-dimple transferred momentum to the water, which is a clear indication that drag is a contributor to the resistance encountered by a spider's leg. Finally, modeling the leg-cum-dimple as a circular cylinder generates values of drag that account for 75&shy;98% of the measured leg force when the dimple is 0 or 1mm deep. These results not only elucidate the primary mechanism of propulsion for D. triton and other similar-sized arthropods, such as adult water striders (Gerridae), but also suggest that the formerly enigmatic locomotion of very small water-walking organisms (e.g. first-instar water striders) can be understood in the same way.

ENERGETIC COST OF GENERATING HORIZONTAL FORCES IN RUNNING 490

Previous studies suggest that generating vertical ground reaction force is the major determinant of the energetic cost of running and that horizontal ground reaction forces have negligible cost. To estimate the energetic cost of generating horizontal propulsive forces, we applied external horizontal forces during steady speed running and measured the subsequent change in the rate of oxygen consumption. Eight subjects ran at 3.3 m/s on a motorized treadmill with eight different levels of externally applied horizontal force (-6, -3, 0,+3, +6, +9, +12, +15% of body weight) applied to their center of mass. Negative forces indicate pulling backwards on the subject and positive forces indicates pulling forwards. We applied these forces via a waist belt attached to springs stretched to the appropriate tension. We found that the energetic cost of running is a parabolic function of applied horizontal force. At the-6% body weight condition, the rate of oxygen consumption was 30.7% (S.E.± 3.1%) greater than for normal running. The minimum rate of oxygen consumption occurred at the +15% body weight condition where it was 32.7%(S.E. ±3.3) lower than for normal running. This observed decrease suggests that generating horizontal propulsive forces accounts for approximately one third of the total energetic cost of normal steady speed running.