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Concept: Reaction


How extinct, non-avian theropod dinosaurs moved is a subject of considerable interest and controversy. A better understanding of non-avian theropod locomotion can be achieved by better understanding terrestrial locomotor biomechanics in their modern descendants, birds. Despite much research on the subject, avian terrestrial locomotion remains little explored in regards to how kinematic and kinetic factors vary together with speed and body size. Here, terrestrial locomotion was investigated in twelve species of ground-dwelling bird, spanning a 1,780-fold range in body mass, across almost their entire speed range. Particular attention was devoted to the ground reaction force (GRF), the force that the feet exert upon the ground. Comparable data for the only other extant obligate, striding biped, humans, were also collected and studied. In birds, all kinematic and kinetic parameters examined changed continuously with increasing speed, while in humans all but one of those same parameters changed abruptly at the walk-run transition. This result supports previous studies that show birds to have a highly continuous locomotor repertoire compared to humans, where discrete ‘walking’ and ‘running’ gaits are not easily distinguished based on kinematic patterns alone. The influences of speed and body size on kinematic and kinetic factors in birds are developed into a set of predictive relationships that may be applied to extinct, non-avian theropods. The resulting predictive model is able to explain 79-93% of the observed variation in kinematics and 69-83% of the observed variation in GRFs, and also performs well in extrapolation tests. However, this study also found that the location of the whole-body centre of mass may exert an important influence on the nature of the GRF, and hence some caution is warranted, in lieu of further investigation.

Concepts: Locomotion, Animal locomotion, Kinematics, Reaction, Theropoda, Classical mechanics, Bird, Dinosaur


AIM: The purpose of this study was to determine the changes in running mechanics that occur when highly trained runners run barefoot and in a minimalist shoe, and specifically if running in a minimalist shoe replicates barefoot running. METHODS: Ground reaction force data and kinematics were collected from 22 highly trained runners during overground running while barefoot and in three shod conditions (minimalist shoe, racing flat and the athlete’s regular shoe). Three-dimensional net joint moments and subsequent net powers and work were computed using Newton-Euler inverse dynamics. Joint kinematic and kinetic variables were statistically compared between barefoot and shod conditions using a multivariate analysis of variance for repeated measures and standardised mean differences calculated. RESULTS: There were significant differences between barefoot and shod conditions for kinematic and kinetic variables at the knee and ankle, with no differences between shod conditions. Barefoot running demonstrated less knee flexion during midstance, an 11% decrease in the peak internal knee extension and abduction moments and a 24% decrease in negative work done at the knee compared with shod conditions. The ankle demonstrated less dorsiflexion at initial contact, a 14% increase in peak power generation and a 19% increase in the positive work done during barefoot running compared with shod conditions. CONCLUSIONS: Barefoot running was different to all shod conditions. Barefoot running changes the amount of work done at the knee and ankle joints and this may have therapeutic and performance implications for runners.

Concepts: Ground reaction force, Analysis of variance, Running, Barefoot running, Reaction, Classical mechanics, Force, Knee


The purpose of this study is to examine the effects of different volume and training surfaces during a short-term plyometric training program on neuromuscular performance. Twenty-nine subjects were randomly assigned to four groups: control group (CG, n=5), moderate volume group (MVG, n=9, 780 jumps), moderate volume hard surface group (MVGHS, n=8, 780 jumps), and high volume group (HVG, n=7, 1560 jumps). A series of tests were performed by the subjects before and after seven weeks of plyometric training. These tests were: measurement of maximum strength (5 maximum repetitions [5RM]), drop jumps (DJ) of varying height ( 20, 40, and 60cm), squat and countermovement jumps (SJ and CMJ, respectively), timed 20m sprint, agility, body weight, and height. The results of the present study suggest that high training volume leads to a significant increase in explosive performance that requires fast stretch shortening cycle (SSC) actions (such as DJ and sprint) in comparison to what is observed after a moderate training volume regimen. Secondly, when plyometric training is performed on a hard training surface (high impact reaction force), a moderate training volume induces optimal stimulus to increase explosive performance requiring fast SSC actions (e.g. DJ), maximal dynamic strength enhancement, and higher training efficiency. Thus, a finding of interest in the study was that after 7 weeks of plyometric training, performance enhancement in maximal strength and in actions requiring fast SSC (such as DJ and sprint) were dependent on the volume of training and the surface on which it was performed. This must be taken into account when using plyometric training on different surfaces.

Concepts: Reaction, Force, Time, Surface, Explosive material, Strength, Differential geometry, Plyometrics


ABSTRACT:: Weight shift during the golf swing has been a topic of discussion among golf professionals; however it is still unclear how weight shift varies in golfers of different performance levels. The main purpose of this study was to examine: 1) the changes in the peak ground reaction forces (GRF) and the timing of these events between high and low handicap golfers and 2) the differences between the leading and trailing legs golfers. Twenty-eight male golfers were recruited and divided intobased on having a low handicap (LHCP, < 9) and or high handicap (HHCP, >9) which defined the experimental groups. Three-dimensional GRF peaks and the timing of the peaks were recorded bilaterally during a golf swing. The golf swing was divided into phases: 1) address to the top of the backswing, 2) top of the backswing to ball contact, and 3) ball contact to the end of follow through. Repeated measures ANOVAs (a=0.05) were completed for each study variablethe magnitude and the timing of peak vertical GRF, peak lateral GRF, and peak medial GRF (a=0.05). The Low handicapLHCP group had a greater transfer of vertical force from the trailing foot to the leading foot in phase 2 than the high handicap groupHHCP. The LHCPLow handicap group also demonstrated earlier timing of peak vertical force throughout the golf swing than the high handicap groupHHCP. The LowLHCP and high handicapHHCP groups demonstrated different magnitudes of peak lateral force. The LHCP Low handicap group also had an earliery timing of peak lateral GRF in phase 2 and earliery timing of peak medial GRF in phase 1 and 2 than the high handicap groupHHCP group. In general, Low handicapLHCP golfers generally demonstrated greater and earlier force generation than high handicapHHCP golfers. It may be relevant to consider both the magnitude of the forces as well as the timing of these events during golf specific training to improve performance. Theise data may identify differences in reveal weight shifting differences that can be addressed by teaching professionals to help their students better understand weight transferload transmission during the golf swing to optimize performance.

Concepts: Handicap, Reaction, Ground reaction force, Par, Professional golfer, Force, Stroke play, Golf


This study compared the ground reaction forces (GRF) and plantar pressures between unloaded and occasional loaded gait. The GRF and plantar pressures of 60 participants were recorded during unloaded gait and occasional loaded gait (wearing a backpack that raised their body mass index to 30); this load criterion was adopted because is considered potentially harmful in permanent loaded gait (obese people). The results indicate an overall increase (absolute values) of GRF and plantar pressures during occasional loaded gait (p < 0.05); also, higher normalized (by total weight) values in the medial midfoot and toes, and lower values in the lateral rearfoot region were observed. During loaded gait the magnitude of the vertical GRF (impact and thrust maximum) decreased and the shear forces increased more than did the proportion of the load (normalized values). These data suggest a different pattern of GRF and plantar pressure distribution during occasional loaded compared to unloaded gait.

Concepts: Shear stress, Reaction, Absolute value, Obesity, Force, Body mass index, Ground reaction force, Mass


Researchers have used screw theory to describe the motion of the knee in terms of instantaneous axes of the knee (IAK). However, how geometric change to the dynamic alignment of IAK may affect stance phase of foot loading has not yet been fully explained. We have tested our informational framework through readily accessible benchmark data (Fregly et al. 2012): muscle contraction and ground reaction force are compounded into a wrench that is reciprocal to the IAK and resolved into component wrenches belonging to the reciprocal screw system. This revealed the special screw system that defines the freedom available to the knee and more precisely revealed how to measure this first order of freedom. After this step, we determined the reciprocal screw system, which involves the theory of equilibrium. Hence, a screw system of the first order will have a screw system of the fifth order as its reciprocal. We established a framework the estimation of reaction of constraints about the knee using a process that is simplified by the judicious generation of IAK for the first order of freedom in equilibrium.

Concepts: Wrench, Newton's laws of motion, Screw, Scientific method, Physics, Force, Ground reaction force, Reaction


Pregnant women experience numerous physical alterations during pregnancy which may place them at an increased risk of falls. The purpose of this study was to examine ground reaction forces (GRFs) during staircase locomotion in pregnant and non-pregnant women. METHODS: Data were collected on 29 pregnant women in their second and third trimesters, and on 40 control women. Subjects walked at their freely chosen speeds during stair ascent and descent. A force plate imbedded in the second stair, but structurally independent of the staircase, was used to collect GRF data (1080Hz). A marker placed on the L3/L4 spinal segment was used to determine ascent and descent velocity from a motion-capture system. In the statistical analyses, trimester (control, second trimester, third trimester) and subject were the independent variables. Stance time and ascent/descent velocity were analyzed with an ANOVA. Mediolateral excursion of the COP during the step was analyzed with an ANCOVA. The GRFs were categorized into anterioposterior, mediolateral, and vertical forces. A two factor MANCOVA (subject, trimester) was performed on each GRF category. Mass and velocity served as covariates in each analysis (a=0.05). RESULTS: The mediolateral excursion of the COP during ascent was greater in the third trimester (p=0.04). The anterioposterior braking impulse was greater in both ascent (p=0.01) and descent (p=0.01) during pregnancy. The vertical GRF loading rate during descent was greater in pregnant women than in controls (p=0.04). CONCLUSION: These alterations are likely related to increased instability during stairway walking and could contribute to increased fall risk during pregnancy.

Concepts: Reaction, Force, Walking, Stairway, Childbirth, Ground reaction force, Trimester, Pregnancy


High-heeled shoes are associated with instability and falling, leading to injuries such as fracture and ankle sprain. Knowledge of the motion of the body’s center of mass (COM) with respect to the center of pressure (COP) during high-heeled gait may offer insights into the balance control strategies and provide a basis for approaches that minimize the risk of falling and associated adverse effects. The study aimed to investigate the influence of the base and height of the heels on the COM motion in terms of COM-COP inclination angles (IA) and the rate of change of IA (RCIA). Fifteen females who regularly wear high heels walked barefoot and with narrow-heeled shoes with three heel heights (3.9cm, 6.3cm and 7.3cm) while kinematic and ground reaction force data were measured and used to calculate the COM and COP, as well as the temporal-distance parameters. The reduced base of the heels was found to be the primary factor for the reduced normalized walking speed and the reduced frontal IA throughout the gait cycle. This was achieved mainly through the control of the RCIA during double-leg stance (DLS). The heel heights affected mainly the peak RCIA during DLS, which were not big enough to affect the IA. These results suggest young adults adopt a conservative strategy for balance control during narrow-heeled gait. The results will serve as baseline data for future evaluation of patients and/or older adults during narrow-heeled gait with the aim of reducing the risk of falling.

Concepts: Ground reaction force, Force, Heel, Walking, High-heeled footwear, Reaction, Newton's laws of motion, Classical mechanics


The purpose of this study was to determine the degree to which subtalar joint pronation resulting from a SP foot affects knee alignment, hip muscle activation and ground reaction force attenuation in female athletes during a broad jump-to-cut maneuver.

Concepts: Synovial joint, Ibn Bajjah, Subtalar joint, Newton's laws of motion, Force, Classical mechanics, Ground reaction force, Reaction


The regular practice of Nordic walking (NW) has increased in recent years, in part thanks to the health benefits described by the scientific literature. However, there is no consensus on the effects of shock-impact absorption during its practice. Purpose: The aim of this study was to compare the levels of impact and ground reaction forces (GRF) between NW and walking (W). Method: Twenty physically active and experienced participants were assessed using a dynamometric platform and accelerometry analysis. Results: The results show statistically significantly higher levels of acceleration in the tibia (12%) and head (21%) during NW compared with W. Equally, GRF were significantly higher (27%) at the instant of strike compared with W, and a reduction of the forces at the instant of takeoff (8%) was observed. Conclusions: During NW, shock impacts and GRF levels increased compared with W, an aspect that should be considered when prescribing health improvement programs.

Concepts: Physics, Walking, Reaction, Nordic walking, Scientific method, Exercise, Classical mechanics, Ground reaction force