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Concept: Ground reaction force


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 the study was to examine the effects of a resistance exercise program on soccer kick biomechanics. Twenty male amateur soccer players were divided in the Experimental (EG) and the Control (CG) group, each consisting of ten players. The EG followed a 10-week resistance exercise program mainly for the lower limb muscles. Maximal instep kick kinematics, electromyography and ground reaction forces (GRFs) as well as maximum isometric leg strength were recorded before and after training. A two-way analysis of variance showed significantly higher ball speed values only for the EG (26.14 ± 1.17 m/sec versus 27.59 ± 1.49 m/sec before and after training, respectively), while no significant differences were observed for the CG. The EG showed a decline in joint angular velocities and an increase in biceps femoris EMG of the swinging leg during the backswing phase followed by a significant increase in segmental and joint velocities and muscle activation of the same leg during the forward swing phase (P < 0.05). The EG also showed significantly higher vertical GRFs and rectus femoris and gastrocnemius activation of the support leg (P < 0.05). Similarly, maximum and explosive isometric force significantly increased after training only for the EG (P < 0.05). These results suggest that increases in soccer kicking performance after a 10-week resistance training program were accompanied by increases in maximum strength as well as an altered soccer kick movement pattern, characterized by a more explosive backward - forward swinging movement and higher muscle activation during the final kicking phase.

Concepts: Exercise, Ground reaction force, Human leg, Football, Strength training, Isometric exercise, Electromyography, Muscle


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


The goal of this investigation was to investigate how walking patterns are affected following muscle-damaging exercise by quantifying both lower limb kinematics and kinetics. Fifteen young women conducted a maximal isokinetic eccentric exercise (EE) muscle damage protocol (5×15) of the knee extensors and flexors of both legs at 60°/s. Three-dimensional motion data and ground reaction forces (GRFs) were collected 24h pre-EE while the participants walked at their preferred self-selected walking speed (SWS). Participants were asked to perform two gait conditions 48h post-EE. The first condition (COND1) was to walk at their own speed and the second condition (COND2) to maintain the SWS (±5%) they had 24h pre-EE. Walking speed during COND1 was significantly lower compared to pre-exercise values. When walking speed was controlled during COND2, significant effects of muscle damage were noticed, among other variables, for stride frequency, loading rate, lateral and vertical GRFs, as well as for specific knee kinematics and kinetics. These findings provide new insights into how walking patterns are adapted to compensate for the impaired function of the knee musculature following muscle damage. The importance to distinguish the findings caused by muscle damage from those exhibited in response to changes in stride frequency is highlighted.

Concepts: Force, Flexion, Kinematics, Human leg, Muscle contraction, Muscle, Ground reaction force, Walking


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


Intramuscular injection of botulinum toxin (botox) into rodent hindlimbs has developed as a useful model for exploring muscle-bone interactions. Botox-induced muscle inhibition rapidly induces muscle atrophy and subsequent bone loss, with the latter hypothesized to result from reduced muscular loading of the skeleton. However, botox-induced muscle inhibition also reduces gravitational loading (as evident by reduced ground reaction forces during gait) which may account for its negative skeletal effects. The aim of this study was to investigate the skeletal effect of botox-induced muscle inhibition in cage control and tail suspended mice, with tail suspension being used to control for the reduced gravitational loading associated with botox. Female C57BL/6J mice were injected unilaterally with botox and contralaterally with vehicle, and subsequently exposed to tail suspension or normal cage activities for 6 weeks. Botox-induced muscle inhibition combined with tail suspension had the largest detrimental effect on the skeleton, causing the least gains in midshaft tibial bone mass, cortical area and cortical thickness, greatest gains in midshaft tibial medullary area, and lowest proximal tibial trabecular bone volume fraction. These data indicate botox-induced muscle inhibition has skeletal effects over and above any effect it has in altering gravitational loading, suggesting that muscle has a direct effect on bone. This effect may be relevant in the development of strategies targeting musculoskeletal health.

Concepts: Microbial toxins, Botulinum toxin, Muscle atrophy, Ground reaction force, Bone, Muscle, Skeleton, Skeletal system


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