Changes In Ground Reaction Forces Research Articles

Origin of directionally tuned responses in lower limb muscles to unpredictable upper limb disturbances.

Unpredictable forces which perturb balance are frequently applied to the body through interaction between the upper limb and the environment. Lower limb muscles respond rapidly to these postural disturbances in a highly specific manner. We have shown that the muscle activation patterns of lower limb muscles are organized in a direction specific manner which changes with lower limb stability. Ankle muscles change their activity within 80 ms of the onset of a force perturbation applied to the hand which is earlier than the onset of changes in ground reaction force, ankle angle or head motion. The latency of the response is sensitive to the perturbation direction. However, neither the latency nor the magnitude of the response is affected by stiffening the arm even though this alters the magnitude and timing of motion of the body segments. Based on the short latency, insensitivity of the change in ankle muscle activation to motion of the body segments but sensitivity to perturbation direction we reason that changes in ankle muscle activation are most likely triggered by sensory signals originating from cutaneous receptors in the hand. Furthermore, evidence that the latency of changes in ankle muscle activation depends on the number of perturbation directions suggests that the neural pathway is not confined to the spinal cord.

Effect of Fatigue Protocols on Lower Limb Neuromuscular Function and Implications for Anterior Cruciate Ligament Injury Prevention Training: A Systematic Review

Background: Approximately two-thirds of anterior cruciate ligament (ACL) tears are sustained during noncontact situations when an athlete is cutting, pivoting, decelerating, or landing from a jump. Some investigators have postulated that fatigue may result in deleterious alterations in lower limb biomechanics during these activities that could increase the risk of noncontact ACL injuries. However, prior studies have noted a wide variation in fatigue protocols, athletic tasks studied, and effects of fatigue on lower limb kinetics and kinematics. Purpose: First, to determine if fatigue uniformly alters lower limb biomechanics during athletic tasks that are associated with noncontact ACL injuries. Second, to determine if changes should be made in ACL injury prevention training programs to alter the deleterious effects of fatigue on lower limb kinetics and kinematics. Study Design: Systematic review; Level of evidence, 4. Methods: A systematic review of the literature using MEDLINE was performed. Key terms were fatigue, neuromuscular, exercise, hop test, and single-legged function tests. Inclusion criteria were original research studies involving healthy participants, use of a fatigue protocol, study of at least 1 lower limb task that involved landing from a hop or jump or cutting, and analysis of at least 1 biomechanical variable. Results: Thirty-seven studies involving 806 athletes (485 female, 321 male; mean age, 22.7 years) met the inclusion criteria. General fatigue protocols were used in 20 investigations, peripheral protocols were used in 17 studies, and 21 different athletic tasks were studied (13 single-legged, 8 double-legged). There was no consistency among investigations regarding the effects of fatigue on hip, knee, or ankle joint angles and moments or surface electromyography muscle activation patterns. The fatigue protocols typically did not produce statistically significant changes in ground-reaction forces. Conclusion: Published fatigue protocols did not uniformly produce alterations in lower limb neuromuscular factors that heighten the risk of noncontact ACL injuries. Therefore, justification does not currently exist for major changes in ACL injury prevention training programs to account for potential fatigue effects. However, the effect of fatigue related to ACL injuries is worthy of further investigation, including the refinement of protocols and methods of analysis.

Changes in ground reaction force during a rebound-jump task after hip strength training for single-sided ankle dorsiflexion restriction.

[Purpose] Lateral ankle sprains are common injuries suffered while playing sports, and abnormal forward- and inward-directed ground reaction force occurs during a jumping task. However, the influence of hip muscle strength training on jumping performance after ankle injuries has not been fully examined. This study thus examined changes in ground reaction force during a rebound-jump task after training to strengthen hip muscles. [Subjects and Methods] Ten of 30 female high school basketball players were assigned as subjects who showed a difference of 7 or more degrees in dorsiflexion ranges between the bilateral ankles. The subjects underwent 12 weeks of training to strengthen hip abductors and external rotators. Comparisons between before and after training were made regarding ground reaction force components, hip and knee joint angles, percentage of maximum voluntary contraction in leg muscles, and muscle strength of hip muscles during the rebound-jump task. [Results] After training, the subjects showed increased strength of external rotator muscles, increased percentage of maximum voluntary contraction in the gluteus medius muscle, decreased inward ground reaction force, and increased flexion angles of the hip and knee joints. [Conclusion] This study suggests that training to strengthen hip muscles may ameliorate the inward ground reaction force in athletes with ankle dorsiflexion restriction.

Ground reaction forces are more sensitive gait measures than temporal parameters in rodents following rotator cuff injury

Gait analysis is a quantitative, non-invasive technique that can be used to investigate functional changes in animal models of musculoskeletal disease. Changes in ground reaction forces following injury have been observed that coincide with differences in tissue mechanical and histological properties during healing. However, measurement of these kinetic gait parameters can be laborious compared to the simpler and less time-consuming analysis of temporal gait parameters alone. We compared the sensitivity of temporal and kinetic gait parameters in detecting functional changes following rotator cuff injury in rats. Although these parameters were strongly correlated, temporal measures were unable to detect greater than 50% of the functional gait differences between injured and uninjured animals identified simultaneously by ground reaction forces. Regression analysis was used to predict ground reaction forces from temporal parameters. This model improved the ability of temporal parameters to identify known functional changes, but only when these differences were large in magnitude (i.e., between injured vs. uninjured animals, but not between different post-operative treatments). The results of this study suggest that ground reaction forces are more sensitive measures of limb/joint function than temporal parameters following rotator cuff injury in rats. Therefore, although gait analysis systems without force plates are typically efficient and easy to use, they may be most appropriate for use when major functional changes are expected.

Elite sprinters push forward

Elite sprinters captivate audiences with their blazing speed and acceleration, pushing off the blocks and propelling themselves to world records and gold medals. With each step, the ground reaction force pushes the runner up and forwards until they reach the finish line. This is why high net horizontal ground reaction force, summed over each step, is the strongest predictor of sprinting performance. Of course, that is the net effect of the ground reaction force impulse: it must push forward if the sprinter is to go forward. The instantaneous ground reaction force at any given point in a runner's step, and at different times while accelerating during a race, is slightly more complicated. After the initial push off the blocks, as the foot makes contact with the ground, there is firstly a braking impulse in which the ground pushes the runner in the reverse direction. Over the course of the step, the ground reaction force changes orientation from backwards to forwards, outstripping that small rearwards component and driving the runner forward. In other words, there are two phases to each step: a braking component and a pushing component.With this in mind, we return to the elite sprinters. How do these outstanding athletes accelerate quickly? This is the question asked by Jean-Benoît Morin and a team of international collaborators working at the French Institute of Sport in Paris. The team noted that there are two possible ways elite sprinters may be increasing their acceleration capacity: they can ‘push more’, generating higher ground reaction forces in the forwards direction, or they can ‘brake less’, minimizing the ground reaction forces in the backwards direction. (Or, they could employ both strategies.)To determine how elite sprinters accelerate so quickly, the team measured the ground reaction force impulses over multiple steps across the length of a 40 m sprint for nine sprinters ranging from international level (elite) to French national level (sub-elite) competitors. By measuring the different components of ground reaction force, the researchers could see which aspects of that force best predicted sprint time.They found that even with a low sample size, there was a high correlation between sprint performance (mean velocity) and both the net horizontal ground reaction impulse and the positive component of the horizontal ground reaction impulse. In other words, the fastest sprinters had a bigger push-off with each step. There was no correlation between the negative ground reaction force impulse and average velocity; so, although the better sprinters were pushing more, they were not necessarily breaking less.

The effects of changes in the sagittal plane alignment of running-specific transtibial prostheses on ground reaction forces.

[Purpose] To verify the effects of sagittal plane alignment changes in running-specific transtibial prostheses on ground reaction forces (GRFs). [Subjects and Methods] Eight transtibial amputees who used running-specific prostheses during sprinting participated. The sprint movements were recorded using a Vicon-MX system and GRF measuring devices. The experiment levels were set as regularly recommended alignment (REG; the normal alignment for the subjects) and dorsiflexion or plantar flexion from the REG. [Results] The subjects were classified into fast (100-m personal best < 12.50 s) and slow (100-m personal best ≥ 12.50 s) groups. In both groups, there were no significant differences in the center of gravity speed; further, the difference in the stance time was significant in the slow group but not in the fast group. Significant differences were observed in the step length for the fast group, whereas the stance time and step rate significantly differed in the slow group. The GRF impulse showed significant differences in the vertical and braking directions in both groups. [Conclusion] The GRFs are affected by sagittal plane alignment changes in running-specific prostheses. Moreover, our results suggest that the change in GRFs along with the altered sagittal plane alignment influenced the step length and step rate.

The Effect of Ankle Kinesio Tape on Ankle Muscle Activity During a Drop Landing.

The use of Kinesio Tape among health care professional has grown recently in efforts to efficiently prevent and treat joint injuries. However, limited evidence exists regarding the efficacy of this technique in enhancing joint stability and neuromuscular control. To determine how Kinesio Tape application to the ankle joint alters forces and muscle activity during a drop-jump maneuver. Single-group pretest- posttest. University laboratory. 22 healthy adults with no previous history of ankle injury. Participants were instrumented with electromyography on the lower-leg muscles as they jumped from a 35-cm platform onto force plates. Test trials were performed without tape (BL), immediately after application of Kinesio Tape to the ankle (KT-I), and after 24 h of continued use (KT-24). Peak ground-reaction forces (GRFs) and time to peak GRF were compared across taping conditions, and the timing and amplitude of muscle activity from the tibialis anterior, peroneus longus, and lateral gastrocnemius were compared across taping conditions. No significant differences in amplitude or timing of GRFs were observed (P > .05). However, muscle activity was observed to decrease from BL to KT-I in the tibialis anterior (P = .027) and from BL to KT-24 in the PL (P = .022). The data suggest that Kinesio Tape decreases muscle activity in the ankle during a drop-jump maneuver, although no changes in GRFs were observed. This is contrary to the proposed mechanisms of Kinesio Tape. Further research might investigate how this affects participants with a history of injury.

Kinetic changes in gait during low magnitude military load carriage

Indian infantry soldiers carry smaller magnitudes of loads for operational requirements. The ground reaction forces (GRFs) and impulse responses of 10 healthy male Indian infantry soldiers were collected while they walked carrying operational loads between 4.2 and 17.5 kg (6.5–27.2% of mean body weight (BW)) and a control condition of no external load (NL). The GRF and impulse components were normalised for BW, and data for each load condition were compared with NL in each side applying one-way analysis of variance followed by Dunnett's post hoc test. Right foot data were compared with corresponding left foot GRF data for all load conditions and NL. There were significant increases in vertical and anteroposterior GRFs with increase in load. Left and right feet GRF data in corresponding load conditions were significantly different in anteroposterior plane. No significant change was observed in the temporal components of support phase of gait. Changes in impulse parameter were observed in the anteroposterior and vertical planes while carrying load greater than 23 and 16.6% of BW for the right foot and left foot, respectively. Result indicates that smaller magnitudes of loads produced kinetic changes proportional to system weight, similar to heavier loads with the possibility of increased injury risk. Observed smaller asymmetric changes in gait may be considered as postural adjustment due to load. Unique physical characteristics of Indian soldiers and the probable design shortcomings of the existing backpack might have caused significant changes in GRF and peak impulse during smaller load carriage.

플랫폼 다이빙 앞으로 서서 앞으로 11/2회전 동작의 운동역학적 분석

This study was to perform the kinetic analysis of forward <TEX>$1\frac{1}{2}$</TEX> somersault on the platform diving. Six men's diving players of the Korea national reserve athletes participated in this study. The variables were analyzed response time, velocity, center of mass (COM), angle, center of pressure (COP) and ground reaction force (GRF) of motion. For measure and analysis of this study, used to synchronized to 4 camcorder and 1 force plate, used to the Kwon3D XP (Ver. 4.0, Visol, Korea) and Kwon GRF (Ver. 2.0, Visol, Korea) for analyzed of variables. The results were as follows; Time factor were observed in maximum knee flexion depending on the extent of use at phase 1 of take-off to execute the somersault. This enabled the subject to secure the highest possible body position in space at the moment of jumping to execute the somersault and prepare for the entry into the water with more ease. Regarding the displacement of COM, all subjects showed rightward movement in the lateral displacement during technical execution. Changes in forward and downward movements were observed in the horizontal and vertical displacements, respectively. In terms of angular shift, the shoulder joint angle tended to decrease on average, and the elbow joints showed gradually increasing angles. This finding can be explained by the shift of the coordinate points of body segments around the rotational axis in order to execute the half-bending movement that can be implemented by pulling the lower limb segments toward the trunk using the upper limb segments. The hip joint angles gradually decreased; this accelerated the rotational movement by narrowing the distance to the trunk. Movement-specific shifts in the COP occurred in the front of and vertical directions. Regarding the changes in GRF, which is influenced by the strong compressive load exerted by the supporting feet, efficient aerial movements were executed through a vertical jump, with no energy lost to the lateral GRF.

Effect of a change in step direction from a forward to a lateral target in response to a sensory perturbation

The aim of the current study was to investigate the response of healthy older and young adults to a change in step direction from a forward to a lateral target in response to a sensory perturbation. Nine healthy older (75.1±6.7years; age range, 65–81years) and nine young adults (27±3.6years; age range, 23–31years) participated in the study. The sensory perturbation was a visual cue presented at random times while subjects stepped over an obstacle from quiet stance. For both young and elderly subjects there was an abrupt change in the slope of both shear ground reaction forces (GRFs) of the stance limb following the perturbation. The slope and peak of the change in GRFs was greater for the young subjects and the onset significantly earlier (205ms compared to 271ms). Changes in the GRFs were accompanied by an increase in bilateral gluteus medius and stance limb soleus activity. A late visual cue resulted in a delayed response for elderly subjects. These data show that a stepping response to a sensory perturbation was both delayed and of less magnitude for older adults which has implications for fall risk.

Kinematic and dynamic gait compensations resulting from knee instability in a rat model of osteoarthritis

IntroductionOsteoarthritis (OA) results in pain and disability; however, preclinical OA models often focus on joint-level changes. Gait analysis is one method used to evaluate both preclinical OA models and OA patients. The objective of this study is to describe spatiotemporal and ground reaction force changes in a rat medial meniscus transection (MMT) model of knee OA and to compare these gait measures with assays of weight bearing and tactile allodynia.MethodsSixteen rats were used in the study. The medial collateral ligament (MCL) was transected in twelve Lewis rats (male, 200 to 250 g); in six rats, the medial meniscus was transected, and the remaining six rats served as sham controls. The remaining four rats served as naïve controls. Gait, weight-bearing as measured by an incapacitance meter, and tactile allodynia were assessed on postoperative days 9 to 24. On day 28, knee joints were collected for histology. Cytokine concentrations in the serum were assessed with a 10-plex cytokine panel.ResultsWeight bearing was not affected by sham or MMT surgery; however, the MMT group had decreased mechanical paw-withdrawal thresholds in the operated limb relative to the contralateral limb (P = 0.017). The gait of the MMT group became increasingly asymmetric from postoperative days 9 to 24 (P = 0.020); moreover, MMT animals tended to spend more time on their contralateral limb than their operated limb while walking (P < 0.1). Ground reaction forces confirmed temporal shifts in symmetry and stance time, as the MMT group had lower vertical and propulsive ground reaction forces in their operated limb relative to the contralateral limb, naïve, and sham controls (P < 0.05). Levels of interleukin 6 in the MMT group tended to be higher than naïve controls (P = 0.072). Histology confirmed increased cartilage damage in the MMT group, consistent with OA initiation. Post hoc analysis revealed that gait symmetry, stance time imbalance, peak propulsive force, and serum interleukin 6 concentrations had significant correlations to the severity of cartilage lesion formation.ConclusionThese data indicate significant gait compensations were present in the MMT group relative to medial collateral ligament (MCL) injury (sham) alone and naïve controls. Moreover, these data suggest that gait compensations are likely driven by meniscal instability and/or cartilage damage, and not by MCL injury alone.

Effect of rollover footwear on metabolic cost of ambulation, lower limb kinematics, kinetics, and EMG related muscle activity during walking

BackgroundFootwear with a curved sole profile shifts the point of contact between the shoe and floor anteriorly compared to a flat shoe. There have been various reports of changes in ground reaction forces, external joint moments, and (rather inconclusively) muscle activity and energy consumption during walking in this “rollover” footwear [1-6]. The aim of this study was to investigate the effect of two types of rollover footwear (one a new prototype) on walking speed, metabolic cost of gait, lower limb kinematics, kinetics and EMG muscle activity.MethodsTwenty subjects walked in four footwear conditions: (1) a flat control shoe; (2) a flat control shoe weighted to match the mass of the new rollover shoe prototype; (3) the new prototype rollover shoe; (4) a MBT shoe. Data relating to metabolic energy and temporal aspects of gait were collected during a 6 minute walk test. Data on the lower limb kinematics, joint moments, muscle activity and foot pressure were collected during walking on a straight 10 metre course.ResultsThe curved sole moved the contact point under the shoe anteriorly during early stance. This altered ankle moments and reduced ankle plantarflexion after initial contact. In mid stance the roll over footwear reduced ankle dorsiflexion and overall the ankle moved less in roll over footwear. Ground reaction force loading rates were increased by the rollover footwear. There were notable elevations in early stance calf EMG activity. Effects on energy cost of ambulation were negligible. Rollover footwear reduced plantar pressures under the heel and forefoot, and redistributed pressure towards the midfoot. Shoe weight had no effect on any parameters.ConclusionRollover footwear is able to modify ankle kinematics and moments and the activity of some ankle musculature. Effects proximal to the leg were small. There were no effects on the temporal or energy aspects of gait.

노르딕워킹의 속도에 따른 하지 관절의 운동역학적인 분석

In this study, 26 normal subjects were studied to compare the biomechanical Analysis of Lower Limbs on Speed of Nordic Walking. The biomechanical variables were determined by performing three-dimensional gait analysis, and the measurements items were spatial and temporal parameters; vertical ground reaction force; and moments of the hip, knee, and ankle joints. The purpose of this study based on the speed of Nordic Walking to the vertical ground reaction force and joint moments of each were analyzed. Nordic Walking with poles while being whether this weight is reduced to load, not the improvement of muscle activity by identify Nordic walking is to allow efficient. The results of the analysis were follows. The spatial parameters of step length, stride length significantly increased with increase in velocity(p<0.001). The temporal parameters of step time, stride time, the duration of double support use, and the duration of single support use also significantly decreased with increase in velocity(p<0.001), but cadence significantly increased(p<0.01). Analysis of the changes in ground reaction force revealed that vertical ground reaction force significantly increased at the initial contact and the terminal stance and decreased at the mid stance with increase in velocity(p<0.001). Moments of the hip and knee joints significantly in creased with increase in velocity whereas that of the ankle joint did not. Gait analysis revealed that weight-bearing decreased and moments of the hip and knee joints increased with increase in velocity(p<0.01). The results of this study may help people perform Nordic walking efficiently and Nordic walking can be used in the gait training of people with an abnormal gait.

Bilateral Changes in Ground Reaction Forces in Patients with Unilateral Anterior Cruciate Ligament Deficiency During Stair Locomotion

ABSTRACTStair locomotion is an important but challenging functional activity for people with lower limb pathology. This study aimed to investigate the bilateral changes in force-bearing on lower limbs during stair locomotion in patients with unilateral ACL deficiency. The ground reaction forces (GRF) were collected from three force platforms: One at ground level in front of a 5-step stair and two on the first two steps respectively. Parameters in vertical and anterior-posterior GRF were extracted and compared between the ACL-deficient (ACLD) and control groups. The ACLD group showed significantly slower stepping cadences in both stair ascent and stepping down to the ground (p &lt; 0.05). The vertical GRF in the ACLD group demonstrated smaller peak forces but larger minimum forces between the two peaks than those in the control group during both stair ascent and descent. Significantly reduced anterior propulsive forces and push-off rates in the late stance were also found in both limbs of the ACLD group (p &lt; 0.05). The slower cadences and reduced force-bearing on the affected limb suggested a protective strategy was adopted. However, the anterior loading parameters in the early stance on the unaffected limb demonstrated different adaptations with significantly larger magnitudes during stair ascent but reduced magnitudes during stair descent (p &lt; 0.05). Similar results were also found in the weight- transferring strategies between legs in consecutive steps with a significantly larger percentage of lift-up forces but a smaller percentage of impact forces on the leading unaffected limb. The results of this study indicated a cautious force-bearing strategy and bilateral adaptation were apparent in the patients with unilateral ACL deficiency. This information may provide a safety guideline for the patients and be helpful for a better use of the stair tasks as part of a rehabilitation program.

Mobility in musculokeletal disease: compromise and compensations: IP76. Why do People with Neurogenic Claudication Like Shopping?

Background: People with lumbar spinal stenosis commonly present with symptoms of pain in the legs which is brought on by walking, a presentation known as neurogenic claudication. The symptoms are often reported as being reduced when walking with a shopping trolley for support, but the mechanism of symptom improvement is not fully understood. This study was designed to explore the effects of pushing a shopping trolley on spinal posture and loading patterns. Methods: An exploratory study of kinematic and kinetic changes in walking with and without a shopping trolley was undertaken with a group of people with symptoms of neurogenic claudication (n = 8) and a group of asymptomatic controls (n = 8). Three-dimensional motion analysis was performed using the Vicon Motion Analysis system using the Vicon Plug-in-Gait protocol for spinal movement analysis. Vertical ground reaction force measures were captured simultaneously using a forceplate embedded in a walkway within the motion analysis capture volume. Changes in lumbar spinal sagittal posture and ground reaction force were measured during walking with and without the shopping trolley. Between group differences in spinal sagittal angle changes and vertical ground reaction force changes were analysed, and correlations between walking speed, ground reaction force and spinal posture were also explored. Results: Subjects with neurogenic claudication and asymptomatic controls walked with a significantly more flexed spinal posture (increase in flexion 3.40 degrees; z = 3.516; P = <.001) when walking with a shopping trolley. The extent of this postural effect was significantly correlated with individual subjects’ height, with taller subjects showing a greater increase in spinal flexion than shorter subjects (rho = −0.65; P = .006). In addition, mean ground reaction forces across the whole gait cycle were reduced in all participants when walking wtih a shopping trolley (-6.9% of bodyweight; z = −3.45; P = .001). However, at the midstance point of the gait cycle, controls showed minimal reliance on the trolley, whereas people with NC showed continued offloading. Conclusions: Both posture and loading are affected by pushing a shopping trolley; however, patients with NC were found to offload the spine throughout the stance phase of gait, whereas asymptomatic controls did not. Taller subjects offloaded to a greater extent than those of a shorter stature when pushing the shopping trolley. Disclosures: The authors have declared no conflicts of interest.

Adaptations of gait and muscle activation in chronic ACL deficiency

The purpose was to investigate whether deviations in gait parameters or muscular activity patterns can be detected in the injured and healthy leg of chronic ACL-deficient subjects. Sixteen medium-level active chronic ACL-deficient patients classified as "copers" (injury duration: 12-240 months, age 17-52 years) and 15 healthy subjects (age 20-33 years) walked at self-selected speed along a 10-m runway with a level force-plate. Gait specific data, ground reaction forces, knee and ankle angles, and EMG were documented. Knee laxity was increased and the functional scores (Lysholm, KOOS) decreased in the ACL- deficient subjects, whereas the Tegner activity level score was normal. Gait speed, stride length and stance time did not differ between ACL-deficient subjects and controls. Ground reaction forces (magnitude and times), as well as knee and ankle angles at selected points during stance and swing phases were normal in the ACL-deficient subjects compared to controls. The total duration of m. tibialis anterior (TA) activity was longer in ACL-deficient subjects than in controls (ACL-deficient injured leg vs. controls, P < 0.05). In addition, the onset of lateral gastrocnemius (LG) muscle activity occurred earlier in ACL-deficient patients (P < 0.03), resulting in a TA-LG cocontraction in the ACL-deficient but not in the control group. In conclusion, chronic, medium-level active ACL-deficient patients showed abnormalities in muscular activity patterns during gait compared to control subjects, whereas there were no detectable changes in ground reaction forces and 3D kinematic data. As the aberrant muscular activity pattern may be of importance for an even gait, it is proposed that EMG recordings may give additional information in the evaluation and rehabilitation of gait when the ACL is absent.

Computer simulation of the effects of shoe cushioning on internal and external loading during running impacts

Biomechanical aspects of running injuries are often inferred from external loading measurements. However, previous research has suggested that relationships between external loading and potential injury-inducing internal loads can be complex and nonintuitive. Further, the loading response to training interventions can vary widely between subjects. In this study, we use a subject-specific computer simulation approach to estimate internal and external loading of the distal tibia during the impact phase for two runners when running in shoes with different midsole cushioning parameters. The results suggest that: (1) changes in tibial loading induced by footwear are not reflected by changes in ground reaction force (GRF) magnitudes; (2) the GRF loading rate is a better surrogate measure of tibial loading and stress fracture risk than the GRF magnitude; and (3) averaging results across groups may potentially mask differential responses to training interventions between individuals.

Muscle Activity During Running At Reduced Body Weight.

Running at reduced body weight (BW) may have rehabilitation advantages in that it may be possible to modulate the stress applied to the lower extremity. A unique treadmill has been designed to apply an upward directed force on the runner such that effective BW can be reduced. For example, the treadmill can be set to have the runner run at 60% of his/her BW. Recently, it has been reported that ground reaction forces (GRFs) are influenced by the variation of BW (Grabowski and Kram, JAB, 2008). The change in GRFs may be an indication that running style changes as BW is varied and we sought to understand the activity of lower extremity muscles as BW is manipulated. PURPOSE: To determine if knee flexor and extensor muscle activity is influenced by 1) magnitude of effective BW and 2) intensity of exercise across BW conditions. METHODS: Subjects (n=5, 26.4±10.6 yrs; 71±14.2 kg, 176 ± 15.4 cm) completed two minutes of running at 100, 90, 75 and 60% of BW at Rating of Perceived Exertion (RPE) levels of 11 (fairly light), 13 (somewhat hard) and 15 (hard). Order of conditions was RPE 11, 13 and 15 from highest (100%) to lowest (60%) BW. Prior to testing, subjects completed a familiarization period where they ran at different speeds and effective BWs. Measurement began by placing electromyography (EMG) electrodes on the belly of the rectus femoris (RF) and biceps femoris (BF). Data were collected (1000 Hz) during the final 30 seconds of each 2 minute run. EMG data were reduced by removing any zero offset, full wave rectifying, and then averaging muscle activity across 30 seconds. Dependent variables (average RF and BF muscle activity) were analyzed using a 3 (RPE: 11, 13, 15) × 4 (BW: 100, 90, 75, 60%) repeated measures analysis of variance. RESULTS: Neither muscle was influenced by the interaction of BW and RPE (p>0.05). RF and BF muscle activity increased across RPE levels (p0.05). CONCLUSIONS: Muscle activity was not influenced by changes in BW when subjects were allowed to adjust treadmill speed to select a specific RPE. In general, subjects selected a faster running speed as BW was reduced and/or RPE increased. The faster running speed per reduction in effective BW offset any potential reduction of muscle demand. The treadmill used in the study was provided by Alter-G, Inc.

The effect of breast support on kinetics during overground running performance

Changes in ground reaction forces that result from different breast support conditions may have implications for sports performance and transmission of forces through the skeleton. The aim of this investigation was to compare kinetic variables and breast motion in a no-bra, everyday-bra and two sports-bra conditions. Following ethical approval, eight female participants with D-cup breasts had retro-reflective markers placed on the left and right nipples, anterior superior iliac spines and clavicles. Five calibrated ProReflex infra-red cameras (100 Hz; Qualisys) measured 3-D displacement of markers and synchronised kinetic data were collected using a force platform (500 Hz, Kistler 9281CA). A repeated measures one-way ANOVA revealed a significantly higher medial impact force in the no-bra condition (0.15 times body weight) compared with the compression sports-bra condition (0.12 times body weight) (F = 3.64 3,21, p = 0.03). Findings suggest that inadequate breast support affects a female's running kinetics, which may have negative physiological consequences on sports performance.

A Simple Mass-Spring Model With Roller Feet Can Induce the Ground Reactions Observed in Human Walking

It has previously been shown that a bipedal model consisting of a point mass supported by spring limbs can be tuned to simulate periodic human walking. In this study, we incorporated roller feet into the spring-mass model and evaluated the effect of roller radius, impact angle, and limb stiffness on spatiotemporal gait characteristics, ground reactions, and center-of-pressure excursions. We also evaluated the potential of the improved model to predict speed-dependent changes in ground reaction forces and center-of-pressure excursions observed during normal human walking. We were able to find limit cycles that exhibited gait-like motion across a wide spectrum of model parameters. Incorporation of the roller foot (R=0.3 m) reduced the magnitude of peak ground reaction forces and allowed for forward center-of-pressure progression, making the model more consistent with human walking. At a fixed walking speed, increasing the limb impact angle reduced the cadence and prolonged stance duration. Increases in either limb stiffness or impact angle tended to result in more oscillatory vertical ground reactions. Simultaneous modulation of the limb impact angle and limb stiffness was needed to induce speed-related changes in ground reactions that were consistent with those measured during normal human walking, with better quantitative agreement achieved at slower speeds. We conclude that a simple mass-spring model with roller feet can well describe ground reaction forces, and hence center of mass motion, observed during normal human walking.