Triaxial Force Plate Research Articles

Hearing Status and Static Postural Control of Collegiate Athletes.

Because of the close proximity of the cochlea, vestibular apparatus, and shared neurovascular structures, the static postural control of athletes who are deaf or hard of hearing (D/HoH) may be different from that of athletes who are hearing. Limited research is available to quantify differences between these athletes. To determine the effect of hearing status and stance condition on the static postural control of athletes. Cross-sectional study. Athletic training facilities. Fifty-five collegiate varsity athletes who were D/HoH (age = 20.62 ± 1.80 years, height = 1.73 ± 0.08 m, mass = 80.34 ± 18.92 kg) and 100 university club athletes who were hearing (age = 20.11 ± 1.59 years, height = 1.76 ± 0.09 m, mass = 77.66 ± 14.37 kg). Participants completed the Modified Clinical Test of Sensory Interaction and Balance on a triaxial force plate. Anteroposterior and mediolateral (ML) center-of-pressure (CoP) velocity, anteroposterior and ML CoP amplitude root mean square, and 95% ellipse sway area were calculated. Athletes who were D/HoH had a larger CoP velocity, larger ML root mean square, and larger sway area than those who were hearing (P values < .01). A significant main effect of stance condition was observed for all postural control variables (P values < .01). During the Modified Clinical Test of Sensory Interaction and Balance, athletes who were D/HoH demonstrated a larger sway area compared with athletes who were hearing. Therefore, individualized baseline assessments of static postural control may be warranted for athletes who are D/HoH as opposed to comparisons with existing normative data.

Reliability and Effectiveness of a Lateral Countermovement Jump for Stratifying Shuffling Performance Amongst Elite Basketball Players.

Though research suggests that basketball players spend approximately 31% of game actions shuffling laterally, limited data are available on the kinetic factors that separate fast and slow shufflers. The purpose of this study was twofold: (1.) Examine the reliability of kinetic metrics from a single-leg Lateral Countermovement Jump (LCMJ) (2.) Determine if kinetic metrics from the LCMJ can stratify above (i.e., "fast") or below (i.e., "slow") median shuffling performance. Twenty professional basketball players participated in the reliability study (21.7 ± 3.5 years, 1.98 ± 0.1 m; 89.9 ± 10.9 kg). One hundred seven professional and thirty-three collegiate basketball players (N = 140) (22.7 ± 3.5 years, 2.0 ± 0.1 m; 98.4 ± 11.9 kg) participated in the experimental study examining the ability of LCMJ kinetics to stratify shuffling performance. Reliability was assessed using Bland-Altman plots, coefficients of variation (CVs), typical errors (TEs), and intraclass correlation coefficients (ICCs). Anthropometric and LCMJ kinetic differences between fast and slow shufflers were assessed with an independent t-test. Four kinetic metrics (peak vertical force, peak lateral force, relative lateral force, and lateral impulse) met within- and between-session reliability thresholds (CV &lt; 10% and ICC &gt; 0.70). Faster shufflers generated significantly more relative lateral force than their slower counterparts (9.51 ± 0.8 Nx/kg vs. 8.9 ± 0.9 Nx/kg, %Diff 6.3, p &lt; 0.00007, ES = 0.70). Basketball practitioners who have access to triaxial force plates may consider adding the LCMJ into their testing battery, as relative lateral force is a reliable metric that can stratify fast and slow shufflers.

Factors influencing static postural control performance of collegiate marching artists

ABSTRACT The purpose of this study was to examine the influence of surface and instrument on static postural control performance of collegiate marching artists. Fifty-seven collegiate marching artists completed single- and double-leg stance balance with and without an instrument on firm and foam surfaces (four conditions per stance). Two, 15-s trials of each condition were performed on a tri-axial force plate. Total, antero-posterior, and medio-lateral centre-of-pressure excursion, and 95% ellipse sway area were collected. Means from the two trials were averaged. Linear mixed-effects models were fit with centre-of-pressure variables as predictors with fixed effects of surface and instrument, adjusting for number of seasons in marching band and type of instrument. Alpha level was set a priori at p ≤ 0.05. Standing on foam surface, regardless of stance, elicited longer excursion and larger sway area (p < 0.05 for all). Although number of seasons participated in marching band did not influence static postural control under single-leg stance, there was a significant effect for double-leg stance (p < 0.01). Marching artists commonly rehearse and perform on unstable surfaces. Understanding static postural control performance under similar conditions may provide novel and important information for injury risk assessment and performance evaluation.

Somatosensory perturbations influence cortical activity associated with single-limb balance performance.

To determine the association between cortical activity and postural control performance changes with differing somatosensory perturbations. Healthy individuals (n = 15) performed a single-limb balance task under four conditions: baseline, unstable surface (foam), transcutaneous electrical nerve stimulation (TENS) applied to the stance-limb knee, and combined foam + TENS. Cortical activity was recorded with electroencephalography (EEG) and postural sway via triaxial force plate. EEG signals were decomposed, localized, and clustered to generate power spectral density in theta (4-7Hz) and alpha-2 (10-12Hz) frequency bands in anatomical clusters. Postural sway signals were analyzed with center of pressure (COP) sway metrics (e.g., area, distance, velocity). Foam increased theta power in the frontal and central clusters (d = 0.77 to 1.16), decreased alpha-2 power in bilateral motor, right parietal, and occipital clusters (d = - 0.89 to - 2.35) and increased sway area, distance, and velocity (d = 1.09-2.57) relative to baseline. Conversely, TENS decreased central theta power (d = - 0.60), but increased bilateral motor, left parietal, and occipital alpha-2 power (d = 0.51-1.40), with similar to baseline balance performance. In combination, foam + TENS attenuated sway velocity detriments and cortical activity caused by the foam condition alone. There were weak and moderate associations between percent increased central theta and occipital activity and increased sway velocity. Somatosensory perturbations changed patterns of cortical activity during a single-limb balance task in a manner suggestive of sensory re-weighting to pertinent sensory feedback. Across conditions decreased cortical activity in pre-motor and visual regions were associated with reduced sway velocity.

External Focus of Attention Influences Cortical Activity Associated With Single Limb Balance Performance.

External focus (EF) of attention leads to improved balance performance. Consideration of the neuromodulatory effects of EF may inform its clinical utility in addressing neuroplastic impairments after musculoskeletal injuries. This study aimed to determine whether electrocortical activity and balance performance changed with attentional foci that prioritized differing sensory feedback and whether changes in electrocortical activity and balance were associated. Individuals who were healthy (n = 15) performed a single-limb balance task under 3 conditions: internal focus (IF), somatosensory focus (EF with a baton [EF-baton]), and visual focus (EF with a laser [EF-laser]). Electrocortical activity and postural sway were recorded concurrently using electroencephalography and a triaxial force plate. Electroencephalographic signals were decomposed, localized, and clustered to generate power spectral density in θ and α-2 frequency bands. Postural sway signals were analyzed with center-of-pressure sway metrics (eg, area, distance, velocity) and knee angle. The relationship between percent change in clustered brain activity and task performance metrics was assessed. Both EF conditions resulted in increased cortical activity and improved balance performance compared with IF. EF-laser had the largest effect, demonstrating increased frontal θ power (d = 0.64), decreased central θ power (d = -0.30), and decreased bilateral motor, bilateral parietal, and occipital α-2 power (d = -1.38 to -4.27) as well as a shorter path distance (d = -0.94) and a deeper (d=0.70) and less variable (d = -1.15) knee angle than IF. Weak to moderate associations exist between increases in cortical activity and improved balance performance (ρ = 0.405-0.584). EF resulted in increased cortical activity associated with cognitive, motor, somatosensory, and visual processing. EF-laser, which prioritized visual feedback, had the largest and broadest effects. Changes in cortical activity resulting from EF were independently associated with improved balance performance. This study demonstrates that goal-oriented attention results in functional increases in brain activity compared with internally directed self-focus. These results suggest EF may target neurophysiologic impairments and improve balance in clinical populations.

Modulations of Inter-Hemispherical Phase Coupling in Human Single Leg Stance

Introduction: Recent findings from neuroimaging studies provided initial insights into cortical contributions to postural control. These studies observed enhanced cortical activation and connectivity when task-difficulty and postural instability increased. However, little attention has been paid to the allocation of cortical networks appearing with a decreasing base of support from bipedal to single leg stance. Therefore, the aim of the present study was to investigate modulations of functional connectivity from bipedal to single leg stance. Experimental procedures: Cortical activity during bipedal and single leg stance (left) was investigated in 15 male subjects using 128 channel mobile electroencephalography (EEG), while standing on a triaxial force plate. Power spectral density was calculated for theta (4–7 Hz), alpha-1 (8–10 Hz) and alpha-2 (10–12 Hz) frequency bands. Estimations of the phase lag index (PLI) were conducted as a measure of functional connectivity. Moreover, postural control was analyzed by the area of sway and sway velocity. Results: The results demonstrated a significantly increased area of sway and decreased alpha-2 power in single leg compared to bipedal stance. Furthermore, PLIs within the alpha-2 frequency band showed significantly decreased inter-hemispherical phase coupling in single leg stance, associated with connections involving the left motor region. Discussion: Altogether, the present findings may indicate modulations of cortical contributions in single leg compared to bipedal stance. The present data suggest that decreased inter-hemispherical functional connectivity, in conjunction with a global increase in cortical excitability, may indicate enhanced alertness and task-specific selective inhibition of motor networks in favor of postural control.

Reliability of Squat Kinetics in Well-Trained Rugby Players: Implications for Monitoring Training.

Appleby, BB, Cormack, SJ, and Newton, RU. Reliability of squat kinetics in well-trained rugby players: implications for monitoring training. J Strength Cond Res 33(10): 2635-2640, 2019-The aim of this study was to determine the within-session reliability in kinetic variables of the squat in well-trained athletes during a typical resistance training protocol. Fifteen subjects completed 2 testing sessions. Session 1 was establishment of 1 repetition maximum (1RM) squat, and session 2 involved 2 sets of 2 maximal effort repetitions of the squat at 70, 80, and 90% of 1RM with 3D motion analysis and ground reaction force (GRF) measurement using 2 in-ground triaxial force plates. Reliability was calculated using typical error ± 90% confidence limits, expressed as the coefficient of variation and intraclass correlation coefficient. The smallest worthwhile change (SWC%), calculated as 0.2 × between-subject SD, was used to determine the smallest important change in performance. Peak GRF and average GRF were found to have acceptable measures of reliability with the combined left and right leg average GRF capable of detecting the SWC. Independent limb contributions were reliable (left and right, or dominant and nondominant). Reliable kinetics can be obtained in back squat performance typical of a resistance training session in well-trained athletes. This suggests that coaches integrating force plate technology within training sessions may effectively capture between 1 and 6 training sets among several athletes, facilitating analysis and intervention on larger data sets.

Eccentric flywheel post-activation potentiation influences swimming start performance kinetics

ABSTRACTThis study aimed to assess the effects of post-activation potentiation in the strength related variables of a kick start. Thirteen competitive swimmers performed three kick starts after a standardized warm up (denoted USUAL) and another after inducing post-activation through five isotonic repetitions on an eccentric flywheel (denoted PAP). A T-test was used to quantify differences between USUAL and PAP warm up. The best trial of each subject achieved by natural conditions (denoted PEAK) was compared with data obtained after PAP. An instrumented starting block with independent triaxial force plates, collected the strength variables related with the impulse at take off. Improvements in the vertical components of force were observed after PAP compared with USUAL, meanwhile no differences were detected on the horizontal components of it. The velocity at take off was higher after PAP compared with USUAL (4.32 ± 0.88 vs 3.93 ± 0.60 m*s-1; p = 0.02). No differences in force or velocity were detected comparing PAP with PEAK (4.13 ± 0.62 m*s-1, p = 0.11). The PAP warm-up increased vertical force and it was transferred to a higher resultant velocity at take-off. This improvement would equal the best result possible obtained in natural conditions after some trials.

Relationship between Take-Off Force Profiles and Single Leg Hop Distance

The anterior single leg hop for distance test (SLHOP) is a functional performance task that is often used to assess return to play criteria following ACL reconstruction. Clinicians primarily assess SLHOP distance, but the way an individual completes the task may be important when determining return to play status. Recent research has indicated that non-uniformity in vertical ground reaction force (VGRF) profiles may be related to pathomechanics during a vertical jump, but little research has been done to explore the relationship between force profiles and SLHOP distance. PURPOSE: To determine the correlation between a force profile ratio and SLHOP distance. METHODS: Twenty-three female collegiate soccer players (19.1±1.5 years; 166.9±7.1 cm; 62.6±8.3 kg) performed three SLHOP tests for maximal distance off a tri-axial force plate. The SLHOP was performed on the dominant limb. Ground reaction force (GRF) data were collected during SLHOP take-off phase to produce force-time curves that were used to calculate the ratio between the instantaneous and average force profile between three events. Force profile ratios were calculated between: A) the minimum GRF after initiation of the countermovement and the peak GRF prior to takeoff, and B) the point during the countermovement when body weight is reached and peak GRF. Ratio A and B where calculated in both the vertical (z) and anterior-posterior (y) axes. All forces were normalized to body weight, and SLHOP distances were normalized to leg length (cm/cm). The strength of the association between each of the aforementioned force profile ratios and maximum SLHOP distance were calculated using Pearson correlation coefficients with an alpha level set a priori at p<0.05. RESULTS: There were no significant correlations between any combination of force profile ratios and SLHOP distance (p-value range:0.19 to 0.62, r range: -0.11 to -0.26). CONCLUSION: Force profile ratios between straight-line and trace-line distance are not significantly related to average or maximum SLHOP distance. Further research should explore the relationship between other phases within the force-time curve, pathomechanics, and SLHOP distance.

Relationship between Single Leg Hop Distance and Descending Phase Force Variables

Anterior single leg hop for distance (SLHOP) is a common functional assessment used to evaluate lower extremity power and dynamic postural stability. Peak force and rate of force development (RFD) have been used to quantify strength and power, but little is known of the relationship between these measures and SLHOP distance. PURPOSE: To determine the relationship between vertical and anterior force variables during the descending phase and SLHOP distance. METHODS: Twenty-three female collegiate soccer players (19.1±1.5 years; 166.9±7.1 cm; 62.6±8.3 kg) performed SLHOP for maximal distance on their dominant limb, taking off a tri-axial force plate. Kinetic data were collected during the descending phase of the SLHOP to identify peak force in the vertical (Fz) and anterior-posterior (Fy) axes and average rate of force development (RFD), the rate of change in the force-time curve. Two start events were used to calculate RFD: 1) RFDnegative: point of maximum center of mass downward acceleration (subject moving down, acceleration negative) and 2) RFDzero: point when ground reaction force equals body weight (subject moving down, acceleration zero). The RFD end event was peak force (near the lowest point of descending). Forces were normalized to body weight and SLHOP distances were normalized to leg length. A linear regression with best subset selection method was performed to identify which combination of variables were most related to SLHOP distance with an alpha level set a priori at p<0.05. RESULTS: Peak Fy (Beta=189.32, 95%CI=133.04, 245.61, p<0.001) and RFDzero in the vertical axis (Beta=-2.94, 95%CI=5.48, -0.40, p=0.03) were significantly related to SLHOP distance (model adjusted R2=0.69). CONCLUSION: Female soccer players who maximized anterior force and minimized vertical RFD during the descending phase of SLHOP achieved greater SLHOP distances. Results suggest maximal SLHOP distance warrants both vertical and anterior force generation. However, these movement strategies may not translate across different jump tasks (i.e. vertical jump, etc.) or different sports. Further research is warranted to examine the relationship between force and RFD profiles and other jump tasks. Future studies should examine the effect of sex and sport on the relationship between SLHOP distance and force variables.

Load-Power Relationship During a Countermovement Jump: A Joint Level Analysis.

Williams, KJ, Chapman, DW, Phillips, EJ, and Ball, N. Load-power relationship during a countermovement jump: A joint level analysis. J Strength Cond Res 32(4): 955-961, 2018-This study aimed to investigate whether hip, knee, and ankle peak power is influenced by the relative load lifted, altering the joint and system load-power relationship during a countermovement jump (CMJ). Twenty-three male national representative athletes (age: 20.3 ± 3.1 years, squat 1 repetition maximum [1RM]: 133.8 ± 24.8 kg) completed 3 CMJs at relative barbell loads of 0, 10, 20, 30, and 40% of an athlete's estimated back squat 1RM. Ground reaction force and joint kinematics were captured using a 16 camera motion capture array integrated with 2 in-ground triaxial force plates. Hip ((Equation is included in full-text article.)= 20%, range 0 > 40%), knee ((Equation is included in full-text article.)= 0%, 0 > 20%), and ankle ((Equation is included in full-text article.)= 40%, 0 > 40%) peak power was maximized at different percentages of absolute strength, with an athlete-dependent variation in load-power profiles observed across all lower-body joints. A decrease in system (body + barbell mass) peak power was significantly (p ≤ 0.05, r = 0.45) correlated with a reduction in knee peak power. Timing of instantaneous system and hip peak power occurred significantly closer to toe-off as load increased. The findings highlight that the generation and translation of lower-body joint power is influenced by external load and athlete-dependent traits. This subsequently alters the load-power profile at a system level, explaining the broad spectrums of loads reported to optimize system power during a CMJ. When training, we recommend that a combination of barbell loads based on assorted percentages of the estimated 1RM be prescribed to optimize joint and system power during a CMJ.

An Innovative Short Arm Centrifuge for Future Studies on the Effects of Artificial Gravity on the Human Body

In July 2013, the German Aerospace Center (DLR) in Cologne, Germany, commissioned its new medical research facility :envihab. One central element of the facility is a new type of short radius centrifuge called DLR-SAHC 1 (formerly known as :enviFuge), which has been developed in collaboration with AMST Systemtechnik GmbH, Ranshofen, Austria. The shift of subjects above heart-level on a short arm centrifuge allows unique studies on, e.g., the cardiovascular regulation in surroundings with a high gradient of artificial gravity. Equipped with the capacity to move the four nacelles along the acceleration axis simultaneously and independently from each other, the centrifuge allows the possibility to perform up to four complex trials in parallel. The maximal acceleration is 6 g at the foot level and each nacelle can accomodate an up to 150kg payload. Additional equipment can be mounted on two payload bays with a capacity of 100kg each. Standard features of the centrifuge include a motion capturing system with six cameras and two triaxial force plates to study the kinematics of physical exercise (e.g., squatting, jumping or vibration training) under increased gravity. Future projects involving SAHC 1 will allow the development and testing of potential countermeasures and training methods against the negative effects of weightlessness in space on human physiology. Due to the centrifuge’s capability to hold heavy equipment, carrying out a variety of non-human life science experiments requiring complex and heavy hardware is also fully feasible.

SAT0420 Ground Reaction Forces in Patients with Knee Osteoarthritis after Intra-Articular Treatments

BackgroundOsteoarthritis of the knee (OA) is the most common disease affecting the musculoskeletal system and one of the most important causes of disability among elderly population. Patients are assessed by...

SAT0349 Assessment of Postural Stability in Patients with Knee Osteoarthritis: A Pilot Study

BackgroundKnee osteoarthritis (OA) is the most common disease affecting human locomotion, and one of the most important causes of disability among the elderly population. In order to assess health status...

Horizontal dynamic forces generated by swaying and jumping

Modern assembly structures must be designed to withstand the dynamic forces generated by their human occupants; the characterization of these forces has become an important research topic. This paper focuses on the measurement and modelling of horizontal dynamic forces, which have previously received much less attention than vertical forces. Horizontal forces resulting from movements such as swaying and jumping were recorded using a tri-axial force plate and characterized using three properties: the impulse shape, the mean of the split impulse and the frequency spectrum. The fundamental force due to swaying occurred at the swaying frequency. However, the fundamental force due to jumping did not always occur at the jumping frequency, indicating that the current design procedure of assuming a horizontal load model similar to the vertical load model is not always appropriate. For some subjects the position of the centre of mass was simultaneously recorded using a motion capture system to establish a relationship between its velocity and the side-to-side force. Finally, a new, semi-empirical analytical model for representing horizontal forces in design calculations was developed. Results from the model show excellent agreement with measurements from the laboratory tests.

Stick–slip phenomena in the friction of human skin

The stick–slip behaviour of the index finger pad sliding on wet, smooth glass was investigated as a function of normal force and sliding velocity in friction measurements using a tri-axial force plate. The friction coefficients during stick–slip (0.53±0.21) were found to be typically 30% lower than those of stationary sliding (0.78±0.35). Stick–slip friction either occurred continuously throughout entire friction measurements or alternated with stationary sliding phases. During stick–slip the amplitude of the friction coefficient generally varied more than ±25% about the mean, whereas stationary sliding was characterized by variations smaller than ±10%. The load-dependence of the overall friction coefficients as well as those for stationary sliding and stick–slip friction was in agreement with the adhesion friction model. In addition, the friction coefficients decreasing with increasing sliding velocity indicated friction in the mixed lubrication regime. Results obtained on stick–slip friction of the finger pad might be useful in connection with the development of artificial skin in robotics and for the control of stick–slip friction between precision tools and soft tissue.

Influence of surface microstructure on the sliding friction of plantar skin against hard substrates

This study investigated how the sliding friction coefficients of wet foot skin depend on the microscopic surface properties of contacted hard substrates. Fourteen subjects conducted repeated slip experiments with one foot on each of 28 different wet surfaces. The friction and normal forces transmitted to the substrates were measured using a tri-axial force plate, and coefficients of friction were determined over periods of stationary sliding. The surface structures of the substrates, characterised by an optical 3D profilometer, varied from roughness asperities in the range of micrometers to macroscopic surface elements in the range of millimetres. The analysis of the load dependence of the skin friction behaviour based on a two-term model provided information on the adhesion and deformation components of friction. Both adhesion and deformation were found to increase with the surface roughness of the substrates. Adhesion mechanisms seemed to predominate on smooth surfaces showing low friction coefficients (<0.3), while the contributions due to skin deformations were found to be up to 0.4 on rough surfaces with high friction coefficients (>0.5). Independent of friction measurements, the analysis of the surface microstructure of the substrates indicated that ploughing as a skin deformation mechanism could contribute around 50% to the deformation component of friction.

Friction of human skin against smooth and rough glass as a function of the contact pressure

The friction behaviour of human skin was studied by combining friction measurements using a tri-axial force plate with skin contact area measurements using a pressure sensitive film. Four subjects carried out friction measurement series, in which they rubbed the index finger pad and the edge of the hand against a smooth and a rough glass surface under dry and wet conditions. The normal loads were varied up to values of 50 N, leading to skin contact pressures of up to 120 kPa. The analysis of the pressure dependence of friction coefficients of skin for contrasting sliding conditions allowed to determine the involved friction mechanisms on the basis of theoretical concepts for the friction of elastomers. Adhesion was found to be involved in all investigated cases of friction between skin and glass. If adhesion mechanisms predominated (skin against smooth glass in the dry condition and skin against rough glass in the wet condition), the friction coefficients were generally high (typically >1) and decreased with increasing contact pressure according to power laws with typical exponents between −0.5 and −0.2. Contributions to the friction coefficient due to viscoelastic skin deformations were estimated to be relatively small (<0.2). In those cases where the deformation component of friction played an important role in connection with adhesion (skin against rough glass in the dry condition), the friction coefficients of skin were typically around 0.5 and their pressure dependence showed weak trends characterised by exponents ranging from −0.1 to +0.2. If hydrodynamic lubrication came into play (skin sliding on smooth glass in the wet condition), the friction coefficients were strongly reduced compared to dry friction (<1), and their decrease with increasing contact pressures was characterised by exponents of <−0.7.

Asymmetry of gait initiation in hemiparetic stroke subjects

Objective: To investigate symmetry of gait initiation in healthy and hemiparetic subjects. Design: Survey Setting: Kinematic laboratory affiliated with a hospital-based department of rehabilitation Patients or Other Participants: Ten healthy and 14 hemiparetic stroke subjects starting five times with their right and left leg, respectively. Main Outcome Measures: Duration of defined periods, step length, center of pressure, and center of mass were recorded and calculated using two triaxial force plates, contact switches, and a video camera system. Results: Healthy subjects displayed a high degree of independence of kinetic and kinematic parameters of the starting limb. Hemiparetic patients showed differences with respect to the starting limb: when starting with the nonaffected leg, the swing period and step length was shorter and the center of pressure displayed a more marked medio-lateral sway with no corresponding initial movement of the center of mass; when starting with the affected leg the movement pattern of the center of pressure and center of mass was comparable to that of normal subjects. Conclusions: The trajectories of the center of pressure and center of mass and the symmetry parameters are in accordance with a higher degree of uncertainty when starting with the non-affected limb in hemiparetic subjects.

Evidence of positive force feedback among hindlimb extensors in the intact standing cat

1. The functional organization of heterogenic reflexes produced by activation of extensor force receptors (Golgi tendon organs) was studied in intact cats during stationary stance. Intramuscular stimulation (200 Hz, 20 ms) of hindlimb extensor muscles via chronically implanted electrodes was used to evoke weak muscle contractions and naturally activate Golgi tendon organ Ib afferents while cats stood unrestrained with each paw on a moveable triaxial force plate. 2. Intramuscular stimulation of every hindlimb extensor muscle tested in this study evoked excitatory responses that were widely distributed among hindlimb extensor muscles. Source and target specializations in the functional organization of this positive force feedback system were also observed. For example, stimulation of ankle extensors typically excited extensors and flexors at the ankle and hip (but not knee), whereas stimulation of hip extensors typically excited only extensors at all three joints. In addition, intramuscular stimulation of either lateral (LG) or medial (MG) gastrocnemius consistently inhibited soleus while exciting other extensors at the ankle and more proximal joints. 3. The electromyographic (EMG) reflex responses described above are attributed to the natural (via muscle contraction) activation of extensor group Ib afferents. Direct activation of intramuscular afferents by the stimulus was unlikely because there was no evidence that Ia afferents, which have the lowest electrical thresholds, were activated. Both the observed inhibition of the synergist, soleus, and the excitation of the antagonist, tibialis anterior, produced by gastrocnemius stimulation are opposite to the reflex effects that would be produced at the ankle by activation of gastrocnemius Ia afferents.(ABSTRACT TRUNCATED AT 250 WORDS)