Normalization of ground reaction forces

The appropriateness of normalizing data, as one method to reduce the effects of a covariate on a dependent variable, should be evaluated. Using ratio, 0.67-nonlinear, and fitted normalizations, the aim of this study was to investigate the relationship between ground reaction force variables and body mass (BM). Ground reaction forces were recorded for 40 female subjects running at 3.7 +/- 0.18 m x s(-1) (mass = 58 +/- 6 kg). The explained variance for mass to forces (peak-impact-vertical = 70%; propulsive-vertical = 27%; braking = 40%) was reduced to <0.1% for mass to ratio normalized forces (i.e., forces/BM1) with statistically significantly different power exponents (p < 0.05). The smaller covariate effect of mass on loading rate variables of 2-16% was better removed through fitted normalization (e.g., vertical-instantaneous-loading rate/ BM(0.69+/-0.93); +/-95% CI) with nonlinear power exponents ranging from 0.51 to 1.13. Generally, these were similar to 0.67 as predicted through dimensionality theory, but, owing to the large confidence intervals, these power exponents were not statistically significantly different from absolute or ratio normalized data (p > 0.05). Further work is warranted to identify the appropriate method to normalize loading rates either to mass or to another covariate. Ratio normalization of forces to mass, as predicted through Newtonian mechanics, is recommended for comparing subjects of different masses.     

Lower-extremity ground reaction forces in collegiate baseball pitchers

The purpose of this study was to investigate ground reaction forces (GRF) in collegiate baseball pitchers and their relationship to pitching mechanics. Fourteen healthy collegiate baseball pitchers participated in this study. High-speed video and force plate data were collected for fastballs from each pitcher. The average ball speed was 35 ± 3 m/sec (78 ± 7 mph). Peak GRFs of 245 ± 20% body weight (BW) were generated in an anterior or braking direction to control descent. Horizontal GRFs tended to occur in a laterally directed fashion, reaching a peak of 45 ± 63% BW. The maximum vertical GRF averaged 202 ± 43% BW approximately 45 milliseconds after stride foot contact. A correlation between braking force and ball velocity was evident. Because of the downward inclination and rotation of the pitching motion, in addition to volume, shear forces may occur in the musculoskeletal tissues of the stride limb leading to many of the lower-extremity injuries seen in this athletic population.     

Control of ground reaction forces by hindlimb muscles during cat locomotion

It has been proposed that biarticular muscles are primarily responsible for the control of the direction of external forces, as their activation is closely related and highly sensitive to the direction of external forces. This functional role for biarticular muscles has been supported qualitatively by experimental evidence, but has never been tested quantitatively for lack of a mathematical/mechanical formulation of this theory and the difficulty of measuring individual muscle forces during voluntary movements. The purposes of this study were: (1) to define rules for muscular coordination based on the control of external forces; (2) to develop a model of the cat hindlimb that allows for the calculation of the magnitude and direction of the ground reaction forces (GRFs) produced by individual hindlimb muscles; and (3) to test if the coordination of mono- and biarticular cat hindlimb muscles is related to the control of the resultant GRF. We measured the GRF, hindlimb kinematics, selected muscle forces and activations during cat locomotion. Then, the measured muscle forces were used as input to the hindlimb model to compute the muscle-induced GRF. We assume that if activation (and possibly force) increased as the muscle-induced component of GRF approximated the resultant GRF, then that muscle was used by the central nervous system (CNS) to help control the direction of the external GRF. During cat walking, medial gastrocnemius (MG) and plantaris (PL) forces increased with increasing proximity to the GRF, while soleus (SOL) forces and vastus lateralis (VL) activations did not. SOL and VL activation were most strongly related to the vertical and parallel (braking/accelerating) component of the GRF, respectively. We concluded from these results that MG and PL are primarily responsible for the control of the direction of the GRF, while SOL primarily functions as an anti-gravity muscle, and VL as an acceleration/deceleration muscle.     

Estimating the complete ground reaction forces with pressure insoles in walking

This study presented a method to estimate the complete ground reaction forces from pressure insoles in walking. Five male subjects performed 10 walking trials in a laboratory. The complete ground reaction forces were collected during a right foot stride by a force plate at 1000Hz. Simultaneous plantar pressure data were collected at 100Hz by a pressure insole system with 99 sensors covering the whole plantar area. Stepwise linear regressions were performed to individually reconstruct the complete ground reaction forces in three directions from the 99 individual pressure data until redundancy among the predictors occurred. An additional linear regression was performed to reconstruct the vertical ground reaction force by the sum of the value of the 99 pressure sensors. Five other subjects performed the same walking test for validation. Estimated ground reaction forces in three directions were calculated with the developed regression models, and were compared with the real data recorded from force plate. Accuracy was represented by the correlation coefficient and the root mean square error. Results showed very good correlation in anterior-posterior (0.928) and vertical (0.989) directions, and reasonable correlation in medial-lateral direction (0.719). The root mean square error was about 12%, 5% and 28% of the peak recorded value. Future studies should aim to generalize the methods or to establish specific methods to other subjects, patients, motions, footwear and floor conditions. The method gives an extra option to study an estimation of the complete ground reaction forces in any environment without the constraints from the number and location of force plates.     

Variability of ground reaction forces during treadmill walking.

The purpose of this study was to investigate whether or not the neuromuscular locomotor system is optimized at a unique speed by examining the variability of the ground reaction force (GRF) pattern during walking in relation to different constant speeds. Ten healthy male subjects were required to walk on a treadmill at 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0 km/h. Three components [vertical (F(z)), anteroposterior (F(y)), and mediolateral (F(x)) force] of the GRF were independently measured for approximately 35 steps consecutively for each leg. To quantify the GRF pattern, five indexes (first and second peaks of F(z), first and second peaks of F(y), and F(x) peak) were defined. Coefficients of variation were calculated for these five indexes to evaluate the GRF variability for each walking speed. It became clear for first and second peaks of F(z) and F(x) peak that index variabilities increased in relation to increments in walking speed, whereas there was a speed (5.5-5.8 km/h) at which variability was minimum for first and second peaks of F(y), which were related to forward propulsion of the body. These results suggest that there is "an optimum speed" for the neuromuscular locomotor system but only for the propulsion control mechanism.     

Force plate for measuring the ground reaction forces in small animal locomotion

The importance of kinetic force plate studies of locomotion in small animals has grown recently with the increasing use of rodent models for studies of musculoskeletal diseases. However, the force plates for use with animals much smaller than a cat are difficult to design and use. Here we present data on a commercially available small force plate that accurately collects whole-body and, in a modified form, single-limb ground reaction forces in mice. The method used here is convenient, inexpensive, and readily adaptable for use with a variety of small species.     

Use of pressure insoles to calculate the complete ground reaction forces

A method to calculate the complete ground reaction force (GRF) components from the vertical GRF measured with pressure insoles is presented and validated. With this approach it is possible to measure several consecutive steps without any constraint on foot placement and compute a standard inverse dynamics analysis with the estimated GRF.     

Rotating horizontal ground reaction forces to the body path of progression

When studying the biomechanics of a transient turn, the orientation of the body will change relative to the orientation of the force plates over the progression of the turn. To express ground reaction forces relative to the body, this study investigated possible origin locations and axis alignments of body reference frames. The gait patterns of 10 subjects were recorded as subjects negotiated a 90 degrees hallway corner. Body reference frames were chosen whose origins were the center of mass (COM) and the pelvis origin (PEL). A finite-difference method was used to align the axes of the reference frames according to the horizontal paths of the COM and PEL. The ground reaction impulses (GRIs) were calculated relative to the COM and PEL reference frames. GRI differences were small between the PEL and COM frames, suggesting that either is acceptable for turning studies. Based on an investigation of finite-difference parameters, the COM frame should be used when using a kinematic sampling rate of 60 Hz. Either frame is acceptable when sampling at higher rates.     

Vertical ground reaction forces diminish in mice after botulinum toxin injection

We examined changes in weight-bearing ability in mice after injection with botulinum toxin type A (BTX) to determine whether BTX can be used to isolate the effects of muscle on bone. As ambulation patterns were previously shown to improve within two weeks post-injection, we hypothesized that BTX injection to the posterior hindlimb would not significantly affect the mouse's ability to bear weight in the affected limb one week post-injection. Female BALB/c mice (N=13, 16-17 week old) were injected with either 20 μL of BTX (1U/100 g) or saline (SAL) in the left posterior hindlimb. Vertical ground reaction forces (GRF), hindlimb muscle cross-sectional area (MCSA), and tibial bone micro-architecture were assessed for 42 d following injection. Peak and average vertical GRF were 11±1% and 23±3% lower, respectively, in the BTX-injected hindlimb within 4d post-injection and remained lower than the SAL-injected hindlimb 14-21 d post-injection (15±4% and 10±2%, respectively). Time between forelimb and hindlimb peaks was 30-40% greater in the BTX-injected hindlimb than SAL-injected hindlimb 4-14 d post-injection. Peak vertical GRF recovered earlier following BTX injection than MCSA or bone volume fraction. These results indicate that weight-bearing ability recovered despite persistent muscle atrophy, and that weight-bearing alone was insufficient to maintain bone in the absence of muscle activity. We suggest that the absence of high-frequency signals typically associated with fast-twitch muscle activity may be contributing to the ongoing degradation of bone after BTX injection.     

Measured and estimated ground reaction forces for multi-segment foot models

Accurate measurement of ground reaction forces under discrete areas of the foot is important in the development of more advanced foot models, which can improve our understanding of foot and ankle function. To overcome current equipment limitations, a few investigators have proposed combining a pressure mat with a single force platform and using a proportionality assumption to estimate subarea shear forces and free moments. In this study, two adjacent force platforms were used to evaluate the accuracy of the proportionality assumption on a three segment foot model during normal gait. Seventeen right feet were tested using a targeted walking approach, isolating two separate joints: transverse tarsal and metatarsophalangeal. Root mean square (RMS) errors in shear forces up to 6% body weight (BW) were found using the proportionality assumption, with the highest errors (peak absolute errors up to 12% BW) occurring between the forefoot and toes in terminal stance. The hallux exerted a small braking force in opposition to the propulsive force of the forefoot, which was unaccounted for by the proportionality assumption. While the assumption may be suitable for specific applications (e.g. gait analysis models), it is important to understand that some information on foot function can be lost. The results help highlight possible limitations of the assumption. Measured ensemble average subarea shear forces during normal gait are also presented for the first time.     

The effectiveness of scaling procedures for comparing ground reaction forces

Various scaling methods are used when attempting to remove the influence of anthropometric differences on ground reaction forces (GRF) when comparing groups. Though commonly used, ratio scaling often results in an over-correction. Allometric scaling has previously been suggested for kinetic variables but its effectiveness in partialing out the effect of anthropometrics is unknown due to a lack of consistent application. This study examined the effectiveness of allometric scaling vertical, braking and propulsive GRF and loading rate for 84 males and 47 females while running at 4.0 m/s. Raw, unfiltered data were ratio scaled by body mass (BM), height (HT), and BM multiplied by HT (BM1HT). Gender specific exponents for allometric scaling were determined by performing a log-linear (for BM and HT individually) or log-multilinear regression (BMHT). Pearson productmoment correlations were used to assess the effectiveness of each scaling method. Ratio scaling by BM, HT, or BM1HT resulted in an over-correction of the data for most variables and left a considerable portion of the variance still attributable to anthropometrics. Allometric scaling by BM successfully removed the effect of BM and HT for all variables except for braking GRF in males and vertical GRF in females. However, allometric scaling for BMHT successfully removed the effect of BM and HT for all reactionary forces in both genders. Based on these results, allometric scaling for BMHT was the most appropriate scaling method for partialing out the effect of BM and HT on kinetic variables to allow for effective comparisons between groups or individuals.     

Force treadmill for measuring vertical and horizontal ground reaction forces

Weconstructed a force treadmill to measure the vertical, horizontal andlateral components of the ground-reaction forces (F_z,F_y,F_x, respectively) and the ground-reaction force moments(M_z,M_y,M_x), respectively exerted bywalking and running humans. The chassis of a custom-built, lightweight(90 kg), mechanically stiff treadmill was supported along its length bya large commercial force platform. The natural frequencies of vibrationwere >178 Hz for F_z and >87Hz for F_y, i.e., well above thesignal content of these ground-reaction forces. Mechanical tests andcomparisons with data obtained from a force platform runway indicatedthat the force treadmill recordedF_z,F_y,M_x andM_y ground-reaction forces andmoments accurately. Although the lowest natural frequency of vibrationwas 88 Hz for F_x, thesignal-to-noise ratios for F_x andM_z were unacceptable. This devicegreatly decreases the time and laboratory space required for locomotionexperiments and clinical evaluations. The modular design allows forindependent use of both treadmill and force platform.

     

The influence of soft tissue movement on ground reaction forces, joint torques and joint reaction forces in drop landings

The aim of this study was to determine the effects that soft tissue motion has on ground reaction forces, joint torques and joint reaction forces in drop landings. To this end a four body-segment wobbling mass model was developed to reproduce the vertical ground reaction force curve for the first 100 ms of landing. Particular attention was paid to the passive impact phase, while selecting most model parameters a priori, thus permitting examination of the rigid body assumption on system kinetics. A two-dimensional wobbling mass model was developed in DADS (version 9.00, CADSI) to simulate landing from a drop of 43 cm. Subject-specific inertia parameters were calculated for both the rigid links and the wobbling masses. The magnitude and frequency response of the soft tissue of the subject to impulsive loading was measured and used as a criterion for assessing the wobbling mass motion. The model successfully reproduced the vertical ground reaction force for the first 100 ms of the landing with a peak vertical ground reaction force error of 1.2% and root mean square errors of 5% for the first 15 ms and 12% for the first 40 ms. The resultant joint forces and torques were lower for the wobbling mass model compared with a rigid body model, up to nearly 50% lower, indicating the important contribution of the wobbling masses on reducing system loading.     

Muscle activity in the leg is tuned in response to ground reaction forces.

During walking and running, the human body reacts to its external environment. One such response is to the impact forces that occur at heel strike. This study tested previous speculation that the levels of muscle activity in the lower extremities are adjusted in response to the loading rate of the impact forces. A pendulum apparatus was used to deliver repetitive impacts to the heels of 20 subjects. Impact forces were of similar magnitude to those experienced during running, but the loading rate was varied by 13% using different materials in the subjects' shoes. Myoelectric patterns were measured in the tibialis anterior, medial gastrocnemius, vastus medialis, and biceps femoris muscles. Wavelet analysis was used to resolve intensity of the myoelectric patterns into time and frequency space. Substantial and significant differences in the myoelectric activity occurred between the impact conditions for the 50 ms before and the 50 ms after impact, reaching 3 ms in timing, 16% in wavelet number, and 154% in the intensity of the muscle activity.     

Transient decreases in forelimb gait and ground reaction forces following rotator cuff injury and repair in a rat model

Due to inadequate healing, surgical repairs of torn rotator cuff tendons often fail, limiting the recovery of upper extremity function. The rat is frequently used to study rotator cuff healing; however, there are few systems capable of quantifying forelimb function necessary to interpret the clinical significance of tissue level healing. We constructed a device to capture images, ground reaction forces and torques, as animals ambulated in a confined walkway, and used it to evaluate forelimb function in uninjured control and surgically injured/repaired animals. Ambulatory data were recorded before (D–1), and 3, 7, 14, 28 and 56 days after surgery. Speed as well as step width and length were determined by analyzing ventral images, and ground reaction forces were normalized to body weight. Speed averaged 22±6 cm/s and was not affected by repair or time. Step width and length of uninjured animals compared well to values measured with our previous system. Forelimbs were used primarily for braking (?1.6±1.5% vs +2.5±0.6%), bore less weight than hind limbs (49±5% vs 58±4%), and showed no differences between sides (49±5% vs 46±5%) for uninjured control animals. Step length and ground reaction forces of the repaired animals were significantly less than control initially (days 3, 7 and 14 post-surgery), but not by day 28. Our new device provided uninjured ambulatory data consistent with our previous system and available literature, and measured reductions in forelimb function consistent with the deficit expected by our surgical model.     

Reducing ground reaction forces in gymnastics’ landings may increase internal loading

The aim of this study was to use a subject-specific seven-link wobbling mass model of a gymnast, and a multi-layer model of a landing mat, to determine landing strategies that minimise ground reaction forces (GRF) and internal forces. Subject-specific strength parameters were determined that defined the maximum voluntary torque/angle/angular velocity relationship at each joint. These relationships were used to produce subject-specific ‘lumped’ linear muscle models for each joint. Muscle activation histories were optimised using a Simplex algorithm to minimise GRF or bone bending moments for forward and backward rotating vault landings. Optimising the landing strategy to minimise each of the GRF reduced the peak vertical and horizontal GRF by 9% for the backward rotating vault and by 8% and 48% for the forward rotating vault, compared to a matching simulation. However, most internal loading measures (bone bending moments, joint reaction forces and muscle forces) increased compared to the matching simulation. Optimising the landing strategy to minimise the peak bone bending moments resulted in reduced internal loading measures, and in most cases reduced GRF. Bone bending moments were reduced by 27% during the forward rotating vault and by 2% during the backward rotating vault landings when compared to the matching simulations. It is possible for a gymnast to modify their landing strategy in order to minimise internal forces and lower GRF. However, using a reduction in GRF, due to a change in landing strategy, as a basis for a reduction in injury potential in vaulting movements may not be appropriate since internal loading can increase.     

Kinetic analysis of ski turns based on measured ground reaction forces

The objective of this study was to devise a method of kinetic analysis of the ground reaction force that enables the durations and magnitudes of forces acting during the individual phases of ski turns to be described exactly. The method is based on a theoretical analysis of physical forces acting during the ski turn. Two elementary phases were defined: (1) preparing to turn (initiation) and (2) actual turning, during which the center of gravity of the skier-ski system moves along a curvilinear trajectory (steering). The starting point of the turn analysis is a dynamometric record of the resultant acting ground reaction force applied perpendicularly on the ski surface. The method was applied to six expert skiers. They completed a slalom course comprising five gates arranged on the fall line of a 26° slope at a competition speed using symmetrical carving turns (30 evaluated turns). A dynamometric measurement system was placed on the carving skis (168 cm long, radius 16 m, data were recorded at 100 Hz). MATLAB procedures were used to evaluate eight variables during each turn: five time variables and three force variables. Comparison of the turn analysis results between individuals showed that the method is useful for answering various research questions associated with ski turns.     

Posturographic analysis through markerless motion capture without ground reaction forces measurement

The ability of the central nervous system to control posture and balance has been used with increasing frequency for the diagnosis and/or treatment evaluation of various neuromuscular diseases. Typically this analysis (Posturographic Analysis) is based on tracking the motion of the center of mass (COM) during quiet standing, however direct measurement of the COM has been commonly approximated using the movement of the center of pressure (COP). The purpose of this study was to apply and validate a new method to track the COM (center of mass) and COP (center of pressure) from a visual hull measured using a markerless motion capture (MMC) method. The method was tested by comparing the calculation of the COP from direct measurements of the COP. The deviations between the methods, below 2 mm, were small relative to the average range of movement guaranteeing a satisfactory signal to noise ratio. This new method requires only kinematic data through MMC method and without the need of a force plate can identify the influence of individual body segments to motion of the COM.