Magnitude Of Ground Reaction Force Research Articles

The Force-Vector Theory Supports Use of the Laterally Resisted Split Squat to Enhance Change of Direction.

Cooley, C, Simonson, SR, and Maddy, DA. The force-vector theory supports use of the laterally resisted split squat to enhance change of direction. J Strength Cond Res 38(5): 835-841, 2024-The purpose of this study was to challenge the conventional change of direction (COD) training methods of the modern-day strength and conditioning professional. A new iteration of the modified single-leg squat (MSLS), the laterally resisted split squat (LRSS), is theorized to be the most effective movement for enhancing COD performance. This study lays out a rationale for this hypothesis by biomechanically comparing the LRSS, bilateral back squat (BS), and MSLS with a COD task (90-degree turn). One repetition maximum (1RM) for LRSS, MSLS, and BS was measured for 23 healthy active female subjects. Peak ground reaction forces (GRF) for the dominant leg were recorded when performing COD and the LRSS, MSLS, and BS at 70% 1RM. Peak frontal plane GRF magnitude and angle were calculated for each task and submitted to repeated measures ANOVA. Peak GRF magnitude was significantly larger for COD (2.23 ± 0.62 body weight) than the LRSS, MSLS, and BS (p ≤ 0.001). Peak GRF angle was not significantly different between COD and the LRSS (p = 0.057), whereas the MSLS and BS (p < 0.001) vector angles were significantly greater than COD. In this application of the force-vector theory, the LRSS more closely matches COD than the MSLS or BS. Thus, the LRSS has the greater potential to enhance COD.

Evaluation of Resultant Ground Reaction Force and Its Velocity Using the Elapsed Time and Peak Reaction Force at 3-Dimensional Direction During Landing Task

The purpose of this study was to analyze the effects of bilateral landing leg and sex on resultant ground reaction force (GRF) and its velocity, using peak vertical GRF and elapsed time during drop landing tasks. A repeated-measures two-way analysis of variance explored the impact of landing legs and sex on the resultant vector, three-dimensional GRF, elapsed time, and velocity in 40 participants (20 males and 20 females). Participants performed drop landings from a 35-cm box. Effects of sex and landing leg were analyzed using a repeated-measures model (two sexes × two legs) on GRF magnitude and velocity. Females displayed shorter elapsed times to peak GRF compared to males in the anterior-posterior and vertical directions. Significant differences emerged between sexes in both magnitude and velocity of resultant and peak vertical GRF, with females exhibiting higher values. This suggests the adoption of distinct landing strategies between sexes. Notably, no significant differences were found in GRF magnitude or velocity between bilateral leg landings. These results indicate that healthy individuals of both sexes utilize different landing strategies during drop landings. This knowledge has potential applications in clinical settings for evaluating impulse force and stress transfer to the musculoskeletal system during landing tasks.

Biomechanical effects of fatigue on lower-body extremities during a maximum effort kettlebell swing protocol

ABSTRACT Kettlebell training provides multiple health benefits, including the generation of power. The primary purpose of this study was to examine the kinematics and kinetics of lower-body joints during a repeated, maximum effort kettlebell swing protocol. Sixteen resistance and kettlebell swing experienced males performed 10 rounds of a kettlebell swing routine (where one round equates to 30s of swings followed by 30s of rest). Kinematic (i.e., swing duration and angular velocities) and kinetic (i.e., normalised sagittal plane ground reaction force, resultant joint moment [RJM] and power) variables were extracted for the early portion and late portion of the round. Average swing duration and the magnitude of normalised ground reaction forces (GRF) increased within rounds, while hip joint power decreased. Changes in swing duration were minimal, but consistent due to an increase in overall fatigue. An increase in the magnitude of GRF was observed at the end of rounds, which is a potential concern for injury. Hip joint power decreased primarily due to a slower angular velocity. This protocol may be an effective routine for those who are resistance trained with kettlebell swing experience, and who want to optimise power in their exercise program.

Adolescents running in conventional running shoes have lower vertical instantaneous loading rates but greater asymmetry than running barefoot or in partial-minimal shoes

ABSTRACT Footwear may moderate the transiently heightened asymmetry in lower limb loading associated with peak growth in adolescence during running. This repeated-measures study compared the magnitude and symmetry of peak vertical ground reaction force and instantaneous loading rates (VILRs) in adolescents during barefoot and shod running. Ten adolescents (age, 10.6 ± 1.7 years) ran at self-selected speed (1.7 ± 0.3 m/s) on an instrumented treadmill under three counter-balanced conditions; barefoot and shod with partial-minimal and conventional running shoes. All participants were within one year of their estimated peak height velocity based on sex-specific regression equations. Foot-strike patterns, peak vertical ground reaction force and VILRs were recorded during 20 seconds of steady-state running. Symmetry of ground reaction forces was assessed using the symmetry index. Repeated-measures ANOVAs were used to compare conditions (α=.05). Adolescents used a rearfoot foot-strike pattern during barefoot and shod running. Use of conventional shoes resulted in a lower VILR (P < .05, dz = 0.9), but higher VILR asymmetry (P < .05) than running barefoot (dz = 1.5) or in partial-minimal shoes (dz = 1.6). Conventional running shoes result in a lower VILR than running unshod or in partial-minimal shoes but may have the unintended consequence of increasing VILR asymmetry. The findings may have implications for performance, musculoskeletal development and injury in adolescents.

Roles of each leg in impulse generation in professional baseball pitchers: preliminary findings uncover the contribution of the back leg towards whole-body rotation

ABSTRACT This study examined the roles of each leg in generating linear and angular impulses during baseball pitching. Professional pitchers (n = 4) pitched from a force plate instrumented mound, and 6–11 successful fastball pitches were used for analyses. We compared linear and angular impulses across the back and front legs. Linear and angular impulses were calculated from ground reaction force (GRF) and moment about each global axis passing through the centre of mass (COM), respectively. Additionally, we analysed measures that control the moment: (1) GRF magnitude, (2) magnitude of the position vector from COM to the centre of pressure and (3) the angle between (1) and (2). We found that the back leg generated forward linear impulse and the front leg generated backward linear impulse for all pitchers. Surprisingly, we found that the back leg generated significantly greater positive angular impulse about a global leftward axis (from the mound towards first base) than did the front leg in all four pitchers. Furthermore, the back leg’s moment about the leftward axis became positive after the magnitude of forward GRF decreased from its maximum, suggesting that the back leg’s role transitioned from generating forward linear momentum to angular momentum.

Estimation of gait events and kinetic waveforms with wearable sensors and machine learning when running in an unconstrained environment

Wearable sensors and machine learning algorithms are becoming a viable alternative for biomechanical analysis outside of the laboratory. The purpose of this work was to estimate gait events from inertial measurement units (IMUs) and utilize machine learning for the estimation of ground reaction force (GRF) waveforms. Sixteen healthy runners were recruited for this study, with varied running experience. Force sensing insoles were used to measure normal foot-shoe forces, providing a proxy for vertical GRF and a standard for the identification of gait events. Three IMUs were mounted on each participant, two bilaterally on the dorsal aspect of each foot and one clipped to the back of each participant’s waistband, approximating their sacrum. Participants also wore a GPS watch to record elevation and velocity. A Bidirectional Long Short Term Memory Network (BD-LSTM) was used to estimate GRF waveforms from inertial waveforms. Gait event estimation from both IMU data and machine learning algorithms led to accurate estimations of contact time. The GRF magnitudes were generally underestimated by the machine learning algorithm when presented with data from a novel participant, especially at faster running speeds. This work demonstrated that estimation of GRF waveforms is feasible across a range of running velocities and at different grades in an uncontrolled environment.

Relationship between ground reaction force in horizontal plane and mechanical energy flow in torso during baseball tee batting

ABSTRACT The purpose of this study was to clarify the relationship between the ground reaction force (GRF) in the horizontal plane and the mechanical energy flow in the torso during baseball tee batting. The batting motion and GRF acting on each foot of 30 collegiate baseball players were recorded using a motion capture system and two force plates. To measure the mechanical energy inflow into the torso from both hip joints, the mechanical power of the torso by the force and torque of both hip joints was calculated. The horizontal GRF variables of the stride foot were significantly correlated with mechanical energy inflow into the lower torso from the hip joint of the stride foot side. Conversely, the horizontal GRF variables of the pivot foot were not significantly correlated with mechanical energy inflow into the lower torso from the hip joint of the pivot foot side. Thus, these results suggest that exploring the posture easily to receive the horizontal GRF by the stride foot (i.e., optimisation of magnitude and/or angle of GRF acting on the stride foot) increases the mechanical energy inflow into the lower torso from the hip joint of the stride foot side during baseball batting.

Peak Loads Associated With High-Impact Physical Activities in Children.

Examine the magnitude of peak ground reaction forces (pGRF) across different jumping activities in children. Eight children between 8 and 12 years (9.63 [1.49]y; 1.42 [0.08]m; 33.69 [4.81]kg), performed 5 trials of a broad jump, countermovement jump, jumping jack, leap jump, and drop jump on a force plate. The pGRF were determined during the landing phase of each activity and expressed in units of body weight (BW). A repeated-measures analysis of variance was employed to assess differences in pGRF across activities. Drop jump exhibited the greatest pGRF (3.09 [0.46]BW) in comparison with the vertical jumping jack (2.56 [0.21]BW; P < .001) and countermovement jump (2.45 [0.22]BW; P = .001), as well as the horizontal broad jump (2.25 [0.2]BW; P = .003), and leap jump (2.01 [0.1]BW; P = .002). Peak loads between 2 and 3.1BW were exhibited across each jump activity, which is moderate compared with magnitudes in most jump interventions seeking to improve bone health. All conditions except drop jump exhibited loading <3BW, suggesting these activities may not produce sufficient loads to improve bone outcomes.

Effects of step frequency during running on the magnitude and symmetry of ground reaction forces in individuals with a transfemoral amputation

BackgroundIndividuals with unilateral transfemoral amputation are prone to developing health conditions such as knee osteoarthritis, caused by additional loading on the intact limb. Such individuals who can run again may be at higher risk due to higher ground reaction forces (GRFs) as well as asymmetric gait patterns. The two aims of this study were to investigate manipulating step frequency as a method to reduce GRFs and its effect on asymmetric gait patterns in individuals with unilateral transfemoral amputation while running.MethodsThis is a cross-sectional study. Nine experienced track and field athletes with unilateral transfemoral amputation were recruited for this study. After calculation of each participant’s preferred step frequency, each individual ran on an instrumented treadmill for 20 s at nine different metronome frequencies ranging from − 20% to + 20% of the preferred frequency in increments of 5% with the help of a metronome. From the data collected, spatiotemporal parameters, three components of peak GRFs, and the components of GRF impulses were computed. The asymmetry ratio of all parameters was also calculated. Statistical analyses of all data were conducted with appropriate tools based on normality analysis to investigate the main effects of step frequency. For parameters with significant main effects, linear regression analyses were further conducted for each limb.ResultsSignificant main effects of step frequency were found in multiple parameters (P < 0.01). Both peak GRF and GRF impulse parameters that demonstrated significant main effects tended towards decreasing magnitude with increasing step frequency. Peak vertical GRF in particular demonstrated the most symmetric values between the limbs from − 5% to 0% metronome frequency. All parameters that demonstrated significant effects in asymmetry ratio became more asymmetric with increasing step frequency.ConclusionsFor runners with a unilateral transfemoral amputation, increasing step frequency is a viable method to decrease the magnitude of GRFs. However, with the increase of step frequency, further asymmetry in gait is observed. The relationships between step frequency, GRFs, and the asymmetry ratio in gait may provide insight into the training of runners with unilateral transfemoral amputation for the prevention of injury.

The Role of Water-Based Exercise on Vertical Ground Reaction Forces in Overweight Children: A Pilot Study

The aquatic environment represents an adequate and safe alternative for children with overweight to exercise. However, the magnitude of the vertical ground reaction force (Fz) during these exercises is unknown in this population. Therefore, our study aimed to compare the Fz during the stationary running exercise between the aquatic and land environments in children with overweight or obesity. The study is characterized as a cross-over study. Seven children, two with overweight and five with obesity (4 boys and 3 girls; 9.7 ± 0.8 years), performed two experimental sessions, one on land and another in the aquatic environment. In both conditions, each participant performed 15 repetitions of the stationary running exercise at three different cadences (60, 80, and 100 b min−1) in a randomized order. Their apparent weight was reduced by 72.1 ± 10.4% on average at the xiphoid process depth. The peak Fz, impulse, and loading rate were lower in the aquatic environment than on land (p &lt; 0.001). Peak Fz was also lower at 80 b min−1 compared to 100 b min−1 (p = 0.005) and loading rate was higher at 100 b min−1 compared to 80 b min−1 (p = 0.003) and 60 b min−1 (p &lt; 0.001) in the aquatic environment, whereas impulse was significantly reduced (p &lt; 0.001) with the increasing cadence in both environments. It can be concluded that the aquatic environment reduces all the Fz outcomes investigated during stationary running and that exercise intensity seems to influence all these outcomes in the aquatic environment.

Effects of walking speed on magnitude and symmetry of ground reaction forces in individuals with transfemoral prosthesis

Individuals with unilateral transfemoral amputation (uTFA) walk asymmetrically. Investigating gait symmetry in ground reaction force (GRF) is critical because asymmetric loading on the residual limb can result in injury. The aim of this study was to investigate the GRF of individuals with uTFA by systematically controlling their walking at eight speeds(2.0–5.5 km/h with increments of 0.5 km/h) on a treadmill. Forty-eight individuals participated in this study, which included 24 individuals with uTFA (K3 and K4) and 24 individuals without amputation. GRFs (anteroposterior, mediolateral, and vertical) of the prosthetic and intact limb steps were collected for the individuals with uTFA and those of the right limb were collected for the control group. Peak force values of the GRF components, temporal parameters, impulses, and their asymmetry ratios were investigated and statistically analyzed. With an increasing walking speed, the magnitude of GRF changed gradually; individuals with uTFA exhibited increased GRF asymmetry in the vertical and mediolateral components, while that of the anteroposterior component remained constant. uTFA individuals typically maintained a constant asymmetry ratio in the mediolateral and anteroposterior (braking and propulsive) GRF impulses across a wide range of walking speeds. This result suggests that individuals with uTFA may cope with various walking speeds by maintaining symmetric mediolateral and anteroposterior impulses. The data provided in this study can serve as normative data for the GRF and its symmetry across a range of walking speeds in individuals with uTFA.

Characteristics of landing impact in athletes who have not returned to sports at the pre-injury competition level after anterior cruciate ligament reconstruction

BackgroundMost patients with anterior cruciate ligament (ACL) injury undergo ACL reconstruction (ACLR) with the expectation of being able to return to sport (RTS) at the same level of the competition as before the injury. The magnitude and asymmetry of landing impact are important post-ACLR functional variables related to increased ACL strain and poor athletic performance. However, the association between the RTS status and landing impact in post-ACLR patients is unknown. ObjectiveTo investigate the association between RTS status and landing impact during single-leg landing in post-ACLR patients. MethodsForty-four patients after primary, unilateral ACLR participated in this study. They had already participated in sports post-ACLR. Questionnaires were used to assess whether the participants achieved the same competitive level of RTS as before the injury. The magnitude and symmetry of the peak vertical ground reaction force (pVGRF) were collected and analysed during single-leg jump landings. Additionally, knee functions (range of motion, laxity, effusion, strength, and single-leg hop distance) were measured. ResultsA total of 28 (64%) patients reported RTS at their pre-injury competition levels. The no-RTS group had a lower pVGRF magnitude on the operated side than the yes-RTS group (P = .019). The no-RTS group had a higher rate of pVGRF asymmetry (50%) than the yes-RTS group (18%) (P = .040). Logistic regression analysis revealed that pVGRF magnitude and asymmetry were significantly associated with the RTS status. Logistic regression analysis adjusted for knee function revealed that the pVGRF magnitude was significantly associated with the RTS status. ConclusionIn patients who are unable to RTS at their pre-injury competition level after ACLR, the pVGRF is lower and more likely to be asymmetrical than in those able to RTS at their pre-injury competition level.

NFL Draft Prep Players Improve 40-Yard Run Times and Foot-Ground Kinetics

Introduction: Linear speed is a discriminant factor between drafted and undrafted American football players into the National Football League. Linear speed is influenced by foot-ground contact time and the magnitude of vertical ground reaction force. The aim of this study was to determine if foot-ground kinetics during speed running could be modified through participating in a 6-week NFL draft preparation camp. Methods: To evaluate foot-ground kinetics, 16 American football players ran on an instrumented treadmill for 5 seconds at 6.5 m/s. Linear speed was measured during a 40-yard (36.6 m) outdoor run. Pre- and post-camp linear speed times, stance-averaged vertical ground reaction forces (vGRF, kg/N), foot-ground contact time (msec), and vertical impulse (kg/N * s) were examined using paired t-tests, p&lt;.05. Results: Linear speed times significantly improved [(pre, 4.8±0.2 vs. post, 4.6±0.2 sec), t(15)=13.8, p&lt;.001)], and foot-ground contact time significantly decreased for the right limb [(pre, 177+3.2 vs. post, 168+2.2 ms), t(15)=2.21, p=.043]. Mean vertical impulse and stance-averaged GRF for both limbs remained unchanged, p&gt;.05. Conclusions: Linear speed and selected foot-ground kinetics are modifiable in NFL draft prep players. Training appears to lower 40-yard run times and foot-ground contact time.

Dynamical determinants enabling two different types of flight in cheetah gallop to enhance speed through spine movement

Cheetahs use a galloping gait in their fastest speed range. It has been reported that cheetahs achieve high-speed galloping by performing two types of flight through spine movement (gathered and extended). However, the dynamic factors that enable cheetahs to incorporate two types of flight while galloping remain unclear. To elucidate this issue from a dynamical viewpoint, we developed a simple analytical model. We derived possible periodic solutions with two different flight types (like cheetah galloping), and others with only one flight type (unlike cheetah galloping). The periodic solutions provided two criteria to determine the flight type, related to the position and magnitude of ground reaction forces entering the body. The periodic solutions and criteria were verified using measured cheetah data, and provided a dynamical mechanism by which galloping with two flight types enhances speed. These findings extend current understanding of the dynamical mechanisms underlying high-speed locomotion in cheetahs.

Ankle Instability Patients Exhibit Altered Muscle Activation of Lower Extremity and Ground Reaction Force during Landing: A Systematic Review and Meta-Analysis.

This review aimed to investigate characteristics of muscle activation and ground reaction force (GRF) patterns in patients with ankle instability (AI). Relevant studies were sourced from PubMed, CINAHL, SPORTDiscus, and Web of Science through December 2019 for case-control study in any laboratory setting. Inclusion criteria for study selection were (1) subjects with chronic, functional, or mechanical instability or recurrent ankle sprains; (2) primary outcomes consisted of muscle activation of the lower extremity and GRF during landing; and (3) peer-reviewed articles with full text available, including mean, standard deviation, and sample size, to enable data reanalysis. We evaluated four variables related to landing task: (1) muscle activation of the lower extremity before landing, (2) muscle activation of the lower extremity during landing, (3) magnitude of GRF, and (4) time to peak GRF. The effect size using standardized mean differences (SMD) and 95% confidence intervals (CI) were calculated for these variables to make comparisons across studies. Patients with AI had a lower activation of peroneal muscles before landing (SMD = -0.63, p < 0.001, CI = -0.95 to -0.31), greater peak vertical GRF (SMD = 0.21, p = 0.03, CI = 0.01 to 0.40), and shorter time to peak vertical GRF (SMD = -0.51, p < 0.001, CI = -0.72 to -0.29) than those of normal subjects during landing. There was no significant difference in other muscle activation and GRF components between the patients with AI and normal subjects (p > 0.05). Altered muscle activation and GRF before and during landing in AI cases may contribute to both recurrent ankle and ACL injuries and degenerative change of articular.

Principal Component Analysis of the Running Ground Reaction Forces With Different Speeds.

Ground reaction force (GRF) is a key metric in biomechanical research, including parameters of loading rate (LR), first impact peak, second impact peak, and transient between first and second impact peaks in heel strike runners. The GRFs vary over time during stance. This study was aimed to investigate the variances of GRFs in rearfoot striking runners across incremental speeds. Thirty female and male runners joined the running tests on the instrumented treadmill with speeds of 2.7, 3.0, 3.3, and 3.7 m/s. The discrete parameters of vertical average loading rate in the current study are consistent with the literature findings. The principal component analysis was modeled to investigate the main variances (95%) in the GRFs over stance. The females varied in the magnitude of braking and propulsive forces (PC1, 84.93%), whereas the male runners varied in the timing of propulsion (PC1, 53.38%). The female runners dominantly varied in the transient between the first and second peaks of vertical GRF (PC1, 36.52%) and LR (PC2, 33.76%), whereas the males variated in the LR and second peak of vertical GRF (PC1, 78.69%). Knowledge reported in the current study suggested the difference of the magnitude and patterns of GRF between male and female runners across different speeds. These findings may have implications for the prevention of sex-specific running-related injuries and could be integrated with wearable signals for the in-field prediction and estimation of impact loadings and GRFs.

Magnitude of vertical ground reaction force during water-based exercises in women with obesity

ABSTRACT Individuals with obesity are recommended to practice physical activity with reduced weight bearing, such as water-based exercises. However, the magnitude of the vertical ground reaction force (Fz) during these exercises is unknown in this population. This study aimed to verify the magnitude of peak Fz (Fzpeak) in women with obesity during different exercises performed in water and on land and at different intensities. Ten adult women with obesity class 1 (body mass index: 31.6 ± 0.57 kg·m−2) completed two randomised sessions, one performed on land and other in water, composed by three exercises (stationary running, frontal kick, butt kick) performed at cadences of 80 b·min−1, 100 b·min−1 and maximal. Fzpeak value was determined in each situation. Data were analysed using repeated-measures three-way ANOVA (α = 0.05). Significant lower Fzpeak values were observed in water (p < 0.001), with no significant differences between the exercises. A significant difference in the Fzpeak was verified between maximal cadence and 80 b·min−1 (p = 0.01). Water-based exercises performed by women with obesity revealed a Fzpeak reduction from land to water corresponding to 41–67%. Also, the three water-based exercises resulted in similar Fzpeak values, while an increase was observed 80 b·min−1 to the maximal cadence.

Anterior-posterior ground reaction forces across a range of running speeds in unilateral transfemoral amputees

ABSTRACT As a fundamental motor pattern, the ability to run at a range of constant speeds is a prerequisite for participating in competitive games and recreational sports. However, it remains unclear how unilateral transfemoral amputees modulate anterior and posterior ground reaction force impulses (GRFIs) in order to maintain constant running speeds. The purpose of this study was to investigate anterior and posterior GRFIs across a wide range of constant running speeds in unilateral transfemoral amputees wearing a running-specific prosthesis. Eleven runners with unilateral transfemoral amputation ran on an instrumented treadmill at 5 different speeds (30%, 40%, 50%, 60%, and 70% of the average velocity of their 100-m personal records). Anterior-posterior ground reaction forces (GRFs) were measured at 1000 Hz over 14 consecutive steps. Impulse, magnitude, and duration of anterior and posterior GRFs were compared between the affected and unaffected limbs at each speed. The net anterior-posterior GRFI, reflecting the changes in horizontal running velocity, was consistently positive (propulsion) in the affected limb and negative (braking) in the unaffected limb at all speeds. Regardless of running speed, unilateral transfemoral amputees maintain constant running speeds not over each step, but over 2 consecutive steps (i.e., one stride).

A comparison of centre of pressure behaviour and ground reaction force magnitudes when individuals walk overground and on an instrumented treadmill

BackgroundInstrumented treadmills facilitate analysis of consecutive strides in ways that typical overground gait data collections cannot. Researchers have quantified differences between joint kinetic measures whilst walking on an instrumented treadmill compared to those walking overground. The reason for such differences has not yet been established. Research QuestionCan we identify the source or sources of these errors by comparing centre of pressure and ground reaction force measurements recorded on a treadmill to those collected overground? MethodsKinematic and kinetic data were recorded while nineteen individuals walked continuously at their self-selected walking speed overground and on a treadmill. Comparisons of the centre of pressure and ground reaction forces were made between the two conditions using 2-tailed paired t-tests and Cohen’s d effect size. ResultsThe results indicated that participants had significantly faster backwards, lateral and medial centre of pressure velocities when walking on a treadmill compared to when they were walking overground. Additionally, participants also had significantly reduced peak propulsive ground reaction forces when walking on a treadmill than walking overground. SignificanceThese results suggest that shear forces caused by the belts sliding over the treadmill force platforms affect the centre of pressure during early stance, and the minimal acceleration of a participant’s centre of mass during treadmill walking results in reduced propulsive force during late stance. Therefore, care should be taken during studies when comparing kinetic gait variables between overground and treadmill walking.

Comparison of methods of derivation of the yank-time signal from the vertical ground reaction force–time signal for identification of movement-related events

Temporal changes in ground reaction force magnitudes reflect movement strategy, and thus underlying muscle activation patterns, during movement tasks. Speculatively, these changes may be observed more readily when the force–time signal is differentiated, yielding the yank-time signal. However, the differentiation process, including the signal filtering used before or after differentiation, can significantly affect the signal-to-noise ratio (SNR) and likelihood of meaningful inference. The aim of the present study was to compare three methods of deriving the yank-time signal: Method 1 derived the yank-time signal using 2nd-order central differentiation subsequent to application of a 4th-order Butterworth filter; Method 2 included the same process as Method 1 but additionally filtered the yank-time data with a Savitzky-Golay smoothing filter; and Method 3 directly and simultaneously derived and smoothed the yank-time signal using a Savitzky-Golay digital differentiation filter. The current analyses revealed Method 2 had the best SNR, followed by Methods 3 and 1, but caused a small loss of signal amplitude. With regards to timing of inflection points in the yank-time data, no significant difference was observed. Therefore, Method 3 led to the best derivation of the yank-time signal due to its efficiency and preservation of signal characteristics and good SNR. Also, a strong association between the first maximum point of the yank-time signal and the start of the downward movement of the body’s centre of mass during a countermovement jump, as identified by 3-D motion analysis, was observed. Thus, subtle events (e.g. start of downward movement) can be easily observed in the yank-time signal.