Effects of back loading on the biomechanics of sit-to-stand motion in healthy children

The goal of the present study was to determine the thus far unstudied effects of back loading on the kinematics and kinetics of sit-to-stand (STS) motion in healthy children. Fifteen children (8 boys, 7 girls, mean age 9.6 years, SD 1.2 years) were tested with no back load and with a back load of 10% and 20% of body weight, respectively. A motion analysis system was used with six infrared cameras and two force plates. Total STS duration did not change; however, differential effects were shown for the durations of its phases. Back loading increased ankle dorsiflexion yielding a greater maximal dorsiflexion angle. Effects on the knee angle were limited except for a significant decrease in final knee flexion. Initial and maximal hip flexion increased but final hip angle did not change. Initial backward pelvic tilt decreased and a shift to forward pelvic tilt occurred at an earlier stage of STS motion. Back loading affected trunk motion: maximal and final forward shoulder tilt increased. Maximal ankle and knee moments and powers increased; however, hip joint kinetics was not affected significantly. Therefore, while maintaining the general pattern of STS motion, participants showed selectively significant adjustments to back loading during its different phases. The main kinematic adjustments were increased trunk flexion and greater ankle dorsiflexion, while the major kinetic adjustment was increased knee extension moment. Increased back loading yielded more pronounced effects, primarily in the ankle. In sum, back loading substantially affected the biomechanics of STS motion even for the lower load level studied. This finding may be of clinical relevance for musculoskeletal disorders, but this needs to be examined.     

Single leg landing movement differences between male and female badminton players after overhead stroke in the backhand-side court

Females showed higher anterior cruciate ligament (ACL) injuries rate on the opposite side of dominant hand compared with males during single leg landing in the backhand-side court after overhead stroke. The purpose of this study was to conduct biomechanics testing including kinematics and kinetics to provide some insights on the ACL injuries risks during single leg landing in the backhand-side court after overhead stroke between females and males. Twenty collegiate badminton players (10 females, 10 males) voluntarily participated in this study. Sagittal plane kinematic and kinetic data of the lower limb, and their ground reaction forces during the single leg landing in the backhand-side court after overhead stroke were collected. Results shown that, at the peak posterior ground reaction force (GRF) moment, the ankle dorsiflexion, knee and hip flexion angles of the female were lower than that of male. Meantime, the knee extension moment of the female was lower than that of males but the hip extension moment of the female was larger compared to males at the peak posterior GRF moment. The peak vertical and posterior GRF of female badminton players were larger than that of males. Decreased hip, knee, and ankle flexion angles at the peak posterior GRF moment and greater peak vertical and posterior GRF may expose female badminton players to the higher risk ACL injuries compared to males during single leg landing after overhead stroke in the backcourt-side. Preventative training programs designed to prevent the ACL injuries rate of female badminton players should take these factors into consideration.     

Restricting ankle dorsiflexion does not mitigate the benefits of external focus of attention on landing biomechanics in healthy females

PurposeRestricted ankle dorsiflexion can promote aberrant biomechanics associated with risk for knee injury during dynamic activities. Attentionally focused instructions have been used to improve high-risk knee biomechanics during landing tasks. Yet, it is unknown whether attentionally focused instruction can effectively improve landing patterns in the presence of a mechanical restriction on the ankle. Therefore, our purpose was to determine whether restricting ankle dorsiflexion by use of bracing mitigated the effects of attentional foci on landing biomechanics in healthy females.MethodsWe used a crossover design to investigate lower extremity biomechanics in 19 healthy females between the ages of 18–35 during a series of jump-landing tasks. Participants completed 6 blocks of 3 jump-landings on separate force platforms in a randomized order based on brace condition (brace, no brace) and mode of attentional foci (neutral, internal focus [IF], external focus [EF]). Attentionally focused instructions were provided immediately prior to 3 practice jump-landings, followed by 5 test jump-landings with self-controlled feedback only.ResultsAnkle bracing decreased peak dorsiflexion and sagittal range of motion (ROM) (mean difference: 5.7–5.8°), and peak inversion and frontal ROM (mean difference: 2.4–3.0°). However, hip flexion ROM (mean difference: 1.8°) increased compared to the no brace condition. Regardless of ankle bracing, EF instruction increased peak hip flexion (mean difference: 4.9°) and hip flexion range of motion (mean difference: 3.8–4.6°), while decreasing peak knee valgus (mean difference: 0.8–1.0°) and knee valgus moment (mean difference: 0.04 Nm/kg). Additionally, EF instruction increased peak hip abduction to a similar degree when braced (mean difference: 3.6–4.0°) and not braced (mean difference: 2.1–2.5°). Lastly, EF instruction increased hip abduction ROM only when braced (mean difference: 2.3–2.4°), but decreased peak knee valgus power only when not braced (mean difference: 0.18 W/kg).ConclusionsOur findings indicate that mechanically restricting ankle dorsiflexion does not mitigate the ability of EF instruction to enhance jump-landing performance by means of improving hip and knee biomechanics in healthy females. However, our findings suggest an improved ability to control the rate of knee valgus loading when not braced. Therefore, we conclude that EF instruction remains a viable clinical strategy to improve landing patterns in the presence of restricted ankle dorsiflexion, yet this approach may be ineffective to reduce the rate of knee joint loading.     

The effect of shoe and floor characteristics on walking kinematics

It is common sense that walking on sand poses challenges to postural control. However, there are no studies quantifying the kinematics of sand walking compared to other types of postural perturbations such as unstable shoes. The aim of the study was to investigate differences in walking kinematics during walking on solid ground, in unstable shoes and on unstable surfaces. Nineteen healthy young adults (23.5 ± 1.5 years) performed three different walking tasks: 1) walking at preferred speed while wearing regular shoes; 2) Walking at preferred speed wearing Masai Barefoot Technology shoes and 3) barefoot walking at preferred speed on a large sand grave. Full-body kinematics were recorded during all conditions using an inertial motion capture system. Basic gait parameters (walking speed, stride length and duration), relative vertical center-of-mass position (rvCOM), and ankle, knee and hip joint angles in the sagittal plane were compared across the tasks through statistical parametric mapping over the course of full walking cycles. Participants presented similar walking speed, as well as stride length and duration across different conditions (p > 0.05). However, walking on sand reduced the rvCOM (p < 0.05), while also requiring greater ankle plantarflexion during stance phase (p < 0.05), as well as greater knee and hip flexion during leg swing and initial contact when compared to the other conditions (p < 0.05). It was concluded that walking on sand substantially changes walking kinematics, and may cause greater postural instability than unstable shoes. Therefore, walking on sand can be an alternative to improve postural control in patients undergoing walking rehabilitation.     

Weighted vest effects on impact forces and joint work during vertical jump landings in men and women

Weighted vest (WV) use during vertical jump landings (VJL) does not appear to alter peak vertical ground reaction forces (GRF) or peak joint torques. However, WV effects on joint work and sex differences during VJL are not well understood. This study assessed WV effects on vertical GRF and sagittal joint work during VJL in men and women. Twelve men and 12 women performed VJL wearing a WV with zero added mass (unloaded) and with 10% body mass (loaded) while GRF and kinematic data were obtained. Mixed-model analyses of variance (α = 0.05) and effect sizes (ES) were used to assess differences between sexes and/or load conditions. Regardless of sex, greater landing height (p < 0.001; ES = 0.37) and peak vertical GRF (p = 0.001; ES 0.51) occurred when unloaded, while greater landing time (p = 0.001; ES = 0.46) and negative lower extremity work (p < 0.001; ES = 0.41) occurred when loaded through greater negative work about the hip (p = 0.001; ES = 0.27) and ankle (p = 0.020; ES = 0.27). No differences in hip (p = 0.753; ES = 0.03), knee (p = 0.588; ES = 0.07), or ankle (p = 0.580; ES = 0.09) joint displacement were detected between loaded and unloaded conditions. Men exhibited greater landing heights (p < 0.001; ES = 2.49) and greater peak vertical GRF than women (p = 0.007; ES = 1.18), though women exhibited greater negative lower extremity work (p < 0.001; ES = 1.98) than men through greater negative knee (p < 0.001; ES = 1.98) and ankle (p = 0.032; ES = 0.94) work. No sex differences were detected for joint angular displacement about the hip (p = 0.475; ES = 0.30), knee (p = 0.666; ES = 0.18), or ankle (p = 0.084; ES = 0.71). These data revealed a unique load accommodation strategy during VJL with a WV characterized by greater lower extremity joint work performed via increased joint torque despite lesser landing height and peak vertical GRF. Women appear to perform greater lower extremity joint work than men during VJL despite lesser landing height and peak vertical GRF. Current and prospective WV users should be aware of their load accommodation strategy during VJL with an external load. Women may consider developing more refined load accommodation strategies for VJL regardless of whether external loading is applied to avoid performing excessive amounts of lower extremity work.     

Synergistic interaction between ankle and knee during hopping revealed through induced acceleration analysis

The forces produced by the muscles can deliver energy to a target segment they are not attached to, by transferring this energy throughout the other segments in the chain. This is a synergistic way of functioning, which allows muscles to accelerate or decelerate segments in order to reach the target one. The purpose of this study was to characterize the contribution of each lower extremity joint to the vertical acceleration of the body’s center of mass during a hopping exercise. To accomplish this, an induced acceleration analysis was performed using a model with eight segments. The results indicate that the strategies produced during a hopping exercise rely on the synergy between the knee and ankle joints, with most of the vertical acceleration being produced by the knee extensors, while the ankle plantar flexors act as stabilizers of the foot. This synergy between the ankle and the knee is perhaps a mechanism that allows the transfer of power from the knee muscles to the ground, and we believe that in this particular task the net action of the foot and ankle moments is to produce a stable foot with little overall acceleration.     

Lower extremity strength and hopping and jumping ground reaction forces in children with neurofibromatosis type 1

The purpose of this study was to (1) extend the research findings of decreased muscular force production in grip strength to the lower extremity strength of children with NF1 and (2) to determine if there was a relationship between isometric strength and functional activities in children with NF1. Force production was assessed using a hand held dynamometer (HHD) and a functional task (hopping and jumping) on a force plate. Data from twenty-six children with NF1 were compared to data from 48 typically developing children of similar sex, weight and height. Children with NF1 demonstrated statistically significant lower force production with HHD (p<0.01) during hip extension, but similar force production for knee extension and ankle plantar flexion compared to the control group. A relationship existed between the ground reaction forces at take-off from both hopping and jumping and the force generated from knee extensor strength in the NF1 group. The addition of a functional task to hand held dynamometry is useful for determining a relationship between common clinical measures and functional activities.     

Arch structure is associated with unique joint work,relative joint contributions and stiffness during landing

To examine lower extremity joint contributions to a landing task in high-(HA) and low-arched (LA) female athletes by quantifying vertical stiffness, joint work and relative joint contributions to landing.MethodsTwenty healthy female recreational athletes (10 HA and 10 LA) performed five barefoot drop landings from a height of 30 cm. Three-dimensional kinematics (240 Hz) and ground reaction forces (960 Hz) were recorded simultaneously. Vertical stiffness, joint work values and relative joint work values were calculated using Visual 3D and MatLab.ResultsHA athletes had significantly greater vertical stiffness compared to LA athletes (p = 0.013). Though no differences in ankle joint work were observed (p = 0.252), HA athletes had smaller magnitudes of knee (p = 0.046), hip (p = 0.019) and total lower extremity joint work values (p = 0.016) compared to LA athletes. HA athletes had greater relative contributions of the ankle (p = 0.032) and smaller relative contributions of the hip (p = 0.049) compared to LA athletes. No differences in relative contributions of the knee were observed (p = 0.255).ConclusionsThese findings demonstrate that aberrant foot structure is associated with unique contributions of lower extremity joints to load attenuation during landing. These data may provide insight into the unique injury mechanisms associated with arch height in female athletes.     

The effect of trunk flexion angle on lower limb mechanics during running

Trunk flexion is an understudied biomechanical variable that potentially influences running performance and susceptibility to injury. We present and test a theoretical model relating trunk flexion angle to stride parameters, joint moments and ground reaction forces that have been implicated in repetitive stress injuries. Twenty-three participants (12 male, 11 female) ran at preferred trunk flexion and three more flexed trunk positions (moderate, intermediate and high) on a custom built Bertec™ instrumented treadmill while kinematic and kinetic data were simultaneously captured. Markers adhered to bony landmarks tracked the movement of the trunk and lower limb. Stride parameters, moments of force and ground reaction force were calculated using Visual 3D (C-Motion ©) software. From preferred to high trunk flexion, stride length decreased 6% (P < 0.001) and stride frequency increased 7% (P < 0.001). Extensor moments at the hip increased 70% (P < 0.001), but knee extensor (P < 0.001) and ankle plantarflexor moments (P < 0.001) decreased 22% and 14%, respectively. Greater trunk flexion increased rate of loading by 29% (P < 0.01) and vertical ground reaction force impact transients by 20% (P < 0.01). Trunk flexion angle during running has significant effects on stride kinematics, lower extremity joint moments and ground reaction force and should be further investigated in relation to running performance and repetitive stress injuries.     

Simple lower extremity two-joint synergy

The purpose of this study was to describe the existence of a simple synergy in the lower extremity. Subjects performed discrete knee flexion or extension movements or ankle plantar or dorsiflexion movements in a sagittal plane, moving one of the joints "as fast as possible." Joint angles and electromyographic (EMG) signals from the biceps femoris, rectus femoris, soleus, and tibialis anterior were recorded. Typically, EMG patterns in both muscle pairs acting at the joints demonstrated the "triphasic" pattern. The knee flexor (biceps femoris) and ankle plantar flexor (soleus) tended to show simultaneous EMG bursts, while the knee extensor (rectus femoris) and ankle dorsiflexor (tibialis anterior) had similar patterns of activation. A two-joint simple synergy previously established for upper extremities seems pertinent for lower extremities as well. Such a synergy is used by the central nervous system to simplify control of the postural component of a motor task.     

Lower limb kinematic variability in dancers performing drop landings onto floor surfaces with varied mechanical properties

Elite dancers perform highly skilled and consistent movements. These movements require effective regulation of the intrinsic and extrinsic forces acting within and on the body. Customized, compliant floors typically used in dance are assumed to enhance dance performance and reduce injury risk by dampening ground reaction forces during tasks such as landings. As floor compliance can affect the extrinsic forces applied to the body, secondary effects of floor properties may be observed in the movement consistency or kinematic variability exhibited during dance performance. The aim of this study was to investigate the effects of floor mechanical properties on lower extremity kinematic variability in dancers performing landing tasks. A vector coding technique was used to analyze sagittal plane knee and ankle joint kinematic variability, in a cohort of 12 pre-professional dancers, through discrete phases of drop landings from a height of 0.2 m. No effect on kinematic variability was observed between floors, indicating that dancers could accommodate the changing extrinsic floor conditions. Future research may consider repeat analysis under more dynamic task constraints with a less experienced cohort. However, knee/ankle joint kinematic variability was observed to increase late in the landing phase which was predominantly comprised of knee flexion coupled with the terminal range of ankle dorsiflexion. These findings may be the result of greater neural input late in the landing phase as opposed to the suggested passive mechanical interaction of the foot and ankle complex at initial contact with a floor. Analysis of joint coordination in discrete movement phases may be of benefit in identifying intrinsic sources of variability in dynamic tasks that involve multiple movement phases.     

A prospective comparison of lower extremity kinematics and kinetics between injured and non-injured collegiate cross country runners

Collegiate cross country runners are at risk for running related injuries (RRI) due to high training volume and the potential for aberrant lower extremity biomechanics. However, there is a need for prospective research to determine biomechanical risk factors for RRI. The purpose of this study was to prospectively compare ankle, knee, and hip kinematics and kinetics and ground reaction force characteristics between injured and non-injured cross country runners over a 14-week season. Biomechanical running analyses were conducted on 31 collegiate-cross country runners using a 3-dimensional motion capture system and force plate prior to the start of the season. Twelve runners were injured and 19 remained healthy during the course of the season. Peak external knee adduction moment (KAM), a surrogate for frontal plane knee loading, and peak ankle eversion velocity were greater in runners who sustained an injury compared to those who did not, and no differences were noted in ground reaction force characteristics, or hip kinematics and kinetics. Reducing the KAM and ankle eversion velocity may be an important aspect of preventing RRI.     

Sit-to-stand: Functional relationship between upper body and lower limb segments

The behaviour of linked body segments during sit-to-stand was the subject of this study which investigated the relationship between the trunk and lower limb segments by varying the initial position of the trunk. Six subjects were videotaped as they stood up with feet on a forceplate from three initial positions: erect sitting, trunk flexed forward 30 deg, and 60 deg. When subjects actively flexed the trunk in the pre-extension phase, the order in which lower limb joints extended was knee, hip, ankle. However, when there was no active flexion, the order of onsets changed, the hip extending first followed by the knee and ankle. An extensor support moment (SM), a summation of extensor moments at hip, knee and ankle, occurred throughout the extension phase. The mean peak value of SM remained invariant in all three conditions despite variability in individual hip, knee and ankle moments. When active trunk flexion was absent, the duration of the extension phase was longer and a high value of SM was sustained for a longer proportion of the phase, indicating that more muscle force was required. The findings support the view that biomechanical characteristics emerge naturally from a functional coupling between segments, according to the demands of the action.     

Ankle dorsiflexion range of motion is associated with kinematic but not kinetic variables related to bilateral drop-landing performance at various drop heights

Limited evidence is available concerning ankle dorsiflexion range of motion (DF ROM) and its relationship with landing performance from varying drop heights. The aim of this investigation was to determine the relationship between ankle DF ROM and both kinetic and kinematic variables measured during bilateral drop-landings from 50%, 100% and 150% of countermovement jump height. Thirty-nine participants were measured for their ankle DF ROM using the weight-bearing lunge test, after which five bilateral drop-landings were performed from 50%, 100% and 150% of maximal countermovement jump height. Normalized peak vertical ground reaction force (vGRF), time to peak vGRF and loading rate was calculated for analysis, alongside sagittal-plane initial contact angles, peak angles and joint displacement for the hip, knee and ankle. Frontal-plane projection angles were also calculated. Ankle DF ROM was not related to normalized peak vGRF, time to peak vGRF or loading rate (P > 0.05), regardless of the drop height. However, at drop heights of 100% and 150% of countermovement jump height, there were numerous significant (P < 0.05) moderate to large correlations between ankle DF ROM and initial contact angles (r = −0.34 to −0.40) and peak angles (r = −0.42 to −0.52) for the knee and ankle joint. Knee joint displacement (r = 0.39–0.47) and frontal-plane projection angle (r = 0.37–0.40) had a positive relationship with ankle DF ROM, which was consistent across all drop heights. Ankle DF ROM influences coordination strategies that allow for the management of vGRF during bilateral drop-landings, with alterations in alignment for the knee and ankle joints at both initial contact and peak angles.     

Age-related differences in movement strategies and postural control during stooping and crouching tasks

While epidemiologic data suggests that one in four older adults have difficulty performing stooping and crouching (SC) tasks, little is known about how aging affects SC performance. This study investigated differences between young and older adults in lower limb kinematics and underfoot center of pressure (COP) measures when performing a series of SC tasks. Twelve healthy younger and twelve healthy older participants performed object-retrieval tasks varying in: (1) initial lift height, (2) precision demand, and (3) duration. Whole-body center of mass (COM), underfoot COP, and hip and knee angular kinematics (maximum angles and velocities) were analyzed. Compared to younger, older participants moved slower when transitioning into and out of pick-up postures that were characterized by less hip and knee flexion. Older participants also showed a diminished ability to adapt to the changing postural demands of each set of tasks. This was especially evident during longer tasks, whereby older individuals avoided high knee flexion crouching postures that were commonly used by younger participants. Older adults also tended to exhibit faster and more frequent COP trajectory adjustments in the anterior–posterior direction. It is likely that limitations in physical characteristics such as lower limb strength and range of motion contributed to these differences.     

An investigation of lower extremity energy dissipation strategies during single-leg and double-leg landing based on sagittal and frontal plane biomechanics

There is limited understanding of the differences in lower extremity energy dissipation strategies between single-leg and double-leg landing maneuvers. This study sought to investigate these differences in sagittal and frontal planes, and explain the differences using kinematics and kinetics. We hypothesized that single-leg and double-leg landing maneuvers involve different lower extremity energy dissipation strategies in both planes. Ten recreational athletes were recruited and instructed to perform double-leg and single-leg landing from 0.60-m height. Force-plates and motion-capture system were used to obtain kinetics and kinematics data respectively. Joint power was taken as product of joint moment and angular velocity. Joint work was computed as integral of joint power over time, whereby negative work represented energy dissipation. In the sagittal plane, the hip and knee showed major contributions to energy dissipation during double-leg landing; the hip and ankle were the dominant energy dissipaters during single-leg landing. In the frontal plane, the hip acted as the key energy dissipater during double-leg landing; the knee contributed the most energy dissipation during single-leg landing. The knee also exhibited greater frontal plane joint ROM, moment and energy dissipation during single-leg landing than double-leg landing. Our findings indicated that different energy dissipation strategies were adopted for double-leg and single-leg landing in sagittal and frontal planes. Considering the prominent frontal plane biomechanics exhibited by the knee during single-leg landing, we expect that this maneuver may have greater likelihood of leading to traumatic knee injuries, particularly non-contact ACL injuries, compared to the double-leg landing maneuver.     

Relation of maximal ankle dorsiflexion angle and passive resistive torque to passive-elastic stiffness of ankle dorsiflexion stretch

The maximal passive ankle dorsiflexion angle and the maximal passive resistive torque at this angle were measured for 81 women 20 to 84 years of age and correlated with the passive-elastic stiffness (stiffness) of an ankle dorsiflexion stretch. Pearson correlation coefficients and multiple regression analyses were used to examine whether the two clinical measurements could predict ankle stiffness. The maximal passive resistive torque showed a moderate correlation with stiffness in the full stretch range (r = .69) and high correlation with stiffness in the last half of the full stretch range (r = .84). The maximal dorsiflexion angle showed a low correlation with stiffness in the full stretch range (r = .27) and in the last half of the full stretch range (r = .36). The maximal passive resistive torque and the dorsiflexion angle together accounted for 54% of the stiffness variance in the full stretch range and 76% of the stiffness variance in the last half of the full stretch range. Thus, the clinical measurements of the maximal passive dorsiflexion angle and the maximal passive resistive torque were directly and significantly related to the ankle dorsiflexion passive-elastic stiffness and good predictors of stiffness in the last half of the passive ankle dorsiflexion stretch.     

Performing the vertical jump: movement adaptations for submaximal jumping

The purpose of this study was to gain insight into the kinematics and kinetics of the vertical jump when jumping for different heights and to investigate movement effectiveness as a criterion for movement control in submaximal jumping. In order to jump high a countermovement is used and large body segments are rotated, both of which consume energy which is not directly used to gain extra jump height. It was hypothesized that the energy used to reach a specified jump height is minimized by limiting the non-effective energy consumed. Standing vertical jumps attempting 100%, 75%, 50%, and 25% of maximal height were performed by a group of 10 subjects. Force and motion data were recorded simultaneously during each performance. We found that jump height increased due to increasing vertical velocity at take off. This was primarily related to an increase in countermovement amplitude. As such, flexion amplitude of the hip joint increased with jump height whereas the ankle and knee joint flexion did not. These findings revealed that for submaximal jumping a consistent strategy was used of maximizing the contribution of distal joints and minimizing the contribution of proximal joints. Taking into account the high inertia of proximal segments, the potential energy deficit due to countermovement prior to joint extension, the advantageous horizontal orientation of the foot segment during stance and the tendon lengths in distal muscles, it was concluded that movement effectiveness is a likely candidate for the driving criterion of this strategy.     

Use of self-organizing maps to study sex- and speed-dependent changes in running biomechanics

BackgroundUp to 79% of runners get injured every year, with higher rates of injuries occurring in females than males. A self-organizing map (SOM) is a type of artificial neural network that can be used to inspect large datasets and study coordination patterns. The purpose of this study was to use an SOM to study the effects of sex and speed on biomechanical coordination patterns.MethodThirty-two healthy runners ran on an instrumented treadmill at their long slow distance speed (LSD) and at speed 30% faster (LSD + 30%). Vertical ground reaction force (vGRF), vertical tibial acceleration, step parameters, electromyograms (EMG) of six lower limb muscles, and joint angles were collected across speeds. Rate of loading (ROL), tibial impact shock (TIS), coupling angle variability (CAV) and movement pattern proportions for hip/knee sagittal and hip frontal / knee sagittal plane couplings, peak EMG, step length, step rate, and knee and ankle joint angle at initial contact were used as an input for the SOM (37 variables).ResultsThe analysis identified four clusters (i.e., running patterns). While males and females showed similar distribution across clusters at LSD (p = .36) and at LSD + 30% (p = .51), females did exhibit a significant (p = .03) shift between clusters as the speed increased from LSD to LSD + 30% whereas males did not (p = .17). The shift was associated with an increase in TIS, ROL, step length, step rate, vastus lateralis EMG, hip flexion/knee extension movement pattern proportion, and a decrease in ST EMG and CAV_IC for hip sagittal/knee sagittal coupling.ConclusionAs running speed increased there was a significant change in the coordination pattern in females, which was characterized by increases in several variables that are purported risk factors for running related injuries.     

Adaptive Dynamics of the Leg Movement Patterns of Human Infants: I. The Effects of Posture on Spontaneous Kicking

This is the first of two articles in which we describe how infants adapt their spontaneous leg movements to changes in posture or to elicitation of behaviors by a mechanical treadmill. In this article, we compare the kinematics of kicks produced by 3-month-old infants in three postures, supine, angled (45 degrees ), and vertical, and examine the changes in muscular and nonmuscular force contributions to limb trajectory. By manipulating posture we were able to assess the sensitivity of the nascent motor system to changes in the gravitational context. The postural manipulation elicited a distinct behavioral and dynamic effect. In the more upright postures, gravitational resistance to motion at the hip was 4 to 10 times greater than resistance met in the supine posture, necessitating larger muscle torques to drive hip flexion. Kicks produced in the vertical posture showed a reduction in hip joint range of motion and an increase in synchronous joint flexion and extension at the hip and knee. At the same time, hip and knee muscle torques were also more highly correlated in kicks performed in the vertical than in the supine or angled posture. This increased correlation between muscle torques at the hip and knee implicates anatomical and energetic constraints-the intrinsic limb dynamics-in creating coordinated limb behavior out of nonspecific muscle activations.