Joint power generation differentiates young and adult sprinters during the transition from block start into acceleration: a cross-sectional study.

The aim of this study was to investigate differences in joint power generation between well-trained adult athletes and young sprinters from block clearance to initial contact of second stance. Eleven under 16 (U16) and 18 under 18 (U18) promising sprinters executed an explosive start action. Fourteen well-trained adult sprinters completed the exact same protocol. All athletes were equipped with 74 spherical reflective markers, while an opto-electronic motion analysis system consisting of 12 infrared cameras (250 Hz, MX3, Vicon, Oxford Metrics, UK) and 2 Kistler force plates (1,000 Hz) was used to collect the three-dimensional marker trajectories and ground reaction forces (Nexus, Vicon). Three-dimensional kinematics, kinetics, and power were calculated (Opensim) and time normalised from the first action after gunshot until initial contact of second stance after block clearance. This study showed that adult athletes rely on higher knee power generation during the first stance to induce longer step length and therefore higher velocity. In younger athletes, power generation of hip was more dominant.

Control of propulsion and body lift during the first two stances of sprint running: a simulation study.

The aim of this study was to relate the contribution of lower limb joint moments and individual muscle forces to the body centre of mass (COM) vertical and horizontal acceleration during the initial two steps of sprint running. Start performance of seven well-trained sprinters was recorded using an optoelectronic motion analysis system and two force plates. Participant-specific torque-driven and muscle-driven simulations were conducted in OpenSim to quantify, respectively, the contributions of the individual joints and muscles to body propulsion and lift. The ankle is the major contributor to both actions during the first two stances, with an even larger contribution in the second compared to the first stance. Biarticular gastrocnemius is the main muscle contributor to propulsion in the second stance. The contribution of the hip and knee depends highly on the position of the athlete: During the first stance, where the athlete runs in a forward bending position, the knee contributes primarily to body lift and the hip contributes to propulsion and body lift. In conclusion, a small increase in ankle power generation seems to affect the body COM acceleration, whereas increases in hip and knee power generation tend to affect acceleration less.

Human-robot interaction: kinematics and muscle activity inside a powered compliant knee exoskeleton.

Until today it is not entirely clear how humans interact with automated gait rehabilitation devices and how we can, based on that interaction, maximize the effectiveness of these exoskeletons. The goal of this study was to gain knowledge on the human-robot interaction, in terms of kinematics and muscle activity, between a healthy human motor system and a powered knee exoskeleton (i.e., KNEXO). Therefore, temporal and spatial gait parameters, human joint kinematics, exoskeleton kinetics and muscle activity during four different walking trials in 10 healthy male subjects were studied. Healthy subjects can walk with KNEXO in patient-in-charge mode with some slight constraints in kinematics and muscle activity primarily due to inertia of the device. Yet, during robot-in-charge walking the muscular constraints are reversed by adding positive power to the leg swing, compensating in part this inertia. Next to that, KNEXO accurately records and replays the right knee kinematics meaning that subject-specific trajectories can be implemented as a target trajectory during assisted walking. No significant differences in the human response to the interaction with KNEXO in low and high compliant assistance could be pointed out. This is in contradiction with our hypothesis that muscle activity would decrease with increasing assistance. It seems that the differences between the parameter settings of low and high compliant control might not be sufficient to observe clear effects in healthy subjects. Moreover, we should take into account that KNEXO is a unilateral, 1 degree-of-freedom device.

From block clearance to sprint running: characteristics underlying an effective transition.

The aim of this study was to characterize the specifics of the sprint technique during the transition from start block into sprint running in well-trained sprinters. Twenty-one sprinters (11 men and 10 women), equipped with 74 spherical reflective markers, executed an explosive start action. An opto-electronic motion analysis system consisting of 12 MX3 cameras (250 Hz; 325,000 pixels) and two Kistler force plates (1000 Hz) was used to collect the three-dimensional (3D) marker trajectories and ground reaction forces (Nexus, Vicon). The 3D kinematics, joint kinetics, and power were calculated (Opensim) and were time normalized to 100% from the first action after gunshot until the end of second stance after block clearance (Matlab). The results showed that during the first stance, power generation at the knee plays a significant role in obtaining an effective transition, representing 31% of power generation in the lower limb, in the absence of preceding power absorption. Furthermore, the sprinter actively searches a more forward leaning position to maximize horizontal velocity. Since success during sprinting from the second stance onwards involves high hip and ankle activation, the above-mentioned three characteristics are specific skills required to successfully conclude the transition from start block into sprint running.

The added value of an actuated ankle-foot orthosis to restore normal gait function in patients with spinal cord injury: a systematic review.

OBJECTIVE: To provide an overview of robot-assisted rehabilitation devices developed for actuation of the ankle-foot complex and their ability to influence the attributes of normal gait in patients with spinal cord injury. METHODS: A search was conducted in MEDLINE, Web of Knowledge, National Academic Research and Collaborations Information System, and Physiotherapy Evidence Data-base (1985-2011), using, "ankle", "foot", "robotics", "orthotics" and "spinal cord injury" as most relevant keywords. Article inclusion was performed in 3 stages; at the level of: (i) title, (ii) abstract and (iii) full text. RESULTS: The actuated ankle-foot orthoses currently available are characterized by several combinations of an actuator and a control mechanism. Both the actuator and the control strategy substantially influence human-machine interaction and therefore the potential of the device to assist in modifying locomotor function and potentially modify the underlying motor control mechanisms. CONCLUSION: Due to small sample sizes, limited studies in patients with spinal cord injury, and limitations in study design, it is difficult to draw firm conclusions on the effect of different types of actuated ankle-foot orthoses. Based on the limited data available, pneumatic artificial muscles in combination with proportional myoelectric control are suggested to have the potential to meet most of the preconditions to restore the attributes of normal gait and therefore facilitate neuroplasticity.

Decrease in postural sway and trunk stiffness during cognitive dual-task in nonspecific chronic low back pain patients, performance compared to healthy control subjects.

STUDY DESIGN: A 2-group experimental design. OBJECTIVE: To investigate the effect of a cognitive dual-task on postural sway of pelvis and trunk during unstable sitting in nonspecific chronic patients with low back pain (CLBP) compared to healthy control subjects. SUMMARY OF BACKGROUND DATA: Higher cognitive systems as well as sensory processes contribute to postural control. An increase in postural sway due to a cognitive dual task could mean more need of cognitive systems to control balance. METHODS: A total of 21 CLBP patients and 21 control subjects were included based on detailed clinical criteria. Every subject was submitted to 2 postural control tests in an unstable sitting position (easy test position: 2 feet on the ground and difficult test position: 1 foot lifted). Both tests were performed with and without cognitive dual-task. A 3-dimensional motion analysis system was used measure angular displacement of trunk and pelvis in the 3 cardinal planes. RESULTS: In the most difficult balance position, postural sway increases in the control group when the cognitive dual-task is added, for 50% of the variables the increase is significant (P between 0.02 and 0.05). On the contrary, postural sway decreases, not significantly, in the CLBP group when the dual-task is added. These findings are the same for trunk as for pelvis deviations. The Pearson correlation coefficient between trunk and pelvis movement from the CLBP group are lower for all 3 movement directions in the dual-task condition (r between 0.441 and 0.988) compared to the single task condition (r between 0.982 and 0.995). CONCLUSION: In nonspecific CLBP patients, a cognitive dual-task reduces both postural sway and trunk stiffness due to the distracting effect of the dual-task. This effect is only visible when the balance task is difficult.

Differences in balance strategies between nonspecific chronic low back pain patients and healthy control subjects during unstable sitting.

STUDY DESIGN: A 2-group experimental design. OBJECTIVE: To investigate differences in postural control strategies of pelvis and trunk movement between nonspecific chronic low back pain (CLBP) patients and healthy control subjects using 3-dimensional motion analysis. SUMMARY OF BACKGROUND DATA: Increased postural sway assessed by center of pressure displacements have been documented in patients with low back pain (LBP). The 3-dimensional movement strategies used by patients with LBP to keep their balance are not well documented. METHODS: Nineteen CLBP patients and 20 control subjects were included based on detailed clinical criteria. Every subject was submitted to a postural control test in an unstable sitting position. A 3-dimensional motion analysis system, equipped with 7 infrared M1 cameras, was used to track 9 markers attached to the pelvis and trunk to estimate their angular displacement in the 3 cardinal planes. RESULTS: The total angular deviation in all 3 directions of pelvis and trunk was higher in the CLBP group compared with the control group. In 4 of the 6 calculated differences, a significant higher deviation was found in the CLBP group (significant P-values between 0.013 and 0.047). Subjects of both groups mostly used rotation compared with lateral flexion and flexion/extension displacements of pelvis and trunk to adjust balance disturbance. The CLBP group showed a high correlation (Pearson: 0.912-0.981) between movement of pelvis and trunk, compared with the control group. CONCLUSION: A higher postural sway and high correlation between pelvis and trunk displacements was found in the LBP group compared with healthy controls.

Reproducibility of postural control measurement during unstable sitting in low back pain patients.

BACKGROUND: Postural control tests like standing and sitting stabilometry are widely used to evaluate neuromuscular control related to trunk balance in low back pain patients. Chronic low back pain patients have lesser postural control compared to healthy subjects. Few studies have assessed the reproducibility of the centre of pressure deviations and to our knowledge no studies have investigated the reproducibility of three-dimensional kinematics of postural control tests in a low back pain population. Therefore the aim of this study was to assess the test-retest reproducibility of a seated postural control test in low back pain patients. METHODS: Postural control in low back pain patients was registered by a three dimensional motion analysis system combined with a force plate. Sixteen chronic low back pain patients having complaints for at least six months, were included based on specific clinical criteria. Every subject performed 4 postural control tests. Every test was repeated 4 times and lasted 40 seconds. The force plate registered the deviations of the centre of pressure. A Vicon-612-datastation, equipped with 7 infra-red M1 camera's, was used to track 13 markers attached to the torso and pelvis in order to estimate their angular displacement in the 3 cardinal planes. RESULTS: All Intraclass Correlation Coefficients (ICC) calculated for the force plate variables did not exceed 0.73 (ranging between 0.11 and 0.73). As for the torso, ICC's of the mean flexion-extension and rotation angles ranged from 0.65 to 0.93 and of the mean lateral flexion angle from 0.50 to 0.67. For the pelvis the ICC of the mean flexion-extension angle varied between 0.66 and 0.83, the mean lateral flexion angle between 0.16 and 0.81 and the mean rotation angle between 0.40 and 0.62. Consecutive data suggest that the low test-retest reproducibility is probably due to a learning effect. CONCLUSION: The test-retest reproducibility of these postural control tests in an unstable sitting position can globally be considered as rather moderate. In order to improve the test-retest reproducibility, a learning period may be advisable at the beginning of the test.

Effects of hallux limitus on plantar foot pressure and foot kinematics during walking.

The effects of hallux limitus on plantar foot pressure and foot kinematics have received limited attention in the literature. Therefore, a study was conducted to assess the effects of limited first metatarsophalangeal joint mobility on plantar foot pressure. It was equally important to identify detection criteria based on plantar pressures and metatarsophalangeal joint kinematics, enabling differentiation between subjects affected by hallux limitus and people with normal hallux function. To further our understanding of the relation between midtarsal collapse and hallux limitus, kinematic variables relating to midtarsal pronation were also included in the study. Two populations of 19 subjects each, one with hallux limitus and the other free of functional abnormalities, were asked to walk at their preferred speed while plantar foot pressures were recorded along with three-dimensional foot kinematics. The presence of hallux limitus, structural or functional, caused peak plantar pressure under the hallux to build up significantly more and at a faster rate than under the first metatarsal head. Additional discriminators for hallux limitus were peak dorsiflexion of the first metatarsophalangeal joint, time to this peak value, peak pressure ratios of the first metatarsal head and the more lateral metatarsal heads, and time to maximal pressure under the fourth and fifth metatarsal heads. Finally, in approximately 20% of the subjects, with and without hallux limitus, midtarsal pronation occurred after heel lift, validating the claim that retrograde midtarsal pronation does occur.

Effects of simulated genu valgum and genu varum on ground reaction forces and subtalar joint function during gait.

The mechanical effects of genu valgum and varum deformities on the subtalar joint were investigated. First, a theoretical model of the forces within the foot and lower extremity during relaxed bipedal stance was developed predicting the rotational effect on the subtalar joint due to genu valgum and varum deformities. Second, a kinetic gait study was performed involving 15 subjects who walked with simulated genu valgum and genu varum over a force plate and a plantar pressure mat to determine the changes in the ground reaction force vector within the frontal plane and the changes in the center-of-pressure location on the plantar foot. These results predicted that a genu varum deformity would tend to cause a subtalar pronation moment to increase or a supination moment to decrease during the contact and propulsion phases of walking. With genu valgum, it was determined that during the contact phase a subtalar pronation moment would increase, whereas in the early propulsive phase, a subtalar supination moment would increase or a pronation moment would decrease. However, the current inability to track the spatial position of the subtalar joint axis makes it difficult to determine the absolute direction and magnitudes of the subtalar joint moments.