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PubMed1995

Combined knee loading states that generate high anterior cruciate ligament forces.

Injuries to the anterior cruciate ligament frequently occur under combined mechanisms of knee loading. This in vitro study was designed to measure levels of ligament force under dual combinations of individual loading states and to determine which combinations generated high force. Resultant force was recorded as the knee was extended passively from 90 degrees of flexion to 5 degrees of hyperextension under constant tibial loadings. The individual loading states were 100 N of anterior tibial force, 10 Nm of varus and valgus moment, and 10 Nm of internal and external tibial torque. Straight anterior tibial force was the most direct loading mechanisms; the mean ligament force was approximately equal to applied anterior tibial force near 30 degrees of flexion and to 150% of applied tibial force at full extension. The addition of internal tibial torque to a knee loaded by anterior tibial force produced dramatic increases of force at full extension and hyperextension. This loading combination produced the highest ligament forces recorded in the study and is the most dangerous in terms of potential injury to the ligament. In direct contrast, the addition of external tibial torque to a knee loaded by anterior tibial force decreased the force dramatically for flexed positions of the knee; at close to 90 degrees of flexion, the anterior cruciate ligament became completely unloaded. The addition of varus moment to a knee loaded by anterior tibial force increased the force in extension and hyperextension, whereas the addition of valgus moment increased the force at flexed positions. These states of combined loading also could present an increased risk for injury. Internal tibial torque is an important loading mechanism of the anterior cruciate ligament for an extended knee. The overall risk of injury to the ligament from varus or valgus moment applied in combination with internal tibial torque is similar to the risk from internal tibial torque alone. External tibial torque was a relatively unimportant mechanism for generating anterior cruciate ligament force.

Aged↗
PubMed1988

A new dynamometer measuring concentric and eccentric muscle strength in accelerated, decelerated, or isokinetic movements. Validity and reproducibility.

A new computerized dynamometer (the SPARK System) is described. The system can measure concentric and eccentric muscle strength (torque) during linear or nonlinear acceleration or deceleration, isokinetic movements up to 400 degrees.s-1, and isometric torque. Studies were performed to assess: I. validity and reproducibility of torque measurements; II. control of lever arm position; III. control of different velocity patterns; IV. control of velocity during subject testing; and, V. intra-individual reproducibility. No significant difference was found between torque values computed by the system and known torque values (p greater than 0.05). No difference was present between programmed and external measurement of the lever arm position. Accelerating, decelerating and isokinetic velocity patterns were highly reproducible, with differences in elapsed time among 10 trials being never greater than 0.001 s. Velocity during concentric and eccentric isokinetic quadriceps contractions at 30 degrees.s-1, 120 degrees.s-1 and 270 degrees.s-1 never varied by more than 3 degrees.s-1 among subjects (N = 21). Over three days of testing, the overall error for concentric and eccentric quadriceps contraction peak torque values for 5 angular velocities between 30 degrees.s-1 and 270 degrees.s-1 ranged from 5.8% to 9.0% and 5.8% to 9.6% respectively (N = 25). The results indicate that the SPARK System provides valid and reproducible torque measurements and strict control of velocity. In addition, the intra-individual error is in accordance with those reported for other similar devices.

Adult↗
PubMed1992

Isokinetic rehabilitation after arthroscopic meniscectomy.

The aim of this study was to assess the effects in humans of early (2 weeks) and delayed (6 weeks) isokinetic strength training in the recovery of muscle strength following an arthroscopic partial meniscectomy. The peak torque developed in the quadriceps and hamstrings and the torque developed at a knee angle of 1.05 rad were evaluated in 16 subjects, pre-operatively (pre-op), and 2, 6, and 10 weeks post-operatively (post-op), on an isokinetic device at four different velocities (1.05, 2.09, 3.14, and 4.19 rad.s-1). The fatigue characteristics of the muscles were evaluated by having the subject perform 15 maximal contractions at 3.14 rad.s-1. Training was done on the same device (three times a week for 1-2 months), beginning either 2 or 6 weeks post-op. A repeated measures analysis of variance demonstrated a time effect but no differences between groups and no interactions. Torques developed by the knee flexors and extensors were significantly smaller 2 weeks post-op than pre-op, at all velocities tested. Torques developed in the quadriceps recovered to their pre-op values by 6 weeks, and further gained significantly in strength from 6 to 10 weeks. Quadriceps torques remained weaker than the contralateral side at 10 weeks. Hamstrings torques were either higher or similar to pre-op values by 6 weeks, and demonstrated increases from 6 to 10 weeks post-op at 1.05 and 4.19 rad.s-1 only. Total work and average power developed by the quadriceps and hamstrings during the fatigue protocol changed with time in a similar manner to torque.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1991

Static and dynamic assessment of the Biodex dynamometer.

The validity and accuracy of the Biodex dynamometer was investigated under static and dynamic conditions. Static torque and angular position output correlated well with externally derived data (r = 0.998 and r greater than 0.999, respectively). Three subjects performed maximal voluntary knee extensions and flexions at angular velocities from 60 to 450 degrees.s-1. Using linear accelerometry, high speed filming and Biodex software, data were collected for lever arm angular velocity and linear accelerations, and subject generated torque. Analysis of synchronized angular position and velocity changes revealed the dynamometer controlled angular velocity of the lever arm to within 3.5% of the preset value. Small transient velocity overshoots were apparent on reaching the set velocity. High frequency torque artefacts were observed at all test velocities, but most noticeably at the faster speeds, and were associated with lever arm accelerations accompanying directional changes, application of resistive torques by the dynamometer, and limb instability. Isokinematic torques collected from ten subjects (240, 300 and 400 degrees.s-1) identified possible errors associated with reporting knee extension torques at 30 degrees of flexion. As a result of tissue and padding compliance, leg extension angular velocity exceeded lever arm angular velocity over most of the range of motion, while during flexion this compliance meant that knee and lever arm angles were not always identical, particularly at the start of motion. Nevertheless, the Biodex dynamometer was found to be both a valid and an accurate research tool; however, caution must be exercised when interpreting and ascribing torques and angular velocities to the limb producing motion.

Adolescent↗
PubMed1987

Ageing and isokinetic plantar flexion.

Isokinetic torques (Cybex II) of the plantar flexors in 25 healthy men were compared at 5 angular velocities (30, 60, 90, 120 and 180 degrees X s-1). The purposes were to compare plantar flexion torques in young and old subjects, and to determine whether the expected decrease was significantly associated with age, physical activity, or aerobic fitness. Four groups were studied: young (21.7 +/- 2.0 years) and older (63.3 +/- 2.8 years), active and sedentary. Measurements of height, weight, % body fat, VO2max, and daily leisure energy expenditure (questionnaire) were determined for each subject. Statistical measures of analysis of variance were used to determine significant differences among groups; product moment correlation and stepwise regression analysis were used to describe the degree of association between the dependent variable of plantar flexion torque and the independent variables at each velocity. A decline in torque was observed as the isokinetic velocity of angular motion increased. Age alone was a significant determinant of plantar flexion torque, whereas at the slowest speed, when VO2max was used as an explanatory variable, age was not a significant determinant of torque. At 30 degrees X s-1 47% of the variance in torque was explained by VO2max while at 180 degrees X s-1 49% of the variance was explained by age.

Adult↗
PubMed2002

Commonalities and differences in control of various drawing movements.

Characteristics of control at the shoulder and elbow during nine types of drawing movements were studied in the present work. The task was to repetitively track a template, depicted on a horizontal table, with the index finger at a cyclic frequency of 1.5 Hz. The templates were a circle, four ovals and four lines of different orientations. The wrist was immobilized and the movement consisted of rotations at the shoulder and elbow joints. The studied movements varied in a wide range with respect to the amplitude of elbow and shoulder movements and relative phase between them. Kinetic analysis included analysis of torque signs, impulses, and timing. It demonstrated that the role of muscle torque in movement production was different at the two joints. During eight out of the nine movement types, the muscle torque at the shoulder accelerated and decelerated this joint and almost completely coped with the influence of the interactive torque arising from elbow motion. Conversely, interactive torque generated by shoulder motion played a dominant role in elbow acceleration and deceleration, whereas muscle torque at the elbow adjusted passive elbow movement to the various template shapes. EMG data were in agreement with the conclusions made from the kinetic analysis. Collectively, these data support the hypothesis that the two joints have different functions in movement production. The shoulder creates a foundation for motion of the entire arm through the interactive torque, and the elbow serves as a fine-tuner of the end-point movement. Control of the shoulder was similar across the eight movement types and the differences in the end-point path were provided by variations in elbow control. The two joints exchanged roles during one movement type, namely, drawing the line tilted right. During this movement, the elbow musculature generated motion at this joint and the shoulder musculature counteracted mechanical influence of this motion on the shoulder position. The findings suggest that during drawing movements, the control strategy exploits intersegmental dynamics of the shoulder-elbow mechanical linkage.

Adolescent↗
PubMed2004

The scaling of motor noise with muscle strength and motor unit number in humans.

Understanding the origin of noise, or variability, in the motor system is an important step towards understanding how accurate movements are performed. Variability of joint torque during voluntary activation is affected by many factors such as the precision of the descending motor commands, the number of muscles that cross the joint, their size and the number of motor units in each. To investigate the relationship between the peripheral factors and motor noise, the maximum voluntary torque produced at a joint and the coefficient of variation of joint torque were recorded from six adult human subjects for four muscle/joint groups in the arm. It was found that the coefficient of variation of torque decreases systematically as the maximum voluntary torque increases. This decreasing coefficient of variation means that a given torque or force can be more accurately generated by a stronger muscle than a weaker muscle. Simulations demonstrated that muscles with different strengths and different numbers of motor units could account for the experimental data. In the simulations, the magnitude of the coefficient of variation of muscle force depended primarily on the number of motor units innervating the muscle, which relates positively to muscle strength. This result can be generalised to the situation where more than one muscle is available to perform a task, and a muscle activation pattern must be selected. The optimal muscle activation pattern required to generate a target torque using a group of muscles, while minimizing the consequences of signal dependent noise, is derived.

Action Potentials↗
PubMed1998

Effects of contraction duration on low-frequency fatigue in voluntary and electrically induced exercise of quadriceps muscle in humans.

The aims of this study were to investigate if low-frequency fatigue (LFF) dependent on the duration of repeated muscle contractions and to compare LFF in voluntary and electrically induced exercise. Male subjects performed three 9-min periods of repeated isometric knee extensions at 40% maximal voluntary contraction with contraction plus relaxation periods of 30 plus 60 s, 15 plus 30 s and 5 plus 10 s in protocols 1, 2 and 3, respectively. The same exercise protocols were repeated using feedback-controlled electrical stimulation at 40% maximal tetanic torque. Before and 15 min after each exercise period, knee extension torque at 1, 7, 10, 15, 20, 50 and 100 Hz was assessed. During voluntary exercise, electromyogram root mean square (EMGrms) of the vastus lateralis muscle was evaluated. The 20-Hz torque:100-Hz torque (20:100 Hz torque) ratio was reduced more after electrically induced than after voluntary exercise (P < 0.05). During electrically induced exercise, the decrease in 20:100 Hz torque ratio was gradually (P < 0.05) reduced as the individual contractions shortened. During voluntary exercise, the decrease in 20:100 Hz torque ratio and the increase in EMGrms were greater in protocol 1 (P < 0.01) than in protocols 2 and 3, which did not differ from each other. In conclusion, our results showed that LFF is dependent on the duration of individual muscle contractions during repetitive isometric exercise and that the electrically induced exercise produced a more pronounced LFF compared to voluntary exercise of submaximal intensity. It is suggested that compensatory recruitment of faster-contracting motor units is an additional factor affecting the severity of LFF during voluntary exercise.

Adult↗
PubMed1994

Reaction forces, tool handle displacements and discomfort ratings during work with shut-off nutrunners.

Reaction forces, hand-arm displacement and subjective ratings were studied during securing of threaded fasteners with nine right-angle nutrunners. The nutrunners had three different preset spindle torques and three different shut-off mechanisms. The tools were tested according to the international ISO method and with a method proposed by the authors in a previous study. Twelve male subjects participated in the study. A force platform measured the reaction forces between the subjects and the floor. Motions of the hand-arm system and shoulder were measured with an optoelectronic measuring system and a goniometer. Strong correlations between motions/ ground reaction forces and ratings were found. The ratings were also strongly correlated with the tool torque impulse, calculated according to ISO 6544, as well as with the time for which the torque exceeded 75% of the preset spindle torque multiplied by the preset spindle torque to produce a 'time-torque value'. The strongest correlation, though, was found with the time-torque value, but the difference was not large enough to propose a change of the ISO standard.

Journal Article↗
PubMed1993

Hand position affects elbow joint load during push-up exercise.

Electromagnetic motion sensors and a piezoelectric force plate were used to record simultaneously upper-extremity motion and forces in nine healthy male subjects during push-ups in six different hand positions. The intersegmental loading pattern on the elbow joint during a push-up exercise was investigated. The importance of hand position on these loads was examined. Peak forces exerted on the elbow joint along the forearm axis averaged 45% of the body weight for the 'normal' hand position and were significantly decreased if hands were positioned either 'apart' or 'superior' from 'normal'. The peak torque in the 'normal' position tended to produce elbow flexion and was 2305.9 N cm, or 56% of the maximal isometric extensor torque, which was significantly different from hands 'apart' and hand 'together' at 29 and 71% of the maximal isometric torque, respectively. The maximum torque about the forearm axis tended to produce forearm pronation with a mean value of 315.4 N cm (35% of the maximal isometric supinator torque). The maximum valgus torque at the elbow opposed by the medial ligamentous structure was 1241 N cm and was significantly increased if the hand was positioned superiorly. The valgus torque increased by 42% for the one-handed push-up under simulated fall condition. The results of this analysis give an insight into the biomechanics of a normal elbow and to its load-carrying capacity. The data may be extrapolated to the design of rehabilitation programs and possibly to the mechanisms of forearm injury during a fall.

Adult↗
PubMed2006

Joint-angle-dependent neuromuscular dysfunctions at the wrist in persons after stroke.

OBJECTIVE: To evaluate the joint-angle-dependent neuromuscular functions at the affected wrist in hemiplegic subjects after stroke while doing isometric maximal voluntary wrist flexion and extension across different wrist angles. DESIGN: We investigated torques during isometric maximal voluntary wrist flexions and extensions at 8 different wrist angles, ranging from -45 degrees to 60 degrees. We used the associated electromyographic activities of 2 agonist and antagonist muscle pairs related to wrist and elbow joints for the analysis of muscular coactivations. We compared the data obtained from poststroke subjects' affected and unaffected sides. SETTING: A research laboratory in a rehabilitation center. PARTICIPANTS: Eleven subjects with hemiplegia after stroke with passive range of motion (ROM) in the wrist from -45 degrees to 60 degrees. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Directly measured torques, torques after normalization during maximal isometric wrist contractions, and normalized moving average electromyographic signals of each muscle at the tested positions. RESULTS: The measured torques of the affected wrists were significantly lower than those of the unaffected wrists at all tested angles during wrist flexion and extension (P<.05). The angle-dependent patterns of the normalized torque across the tested wrist angles varied from those of the unaffected wrists (2-way analysis of variance, P<.05). There were decreases in normalized torques during both flexion and extension at the extended positions in the affected group (P<.05). Abnormal cocontractions were found in agonist and antagonist muscle pairs related to wrist and elbow joints, and between the elbow flexor and wrist extensor when subjects did the wrist contractions on the paretic side, especially at the wrist extended positions. CONCLUSIONS: Wrist muscle weakness was distributed unevenly across the selected wrist ROM on the affected side, as represented by the varied patterns of the normalized torque-angle relationship, compared with the unaffected wrists. There were reductions in the selective control of muscle coactivating synergies both single-jointly and cross-jointly in the impaired nervous system during wrist contractions; the extent of these reductions was also related to the wrist angle configuration.

Adult↗
PubMed2005

Biomechanical and histomorphometric study on the bone-screw interface of bioactive ceramic-coated titanium screws.

The purpose of this study was to compare the osseointegration of 4 different kinds of bioactive ceramic-coated screws with uncoated screws by biomechanical and histomorphometric analysis. Calcium pyrophosphate (CPP), apatite-wollastonite 1:3 glass ceramic (W3G), apatite-wollastonite 1:1 glass ceramic (WAG) and bioactive CaO-SiO2-B2O3 glass ceramic (CSG) coatings were prepared and coated by the dipping method. Coated and uncoated titanium screws were inserted into the tibia of 18 adult mongrel male dogs for 2, 4, and 8 weeks. The insertion torques, radiographs, undecalcified histology, histomorphometric analysis, and extraction torques were evaluated. No difference of insertion torque was found among the five screw types. At 2 and 4 weeks after implantation, the extraction torque of ceramic-coated screws was significantly higher than that of uncoated screws (p < or = 0.0001). At 8 weeks, the extraction torques of CPP-, W3G-, and WAG-coated screws were significantly higher than those of CSG-coated and uncoated screws (p<0.0001). At 2, 4, and 8 weeks, the extraction torques of 4 different ceramic-coated screws were significantly higher than the corresponding insertion torque. Strong fixation was observed even at 2 weeks in the CPP-, W3G- and WAG-coated screws. The bone-screw contacts of the 4 different coated screws at 2 and 4 weeks were statistically higher than that of the uncoated screws, and the bone-screw contacts of the CPP-, W3G- and WAG-coated screws at 8 weeks were also statistically higher than that of the uncoated screws. The fixation strength was increased by the presence of osteoconductive coating materials, such as CPP, W3G, and WAG, which enabled to achieve higher fixation strength even as early as 2-8 weeks after the insertion.

Animals↗
PubMed1989

A model for bacterial flagellar motor: free energy transduction and self-organization of rotational motion.

A bacterial flagellar motor is an energy transducing molecular machine which shows some attractive characteristics. First, this motor is driven by a protonmotive force (PMF) across the membrane, two components of which, electric potential delta psi and chemical potential -(2.3RT/F)delta pH, are equivalently transduced to the mechanical work of the motor rotation. Second, a PMF threshold for rotation is observed. Third, this motor can rotate reversibly either counterclockwise (CCW) or clockwise (CW) at almost the same speed. To clarify the osmomechanical coupling of this motor, these characteristics must be explained consistently at the molecular level. In this paper, in order to allow quantitative analyses of the above characteristics, a theoretical model of a bacterial flagellar motor is constructed assuming that the torque generating sites are electrodes which can be charged by protons and that the electrostatic interaction between the electrodes generates the rotation torque. Electrode reaction reasonably derives the equivalence of delta psi and -(2.3RT/F)delta pH. In this model, rates of charging and discharging of protons are influenced by the motor rotation rate, so that the torque generating sites co-operatively work through the motor rotation. We named this kind of co-operativity among them "dynamic co-operativity" in torque generation. This co-operativity causes autocatalytic generation of motor torque and the existence of the rotation threshold. In this model, the appearance of the stable rotational states can be described by phase transition caused by the dynamic co-operativity among torque generating sites. According to this model, the flagellar motor has two stable rotational states corresponding to CCW and CW, which show the same torques. The motor selects one direction from them to rotate, and that is self-organization of rotational motion. Interpretation of the transition between the two stable rotational states as the chemotactic reversals of the flagellar motor is also discussed.

Bacterial Physiological Phenomena↗
PubMed1997

Fatigue-related EMG responses of trunk muscles to a prolonged, isometric twist exertion.

OBJECTIVE: To determine the responses of the trunk muscles to fatigue during sustained, isometric axial torque exertions. DESIGN: Electromyographic (EMG) amplitude and frequency parameters were used to investigate the effects of prolonged contractions on the recruitment patterns of trunk muscles, with special emphasis on those antagonistic and stabilizing muscles not contributing directly to the required axial torque. BACKGROUND: High levels of muscle coactivity have been observed during axial torque generation in the trunk. It has been suggested that this serves to enhance postural stability of the spine during potentially injurious twist exertions. Muscle fatigue decreases force-generating capacity but the resulting effects on muscle recruitment, and the subsequent consequences for postural stability, have not been explored previously during the production of axial torques in the spine. METHODS: Eleven male and 11 female subjects maintained an isometric axial torque exertion to the left (40% of maximum) until volitional exhaustion in an upright standing posture. Maximum axial torques were measured before and after the trial. The average EMG amplitude (AEMG) and mean power frequency (MPF) of seven bilateral trunk muscles (representing agonists, antagonists and trunk stabilizers) were continuously monitored throughout the trial. RESULTS: The average decrease in maximum torque was 18.5% and the average endurance time was 163 +/- 52 s. Evidence of muscle fatigue was provided by significant MPF decreases in 12 of the 14 muscles monitored (P < 0.05) with a overall average decrease of 20.4%. There was a significant increase in AEMG with time for 11 of the 14 muscles monitored (P < 0.05). These increases in activation were linked to increases in muscle force for most of the antagonistic and stabilizing muscles. Gender effects were rarely observed. CONCLUSIONS: Fatigue results in an change in the recruitment patterns of trunk muscles. Muscles serving as antagonists and trunk stabilizers during prolonged axial twist exertions increased their force levels in response to fatigue.

Journal Article↗
PubMed1998

Changes in muscle strength properties caused by harvesting of autogenous semitendinosus tendon for reconstruction of contralateral anterior cruciate ligament.

A prospective study was conducted of how the muscle strength of the donor knee is affected by harvesting of the autogenous semitendinosus tendon (St) for use as a substitute graft material in cruciate ligament reconstruction. There were 25 patients from whom only the St was harvested from the contralateral (i.e., healthy/donor) knee. Using a Biodex System II (Biodex, New York, NY), the strength of the donor knee was measured during both extension and flexion, both before and 12 months after the tendon harvesting procedure. A comparative study was made of the preharvest and postharvest values for the peak torque and peak torque angle in the isokinetic contraction. There were no statistically significant differences between the preharvest and postharvest peak torque values of the donor knee. However, the peak torque angle decreased significantly after the tendon harvest; the range of the mean decrease was from 11.7 degrees to 15.0 degrees. This indicates that there was a change to a small flexion angle (P < .05). After the tendon harvest, regardless of the applied angular velocity, more than 80% of the cases showed a change of torque curve shape in which there was no peak in the latter half, and the position of the peak was shifted to the left. In conclusion, the results of this study indicate that harvesting of the autogenous St does not affect the peak torque, but the peak torque angle during flexion of the donor knee is reduced.

Adolescent↗
PubMed2007

Resistance training and locomotor recovery after incomplete spinal cord injury: a case series.

STUDY DESIGN: Longitudinal intervention case series. OBJECTIVE: To determine if a 12-week resistance and plyometric training program results in improved muscle function and locomotor speed after incomplete spinal cord injury (SCI). SETTING: University research setting. METHODS: Three ambulatory individuals with chronic (18.7+/-2.2 months post injury) motor incomplete SCI completed 12 weeks of lower extremity resistance training combined with plyometric training (RPT). Muscle maximum cross-sectional area (max-CSA) of the knee extensor (KE) and plantar flexor (PF) muscle groups was determined using magnetic resonance imaging (MRI). In addition, peak isometric torque, time to peak torque (T (20-80)), torque developed within the initial 220 ms of contraction (torque(220)) and average rate of torque development (ARTD) were calculated as indices of muscle function. Maximal as well as self-selected gait speeds were determined pre- and post-RPT during which the spatio-temporal characteristics, kinematics and kinetics of gait were measured. RESULTS: RPT resulted in improved peak torque production in the KE (28.9+/-4.4%) and PF (35.0+/-9.1%) muscle groups, as well as a decrease in T(20-80), an increased torque(220) and an increase ARTD in both muscle groups. In addition, an increase in self-selected (pre-RPT=0.77 m/s; post-RPT=1.03 m/s) and maximum (pre-RPT=1.08 m/s; post-RPT=1.47 m/s) gait speed was realized. Increased gait speeds were accompanied by bilateral increases in propulsion and hip excursion as well as increased lower extremity joint powers. CONCLUSIONS: The combination of lower extremity RPT can attenuate existing neuromuscular impairments and improve gait speed in persons after incomplete SCI.

Adolescent↗
PubMed2004

[High field MR imaging: magnetic field interactions of aneurysm clips, coronary artery stents and iliac artery stents with a 3.0 Tesla MR system].

PURPOSE: To evaluate magnetic field interactions of commonly used biomedical implants at 3.0 Tesla. MATERIALS AND METHODS: Fourteen aneurysm clips designed for permanent placement in intracranial aneurysms, 19 coronary artery stents and 20 iliac artery stents were evaluated in an actively shielded compact 3.0 T MR system (Intera, Philips Medical Systems, Best, The Netherlands, length of magnet 1.57 m). The magnetic deflection forces (translational movement) were evaluated as follows: The implants were suspended by a fine string and placed in the magnet bore at the location of the maximum magnetic field gradient. The translational forces F (z) were calculated from the measured angle of deflection from the vertical axis. The magnetic field-induced torque (rotational forces) was evaluated as follows: Each implant was placed in the center of the magnetic bore parallel to the static magnetic field B0 (position 0 degrees ). Any possible displacement of the implant was noted on a millimeter scale and any torque qualitatively evaluated using a 5 point grading scale (0: no torque; + 4: very strong torque). The implant was turned in steps of 45 degrees, and the procedure was repeated to encompass a full 360 degrees rotation. RESULTS: In 52 of the 53 devices tested, the deflection force (deflection angle: range 0-21 degrees, translational force: range 0-3.8 mN) was less than the gravitational force (i.e., the implant's weight). These devices (n = 52/53) did not show any alignment to or rotation in the magnetic field at any of the various 45 degrees -increment positions corresponding to a qualitative torque evaluation of grade 0/4. One device (n = 1/53), an iliac artery stent made of stainless steel (Zenith, Cook, Mönchengladbach, BRD), was found to have deflection forces (deflection angle 88 degrees translational force 299 mN) greatly exceeding the gravitational force as well as a pronounced torque (grade 4/4). CONCLUSION: Out of 53 biomedical implants evaluated for magnetic field interactions at 3.0 T, one iliac artery stent made of stainless steel was found to be potentially unsafe based on ASTM criteria. MR imaging at 3.0 Tesla may be performed safely in patients with any of the other 52 different implants evaluated in this study with respect to magnetic field translational attraction and torque.

Alloys↗
PubMed1994

Adaptive Dynamics of the Leg Movement Patterns of Human Infants: II. Treadmill stepping in Infants and Adults.

Infant treadmill steps have many temporal and kinematic similarities to adult walking. Kinematic similarities can result from different patterns of underlying torque, however. In this study, we used inverse dynamics to compare the patterns and contributions of active (muscle) and passive (gravity and motion-dependent) torques in the swing phase of treadmill stepping in 7-month-old infants and adults. Results indicated that adults consistently used muscle torque to initiate and terminate swing, but that passive torques accounted for leg motion during most of the swing phase. Infants, in contrast, displayed multiple patterns of torque contributions during swing. In the most frequently occurring infant pattern, muscle torque remained flexor throughout swing and joint reversals were due to the dominant passive gravitational torque. The kinetic data suggest that the temporally and kinematically similar treadmill steps produced by adults and infants do not emanate from a unique set of neural commands to the muscles, but from a flexible interplay between multiple internal as well as external elements. These data suggest that the intrinsic dynamics of the human system provide a medium out of which, given a supportive context, stable patterns can emerge spontaneously. During development, voluntary controlled movement patterns must build on these intrinsic dynamics.

Journal Article↗