The genetics of alcoholism: 1997

The relationship between trunk muscle morphology as measured on transverse magnetic resonance images and isokinetic lifting, psychophysical lifting, and static back muscle endurance testing was examined in 110 men, ages 35-67 years (mean, 48 years), who had been chosen based on their exposure to a wide variety of occupational and leisure-time physical activities. The computed T2-relaxation times and the T2-weighted and proton density-weighted signal intensities of the erector spinae, quadratus lumborum, and psoas major muscles had almost no association with any of the strength tests. The cross-sectional areas of the muscles had good correlations with isokinetic lifting strength (r = 0.46-0.53). They did not correlate well with psychophysical lifting and static back muscle endurance. Other characteristics or neurological or psychological factors may have more influence on those tests.     

The relationship between EMG activity and extensor moment generation in the erector spinae muscles during bending and lifting activities

The relationship between EMG activity and extensor moment generation in the erector spinae muscles was investigated under isometric and concentric conditions. The full-wave rectified and averaged EMG signal was recorded from skin-surface electrodes located over the belly of the erector spinae at the levels of T10 and L3, and compared with measurements of extensor moment. The effects of muscle length and contraction velocity were studied by measuring the overall curvature (theta) and rate of change of curvature (d theta/dt) of the lumbar spine in the sagittal plane, using the '3-Space Isotrak' system. Isometric contractions were investigated with the subjects pulling up on a load cell attached to the floor. Hand height was varied to produce different amounts of lumbar flexion, as indicated by changes in lumbar curvature. The extensor moment was found to be linearly related to EMG activity, and the 'gradient' and 'intercept' of the relationship were themselves dependent upon the lumbar curvature at the time of testing. Concentric contractions were investigated with the subjects extending from a seated toe-touching position, at various speeds, while the torque exerted on the arm of a Cybex dynamometer was continuously measured. Under these conditions the EMG signal (E) was higher than the isometric signal (E0) associated with the same torque. E and E0 were related as follows: E0 = E/(1 + A d theta/dt), where A = 0.0014 exp (0.045P) and P = percentage lumbar flexion. This equation was used to correct the EMG data for the effect of contraction velocity. The corrected data were then used, in conjunction with the results of the isometric calibrations, to calculate the extensor moment generated by the erector spinae muscles during bending and lifting activities. The extensor moment can itself be used to calculate the compressive force acting on the lumbar spine.     

Interrater reliability of six tests of trunk muscle function and endurance

Some studies have shown a relationship between trunk muscle strength and low back pain. Measures of trunk muscle strength and endurance, which are feasible in the clinical setting, are needed. The purpose of this study was to determine interrater reliability of six tests of abdominal and trunk extensor muscle strength and endurance. The tests included abdominal and extensor dynamic endurance, hand-held dynamometry of isometric flexion and extension, and abdominal and extensor static endurance. Thirty-nine healthy workers were recruited as subjects. Each was tested by three raters on 3 days within 1 week. Intraclass correlation coefficients (ICC) and the standard error of measurement (SEM) were calculated: abdominal dynamic endurance ICC = .89, SEM = 8 repetitions; extensor dynamic endurance ICC = .78, SEM = 9 repetitions; abdominal isometric force ICC = .25, SEM = 60 N; extensor isometric force ICC = .24, SEM = 68 N; abdominal static endurance ICC = .51, SEM = 35 seconds; extensor static endurance ICC = .59, SEM = 20 seconds. The dynamic endurance tests had acceptable interrater reliability. For the others, reliability was poor and the SEMs were large.     

Importance of the intersegmental trunk muscles for the stability of the lumbar spine. A biomechanical study in vitro

STUDY DESIGN: A biomechanical study was performed to determine the consequences of a simulation of muscle forces on the loads imposed on the functional spinal units. OBJECTIVES: No biomechanical study has investigated the effect of incorporation of agonist and antagonist muscle forces on the loading of functional spinal units. SUMMARY OF BACKGROUND DATA: Spinal disorders and low back pain are increasingly becoming a worldwide problem. Traditional conservative therapies are intended to strengthen the muscles of the trunk using a judicious regimen of physical exercises. METHODS: Eighteen whole, fresh-frozen human cadaveric lumbar spine specimens (L2-S2; average age, 53.4 years) were tested in a spine tester using pure flexion-extension, lateral bending, and axial moments. The effects of coactivation of psoas and multifidus muscles on L4-L5 mobility were simulated in vitro by applying two pairs of corresponding force vectors to L4. The segmental stability was defined by the correlation of an applied moment to the resultant deformation as shown in load-displacement curves, and the range of motion was defined as the angular deformation at maximum load. RESULTS: The coactivation of muscles was accompanied by a 20% decrease in the range of motion (i.e., a significant increase in stability) during lateral bending and axial moments. Application of flexion-extension moments and muscle coactivation resulted in a 13% increase in the sagittal range of motion. CONCLUSIONS: The action of the intersegmental agonist and antagonist muscles biomechanically increases the overall stiffness (stability) of the intervertebral joints in axial torque and lateral bending, whereas it may destabilize the segment in flexion.     

Effects of detraining subsequent to strength training on neuromuscular function in patients with inflammatory arthritis

The effects of detraining subsequent to strength training on neuromuscular function were examined in 39 recent-onset rheumatoid arthritis (RA) patients. Eighteen age- and sex-matched healthy people (H) served as controls. Patients were randomly allocated either to the experimental group (PE), who carried out progressive strength training for 6 months, or to the control group (PC), who maintained only their habitual physical activities. After 6 months, PE returned to their earlier physical activities and strength training was terminated. At baseline, the maximal strength of the trunk extensors (not significant), grip strength and maximal dynamic strength and the shape of the force-time curve of the knee extensors were lower in PE and PC (P < 0.05-0.001) than in H. Strength training in PE led to remarkable increases (P < 0.05-0.001) in the maximal strength of all muscle groups without changes in the shape of the force-time curve. The increases in muscle strength in PE obtained by strength training were lost to a great degree during the detraining period for the isometric trunk extension (P < 0.01) and flexion (P < 0.01) strength and for the dynamic knee extension strength (P < 0.05), but not for the grip strength. In PC, trunk extension and flexion strength decreased significantly throughout the study period. At the post-test, all the strength values in both patient groups were much lower than in H. RA is a chronic disease which seems to need continuous physical exercise with sufficient intensity to minimize/prevent the loss of muscle strength and functional capacity.     

Electromyographical study on surgeons in urology. I. Influence of the operating technique on muscular strain

An electromyographical field study was performed in the operating theatre on four surgeons during 15 urological operations. In the course of the operations two endoscopic techniques were applied alternately: (1) direct endoscopy, and (2) monitor endoscopy. During direct endoscopy the surgeon looks into the urethra and the bladder via an endoscope. In monitor endoscopy, by contrast, the operating area is observed via a video system consisting of a camera mounted on top of the endoscope and a monitor. During the operations surface electromyograms were derived from both trapezius muscles, the right deltoideus muscle and the left erector spinae muscle. An activity code describing the surgeons' activity was additionally recorded. Analysis of the activity recording reveals that the monitor endoscopic method is preferred in the first third of the operations, whereas preference is given to the direct method in the last third. The electromyographical measurements indicate that during monitor endoscopy the myoelectrical activity of both trapezius muscles and of the right deltoideus muscle is significantly reduced in comparison with the activity for the direct endoscopic technique. A significant influence of the surgical technique on the myoelectrical activity could not be established for the erector spinae muscle. The electromyographical findings reveal that the activity of the shoulder musculature required for the maintenance of the posture and the performance of the operation is significantly lower in monitor endoscopy than in direct endoscopy. Consequently, a clear reduction in muscular strain can be achieved during the performance of endoscopic operations in urology if a video system is employed.     

Maximal isometric force and muscle cross-sectional area of the forearm in fencers

The maximal isometric force (MIF) of a muscle is directly related to its cross-sectional area (CSA). Strength training produces an increase in muscular force while muscular hypertrophy becomes appreciable at a later time; in asymmetric sports, training causes significant increases in force and muscular mass of the dominant limb of the athlete. The aim of this study was to analyse the differences in muscular force and trophism between the dominant and non-dominant forearms in fencers and in controls. The data of 17 male distance runners (age 21.4 +/- 2.4 years, body mass 74.0 +/- 5.0 kg, height 180 +/- 6 cm) were compared with those of 58 male fencers (age 23.0 +/- 6.7 years, body mass 71.9 +/- 9.3 kg, height 178 +/- 7 cm) drawn from the ranking lists of the National Fencing Committee. They trained for a mean of 11.4 +/- 6.0 (range 2-36) years, commencing at 10.7 +/- 4.5 years of age. Cross-sectional area (muscle plus bone) was estimated in the dominant and non-dominant forearm using a simplified anthropometric method. Maximal isometric force was determined using a mechanical handgrip dynamometer. The differences in CSA and isometric force between the two limbs and between fencers and controls were tested using paired and unpaired Student's t-tests, respectively. Significant differences in CSA and maximal force were observed between the dominant and non-dominant forearm in fencers (both P < 0.001) and in controls (P < 0.005 and P < 0.001, respectively). The fencers showed a greater CSA (P < 0.001) and force (P < 0.001) in the dominant forearm compared with the control group. Furthermore, the differences between the dominant and non-dominant limb of the fencers were significantly greater than the differences between the dominant and non-dominant limb of the controls (P < 0.001 for CSA and P < 0.05 for force). No significant differences in stress ratio (force/CSA) were obtained in either group.(ABSTRACT TRUNCATED AT 250 WORDS)     

Torso muscle moment arms at intervertebral levels T10 through L5 from CT scans on eleven male and eight female subjects

Moment arms for eight pairs of torso muscles were estimated based on data obtained from 19 sets of computed tomography (CT) scans. Muscle centroid locations of the rectus abdominis, external oblique, internal oblique, transversus abdominis, latissimus dorsi, psoas, quadratus lumborum, and the erector spinae mass were identified and digitized relative to vertebral body centers, which were also determined from the scans of the eight females and eleven males. Muscle moment arms were then calculated as the distance between the muscle and vertebral body centroids. The centroids for each torso muscle were plotted at each intervertebral level from T10-11 to L4-L5. When these sections were ordered in a cephalocaudad manner, the centroid-line paths, essential to the determination of muscle force lines-of-action, could be traced. Finally, anthropometric variables such as height, weight, torso depth, breadth, and age were regressed against the moment arm group means to develop several prediction equations that would determine moment arm lengths based on these anthropometric variables.     

Long-term clinical outcome of extracorporeal shock wave lithotripsy monotherapy for staghorn calculi

The thickness (TBmt) and fiber pennation angle (TBpen) of triceps brachii as well as isokinetic force developed during elbow extension were measured in Olympic athletes to investigate the relationship between muscle fiber pennation and force generation capability. The subjects were male members of the 1996 Japanese Olympic team who competed in seven different events; 9 wrestlers, 16 soccer players, 11 sprinters, 5 judo athletes, 7 gymnasts, 9 rowers and 18 baseball players. The TBmt and TBpen, measured by a B-mode ultrasound, ranged between 29 mm and 50 mm and between 11 degrees and 30 degrees, respectively, and on average were larger in the judo athletes, wrestlers and gymnasts compared to the other groups. A significant correlation (r=0.580, p < 0.05) was found between TPpen and TBmt per unit of the upper arm length, and so the observed event-related differences in TBpen tended to reflect the differences in TBmt. The isokinetic forces relative to the cross-sectional area (CSA) estimated from TBmt, measured at two constant velocities of 1.05 rad/s (F1.05/CSA) and 3.14 rad/s (F3.14/ CSA), were negatively correlated to the CSA; r=-0.617 (p < 0.05) for F1.05/CSA and r=-0.635 (p < 0.05) for F3.14/CSA. In addition, low but significant negative correlations existed between TBpen and both F1.05/CSA (r=-0.365, p < 0.05) and F3.14/ CSA (r=-0.336, p <0.05). Even when the effect of TBpen was statistically normalized, the F1.05/CSA and F3.14/CSA were still negatively correlated to the CSA, r=-0.530 (p < 0.05) for F1.05/ CSA and r=-0.561 (p < 0.05) for F3.14/CSA. Therefore, at least in the Olympic athletes tested in this study, the magnitude of the pennation angles reflects muscle size, but it does not seem to be a factor that explains extensively the lower F/CSA in athletes with large muscle size.     

Inefficient muscular stabilization of the lumbar spine associated with low back pain. A motor control evaluation of transversus abdominis

STUDY DESIGN: The contribution of transversus abdominis to spinal stabilization was evaluated indirectly in people with and without low back pain using an experimental model identifying the coordination of trunk muscles in response to a disturbances to the spine produced by arm movement. OBJECTIVES: To evaluate the temporal sequence of trunk muscle activity associated with arm movement, and to determine if dysfunction of this parameter was present in patients with low back pain. SUMMARY OF BACKGROUND DATA: Few studies have evaluated the motor control of trunk muscles or the potential for dysfunction of this system in patients with low back pain. Evaluation of the response of trunk muscles to limb movement provides a suitable model to evaluate this system. Recent evidence indicates that this evaluation should include transversus abdominis. METHODS: While standing, 15 patients with low back pain and 15 matched control subjects performed rapid shoulder flexion, abduction, and extension in response to a visual stimulus. Electromyographic activity of the abdominal muscles, lumbar multifidus, and the surface electrodes. RESULTS: Movement in each direction resulted in contraction of trunk muscles before or shortly after the deltoid in control subjects. The transversus abdominis was invariably the first muscle active and was not influenced by movement direction, supporting the hypothesized role of this muscle in spinal stiffness generation. Contraction of transversus abdominis was significantly delayed in patients with low back pain with all movements. Isolated differences were noted in the other muscles. CONCLUSIONS: The delayed onset of contraction of transversus abdominis indicates a deficit of motor control and is hypothesized to result in inefficient muscular stabilization of the spine.     

Does acute exposure to the electromagnetic field emitted by a mobile phone influence visual evoked potentials? A pilot study

This study was undertaken to determine muscle strength of trunk flexion-extension in hemiplegic patients after stroke compared with that of normal controls. The design consisted of a nonrandomized control trial in a secondary care setting (a rehabilitation unit at a hospital facility). The subjects included 25 post-stroke male hemiplegic patients and 25 male healthy controls. The maximal peak torques of trunk flexion-extension at angular velocities of 0 degrees (isometric contraction), 60 degrees, 120 degrees, and 150 degrees/s were measured by using an isokinetic dynamometer (Cybex Trunk Extension-Flexion Unit, Cybex, Ronkonkoma, NY). Peak torque of trunk flexion and extension in hemiplegic patients was significantly smaller than that of healthy controls (P < 0.05), except isometric trunk flexion (P > 0.05). The weakness of trunk flexion-extension muscles in hemiplegic patients might be accounted for by the bilateral innervation from the motor cortex, the insufficient use of high threshold motor units, and disuse atrophy.     

EMG responses to maintain stance during multidirectional surface translations

To characterize muscle synergy organization underlying multidirectional control of stance posture, electromyographic activity was recorded from 11 lower limb and trunk muscles of 7 healthy subjects while they were subjected to horizontal surface translations in 12 different, randomly presented directions. The latency and amplitude of muscle responses were quantified for each perturbation direction. Tuning curves for each muscle were examined to relate the amplitude of the muscle response to the direction of surface translation. The latencies of responses for the shank and thigh muscles were constant, regardless of perturbation direction. In contrast, the latencies for another thigh [tensor fascia latae (TFL)] and two trunk muscles [rectus abdominis (RAB) and erector spinae (ESP)] were either early or late, depending on the perturbation direction. These three muscles with direction-specific latencies may play different roles in postural control as prime movers or as stabilizers for different translation directions, depending on the timing of recruitment. Most muscle tuning curves were within one quadrant, having one direction of maximal activity, generally in response to diagonal surface translations. Two trunk muscles (RAB and ESP) and two lower limb muscles (semimembranosus and peroneus longus) had bipolar tuning curves, with two different directions of maximal activity, suggesting that these muscle can play different roles as part of different synergies, depending on translation direction. Muscle tuning curves tended to group into one of three regions in response to 12 different directions of perturbations. Two muscles [rectus femoris (RFM) and TFL] were maximally active in response to lateral surface translations. The remaining muscles clustered into one of two diagonal regions. The diagonal regions corresponded to the two primary directions of active horizontal force vector responses. Two muscles (RFM and adductor longus) were maximally active orthogonal to their predicted direction of maximal activity based on anatomic orientation. Some of the muscles in each of the synergic regions were not anatomic synergists, suggesting a complex central organization for recruitment of muscles. The results suggest that neither a simple reflex mechanism nor a fixed muscle synergy organization is adequate to explain the muscle activation patterns observed in this postural control task. Our results are consistent with a centrally mediated pattern of muscle latencies combined with peripheral influence on muscle magnitude. We suggest that a flexible continuum of muscle synergies that are modifiable in a task-dependent manner be used for equilibrium control in stance.     

Relative cross-sectional areas of upper and lower extremity muscles and implications for force prediction

The purposes of this investigation were to examine the reduction in variability when muscle size is expressed relative to a functional group and its effect on improving muscle force prediction. Fourteen subjects performed maximum voluntary contractions (MVC) of elbow flexion and knee extension and had CT scans taken of the thigh and upper arm. The variation of relative cross-sectional area (RCSA) was significantly lower than CSA (p < 0.01). No significant differences were found between male and female RCSAs. A method of predicting individual muscle forces based on an MVC and RCSA is presented that reduced prediction errors to a third of those predicted on the basis of CSA and a specific tension or stress value of muscle.     

Trunk muscle activation and cocontraction while resisting applied moments in a twisted posture

Previous studies of twisting have revealed substantial cocontraction of agonist and antagonist muscles within the torso when torsional moments are generated. The objective of the current study was to quantify the activations and cocontraction of eight trunk muscles as subjects maintained an axially rotated trunk posture and resisted external applied bending moments. Ten subjects twisted their torsos 25 degrees to the right (clockwise) and resisted 20 and 40 Nm bending moments from 12 directions. The moment directions were in a transverse plane and labelled clockwise as viewed from above, ranging from 0 degrees (mid-saggital, anterior) to 330 degrees, in 30 degrees increments. RMS EMG amplitude data were collected using surface electrodes and normalized to maximal voluntary contractions. Significant changes were observed in the muscle responses due to the interaction of the moment direction and moment magnitude for six of the eight muscles tested. Comparison of the present data with that collected previously in neutral postures indicated: (1) a large increase in the activation levels of the right erector spinae and the left external oblique muscles; and (2) a counter-clockwise shift in the moment direction at which the peak activation of these same muscles occurs. Analysis of the relative activation levels (RALs), constructed from the NEMG data to quantify the cocontraction, indicated that the changes in cocontraction were more robust in response to changes in the bending moment's direction as opposed to changes in bending moment's magnitude.     

Co-contraction of lumbar muscles during the development of time-varying triaxial moments

The recruitment and co-contraction of lumbar muscles were investigated during the voluntary development of slowly and rapidly varying trunk flexion and extension, lateral bending, and axial twisting moments. Myoelectric signals were recorded from 14 lumbar muscles in nine young men during maximum voluntary exertions and cyclic isometric exertions. System identification techniques were used to calibrate dynamic models of the relationship between myoelectric signals and force. To assess co-contraction, the predicted muscle forces were subdivided into a task-moment set of muscle forces that minimally satisfied moment equilibrium and a co-contraction set of muscle forces that produced zero net moment. The sum of co-contraction muscle forces was used to quantify the degree of co-contraction present. Co-contraction was largely dependent on the direction of exertion and relatively less dependent on the subject or the rate of exertion. Co-contractions were estimated to contribute approximately 16-19% to the sum of muscle forces at a lumbar cross section during attempted extension of the trunk. Estimated co-contractions during attempted lateral bending and axial twisting were two to three times greater, which demonstrates that co-contraction is a major determinant of spinal loading in these tasks. This analysis suggests that substantial contractions of lumbar muscles, especially during asymmetric exertions, are used for reasons other than equilibrating moments at the L3-L4 level.     

Induction of sister chromatid exchange in the presence of gadolinium-DTPA and its reduction by dimethyl sulfoxide

In order to quantify muscle strength in recessive myotonia congenita (MC) the peak and average peak torques (PT and APT, respectively) of knee flexion and extension of 19 MC patients were measured at speeds of 60 degrees/s and 200 degrees/s. Muscle endurance was measured at a speed of 200 degrees/s. No differences were found between the patient and control groups for PT and APT values for flexion at the high speed, nor were there any differences between the patients and the controls for PT and APT measured at the low speed for knee flexion and extension or muscle endurance at the high speed. However, PT and APT values of the patients for extension at the high speed, 100 +/- 41 Nm (mean +/- SD) and 95 +/- 39, were significantly lower than those of the controls, 129 +/- 43 and 124 +/- 42, respectively. There was no correlation between muscle strength or endurance and disease severity. The muscle strength of the patient group was diminished (p < 0.0001) during the beginning of the measurement at the high speed. The results suggest that after myotonic inhibition subsides the muscle strength of MC patients ranges within normal limits except in rapid and powerful motor activities.     

Fatigue analysis of human reinnervated muscle after microsurgical nerve repair

The reinnervated elbow flexors, biceps, and brachialis muscles were compared with the elbow flexors on the healthy opposite side in terms of muscle strength and fatigue in 10 patients who sustained sequelae of a unilateral posttraumatic brachial plexus palsy. The patients had recovered an active elbow flexion against resistance after microsurgical nerve repair. The patients were reviewed with an average postoperative followup of 12 years (range, 7.5-16 years). Despite a statistically significant difference in maximum isometric force, this study showed that after peripheral nerve repair, a partially reinnervated muscle has the same characteristics of fatigue and endurance as a normally innervated muscle, if these muscles exert the same percentage of their own maximum force.     

Wrist action affects precision grip force

When moving objects with a precision grip, fingertip forces normal to the object surface (grip force) change in parallel with forces tangential to the object (load force). We investigated whether voluntary wrist actions can affect grip force independent of load force, because the extrinsic finger muscles cross the wrist. Grip force increased with wrist angular speed during wrist motion in the horizontal plane, and was much larger than the increased tangential load at the fingertips or the reaction forces from linear acceleration of the test object. During wrist flexion the index finger muscles in the hand and forearm increased myoelectric activity; during wrist extension this myoelectric activity increased little, or decreased for some subjects. The grip force maxima coincided with wrist acceleration maxima, and grip force remained elevated when subjects held the wrist in extreme flexion or extension. Likewise, during isometric wrist actions the grip force increased even though the fingertip loads remained constant. A grip force "pulse" developed that increased with wrist force rate, followed by a static grip force while the wrist force was sustained. Subjects could not suppress the grip force pulse when provided visual feedback of their grip force. We conclude that the extrinsic hand muscles can be recruited to assist the intended wrist action, yielding higher grip-load ratios than those employed with the wrist at rest. This added drive to hand muscles overcame any loss in muscle force while the extrinsic finger flexors shortened during wrist flexion motion. During wrist extension motion grip force increases apparently occurred from eccentric contraction of the extrinsic finger flexors. The coactivation of hand closing muscles with other wrist muscles also may result in part from a general motor facilitation, because grip force increased during isometric knee extension. However, these increases were related weakly to the knee force. The observed muscle coactivation, from all sources, may contribute to grasp stability. For example, when transporting grasped objects, upper limb accelerations simultaneously produce inertial torques at the wrist that must be resisted, and inertial loads at the fingertips from the object that must be offset by increased grip force. The muscle coactivation described here would cause similarly timed pulses in the wrist force and grip force. However, grip-load coupling from this mechanism would not contribute much to grasp stability when small wrist forces are required, such as for slow movements or when the object's total resistive load is small.     

Interactions between vestibular and proprioceptive inputs triggering and modulating human balance-correcting responses differ across muscles

Interactions between proprioceptive and vestibular inputs contributing to the generation of balance corrections may vary across muscles depending on the availability of sensory information at centres initiating and modulating muscle synergies, and the efficacy with which the muscle action can prevent a fall. Information which is not available from one sensory system may be obtained by switching to another. Alternatively, interactions between sensory systems and the muscle to which this interaction is targeted may be fixed during neural development and not switchable. To investigate these different concepts, balance corrections with three different sets of proprioceptive trigger signals were examined under eyes-open and eyes-closed conditions in the muscles of normal subjects and compared with those of subjects with bilateral peripheral vestibular loss. The different sets of early proprioceptive inputs were obtained by employing three combinations of support surface rotation and translation, for which ankle inputs were nulled, normal or enhanced, the knees were either locked or in flexion, and the trunk was either in flexion or extension. Three types of proprioceptive and vestibulospinal interactions were identified in muscles responses. These interactions were typified by the responses of triceps surae, quadriceps, and paraspinal muscles. The amplitudes of stretch responses at 50 ms after the onset of ankle flexion in triceps surae muscles were related to the velocity of ankle stretch. The amplitude of balance-correcting responses at 100 ms corresponded more with stretch of the biarticular gastrocnemius when the knee was re-extended at 60 ms. Absent stretch reflexes at 50 ms in triceps surae with nulled ankle inputs caused a minor, 12-ms delay in the onset of balance-correcting responses in triceps surae muscles. Vestibular loss caused no change in the amplitude of balance-correcting responses, but a negligible decrease in onset latency in triceps surae even with nulled ankle inputs. Stretch responses in quadriceps at 80 ms increased with the velocity of knee flexion but were overall lower in amplitude in vestibular loss subjects. Balance-correcting responses in quadriceps had amplitudes which were related to the directions of initial trunk movements, were still present when knee inputs were negligible and were also altered after vestibular loss. Stretch and unloading responses in paraspinals at 80 ms were consistent with the direction of initial trunk flexion and extension. Subsequent balance-correcting responses in paraspinals were delayed 20 ms in onset and altered in amplitude by vestibular loss. The changes in the amplitudes of ankle (tibialis anterior), knee (quadriceps) and trunk (paraspinal) muscle responses with vestibular loss affected the amplitudes and timing of trunk angular velocities, requiring increased stabilizing tibialis anterior, paraspinal and trapezius responses post 240 ms as these subjects attempted to remain upright. The results suggest that trunk inputs provide an ideal candidate for triggering balance corrections as these would still be present when vestibular, ankle and knee inputs are absent. The disparity between the amplitudes of stretch reflex and automatic balance-correcting responses in triceps surae and the insignificant alteration in the timing of balance-correcting responses in these muscles with nulled ankle inputs indicates that ankle inputs do not trigger balance corrections. Furthermore, modulation of balance corrections normally performed by vestibular inputs in some but not all muscles is not achieved by switching to another sensory system on vestibular loss. We postulate that a confluence of trunk and upper-leg proprioceptive input establishes the basic timing of automatic, triggered balance corrections which is then preferentially weighted by vestibular modulation in muscles that prevent falling. (ABSTRACT TRUNCATED)     

A dynamic model for simulating movements of the elbow, forearm, an wrist

We developed a dynamic model of the upper extremity to simulate forearm and wrist movements. The model is based on the skeletal structure of the arm and is capable of elbow flexion/extension, forearm pronosupination, and wrist flexion/extension and radial/ulnar deviation movements. Movements are produced by activation of a Hill-type model of muscle, and limits on joint motion are imposed by passive moments modeled after experimental results. We investigated the muscle output force sensitivity, as well as wrist flexion/extension motion sensitivity to parameter variations. The tendon slack length and muscle fiber length were found to have the greatest influence on muscle output and flexion/extension wrist motion. The model captured the direction of the moment vectors at the wrist well, but predicted much higher moments than were measured by stimulating the paralyzed muscles of one tetraplegic subject.