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PubMed2003

Functional significance of stiffness in adaptation of multijoint arm movements to stable and unstable dynamics.

This study compared the mechanisms of adaptation to stable and unstable dynamics from the perspective of changes in joint mechanics. Subjects were instructed to make point to point movements in force fields generated by a robotic manipulandum which interacted with the arm in either a stable or an unstable manner. After subjects adjusted to the initial disturbing effects of the force fields they were able to produce normal straight movements to the target. In the case of the stable interaction, subjects modified the joint torques in order to appropriately compensate for the force field. No change in joint torque or endpoint force was required or observed in the case of the unstable interaction. After adaptation, the endpoint stiffness of the arm was measured by applying displacements to the hand in eight different directions midway through the movements. This was compared to the stiffness measured similarly during movements in a null force field. After adaptation, the endpoint stiffness under both the stable and unstable dynamics was modified relative to the null field. Adaptation to unstable dynamics was achieved by selective modification of endpoint stiffness in the direction of the instability. To investigate whether the change in endpoint stiffness could be accounted for by change in joint torque or endpoint force, we estimated the change in stiffness on each trial based on the change in joint torque relative to the null field. For stable dynamics the change in endpoint stiffness was accurately predicted. However, for unstable dynamics the change in endpoint stiffness could not be reproduced. In fact, the predicted endpoint stiffness was similar to that in the null force field. Thus, the change in endpoint stiffness seen after adaptation to stable dynamics was directly related to changes in net joint torque necessary to compensate for the dynamics in contrast to adaptation to unstable dynamics, where a selective change in endpoint stiffness occurred without any modification of net joint torque.

Adaptation, Physiological↗

PubMed2005

Maintaining rotational equilibrium during object manipulation: linear behavior of a highly non-linear system.

We address issues of simultaneous control of the grasping force and the total moment of forces applied to a handheld object during its manipulation. Six young healthy male subjects grasped an instrumented handle and performed its cyclic motion in the vertical direction. The handle allowed for setting different clockwise (negative) or counterclockwise torques. Three movement frequencies: 1, 1.5 and 2 Hz, and five different torques: -1/3, -1/6, 0, 1/6 and 1/3 Nm, were used. The rotational equilibrium was maintained by two means: (1) Concerted changes of the moments produced by the normal and tangential forces, specifically anti-phase changes of the moments during the tasks with zero external torque and in-phase changes during the non-zero-torque tasks, and (2) Redistribution of the normal forces among individual fingers such that the agonist fingers--the fingers that resist external torque--increased the force in phase with the acceleration, while the forces of the antagonist fingers--those that assist the external torque--especially, the fingers with the large moment arms, the index and little fingers, stayed unchanged. The observed effects agree with the principle of superposition--according to which some complex actions, for example, prehension, can be decomposed into elemental actions controlled independently--and the mechanical advantage hypothesis according to which in moment production the fingers are activated in proportion to their moment arms with respect to the axis of rotation. We would like to emphasize the linearity of the observed relations, which was not prescribed by the task mechanics and seems to be produced by specific neural control mechanisms.

Adult↗

PubMed1997

Standing strength training of the ankle plantar and dorsiflexors in older women, using concentric and eccentric contractions.

Many studies have reported strength gains in older adults following high-intensity resistance training. However, the muscle contraction types examined have been primarily isometric (static) or concentric (CONC; shortening). Less is known about how eccentric (ECC) strength in older adults responds to training or about the efficacy of ECC contractions as training stimuli in these subjects, even though muscle contractions of this type are performed in most training regimens and daily physical activities. In this study, 15 physically active, healthy older women [68 (5) years; mean (SD)] completed an 8-week resistance training program of two sessions per week. Training consisted of three sets of eight repetitions of CONC ankle plantar flexion (PF) and ECC dorsiflexion (DF), at greater than 80% of the initial peak torque, in a standing position only. Subjects were tested in standing and supine positions for: (1) strength over a range of 10 degrees DF to 20 degrees PF for both CONC and ECC; DF and PF (2) passive resistive torque of the plantar flexors at 6 degrees/s; and (3) DF and PF rate of torque development. All strength testing and training was done at 30 degrees/s. Significant increases (P < 0.01) were found for both CONC DF (increases 30%) and ECC DF (increases 17%) peak torque in the standing position. No significant changes occurred for DF strength as measured with the subjects in the supine position, PF strength in either position, passive resistive torque, or rate of torque development. In summary, strength gains occurred only in the dorsiflexors, which were trained using ECC contractions. Improvements in DF strength were specific to the position of training, which has implications for the transferability of strength gains to functional tasks such as maintaining gait.

Aged↗

PubMed1986

Effect of muscular activity on valgus/varus laxity and stiffness of the knee.

Quantitative changes in valgus/varus knee stability with different levels of muscular activity were determined for five subjects. A specially designed machine was used to measure resistance to angulation in the frontal plane. This device held the thigh stationary, the knee straight, an cycled the leg from side to side at a constant rate between present moment limits. Resistance to this forced valgus/varus motion was measured simultaneously with torque about the knee in the sagittal plane. Muscle activity was monitored by electromyography (EMG). Direct comparison of moment-rotation characteristics allowed changes in stability to be quantified as a function of extension and flexion torque. Extension torques less than 20% of the maximum increased varus stability more than valgus stability. Flexion torques of the same relative magnitude increased valgus stability more than varus stability. Comparison with the literature suggested that prevention of opening of the lateral side of the joint under varus loading was responsible for increased varus stability with increasing torque, both with extension and flexion torques.

Electromyography↗

PubMed1996

Non-invasive measurement of the input-output properties of peripheral nerve stimulating electrodes.

A non-invasive method was developed to determine the input-output (I/O) properties of peripheral nerve stimulating electrodes. An apparatus was fabricated to measure the 3-dimensional (3-D) isometric torque generated at the cat ankle joint by electrical activation of the sciatic nerve. The performance of the apparatus was quantified, and the utility of the method was demonstrated by measuring the recruitment properties of multiple contact nerve cuff electrodes. Torque-twitch waveforms, recruitment curves of peak torque as a function of stimulus current amplitude, and 2-D joint torque vectors were used to analyze the recruitment properties of the cuff. The peak of the twitch torque was an accurate measure of excitation even for muscles having fibers with varying speeds of contraction. The evoked twitch waveforms and torque vectors generated by selective stimulation of individual nerve branches with a hook electrode were compared to those produced by stimulation of the nerve trunk with the cuff electrode. These data allowed determination of the regions of the nerve trunk that were activated by different electrode geometries and stimulus parameters. The positional stability of electrode recruitment properties could be quantified by measuring I/O characteristics at different limb positions. The methods described are useful for characterization of neural stimulating electrodes and for studies of motor system physiology.

Animals↗

PubMed2004

Changes in mechanical properties of human plantar flexor muscles in ageing.

Changes in contractile and elastic properties of human plantar flexor muscles in ageing, were investigated in 12 young (19-24 years, YG) and 11 old (61-74 year, OG) men. Maximal isometric and concentric voluntary torques, at several angular velocities, were measured to construct torque-angular velocity relationship. This led to the calculation of an index of maximal shorting velocity (VImax) at low torque. Two methods were then used to calculate musculotendinous (MT, quick-release movements) and musculoarticular (MA, sinusoidal perturbations) stiffness. In both cases, stiffness was linearly related to torque, leading to the calculation of a stiffness index (SI) as the slope of the stiffness-torque relationship: SI(MT) and SI(MA), respectively. MA stiffness under passive conditions (Kp) was also determined. Surface electromyograms were useful to control agonist and antagonist myoelectrical activities. As expected, maximal isometric (P<0.005) and concentric torques (P<0.05) as well as VImax(p<0.05) were lower in OG compared to YG. SI(MT) values were higher for OG compared to YG (P<0.05) leading to a mean difference of 55%, whereas SI(MA) and Kp were not significantly different between the two groups. Thus, older men were weaker and exhibited higher SI(MT) values. These impairments seem to be principally due to muscular atrophy and modifications in both muscle fibre-type distribution and fibre composition, in ageing. Invariance of SI(MA) and Kp would suggest an adaptive mechanism in articular structures to avoid the continuous integration of the ankle joint stiffness by the central nervous system, what may simplify most daily motor tasks.

Adult↗

PubMed2005

Is the force-length relationship a useful indicator of contractile element damage following eccentric exercise?

Eccentric exercise has been shown to have a measurable effect on the force-length relationship (FLR), as peak force is shifted to longer muscle lengths following exercise. Recently, this shift in the FLR has been proposed as a "simple, reliable indicator" for assessing contractile element damage following eccentric exercise. However, eccentric exercise causes fatigue and damage, and there is evidence that fatigue alone may also cause a shift in the FLR. The purpose of this paper was to assess the role of fatigue on the FLR (as measured by a torque-joint angle relationship) following isometric and eccentric exercise in the New Zealand white (NZW) rabbit. Six NZW rabbits were divided into two groups for eccentric or isometric contractions of the hindlimb dorsiflexor muscles. Pre- and post-exercise torque-joint angle relationships were measured, and the shift from the pre- to the post-exercise relationship was measured as the change in joint angle at which peak torque was produced. Eccentric exercise resulted in a rightward shift of seven degrees; isometric exercise, which is thought to not cause damage, resulted in a shift of four degrees. Furthermore, torque production was reduced to a greater extent at short compared to long muscle lengths for the eccentric and isometric exercise, resulting in a post-exercise torque-joint angle relationship that was altered in shape. We conclude from these results, that the shift in peak torque may not be a simple and reliable indicator of muscle damage, but is caused by a combination of damage and post-exercise fatigue.

Adaptation, Physiological↗

PubMed1996

Mechanical and physical responses to stretching with and without preisometric contraction in human skeletal muscle.

OBJECTIVE: To examine electromyography (EMG) activity, passive torque, and stretch perception during static stretch and contract-relax stretch. DESIGN: Two separate randomized crossover protocols: (1) a constant angle protocol on the right side, and (2) a variable angle protocol on the left side. SUBJECTS: 10 male volunteers. INTERVENTION: Stretch-induced mechanical response in the hamstring muscles during passive knee extension was measured as knee flexion torque (Nm) while hamstring surface EMG was measured. Final position was determined by extending the knee to an angle that provoked a sensation similar to a stretch maneuver. Constant angle stretch: The knee was extended to 10 degree below final position, held 10sec, then extended to the final position and held for 80 sec. Variable angle stretch: The knee was extended from the starting position to 10 degrees below the final position, held 10sec, then extended to the onset of pain. Subjects produced a 6-sec isometric contraction with the hamstring muscles 10 degrees below the final position in the contract-relax stretch, but not in the static stretch. MAIN OUTCOME MEASURES: Passive torque, joint range of motion, velocity, and hamstring EMG were continuously recorded. RESULTS: Constant angle contract-relax and static stretch did not differ in passive torque or EMG response. In the final position, passive torque declined 18% to 21% in both contract-relax and static stretch (p<.001), while EMG activity was unchanged. In the variable angle protocol, maximal joint angle and corresponding passive torque were significantly greater in contract-relax compared with static stretch(p<.01), while EMG did not differ. CONCLUSION: At a constant angle the viscoelastic and EMG response was unaffected by the isometric contraction. The variable angle protocol demonstrated that PNF stretching altered stretch perception.

Adult↗

PubMed1999

Evaluation of the cushion setting on performance of a Biodex II dynamometer.

OBJECTIVES: To compare the effects of the soft cushion setting with those of the hard cushion setting of a Biodex II isokinetic dynamometer on peak velocity and peak torque with six loads at 23 preset speeds. DESIGN: Mixed within- and between-groups, repeated-measures design. SETTING: Research laboratory of a physical therapy education program. INTERVENTIONS: Each load was strapped onto the right knee attachment of the dynamometer and was dropped through a 100 degrees arc of motion five times at each preset speed using the soft and hard cushion settings. MAIN OUTCOME MEASURES: Peak velocity, peak torque, time to peak velocity, time to peak torque. RESULTS: Peak velocity was significantly influenced by the cushion setting, the weight dropped, and the preset speed. Maximal peak velocity achieved was 400 degrees/sec. Peak torque was influenced significantly by the cushion setting, the load, and the preset speed. Maximal peak torque measured was 34.99 kg-m. CONCLUSION: The rotating shaft of Biodex II dynamometer moved at a slower speed through the entire arc of motion at the soft cushion setting and often failed to achieve a constant, preset speed. Peak torque measured using the soft cushion setting was more variable and therefore questionable.

Equipment Design↗

PubMed2003

[Relationship between gait speed and strength parameters in multiple sclerosis].

INTRODUCTION: Recent studies have focused on correlation between strength and gait parameters in hemiplegia, suggesting the interest for strength training in patients with central nervous system lesions. The aim of this study was to evaluate this correlation in multiple sclerosis (MS) patients with special regard to the different clinical forms including proprioceptive loss or cerebellar ataxia. PATIENTS AND METHOD: Gait speed and muscular function were performed in 27 patients with moderate affected gait (EDDS < 6). Gait speed was evaluated with Locometre and peak-torques of quadriceps and hamstrings were evaluated with isokinetic dynamometer. Patients were separated in three groups related to their deficiency: spastic group (8 patients), spastic with proprioceptive loss (12 patients) and spastic with cerebellar ataxia (7 patients). Gait parameters were evaluated in 10 healthy subjects as control group. RESULTS: Gait speeds (spontaneous and maximal) and peak torques of quadriceps and hamstring were similar in the three groups. In the whole patients group, gait speed was reduced and related to hamstring peak torque (r = 0.56 at spontaneous speed and 0.51 at high speed) but not with quadriceps peak torque. Patients with proprioceptive loss exhibited not only a higher correlation between gait speed and hamstring torque (r = 0.76 and 0.65 respectively) than other patients but also with quadriceps torque (r = 0.66 and 0.59 respectively) when patients in other groups did not. CONCLUSION: As it was previously pointed out in hemiplegic patients, MS patients exhibit some correlation between gait speed and muscle strength, mainly with hamstrings. These correlations can change in special sensory conditions suggesting that patients with sensory loss use different muscular strategies to maintain gait speed. Strength training may therefore be discussed in MS including specific modalities as a function of clinical parameters.

Biomechanical Phenomena↗

PubMed1999

Comparison of the effects of viscoelastic agents on clinical properties of the Unfolder lens injection system.

PURPOSE: To evaluate the effect of 7 viscoelastic materials on the physical properties of the Unfolder lens injection system. SETTING: John A. Moran Eye Center, University of Utah School of Medicine, Salt Lake City, Utah, USA. METHODS: New sterile SI-40NB intraocular lenses (IOLs) were loaded into the Unfolder (AMO PhacoFlex II SofTip insertion system (Allergan) using 7 viscoelastic materials: sodium hyaluronate 1.0% (Healon, Provisc), sodium hyaluronate 1.4% (Healon GV), sodium hyaluronate 1.6% (Amvisc Plus), hydroxypropyl methylcellulose 2.0% (Occucoat), sodium chondroitin 4.0%--sodium hyaluronate 3.0% (Viscoat), and hyaluronate 3.0% (Vitrax). The IOLs were then injected after 10 or 120 seconds in the chamber (chamber dwell time) and 10, 60, or 180 seconds in the barrel of the Unfolder cartridge (barrel dwell time). Torque values (g/cm) required to extrude the lenses were measured with a torque gauge, and the cartridges and IOLs were inspected for damage. RESULTS: Longer chamber dwell times resulted in lower torque values to extrude the IOLs (P < .001). Conversely, longer barrel dwell times resulted in higher torque requirements for IOL extrusion (P < .001). Furthermore, rates of adverse events, such as nondelivery, IOL damage, or Unfolder cartridge damage, were higher with shorter chamber dwell times and longer barrel dwell times (P < .001). However, IOL damage was avoided when the manufacturer's recommended chamber and barrel times were used. CONCLUSION: Increasing chamber dwell time and decreasing barrel dwell time resulted in lower torque values to extrude IOLs from the Unfolder and increased successful use. Moreover, dispersive viscoelastic agents tended to be more forgiving of changing dwell times and therefore had lower torque values and adverse event rates overall.

Chondroitin↗

PubMed2001

Influence of contraction force and speed on muscle fiber conduction velocity during dynamic voluntary exercise.

Before using electromyographic (EMG) variables such as muscle fiber conduction velocity (MFCV) and the mean or median frequency (MDF) of an EMG power spectrum as indicators of muscular fatigue during dynamic exercises, it is necessary to determine the influence of a joint angle, contraction force and contraction speed on the EMG variables. If these factors affect the EMG variables, their influence must be removed or compensated for before discussing fatigue. The vastus lateralis of eight normal healthy male adults was studied. EMG signals during non-fatiguing dynamic knee extension exercises were detected with a three-bar active surface electrode array. EMG variables were calculated from the detected signals and compared with the angle of the knee joint, the extension torque and the extension speed. The extension torque was set at four levels with 10% intervals between 40 and 70% of the maximum voluntary contraction. The extension speed was set at five levels with 60 degrees /s intervals between 0 and 240 degrees /s. Because the joint angle unsystematically affected the MFCV, EMG variables at a given joint angle were extracted for comparison. The influence of the extension torque and speed on the extracted EMG variables was clarified with an ANOVA and a regression analysis. The statistical analyses showed that MFCV increased with the extension torque but did not depend on the extension speed. In contrast, MDF was independent of the extension torque but was dependent on the extension speed. MDF thus showed a behavior different from that of MFCV. It became clear that if MFCV is used as an indicator of muscular fatigue during dynamic exercises, it is at least necessary to extract MFCV at a predetermined joint angle and then remove the influence of extension torque on MFCV.

Action Potentials↗

PubMed2000

Time-course analysis of stretch reflexes in hemiparetic subjects using an on-line spasticity measurement system.

Spasticity after a stroke is usually assessed in a score form by subjectively determining the resistance of a joint to an externally imposed passive movement. This work presents a spasticity measurement system for on-line quantifying the stretch reflex of paretic limbs. Four different constant stretch velocities in a ramp-and-hold mode are used to elicit the stretch reflex of the elbow joint in spastic subjects. The subjects are tested at supine position with the upper limb stretched towards the ground, in contrast with the horizontally stretched movement used in other studies. By subtracting the baseline torque, reflex torque measured at a selected low stretch velocity of 5 deg/sec, the influence of gravity torque and inertial in vertical stretching mode can be minimized. The averaged speed-dependent reflex torque (ASRT), defined as the measured torque deviated from the baseline torque, is used for quantifying the spastic hypertonia. Four subjects having incurred cerebrovascular accident (CVA) are recruited for time-course study in which the measurements are taken at 72 hours, one week, one month, three months, and six months after onset of stroke. During the development of spasticity, the changes of ASRT and velocity sensitivity of ASRT of the involved and the intact elbow joints are discussed.

Elbow Joint↗

PubMed2003

Technique effects on upper limb loading in the tennis serve.

The purpose of this study was to compare the shoulder and elbow joint loads during the tennis serve. Two synchronised 200 Hz video cameras were used to record the service action of 20 male and female players at the Sydney 2000 Olympics. The displacement histories of 20 selected landmarks, were calculated using the direct linear transformation approach. Ball speed was recorded from the stadium radar gun. The Peak Motus system was used to smooth displacements, while a customised inverse-dynamics program was used to calculate 3D shoulder and elbow joint kinematics and kinetics. Male players, who recorded significantly higher service speeds (male = 183 km hr(-1): female = 149 km hr(-1)) recorded significantly higher normalised and absolute internal rotation shoulder torque at the position when the arm was maximally externally rotated (MER) (male = 4.6% and 64.9 Nm: female = 3.5% and 37.5 Nm). A higher absolute elbow varus torque (67.6 Nm) was also recorded at MER, when compared with the female players (41.3 Nm). Peak normalised horizontal adduction torque (male = 7.6%: female = 6.5%), normalised shoulder compressive force (male = 79.6%: female = 59.1%) and absolute compressive force (male = 608.3 N: female = 363.7 N), were higher for the male players. Players who flexed at the front knee by 7.6 degrees, in the backswing phase of the serve, recorded a similar speed (162 km hr(-1)), and an increased normalised internal rotation torque at MER (5.0%), when compared with those who flexed by 14.7 degrees. They also recorded a larger normalised varus torque at MER (5.3% v 3.9%) and peak value (6.3% v 5.2%). Players who recorded a larger knee flexion also recorded less normalised and absolute (4.3%, 55.6 Nm) peak internal rotation torque compared with those with less flexion (5.6%, 63.9 Nm). Those players who used an abbreviated backswing were able to serve with a similar speed and recorded similar kinetic values. Loading on the shoulder and elbow joints is higher for the male than female players, which is a reason for the significantly higher service speed by the males. The higher kinetic measures for the group with the lower knee flexion means that all players should be encouraged to flex their knees during the backswing phase of the service action. The type of backswing was shown to have minimal influence on service velocity or loading of the shoulder and elbow joints.

Acceleration↗

PubMed1999

Surface electrical stimulation of skeletal muscle after spinal cord injury.

STUDY DESIGN: Survey. OBJECTIVE: Examine muscle contractile activity during electrical stimulation (ES) after spinal cord injury (SCI). SETTING: General community of Athens, Georgia, USA. METHODS: Eight clinically complete SCI adults (C6 to T12) 4+/-1 (mean+/-SE) years post injury and eight able-bodied adults were studied. Surface ES was applied to the left m. quadriceps femoris for three sets of 10, 1 s isometric actions (50 Hz trains, 400 micros biphasic pulses, 50 micros phase delay, 1 s: 1 s duty cycle) with 90 s of rest between sets. Current was set to evoke isometric torque that was (1) sufficient to elicit knee extension with 2.3 kg attached to the ankle (low level ES), and (2) intended to equal 30% (mid level ES) or 60% of maximal voluntary torque of able-bodied adults (high level ES, able-bodied only). The absolute and relative cross-sectional area (CSA) of m. quadriceps femoris that was stimulated as reflected by contrast shift in magnetic resonance images and torque were measured. RESULTS: Six+/-2, 20+/-2 and 38+/-4% of the average CSA of m. quadriceps was stimulated during low, mid and high level ES, respectively, for able-bodied. Corresponding values for SCI for low and mid level ES were greater (61+/-12 and 92+/-7%, P = 0.0002). Torque was related to the CSA (cm2) of stimulated muscle (Nm = 3.53 x stimulated CSA+13, r2 = 0.68, P = 0.0010), thus ES of a greater per cent of m. quadriceps femoris in SCI was attributed to their smaller muscle (24+/-3 vs 73+/-5 cm2, P = 0.0001). The decline in torque ranged from 9+/-l to 15+/-4% within and over sets for low, mid or high level ES in able-bodied. SCI showed greater (P = 0.0001) fatigue (19+/-3 to 47+/-6%). CONCLUSION: The territory of muscle activation by surface electrical stimulation varies among SCI patients. Given sufficient current, a large portion of the muscle of interest can be stimulated. The resulting torque is modest, however, compared to that attainable in able-bodied individuals due to the small size and limited fatigue resistance of skeletal muscle years after spinal cord injury.

Adolescent↗

PubMed1986

Influence of subtypes of fast-twitch fibers on isokinetic strength in untrained men.

The purpose of the present study was to investigate the influence of the subtypes of fast-twitch (FT) fibers on the peak torque per unit cross-sectional area in working muscle. Muscle biopsies were taken from the right vastus lateralis of nine healthy untrained men. Using staining for myofibrillar ATPase after preincubation at pH 4.3 and 4.6, muscle fibers were classified as slow-twitch (ST), FTa, and FTb. Cross-sectional area of the leg extensor muscles was measured by ultrasonic method. Maximum voluntary strength for the leg extensor muscles was determined as peak torques with constant angular velocity using an isokinetic dynamometer. The peak torques at various angular velocities (30, 60, 120, and 180 degrees/s) indicated statistically significant positive correlations to the percent number and area of FTa fibers in m. vastus lateralis. Other fiber types, such as ST or FTb, however, were not correlated significantly to the peak torques at all angular velocities. A significant positive correlation of 0.766 (180 deg/s) was observed between peak torque per unit cross-sectional area of the leg extensor muscle and the percent number of FTa fibers. These results suggest that in untrained men, the percent number of FTa fibers plays a more important role in producing high peak torques than the percent number of FTb and ST and that even if muscle cross-sectional area is equal, a muscle with a high percent number of FTa fibers can exert higher isokinetic strength.

Adolescent↗

PubMed1982

Analysis of "invariant characteristics" in the motor control of down's syndrome and normal subjects.

Following Asatryan and Fei'dman (1965), two experiments were conducted to describe the so-called invariant mechanical properties underlying movement control in Down's syndrome and normal subjects. The invariant characteristic is a curve on a graph of joint torque versus joint angle. The task required subjects to maintain a steady joint angle against an external load (torque). Torque was systematically changed via partial unloading in order to obtain torque by length (joint angle) functions at three separate initial joint angles. Instructions required subjects "not to intervene" when unloading occurred in Experiment 1 and to "tense" their muscles prior to unloading in Experiment 2. Both normal and Down's syndrome groups revealed systematic torque by length functions that might be expected according to a simple mass-spring system model. Although the gross organization of movement in Down's syndrome subjects was nearly the same as normals, important differences between the two groups were found. Down's syndrome subjects revealed underdamped motions relative to normals (as shown by differences in the degree of oscillation about the final equilibrium position) and were less able to regulate stiffness (as shown by differences in slope of the torque by angle functions in Experiment 2). We promote the notion that damping and stiffness may be sensitive indices of hypotonia-the most common description of neuromuscular deficiency in Down's syndrome

Journal Article↗

PubMed1994

An evaluation of the length-tension relationship in elderly human ankle dorsiflexors.

Aging is thought to be associated with a decreased elasticity of skeletal muscle, which may be predicted to affect the optimal length at which peak tension is developed. This was assessed in the present study, in which we examined the effect of aging on the muscle length-tension relationship in the right ankle dorsiflexors of 60 subjects aged 20-40 years (M = 25.3; 15 males, 15 females) and 60-80 years (M = 68.8; 15 males, 15 females). Evoked contractile properties, 1-sec tetanic contractions (at 20, 50, and 80 Hz), and maximal voluntary contractions (MVC) were measured at 10 joint angles (15 degrees dorsiflexion to 30 degrees plantarflexion, in 5 degrees increments). Peak twitch torque occurred at the extreme of plantarflexion (30 degrees P) in both elderly and young adults, and although males had significantly greater twitch torques than females, there was no difference between the elderly and young adults. Maximum tetanic torque and MVC torque occurred at 30 degrees P and 20 degrees P, respectively, but in this case the young adults were significantly stronger than the elderly adults, and the males stronger than the females at all joint angles. There was no difference in the torque-angle relationship between elderly and young adults in any of the evoked or voluntary measures. At each of the three frequencies, the rise time of tetanic torque was also similar between elderly and young adults. These results suggest that any age-related change in the elastic properties of the ankle dorsiflexors does not affect the length (as inferred by joint angle)-tension relationship in this muscle group.

Adult↗