Relationship between short latency stretch reflex and fascicle behavior in the soleus muscle in vivo

Objectives:Stretch reflex responses were considered to be affected by the velocity of muscle fiber lengthening and angular velocity. However, the results of previous studies in vivo and in vitro are inconsistent in this regard. The purpose of the present study was to investigate the effects of the velocity of fascicle lengthening on the amplitude of the stretch reflex for each trial with a high angular velocity and wide range of motion.Methods:Thirteen healthy men volunteered for this study. While the ankle was passively moved from 100 to 80 deg at five different angular velocities (100, 200, 300, 500, and 600 deg⋅s^-1), the velocity of fascicle lengthening in the soleus muscle was measured using ultrasonography. In addition, the amplitude of the short latency stretch reflex in the soleus muscle was also measured.Results:As angular velocity increased, the amplitude of the stretch reflex and velocity of fascicle lengthening significantly increased (both p<0.001). For each trial in all subjects, the amplitude of the stretch reflex was not correlated with the velocity of fascicle lengthening at any of the angular velocities.Conclusion:In conclusion, the stretch reflex size is not related to the fascicle behavior in each trial.     

Effect of hip joint angle on concentric knee extension torque

This study tested the hypothesis that the effect of hip joint angle on concentric knee extension torque depends on knee joint angle during a single knee extension task. Twelve men performed concentric knee extensions in fully extended and 80° flexed hip positions with maximal effort. The angular velocities were set at 30° s⁻¹ and 180° s⁻¹. The peak torque and torques attained at 30°, 50°, 70° and 90° (anatomical position = 0°) of the knee joint were compared between the two hip positions. Muscle activations of the vastus lateralis, medialis, rectus femoris and biceps femoris were determined using surface electromyography. The peak torque was significantly greater in the flexed than in the extended hip position irrespective of angular velocity. The torques at 70° and 90° of the knee joint at both angular velocities and at 50° at 180° s⁻¹ were significantly greater in the flexed than in the extended hip position, whereas corresponding differences were not found at 30° (at either angular velocity) and 50° (at 30° s⁻¹) of the knee joint. No effect of hip position on muscle activation was observed in any muscle. These results supported our hypothesis and may be related to the force–length and force–velocity characteristics of the rectus femoris.     

Muscle activation and the isokinetic torque-velocity relationship of the human triceps surae

The influence of muscle activation and the time allowed for torque generation on the angle-specific torque-velocity relationship of the triceps surae was studied during plantar flexion using supramaximal electrical stimulation and a release technique on six male subjects [mean (SD) age 25 (4) years]. Torque-velocity data were obtained under different levels of constant muscle activation by varying the stimulus frequency and the time allowed for isometric torque generation prior to release and isokinetic shortening. To eliminate the effects of the frequency response on absolute torque the isokinetic data were normalized to the maximum isometric torque values at 0.44 rad. There were no significant differences in the normalized torques generated at any angular velocity using stimulus frequencies of 20, 50 or 80 Hz. When the muscle was stimulated at 50 Hz the torques obtained after a 400 ms and 1 s pre-release isometric contraction did not differ significantly. However, with no pre-release contraction significantly less torque was generated at all angular velocities beyond 1.05 rad · s^–1 when compared with either the 200, 400 ms or 1 s condition. With a 200 ms pre-release contraction significantly less torque was generated at angular velocities beyond 1.05 rad · s^–1 when compared with the 400 ms or 1 s conditions. It would seem that the major factor governing the shape of the torque-velocity curve at a constant level of muscle activation is the time allowed for torque generation.     

Regulation of wrist stiffness by the stretch reflex

In restoring the angular position after a displacement, the role of the muscle stretch reflex was investigated by comparing the restored angular torques and angular positions in the wrist under ischaemic and non-ischaemic conditions in normal human subjects. The wrist compliance (COM), defined as the dynamic relation between the angular position and the angular torque of the joint, was calculated to quantify the changes in the restoration of a displacement after abolishing the stretch reflex by ischaemia. The elasticity from the COM-function was found to be single most important factor controlled by the stretch reflex. The elasticity that equals the static stiffness of the system increased by more than 100%, from 0.21 Nm degree-1 with abolished reflex to 0.45 Nm degree-1 with intact reflex. Our results have shown that the stretch reflex assists in the rapid return of the limb to its original position after a mechanical displacement. When the reflex was blocked by ischaemia, the perturbation displaced the limb further away from the initial position.     

THE MECHANISM OF DAYLENGTH PERCEPTION IN THE RED ALGA ACROSYMPHYTON PURPURIFERUM1

The red alga Acrosymphyton purpuriferum (J. Ag.) Sjöst. (Dumontiaceae) is a short day plant in the formation of its tetrasporangia. Tetrasporogenesis was not inhibited by 1 h night-breaks when given at any time during the long (16 h) dark period (tested at 2 h intervals). However, tetrasporogenesis was inhibited when short (8 h) main photoperiods were extended beyond the critical daylength with supplementary light periods (8 h) at an irradiance below photosynthetic compensation. The threshold irradiance for inhibition of tetrasporogenesis was far lower when supplementary light periods preceded the main photoperiod than when they followed it (<0.05 μmol·m⁻²·s⁻¹ vs. 3 μmol·m⁻²·s⁻¹). The threshold level also depended on the irradiance given during the main photoperiod and was higher after a main photoperiod in bright light than after one in dim light (threshold at 3 μmol·m⁻²·s⁻¹ after a main photoperiod at ca. 65 μmol·m⁻²·s⁻¹ vs. threshold at <0.5 μmol·m⁻²·s⁻¹ after a main photoperiod at ca. 35 μmol·m⁻²·s⁻¹). The spectral dependence of the response was investigated in day-extensions (supplementary light period (8 h) after main photoperiod (8 h) at 48 μmol·m⁻²·s⁻¹) with narrow band coloured light. Blue light (λ= 420 nm) was most effective, with 50% inhibition at a quantum-dose of 2.3 mmol·m⁻². However, yellow (λ= 563 nm) and red light (λ= 600 nm; λ= 670 nm) also caused some inhibition, with ca. 30% of the effectiveness of blue light. Only far-red light (λ= 710 nm; λ= 730 nm) was relatively ineffective with no significant inhibition of tetrasporogenesis at quantum-doses of up to 20 mmol·m⁻².     

An evaluation of different protocols for measuring the force-velocity relationship of the human quadriceps muscles

A modified Cybex II isokinetic dynamometer was used to evaluate the problems associated with measuring the concentric force-velocity characteristics of human knee extensor muscles. Three contraction protocols were investigated, simple voluntary contractions (VC); releases from maximal voluntary isometric contractions (VR) and releases from. isometric femoral nerve stimulated contractions (FNR). Percutaneous stimulation of the quadriceps was unsuitable for dynamic contractions as the proportion of the muscle activated varied with the angle of knee flexion. Isometric length-tension relationships and isokinetic contractions at seven angular velocities between 0.5 and 5.2 rad · s^–1 were recorded in five subjects. During isometric and slow dynamic contractions the voluntary forces were often greater than those obtained by femoral nerve stimulation, probably due to subjects stretching the rectus femoris during voluntary manoeuvres. It was found that the VC protocol produced acceptable isokinetic force recordings only at velocities below 3.1 rad · s^–1 in most subjects whilst VR contractions resulted in unexpectedly low forces at velocities below 1.57 rad · s^–1. Of the three techniques employed, FNR, although uncomfortable for subjects, provided the most accurate and reliable method of measuring force-velocity characteristics of knee extensor muscles. FNR contractions produced a force-velocity curve which showed a smooth decline in force with increasing velocity up to 5.2 rad · s^–1. VC contractions appear to be an acceptable alternative for testing the muscles provided the angular velocity is less than 3.1 rad · s^–1 and the subjects can be prevented from stretching the rectus femoris during the movement.     

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.     

Muscular torque generation during imposed joint rotation: torque-angle relationships when subjects' only goal is to make a constant effort

It is a reasonable expectation that voluntarily activated spinal motoneurons will be further excited by increases in spindle afferent activity produced by muscle stretch. Human motor behavior attributed to tonic stretch reflexes and to reflexes recruited by relatively slow joint rotation has been reported from several laboratories. We reinvestigated this issue by rotating the elbow joint over the central portion of its range while subjects focused on keeping their elbow flexion effort constant at one of three different levels and made no attempt to control the position, speed or direction of movement of their forearm. There is evidence that subjects' voluntary motor status is constant under these conditions so that any change in torque would be of involuntary origin. On average, torques rose somewhat and then fell as the elbow was flexed through a range of 80° at 10, 20 and 60°/s and a similar pattern occurred during elbow extension; i.e., both concentric and eccentric torque-angle profiles had roughly similar shapes and neither produced consistent stabilizing cross-range stiffness. The negative stiffness (rising torque) during the early part of a concentric movement and the negative stiffness (falling torque) during the later part of an eccentric movement would not have occurred if a stabilizing stretch reflex had been present. Positive stiffness rarely gave rise to torque changes greater than 20% in either individual or cross-subject averaged data. When angular regions of negative stiffness are combined with regions of low positive stiffness (torque change 10% or less), much of the range of motion was not well stabilized, especially during eccentric movements. The sum of the EMGs from biceps brachii, brachioradialis and brachialis showed a pattern opposite to that expected for a stretch reflex; there was an upward trend in the EMG as the elbow was flexed and a downward trend as the elbow was extended. There was little change in the shape of this EMG-angle relationship with either direction or velocity. The individual EMG-angle relationships were distinctive for each of these three elbow flexor muscles in four of the six subjects; in the remaining two, biceps was distinctive, but brachioradialis and brachialis appeared to be coupled. Although the EMGs of individual muscles were modulated over the angular range, no consistent stretch reflexes could be seen in the individual records. Thus, we could find no clear evidence for stretch reflex stabilization of human subjects maintaining a constant effort. Rather, muscle torque appears to be reflexly modulated across a much used portion of the elbow's angular range so that any appreciable stabilizing stiffness that is sustained for more than fractions of a second is associated with a change in effort.     

Muscular torque generation during imposed joint rotation: torque-angle relationships when subjects' only goal is to make a constant effort

It is a reasonable expectation that voluntarily activated spinal motoneurons will be further excited by increases in spindle afferent activity produced by muscle stretch. Human motor behavior attributed to tonic stretch reflexes and to reflexes recruited by relatively slow joint rotation has been reported from several laboratories. We reinvestigated this issue by rotating the elbow joint over the central portion of its range while subjects focused on keeping their elbow flexion effort constant at one of three different levels and made no attempt to control the position, speed or direction of movement of their forearm. There is evidence that subjects' voluntary motor status is constant under these conditions so that any change in torque would be of involuntary origin. On average, torques rose somewhat and then fell as the elbow was flexed through a range of 80 degrees at 10, 20 and 60 degrees/s and a similar pattern occurred during elbow extension; i.e., both concentric and eccentric torque-angle profiles had roughly similar shapes and neither produced consistent stabilizing cross-range stiffness. The negative stiffness (rising torque) during the early part of a concentric movement and the negative stiffness (falling torque) during the later part of an eccentric movement would not have occurred if a stabilizing stretch reflex had been present. Positive stiffness rarely gave rise to torque changes greater than 20% in either individual or cross-subject averaged data. When angular regions of negative stiffness are combined with regions of low positive stiffness (torque change 10% or less), much of the range of motion was not well stabilized, especially during eccentric movements. The sum of the EMGs from biceps brachii, brachioradialis and brachialis showed a pattern opposite to that expected for a stretch reflex; there was an upward trend in the EMG as the elbow was flexed and a downward trend as the elbow was extended. There was little change in the shape of this EMG-angle relationship with either direction or velocity. The individual EMG-angle relationships were distinctive for each of these three elbow flexor muscles in four of the six subjects; in the remaining two, biceps was distinctive, but brachioradialis and brachialis appeared to be coupled. Although the EMGs of individual muscles were modulated over the angular range, no consistent stretch reflexes could be seen in the individual records. Thus, we could find no clear evidence for stretch reflex stabilization of human subjects maintaining a constant effort. Rather, muscle torque appears to be reflexly modulated across a much used portion of the elbow's angular range so that any appreciable stabilizing stiffness that is sustained for more than fractions of a second is associated with a change in effort.     

Theoretical considerations for muscle-energy savings during distance running

We have recently demonstrated that the triceps surae muscles energy cost (EC_TS) represents a substantial portion of the total metabolic cost of running (E_run). Therefore, it seems relevant to evaluate the factors which dictate EC_TS, namely the amount and velocity of shortening, since it is likely these factors will dictate E_run. E_run and triceps surae morphological and AT mechanical properties were obtained in 46 trained and elite male and female distance runners using ultrasonography and dynamometry. EC_TS (J·stride⁻¹) at the speed of lactate threshold (sLT) was estimated from AT force and crossbridge mechanics and energetics. To estimate the relative impact of these factors on EC_TS, mean values for running speed, body mass, resting fascicle length (L_f), Achilles tendon stiffness and moment arm and maximum isometric plantarflexion torque were obtained. EC_TS was calculated across a range (mean ± 1 sd) of values for each independent factor. Average sLT was 233 m·min⁻¹. At this speed, EC_TS was 255 J·stride⁻¹. Estimated fascicle shortening velocity was 0.08 V_max and the level of muscle activation was 84.7% of maximum isometric torque. Compared to the EC_TS calculated from the lowest range of values obtained for each independent factor, higher AT stiffness was associated with a 39% reduction in EC_TS, 81% reduction in fascicle shortening velocity and a 31% reduction in muscle activation. Longer AT moment arms and elevated body masses were associated with an increase in EC_TS of 18% and 23%, respectively. These results demonstrate that a low EC_TS is achieved primarily from a high AT stiffness and low body mass, which is exemplified in elite distance runners.     

Induced swimming modified the antioxidant status of gilthead seabream (Sparus aurata)

Swimming has relevant physiological changes in farmed fish, although the potential link between swimming and oxidative stress remains poorly studied. We investigated the effects of different medium-term moderate swimming conditions for 6 h on the antioxidant status of gilthead seabream (Sparus aurata), analyzing the activity of enzymes related to oxidative stress in the liver and skeletal red and white muscle. Forty fish were induced to swim individually with the following conditions: steady low (SL, 0.8 body length (BL)·s⁻¹), steady high (SH, 2.3 BL·s⁻¹), oscillating low (OL, 0.2–0.8 BL·s⁻¹) and oscillating high (OH, 0.8–2.3 BL·s⁻¹) velocities, and a non-exercised group with minimal water flow (MF, < 0.1 BL·s⁻¹). All swimming conditions resulted in lower activities of superoxide dismutase (SOD), glutathione reductase (GR), and glutathione-S-transferase (GST) in the liver compared to the MF group, while steady swimming (SL and SH) led to higher reduced glutathione/oxidized glutathione ratio (GSH/GSSG) compared to the MF condition. Swimming also differently modulated the antioxidant enzyme activities in red and white muscles. The OH condition increased lipid peroxidation (LPO), catalase (CAT) and glutathione peroxidase (GPx) activities in the red muscle, decreasing the GSH/GSSG ratio, whereas the SL condition led to increased GSH. Oscillating swimming conditions (OL and OH) led to lower CAT activity in the white muscle, although GPx activity was increased. The GSH/GSSG ratio in white muscle was increased in all swimming conditions. Liver and skeletal muscle antioxidant status was modulated by exercise, highlighting the importance of adequate swimming conditions to minimize oxidative stress in gilthead seabream.     

Effect of creatine supplementation during rapid body mass reduction on metabolism and isokinetic muscle performance capacity

Well-trained subjects (n=6) were studied before and after losing a mean 3.0%–4.3% of body mass to determine whether muscle performance could be maintained or even enhanced by dietary creatine supplementation. During a 5-day period of loss of mass the subjects were randomly assigned to a creatine or placebo supplemented diet. All the subjects were measured before and after loss of mass on both supplements for isokinetic peak torque (PT) and work at peak torque (W _PT) of knee extensors, also for intermittent high intensity working capacity of the same muscle group. The latter test consisted of submaximal isokinetic knee extensions at an angular velocity of 1.57 rad · s⁻¹ for 45 s at the rate of 30 contractions each min (submaximal work, W _{s max} ) followed by 15-s maximal effort (maximal work, W _max ). Total duration of the test was 3 min. Haematocrit was measured and haemoglobin, ammonia, lactate, glucose and urea concentrations were analysed in blood samples obtained at rest and after cessation of muscle performance tests. The results indicated that creatine supplementation in comparison with placebo treatment during rapid body mass reduction may help to maintain muscle PT and W _PT at high angular velocities, not influencing W _max and the rate of fatigue development during W _max , but affecting adversely W _{s max} . Within the limitations of the present study the reasons for the partially detrimental effect of creatine administration remain obscure, but it is suggested that impaired creatine uptake in muscle during body mass loss as well as creatine induced changes in muscle glucose and glycogen metabolism may be involved. Accepted: 18 December 1997     

Human skeletal muscle fibre types and force: velocity properties

It has been reported that there is a relationship between power output and fibre type distribution in mixed muscle. The strength of this relationship is greater in the range of 3–8 rad · s^–1 during knee extension compared to slower or faster angular knee extensor speeds. A mathematical model of the force: velocity properties of muscle with various combinations of fast- and slow-twitch fibres may provide insight into why specific velocities may give better predictions of fibre type distribution. In this paper, a mathematical model of the force: velocity relationship for mixed muscle is presented. This model demonstrates that peak power and optimal velocity should be predictive of fibre distribution and that the greatest fibre type discrimination in human knee extensor muscles should occur with measurement of power output at an angular velocity just greater than 7 rad · s^–1. Measurements of torque: angular velocity relationships for knee extension on an isokinetic dynamometer and fibre type distribution in biopsies of vastus lateralis muscles were made on 31 subjects. Peak power and optimal velocity were determined in three ways: (1) direct measurement, (2) linear regression, and (3) fitting to the Hill equation. Estimation of peak power and optimal velocity using the Hill equation gave the best correlation with fibre type distribution (r > 0.5 for peak power or optimal velocity and percentage of fast-twitch fibres). The results of this study confirm that prediction of fibre type distribution is facilitated by measurement of peak power at optimal velocity and that fitting of the data to the Hill equation is a suitable method for evaluation of these parameters.     

Dynamic simulation and Doppler Ultrasonography validation of blood flow behavior in Abdominal Aortic Aneurysm

Criteria for rupture prediction of Abdominal Aortic Aneurysm (AAA) are based only on the diameter of AAA. This method does not consider complex hemodynamic forces exerted on AAA wall. The methodology used in our study combines Computer-Aided Design (CAD) with Computational Fluid Dynamics (CFD). Three-dimensional vascular structures reconstructions were based on Computed Tomography (CT) images and CAD. CFD theory was used for mathematical modeling and simulations. In this way, dynamic behavior of blood flow in bounded three-dimensional space was described. Doppler Ultrasonography (US) was used for model results validation. All simulations were based on medical investigation of 4 patients (male older than 65 years) with diagnosed AAA. Good correspondence between computed velocities in AAA and measured values with Doppler US (Patient 1 0.60 m·s⁻¹ versus 0.61 m·s⁻¹, Patient 2 0.80 m·s⁻¹ versus 0.80 m·s⁻¹, Patient 3 0.75 m·s⁻¹ versus 0.78 m·s⁻¹, Patient 4 0.50 m·s⁻¹ versus 0.49 m·s⁻¹) was noticed. The good agreement between measured and simulated velocities validates our methodology and the other data available from simulations (eg. von Misses stress) could be used to provide useful information about the possibility of AAA rupture.     

Roles of aorta,ostia and tracheae in heartbeat and respiratory gas exchange in pupae of Troides rhadamantus Staudinger 1888 and Ornithoptera priamus L. 1758 (Lepidoptera,Papilionidae)

In non-diapausing pupae of the two birdwing butterfly species Troides rhadamantus and Ornithoptera priamus (Lepidoptera, Papilionidae) heart activity and CO₂ release rates were measured simultaneously within the initial half of pupal development. Heartbeat patterns in these pupae consist of three different types of activity: Continuous forward-pulse periods of different duration with a frequency range of about 0.25–0.52 s⁻¹, continuous backward-pulse periods with lower frequencies (0.15–0.29 s⁻¹) and intermittent backward-pulse periods when short series of three to 10 single heartbeats at frequencies of 0.12–0.35 s⁻¹ alternated with heart pauses of 2–10 min. CO₂ release was discontinuous (CFO-type) from about four to 12 days after pupation in Troides rhadamantus and from about four to 18 days in Ornithoptera priamus. Mean CO₂ release rates were very low in both species (10–30 nmol g⁻¹ min⁻¹). After this period, heart pauses occurred more frequently, probably indicating the onset of metamorphosis and the beginning partial histolysis of the heart. Infrared-optical and thermometrical measurements of heartbeat indicated that haemolymph transport within the dorsal vessel in forward direction is more effective than in backward direction. This is deduced from the higher heartbeat frequency and heartbeat amplitude of the forward pulsations. Results from ultrasonic doppler velocimetry suggest that haemolymph flow velocity is highest during the relatively long diastasis of 2–3 s (30–40 mm s⁻¹), while minimum particle speed (about 20 mm s⁻¹) is at the end of systole and the beginning of diastole. This would mean that haemolymph velocity is highest between two consecutive peristaltic waves. In contrast to the haemolymph velocity, the speed of the peristaltic wave measured with the infrared transmission technique was lower (about 8.4–22 mm s⁻¹ in Troides, 10–23 mm s⁻¹ in Ornithoptera) and remained constant during forward pulse periods. During backward beating the speed was lower (8–20 mm s⁻¹ in Troides, 9–17 mm s⁻¹ in Ornithoptera) and decreased during backward pulse periods. During day two to seven in Troides and day three to nine in Ornithoptera, spiracular opening periods coincided with changes in heartbeat direction from backward to forward pulsations. A possible influence is the more efficient convective haemolymph mixing in the haemocoel during forward heartbeat. The mixing allows to bring the haemolymph in close contact with the tracheal system where the discharge of CO₂ takes place. Heartbeat may therefore serve for shortening the diffusion pathways for a rapid transition into the tracheal system during the open period of the spiracles.     

The effect of isokinetic dynamometer deceleration phase on maximum ankle joint range of motion and plantar flexor mechanical properties tested at different angular velocities

During range of motion (max-ROM) tests performed on an isokinetic dynamometer, the mechanical delay between the button press (by the participant to signal their max-ROM) and the stopping of joint rotation resulting from system inertia induces errors in both max-ROM and maximum passive joint moment. The present study aimed to quantify these errors by comparing data when max-ROM was obtained from the joint position data, as usual (max-ROM_POS), to data where max-ROM was defined as the first point of dynamometer arm deceleration (max-ROM_ACC). Fifteen participants performed isokinetic ankle joint max-ROM tests at 5, 30 and 60° s⁻¹. Max-ROM, peak passive joint moment, end-range musculo-articular (MAC) stiffness and area under the joint moment-position curve were calculated. Greater max-ROM was observed in max-ROM_POS than max-ROM_ACC (P < 0.01) at 5 (0.2 ± 0.15%), 30 (1.8 ± 1.0%) and 60° s⁻¹ (5.9 ± 2.3%), with the greatest error at the fastest velocity. Peak passive moment was greater and end-range MAC stiffness lower in max-ROM_POS than in max-ROM_ACC only at 60° s⁻¹ (P < 0.01), whilst greater elastic energy storage was found at all velocities. Max-ROM and peak passive moment are affected by the delay between button press and eventual stopping of joint rotation in an angular velocity-dependent manner. This affects other variables calculated from the data. When high data accuracy is required, especially at fast joint rotation velocities (≥30° s⁻¹), max-ROM (and associated measures calculated from joint moment data) should be taken at the point of first change in acceleration rather than at the dynamometer’s ultimate joint position.     

Maximal and submaximal isometric torque is elevated immediately following highly controlled active stretches of the hamstrings

Hamstring strain rehabilitation programs with an eccentric bias are effective but have a low adherence rate. Post-stretch isometric (PS-ISO) contractions which incorporate a highly controlled eccentric contraction followed by an isometric contraction resulting in elevated torque during following stretch, compared with isometric contractions at the same joint angle. This study measured torque, activation and musculotendinous unit behaviour of the hamstrings during PS-ISO contractions of maximal and submaximal levels using two stretch amplitudes. Ten male participants (24.6 years ± 2.22 years) completed maximal and submaximal baseline isometric contractions at 90°, 120° and 150° knee flexion and PS-ISO contractions of maximal and submaximal intensity initiated at 90° and 120° incorporating active stretch of 30° and 60° at 60°·s⁻¹. Torque and muscle activation of the knee flexors were simultaneously recorded. Musculotendinous unit behaviour of the biceps femoris long head was recorded via ultrasound during all PS-ISO contractions. Compared with baseline, torque was 8% and 39% greater in the maximal and submaximal PS-ISO conditions respectively with no change in muscle activation. The biceps femoris long head muscle lengthened during all PS-ISO contractions. PS-ISO contractions may be beneficial where the effects of highly controlled eccentric contractions and elevated isometric torque are desired, such as hamstring rehabilitation.     

Relationship between neuromuscular fatigue and spasticity in chronic stroke patients: A pilot study

IntroductionThe aim of this study was to assess the effects of neuromuscular fatigue on stretch reflex-related torque and electromyographic activity of spastic knee extensor muscles in hemiplegic patients. The second aim was to characterize the time course of quadriceps muscle fatigue during repetitive concentric contractions.MethodsEighteen patients performed passive, isometric and concentric isokinetic evaluations before and after a fatigue protocol using an isokinetic dynamometer. Voluntary strength and spasticity were evaluated following the simultaneous recording of torque and electromyographic activity of rectus femoris (RF), vastus lateralis (VL) and biceps femoris (BF).ResultsIsometric knee extension torque and the root mean square (RMS) value of VL decreased in the fatigued state. During the fatigue protocol, the normalized peak torque decreased whereas the RMS of RF and BF increased between the first five and last five contractions. There was a linear decrease in the neuromuscular efficiency-repetitions relationships for RF and VL. The peak resistive torque and the normalized RMS of RF and VL during passive stretching movements were not modified by the fatigue protocol for any stretch velocity.DiscussionThis study showed that localized quadriceps muscle fatigue caused a decrease in voluntary strength which did not modify spasticity intensity. Changes in the distribution of muscle fiber type, with a greater number of slow fibers on the paretic side, may explain why the stretch reflex was not affected by fatigue.     

Modeling of a simple motor task in man: intentional arrest of an ongoing movement

Normal subjects and cerebellar patients were instructed to arrest “as soon as possible” a ballistically initiated flexion movement of the forearm. The intentional actions consist essentially of a downward torque, the peak value of which has almost a constant latency (about 200 msec) from the beginning of the movement. A variable number of oscillations precede the arrest of the movement, the characteristics of which depend on the initial velocity of the flexion and on the mass with which the forearm is loaded. The motor commands responsible for the intentionally produced downward torque are controlled centrally as to leave the ratio between the peak values of the angular velocity which precede and follow the peak of the torque almost constant, under all conditions. To describe the oscillations a simple analytical model was proposed which includes the mechanical as well as the reflex factors, the latter under the form of a delayed velocity term. The satisfactory fitting of this model to the experimental findings permitted to establish the following points:

1. The oscillations are sustained by both a mechanical and a reflex stiffness. The contribution of the reflex loop is however quantitatively dominant since it accounts for about 75% of the inertial torque. It is fairly constant over the range of frequency of the oscillations considered.
2. Under the imposed experimental conditions angular velocity appears to be the parameter of the movement which is predominantly sensed and fed back by the reflex loop.

Data were also presented on the performance of the motor task by patients who underwent surgical ablations of the cerebellar cortex. Comparison of these results with those of normal subjects strongly supports the hypothesis that cerebellar-related activities are instrumental in determining the sensitivity of the stretch reflex to angular velocity.