Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “REFLEX”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,711 records · Page 95Linked to original sources
PubMedDate not supplied

Acoustic reflex averaging.

Signal averaging techniques have been applied to acoustic reflex measurement in order to meet the need for better temporal resolution and more accurate threshold delineation. We describe an approach to reflex measurement based on a signal averaging technique designed to examine both threshold and suprathreshold characteristics of the acoustic reflex. Results indicate that: (1) many supposed reflex threshold and latency aberrations are actually amplitude aberrations that are inappropriately classified because of instrumentation constraints; (2) reflex amplitude and waveform morphology can be recorded with appropriate fidelity using a signal averaging technique; (3) problems due to absolute amplitude variability can be minimized by using an index technique to assess amplitude relationships; (4) amplitude indices are sensitive indicators of neural pathology; (5) signal averaging and suprathreshold measurement of reflex amplitude and waveform morphology promise to enhance the sensitivity of acoustic reflex measurement.

Adult↗
PubMed2005

Reversed ipsilateral acoustic reflex: a study on subjects treated with muscle relaxants.

OBJECTIVES: To rule out any possible involvement of the middle ear muscles in the genesis of the reversed ipsilateral acoustic reflex (RIAR). DESIGN: Prospective study to evaluate the effects of muscle relaxants on the RIAR of otosclerotic ears as well as on the acoustic reflex of individuals with normal middle ear function. Admittance recording during ipsilateral acoustic stimulation was performed in patients undergoing pharmacological treatment for surgical procedures. Fentanyl, propofol, and a muscle relaxant were sequentially administered intravenously. Ipsilateral acoustic reflexes were recorded before and after each drug injection. Three patients were affected from otosclerosis, whereas 14 individuals had normal middle ear function. Moreover, the ipsilateral acoustic reflex obtained in normal subjects after their treatment with muscle relaxants was compared with that of 10 otosclerotic patients who were not treated pharmacologically. RESULTS: The RIAR of three otosclerotic ears was not inhibited by muscle relaxants as well as by fentanyl and propofol. Moreover, muscle relaxants, when administered in normal subjects, always induced the block of the stapedial reflex that was replaced by a reversed reflex strictly similar to the RIAR of the 10 otosclerotic patients not treated pharmacologically. Propofol could also induce, in most of the cases, the reduction and in some occasion even the reversal of the stapedial reflex, whereas fentanyl did not affect it significantly. CONCLUSION: The RIAR does not appear to be related to the contraction of the middle ear muscles.

Acoustic Stimulation↗
PubMed1995

Hemodynamic and autonomic reflex effects of chronic N-type Ca2+ channel blockade with omega-conotoxin GVIA in conscious normotensive and hypertensive rabbits.

The effects of chronic administration of omega-conotoxin GVIA (omega-CTX), an N-type Ca2+ channel blocker, on hemodynamics and autonomic reflexes were studied in conscious normotensive (sham) and hypertensive (wrap) New Zealand white rabbits. During surgery, a pulsed Doppler-flow probe was implanted around the lower abdominal aorta, and both kidneys were wrapped in cellophane (wrap) or left undisturbed (sham). Rabbits were studied 4 weeks later on 5 consecutive days. On days 1-4, hemodynamics, the baroreceptor-heart-rate (HR) reflex induced by drugs and the Bezold-Jarisch-like reflex evoked by serotonin, were measured before and 2 h after administration of omega-CTX (10 micrograms/kg i.v. bolus). On day 5, hemodynamics and reflexes were again assessed, but no further omega-CTX was given. On day 1, omega-CTX caused falls in mean arterial pressure (MAP) of 17 +/- 3 and 27 +/- 5 mm Hg in sham (n = 6) and wrap (n = 11) rabbits, respectively, with tachycardia. Hindquarter vascular conductance (HVC) increased > 35% in both groups. On days 2-4, omega-CTX still caused falls in MAP; however it was progressively less than on day 1. HR and HVC did not change. After omega-CTX on day 1, there was marked attenuation of the sympathetic components and decrease in the vagal components of the baroreceptor-HR reflex curves, with HR range decreasing by > 61% in sham and wrap rabbits. These curves remained similar over the next 4 days, and were unaffected by further omega-CTX. However, omega-CTX had no effect on the vagally mediated Bezold-Jarisch-like reflex. In auxiliary experiments in vitro, omega-CTX (1 nM) inhibited sympathetic, but not vagal, responses to electrical nerve stimulation of rabbit-isolated right atria. Thus, omega-CTX is a potent hypotensive agent in normotensive and hypertensive rabbits, predominantly via a peripheral sympatholytic action with no effect on vagal-cardiac efferent activity. However, it may affect the vagal component of the baroreceptor-HR reflex by an unknown central mechanism.

Animals↗
PubMed2000

Deterioration of the pharyngo-UES contractile reflex in the elderly.

OBJECTIVES/HYPOTHESIS: Deterioration of aerodigestive tract reflexes such as the esophagoglottal and pharyngoglottal closure reflexes and pharyngeal swallow has been documented in the elderly. However, the effect of aging on the contractile response of the upper esophageal sphincter (UES) to pharyngeal water stimulation has not been studied. The aim of this study was to characterize the pharyngo-UES reflex in the healthy elderly. METHODS: We studied nine healthy elderly (77 +/- 1 y [SD]; four men, five women) and nine healthy young volunteers (26 +/- 2 y [SD]; four men, five women). AUES sleeve sensor was used to measure the pressure. We tested pharyngeal stimulation induced by rapid pulse and slow continuous injection of water. RESULTS: The volume of water required to stimulate the pharyngo-UES contractile reflex by rapid pulse injection in the elderly (0.5 +/- 34 0.1 mL) was significantly higher than that in the young (0.1 +/- 0.02 mL) (P < .05). In contrast to young subjects, there was no pressure increase in resting UES pressure observed in the elderly for continuous pharyngeal water infusion. In both young and elderly, the threshold volume for the pharyngo-UES contractile reflex was significantly lower than that for pharyngeal swallows. CONCLUSIONS: The pharyngo-UES contractile reflex deteriorates with aging. This deterioration is primarily due to abnormalities of the afferent limb of the reflex.

Aged↗
PubMed1993

Intraoperative long-latency reflex activity in idiopathic scoliosis demonstrates abnormal central processing. A possible cause of idiopathic scoliosis.

Segmental reflex regulation in 37 patients with idiopathic scoliosis and 8 patients with nonidiopathic scoliosis was studied by recording evoked reflex muscle potentials from four muscle groups in each lower extremity during partial neuromuscular blockade. Effects on reflex activity mediated through descending systems arising in the brain stem were investigated by recording from proximal-distal and flexor-extensor muscles. Ipsilateral and contralateral long-latency complex polysynaptic activity was present in all 37 patients with idiopathic scoliosis. This reflex activity was absent in eight nonidiopathic scoliosis patients. Long-latency reflex activity may represent segmental disinhibition. The presence of long-latency reflex activity in patients with idiopathic scoliosis and the absence of this activity in nonidiopathic scoliosis patients with curves of equal magnitude demonstrates that the curve per se is not responsible for the activity. This would imply that abnormal reflex processing may play a role in the development of the spinal deformity in patients with idiopathic scoliosis.

Adolescent↗
PubMed2001

Correlation between a positive Hoffmann's reflex and cervical pathology in asymptomatic individuals.

STUDY DESIGN: Asymptomatic patients with a positive Hoffmann's reflex were prospectively studied with cervical radiographs and magnetic resonance imaging. OBJECTIVES: To determine a relationship between a positive Hoffmann's reflex and cervical pathology in asymptomatic patients and to evaluate if further work-up was necessary in this patient population. SUMMARY OF BACKGROUND DATA: A positive Hoffmann's reflex usually implies an upper motor neuron lesion from spinal cord compression. Although this reflex is commonly tested, the significance of this reflex in asymptomatic patients is not known. METHODS: Sixteen patients without cervical pain or radiculopathy and a positive Hoffmann's reflex were prospectively studied with cervical radiographs and magnetic resonance imaging. Positive findings were correlated with a detailed neurologic examination. RESULTS: All 16 patients were asymptomatic. Fourteen patients (87.5%) demonstrated spondylosis on cervical radiographs. The magnetic resonance imaging studies showed pathologic findings in all 16 patients. Fifteen patients (94%) had cervical involvement with cord compression from a herniated nucleus pulposus. The remaining patient had a T5-T6 thoracic disc with resultant compression. No treatment was instituted, and the clinical course of each patient was not affected. CONCLUSIONS: Although the presence of cervical cord impingement is extremely high in these patients, no treatment was rendered specifically to address the cervical pathology. Therefore, the presence of a positive Hoffmann's reflex in asymptomatic patients strongly suggests underlying cervical pathology, but it does not warrant further evaluation with either cervical radiographs or magnetic resonance imaging since the management and clinical course are not affected by positive studies.

Adult↗
PubMed2003

Childhood otitis media and electrically elicited stapedius reflexes in adult cochlear implantees.

BACKGROUND: Electric stapedius reflex thresholds are helpful in programming cochlear implants, but only approximately two-thirds of patients have identifiable reflexes. HYPOTHESIS: Childhood otitis media correlates with absent stapedius reflexes or with high electric stimulation needed to elicit a reflex in cochlear implantees. STUDY POPULATION: Twenty-five adults with acquired hearing loss who underwent implantation with the MED-EL COMBI 40+ standard electrode array. STUDY PARAMETERS: The extent of temporal bone pneumatization, an indicator of childhood otitis media, was measured from preoperative computed tomographic images. Clinical units (in microamperes) needed to elicit a contralateral stapedius reflex, or maximum stimulation tried, were recorded. RESULTS: No definite association of pneumatization volume with intensity of stimulation was observed. The null hypothesis of no association cannot be rejected. CONCLUSION: Childhood otitis media does not seem to explain absent electric stapedius reflexes and the wide range of clinical units needed for maximum comfortable loudness level. Remaining potential explanations probably include the wide range of cochlear neurons that can be electrically stimulated, and that the maximum tolerable stimulation is too low to elicit a stapedius reflex.

Adult↗
PubMed2005

Effect of noxious electrical stimulation of the peroneal nerve on stretch reflex activity of the hamstring muscle in rats: possible implications of neuronal mechanisms in the development of tight hamstrings in lumbar disc herniation.

STUDY DESIGN: The effect of noxious electrical stimulation of the peroneal nerve on the stretch reflex electromyogram activity of the hamstring muscle (semitendinous) was studied. OBJECTIVE: To verify the following hypothetical mechanisms underlying tight hamstrings in lumbar disc herniation: stretch reflex muscle activity of hamstrings is increased by painful inputs from an injured spinal nerve root and the increased stretch reflex muscle activity is maintained by central sensitization. SUMMARY OF BACKGROUND DATA: It is reported that stretch reflex activity of the trunk muscles is induced by noxious stimulation of the sciatic nerve and maintained by central sensitization. METHODS: In spinalized rats (transected spinal cord), the peroneal nerve was stimulated electrically as a conditioning stimulus. Stretch reflex electromyogram activity of the semitendinous muscle was recorded before and after the conditioning stimulus. RESULTS: Even after electrical stimulation was terminated, an increased stretch reflex activity of the hamstring muscle was observed. CONCLUSIONS: It is likely that a central sensitization mechanism at the spinal cord level was involved in the increased reflex activity. Central sensitization may play a part in the neuronal mechanisms of tight hamstrings in lumbar disc herniation.

Animals↗
PubMed2005

Reflex muscle contraction in the unaware occupant in whiplash injury.

STUDY DESIGN: Computer modeling and parametric analysis were used to determine the effect of reflex contraction of the neck muscles in the unaware occupant in whiplash. OBJECTIVE: To delineate effects of reflex contraction on spinal segmental kinematics during the retraction phase. SUMMARY OF BACKGROUND DATA: The ability of reflex neck muscle contraction to mitigate whiplash injury in the unaware occupant remains unclear. Analyzing relative timing between electromyographic and head-neck kinematics, previous investigators theorized that muscle contraction alters spinal kinematics, decreasing injury likelihood. Other investigators suggested that injury occurs during the initial (retraction) phase of head-neck kinematics, before significant muscle force generation. METHODS: Computer modeling was used to determine reflex contraction effects on segmental angulations, implementing parametric analysis techniques to vary reflex delay and impact severity. RESULTS: Shorter reflex delays had a greater effect on segmental angulations later in the event and at lower impact severities. However, the magnitude of this effect, particularly at higher impact severities and during maximum cervical S-curvature (factors implicated in the whiplash injury mechanism) was minimal, altering segmental angulations by a maximum of 19%. CONCLUSIONS: Because reflex contraction did not substantially alter spinal kinematics, muscle contraction likely does not initiate in sufficient time to mitigate whiplash injuries that may occur during the retraction phase.

Cervical Vertebrae↗
PubMed1997

Simultaneous and nonlinear identification of mechanical and reflex properties of human elbow joint muscles.

The naturally coexisting intrinsic mechanical and reflex properties of the human elbow joint were identified simultaneously using nonlinear, time-delay, continuous-time, and dynamic models. Angular random perturbations of small amplitude and low bandwidth were applied to the joint using a computer-controlled servomotor, while the subject maintained various levels of mean background muscle torque. Joint neuromuscular dynamics were identified from the measured elbow angle and torque. Stretch reflexes were modeled nonlinearly with both dynamic and static reflex gains. A continuous-time system identification method was developed to estimate parameters of the nonlinear models directly from sampled data while retaining realistic physical or physiological interpretations. Results from six subjects showed that dynamic stretch reflex gains, joint stiffness, and viscosity generally increased with mean background muscle torque; and that dynamic stretch reflex gain was higher during muscle stretch than that during muscle shortening. More importantly, the study provided realistic simultaneous estimates of the relative contributions of intrinsic mechanical and reflex actions to net joint torque. In particular, reflexively-mediated stiffness generated a significant portion of the total joint stiffness and the percentage varied systematically with background muscle torque.

Biomechanical Phenomena↗
PubMed2005

Reflex regulation of antagonist muscles for control of joint equilibrium position.

A systems model of spinal neuro-musculo-skeletal system (alpha - gamma model) is developed to investigate the plausible roles of spinal proprioceptive feedback in movement control. The model is composed of a joint, a pair of antagonist muscles, length and velocity feedback from muscle spindle, as well as spinal stretch reflex, reciprocal inhibition and recurrent inhibition of Renshaw cells. A descending command modulates the background activation of alpha motoneuron pools in combination with these reflex activities. A static gamma command controls the fusimotor contraction of the spindle. Simulation results reveal that the equilibrium joint angle is linearly correlated to the level of static gamma fusimotor activity of the spindle for a wide range of external loading conditions and reflex gains, suggesting that these spinal reflexes may contribute to regulate the equilibrium position of the joint. Sensitivity analysis further shows that reflex gains and other central commands alter the quasi-linear relation in regular fashions. The reciprocal inhibition gain changes the slope of the linear theta(eq) - gamma curve; and the descending alpha excitation, the stretch reflex gain, and the external load all shift the theta(eq) - gamma curve in parallel. These results imply that reflex gains and descending alpha commands may be coordinated to maintain a unique theta(eq) - gamma curve while providing the flexibility to counteract external loads, to execute a movement, or to regulate additional muscle variables. Dynamic simulation suggests that control of a class of movements can be achieved with a triphasic, alpha pulse and a continuous gamma signal. The model study supports the notion of a dual strategy for controlling trajectories via a feedforward alpha command and for regulating the final equilibrium positions via a feedback gamma command.

Adaptation, Physiological↗
PubMedDate not supplied

Does the baroreceptor-heart rate reflex indicate the capacity of the arterial baroreceptors to control blood pressure?

1. We have examined the question asked in the title by studying: the carotid baroreceptor reflex in man with a variable-pressure neck chamber; the carotid baroreceptor reflex in conscious rabbits with a variable-pressure capsule around the carotid bifurcation; and the barorecptor-heart rate reflex. 2. At rest, the association between the sensitivities of the carotid baroreceptor-blood pressure and -heart rate reflexes, and of these with the baroreceptor-heart rate reflex, is weak. Exercise causes dissociation between control of blood pressure and heart rate. In hypertension, depression of the baroreceptor-heart rate reflex does not reflect the altered characterisitics of the carotid sinus-blood pressure reflex.

Animals↗
PubMedDate not supplied

Participation of prostanoids in chemical activation of the pericardial pressor reflex in dogs.

Experiments were performed on anaesthetized, open-chest dogs to determine the reflex effects on systemic blood pressure and heart rate produced by stimulation of the parietal pericardium with bradykinin, prostacyclin, prostaglandin E2 (PGE2), prostaglandin D2 (PGD2) and arachidonic acid. Pericardial application of bradykinin (1 microgram) consistently elicited reflex increases in blood pressure and heart rate, whereas application of prostanoids or arachidonic acid in doses up to 10 micrograms failed to produce any cardiovascular responses. Indomethacin, applied either directly to the parietal pericardium (1 microgram/ml) or given intravenously (5 mg/kg) caused a long lasting reduction of the reflex responses to bradykinin. The reflex effects of bradykinin could be temporarily restored by treatment of the pericardium with either prostacyclin (0.1 microgram/min) or PGE2 (0.1 microgram/min). PGD2 (0.1-1 microgram/min) did not influence the bradykinin induced pericardial reflex. Superfusion of arachidonic acid (3 micrograms/min) over the pericardium amplified the reflex effects of bradykinin when given before, but not when given after indomethacin treatment. The results indicate that locally formed prostanoids, specifically prostacyclin and PGE2, can facilitate activation of the pericardial pressor reflex by bradykinin. The findings may be relevant to the changes in cardiovascular activity occurring during pericardial inflammation.

Animals↗
PubMed1998

Evidence that a transcortical pathway contributes to stretch reflexes in the tibialis anterior muscle in man.

1. In human subjects, stretch applied to ankle dorsiflexors elicited three bursts of reflex activity in the tibialis anterior (TA) muscle (labelled M1, M2 and M3) at mean onset latencies of 44, 69 and 95 ms, respectively. The possibility that the later of these reflex bursts is mediated by a transcortical pathway was investigated. 2. The stretch evoked a cerebral potential recorded from the somatosensory cortex at a mean onset latency of 47 ms in nine subjects. In the same subjects a compound motor-evoked potential (MEP) in the TA muscle, evoked by magnetic stimulation of the motor cortex, had a mean onset latency of 32 ms. The M1 and the M2 reflexes thus had too short a latency to be caused by a transcortical pathway (minimum latency, 79 ms (47 + 32)), whereas the later part of the M2 and all of the M3 reflex had a sufficiently long latency. 3. When the transcranial magnetic stimulation was timed so that the MEP arrived in the TA muscle at the same time as the M1 or M2 reflexes, no extra increase in the potential was observed. However, when the MEP arrived at the same time as the M3 reflex a significant (P < 0.01) extra-facilitation was observed in all twelve subjects investigated. 4. Peaks evoked by transcranial magnetic stimulation in the post-stimulus time histogram of the discharge probability of single TA motor units (n = 28) were strongly facilitated when they occurred at the same time as the M3 response. This was not the case for the first peaks evoked by electrical transcranial stimulation in any of nine units investigated. 5. We suggest that these findings are explained by an increased cortical excitability following TA stretch and that this supports the hypothesis that the M3 response in the TA muscle is - at least partly - mediated by a transcortical reflex.

Adult↗
PubMed1999

Pulmonary oedema produced by scorpion venom augments a phenyldiguanide-induced reflex response in anaesthetized rats.

1. The involvement of pulmonary oedema produced by scorpion venom in augmenting a phenyldiguanide (PDG)-induced reflex response was evaluated in urethane-anaesthetized rats. 2. PDG-induced bradycardiac, hypotensive and apnoeic responses, expressed as time-response area, exhibited similarities before or after venom treatment. Hence, the time-response area of bradycardia was taken as a reflex parameter. Pulmonary oedema was determined by physical evaporation and histological methods. 3. Exposure to Indian red scorpion (Buthus tamulus, BT; i.v.) venom for 30 min increased the pulmonary water content (P < 0.05; Student's t test) and augmented the PDG-induced bradycardiac reflex response by more than 2 times (P < 0.001). The increase of pulmonary water content was maximal with 100 microg kg-1 of venom and the augmentation was maximal with 10 microg kg-1. In a separate series of experiments, the venom (100 microg kg-1)-induced pulmonary oedema was confirmed by histological and physical methods. In this group also, the venom augmented the reflex to the same magnitude. 4. Pulmonary oedema (physical and histological) and augmentation of the bradycardiac reflex response after BT venom (100 microg kg-1; i.v.) were absent in animals pretreated with aprotinin, a kallikrein-kinin inhibitor (6000 KIU; i. v.). 5. Ondansetron (10 microg kg-1; i.v.), a 5-HT3 receptor antagonist, failed to block the venom-induced pulmonary oedema (physical and histological) but blocked the venom-induced augmentation of the reflex. 6. The results of this study indicate that the venom-induced augmentation of the PDG reflex is associated with pulmonary oedema involving kinins utilizing 5-HT3 receptors.

Anesthetics, Intravenous↗
PubMed1999

Prolonged enhancement of the micturition reflex in the cat by repetitive stimulation of bladder afferents.

1. Prolonged modulation of the parasympathetic micturition reflex was studied in cats anaesthetized by alpha-chloralose. Reflex discharges were recorded from a thin pelvic nerve filament to the bladder and evoked by stimulation of the remaining ipsilateral bladder pelvic nerves or urethral branches of the pudendal nerve. 2. Stimulation of bladder or urethral afferents at Adelta intensity evoked micturition reflexes with a latency of 90-120 ms. Such reflexes were much enhanced following repetitive conditioning stimulation of the same afferents at 20 Hz for 5 min. 3. The reflex enhancement lasted more than 1 h after the conditioning stimulation. The effect was not prevented by a preceding complete transection of the sympathetic supply to the bladder. A prolonged suppression of the reflex was obtained after conditioning stimulation of afferents in the dorsal clitoris nerves. 4. It is proposed that the prolonged modulations of the micturition reflex represent physiological adaptive processes, which preserve a flawless function of the bladder during life. The observations provide a theoretical explanation for the beneficial effect of electric nerve stimulation in patients with voiding disorders.

Animals↗
PubMed2001

Serotonin 5-HT(2) receptor activation induces a long-lasting amplification of spinal reflex actions in the rat.

1. C-fibre activation induces a long-term potentiation (LTP) in the spinal flexion reflex in mammals, presumably to provide enhanced reflexive protection of damaged tissue from further injury. Descending monoaminergic pathways are thought to depress sensory input but may also amplify spinal reflexes; the mechanisms of this modulation within the spinal cord remain to be elucidated. 2. We used electrical stimulation of primary afferents and recordings of motor output, in the rat lumbar spinal cord maintained in vitro, to demonstrate that serotonin is capable of inducing a long-lasting increase in reflex strength at all ages examined (postnatal days 2-12). 3. Pharmacological analyses indicated an essential requirement for activation of 5-HT(2C) receptors while 5-HT(1A/1B), 5-HT(7) and 5-HT(2A) receptor activation was not required. In addition, primary afferent-evoked synaptic potentials recorded in a subpopulation of laminae III-VI spinal neurons were similarly facilitated by 5-HT. Thus, serotonin receptor-evoked facilitatory actions are complex, and may involve alterations in neuronal properties at both motoneuronal and pre-motoneuronal levels. 4. This study provides the first demonstration of a descending transmitter producing a long-lasting amplification in reflex strength, accomplished by activating a specific serotonin receptor subtype. It is suggested that brain modulatory systems regulate reflex pathways to function within an appropriate range of sensori-motor gain, facilitating reflexes in behavioural situations requiring increased sensory responsiveness.

Animals↗
PubMed1997

Soleus muscle length, stretch reflex excitability, and the contractile properties of muscle in children and adults: a study of the functional joint angle.

The influence of the joint angle on stretch reflex excitability of the soleus muscle at the ankle has been studied in 22 children aged 3.9 to 13.6 years and 9 adults aged 19 to 70 years. For all subjects, reflex EMG and mechanical twitch torque gain were trivial at resting plantar flexion. The reflex EMG gain reached a maximum between -15 degrees and -10 degrees of plantar flexion beyond the neutral angle, 0 degrees, defined as the foot at right angle to the tibia, diminishing steeply with further dorsiflexion. The reflex mechanical gain rose to a peak between 0 degrees and +10 degrees of dorsiflexion beyond neutral, declining steeply thereafter. By contrast, axonally stimulated muscle twitch torque increased serially up to +30 degrees dorsiflexion beyond neutral. For the soleus muscle, the optimal reflex neuromechanical angle lies approximately midway between the angle for optimal reflex EMG gain (in mild plantar flexion, at which the largest and strongest motor units can be activated) and the optimal muscle mechanical angle (at the extreme of soleus muscle dorsiflexion). These studies confirm that the excitability of the spinal alpha motor neuron pool in vivo is strongly influenced by muscle length and explain the variability in reflex excitability within and between subjects, if the joint angle is not controlled. They also indicate how posture influences movement, agreeing with the known function of the soleus muscle in the stance phase of gait and the modulation of motor unit recruitment during voluntary alternating movements at the ankle. Soleus muscle twitch characteristics show a fivefold to eightfold increase in peak force associated with a tenfold reduction in compliance in the first two decades of life and an apparent speeding up of twitch time in the first decade.

Adolescent↗