Gastrocnemius Contracture Research Articles

The Effect Of Isolated Gastrocnemius Contracture And Surgical Gastrocnemius Recession On Strength And Function

Isolated gastrocnemius contracture (IGC), characterized as reduced ankle dorsiflexion with full knee extension, has been linked to foot injuries limiting function and activities of daily living. Patients with IGC may be unable to produce normal plantarflexion force. Gastrocnemius recession surgery has been shown to increase function and dorsiflexion range of motion, but the impact on force production is unknown. PURPOSE: To determine the effect of gastrocnemius recession surgery on function and force production. METHODS: 7 legs with IGC (51 years, 83.78 kg, 1.69 m) and 23 control legs (48 years, 74.19 kg, 1.72 m) performed isometric plantarflexion at maximum ankle dorsiflexion with full knee extension. Subjects with IGC were tested pre- and 3 months post-surgery at their maximum ankle dorsiflexion angle. The Foot and Ankle Ability Measures (FAAM) questionnaire was used to evaluate function. Passive ankle dorsiflexion range of motion (PROM) was measured with the knee in full extension using a bi-plane goniometer. RESULTS: Post-surgery, subjects with IGC had significantly (p < 0.01) increased self reported function (91% vs. 59%) and global rating (86% vs. 48%) scores, but these were slightly lower than scores reported by control subjects. PROM significantly (p < 0.001) increased post-surgery from 0° to 13°, and was not different from control subjects (14°). There was a 10% increase in isometric plantarflexion strength post-surgery, however significant (p < 0.05) weakness persisted compared to control subjects (∼ 35%). CONCLUSION: Gastrocnemius recession surgery is a beneficial treatment option for patients with IGC. Increased function, dorsiflexion range of motion, and plantarflexion strength were observed post-surgery. Although improvements were noted following gastrocnemius recession, subjects were still weaker than controls and therefore may benefit from organized post-surgical rehabilitation. Supported in part by Synthes, Inc. and NIHR01AR040408.

Insertional Achilles Tendinopathy: Surgical Options

Insertional Achilles tendinopathy is a chronic condition in which patients describe functional impairment, pain, and swelling at the tendon insertion. Treatment is primarily nonsurgical with physical therapy, nonsteroidal anti-inflammatory drugs, rest, heel lifts, or heel sleeves; but for patients who fail nonsurgical management, surgical debridement of the Achilles tendon at its insertion can provide relief. The goal of surgery is to remove the diseased tendon and intratendinous calcifications, as well as to address associated pathology, including inflamed retrocalcaneal bursa, prominence of the posterior-superior calcaneal tuberosity, and gastrocnemius contracture. This article presents the author's preferred technique for surgical treatment of insertional Achilles tendinopathy and discusses the rationale behind this technique.

Effects of Static Posture on Lower Limb Vascular Parameters in Women with Varicose Veins

PURPOSE: The goal of this study was to assess the effects of maintaining a static posture on lower limb vascular parameters in women presenting varicose veins but no other circulatory problems. METHODS: Subjects (30 to 60 years old women) were divided into 2 groups: the experimental group was composed of women with varicose veins (n=10) and the control group included women without varicose veins (n=10). All subjects had to maintain 4 different static postures for 20 min: sitting, 2 sit-to-stand, and standing postures. The following variables were measured during each posture: lower limb subcutaneous blood perfusion, transcuteneous O2 and CO2 partial pressure, foot temperature and volume, surface EMG of the gastrocnemius, lower limb joint angles, heart/foot distance, O2 consumption, heart rate, and perceived comfort. Physiological variables were compared to perceived comfort in order to assess if the subjects could perceive the physiological changes occurring in their lower limbs during static postures. A mixed model factorial analysis of variance using the independent groups and the repeated postures was used to reveal statistical significant differences. Pearson correlaltions were also calculated between all variables. RESULTS: Correlations amongst physiological variables ranged from low (r=0.52 perfusion vs. foot oedema) to high (r=0.97 heart/foot distance vs. heart rate). Perceived comfort at the feet was highly correlated with the physiological variables (r ranging from 0.75 to 0.99). CONCLUSIONS: The results reveal that the heart/foot distance and the gastrocnemius muscle contraction are most important because of their strong relationship to blood perfusion velocity. Perceived comfort appeared to be more closely related to muscle contraction and energy expenditure than perfusion. Accordingly, subjects had a tendency to adopt a posture less demanding physically without perceiving the differences associated with perfusion, perhaps increasing the risk of circulatory problems in women working in static postures.

Die endoskopische Verlängerungsoperation des Musculus gastrocnemius zur Behandlung des Gastrocnemius equinus

Endoscopic gastrocnemius recession is a new technique to treat gastrocnemius equinus. Smaller incisions and the ability to perform the technique in a supine position are purported advantages. This study was designed to evaluate the results and possible complications of this new technique. 47 patients undergoing 54 endoscopic gastrocnemius recessions were followed in a prospective study. Pre-operative criteria were a lack of ankle dorsiflexion with the knee extended to create a 90 degrees relationship of the foot to the leg in symptomatic patients. Pre- and postoperative ankle dorsiflexion were assessed, as were complications such as infection, nerve injury, haematoma, over-lengthening and poor cosmesis. Mean age of the patients was 49.4 years. Post-operative follow-up from the index procedure was 27 months (range: 12 - 62 months). Pre-operative dorsiflexion was - 8 +/- 4 degrees; post-operative this improved significantly to 7 +/- 4 degrees (p = 0.00001). Most patients had additional reconstructive procedures. There were no infections; six limbs had lateral foot or leg dysaesthesia, one haematoma, one over-lengthened gastrocnemius while six limbs has an unacceptable cosmesis (due to tenting of the skin). Two diabetic patients required additional Achilles tendon lengthening in subsequent surgery. Patients with pre-operative dorsiflexion of - 10 degrees or more had an untoward result, but this was not statistically significant (p = 0.13). Lateral foot dysaesthesia was observed more often in patients having a combined procedure with calcaneal osteotomy (p = 0.03). The endoscopic gastrocnemius recession procedure can significantly increase ankle dorsiflexion, but has potential complications including lateral dysaesthesia (11 %), and unacceptable cosmesis (11 %) as the most common. Modifications to the technique may be needed. Overall, the endoscopic technique appears promising in decreasing gastrocnemius contracture.

The Mechanics Behind the Image: Foot and Ankle Pathology Associated with Gastrocnemius Contracture

Contracture of the gastrocnemius musculature is a prevalent finding in the setting of foot and ankle pathology. Tightness of the posterior musculotendinous structures in the leg limits ankle range of motion and affects an equinus posture of the foot. Increased contact pressures are generated in the plantar foot with weightbearing. The resultant overload of the ligaments and the intrinsic muscles of the midfoot and forefoot is manifest in a variety of pathologic processes. The altered mechanics contributes to, among other conditions, ankle impingement, plantar fasciitis, midfoot arthritis, posterior tibial tendon dysfunction, forefoot overload, diabetic ulceration, and Charcot arthropathy. Effective management of these conditions includes addressing the underlying gastrocnemius contracture as well as the related foot and ankle pathology. Here we describe the underlying biomechanical abnormalities and radiographic findings in these pathological conditions of the foot and ankle associated with gastroequinus contracture. An awareness and understanding of the pathomechanics should enable the radiologist to better appreciate the form and function associated with the image.

Evidence for chemicals sensitizing discharge from skeletal muscle afferents supporting cardiorespiratory reflexes during simulated exercise

the chemical metabolites in skeletal muscle that sensitize the activity of small-diameter afferents and support increases in cardiorespiratory function during exercise have been the subject of much investigation with a variety of methods. The chemicals that change within muscle with exercise have

The Effect of Triceps Surae Contracture Force on Plantar Foot Pressure Distribution

Triceps surae contractures have been associated with foot and ankle pathology. Achilles tendon contractures have been shown to shift plantar foot pressure from the heel to the forefoot. The purpose of this study was to determine whether isolated gastrocnemius contractures had similar effects and to assess the effects of gastrocnemius or soleus contracture on midfoot plantar pressure. Ten fresh frozen cadaver below-knee specimens were loaded to 79 pounds (350 N) plantar force with the foot unconstrained on a 10-degree dorsiflexed plate. Combinations of static gastrocnemius or soleus forces were applied in 3-lb increments and plantar pressure recordings were obtained for the hindfoot, midfoot, and forefoot regions. The percentage of plantar force borne by the forefoot and midfoot increased with triceps surae force, while that borne by the hindfoot decreased (p<or=0.005). Increasing gastrocnemius force had similar results. Increasing triceps surae force from 0 to 21 lbs (93 N) increased average percent forefoot and midfoot force 59% and 38%, respectively, and reduced average percent hindfoot force 18%. Increasing gastrocnemius force from 0 to 18 lbs increased average percent forefoot and midfoot force 50% and 32%, respectively, and reduced average percent hindfoot force 16%. For a given triceps surae force, there was no statistical difference in pressure distribution noted between different combinations of gastrocnemius and soleus force. In a static model, increased triceps surae or isolated gastrocnemius force shifted weightbearing plantar pressure from the hindfoot to the midfoot and forefoot. Similar results were noted whether the triceps surae force was applied through the gastrocnemius or soleus or both. The results of this study are consistent with the clinical association of triceps surae contracture with foot and ankle disorders including diabetic foot ulcers and metatarsalgia. The similar effects with triceps surae force application through the gastrocnemius or soleus suggest that patients with isolated gastrocnemius contractures may obtain similar clinical benefits with potentially less morbidity after gastrocnemius aponeurosis lengthening as compared to Achilles tendon lengthening.

Biomechanical characterization and clinical implications of artificially induced toe-walking: Differences between pure soleus, pure gastrocnemius and combination of soleus and gastrocnemius contractures

The purpose of this study was to characterize biomechanically three different toe-walking gait patterns, artificially induced in six neurologically intact subjects and to compare them to selected cases of pathological toe-walking. The subjects, equipped with lightweight mechanical exoskeleton with elastic ropes attached to the left leg's heel on one end and on shank and thigh on the other end in a similar anatomical locations where soleus and gastrocnemius muscles attach to skeleton, walked at speed of approximately 1 m/s along the walkway under four experimental conditions: normal walking (NW), soleus contracture emulation (SOL), gastrocnemius contracture emulation (GAS) and emulation of both soleus and gastrocnemius contractures (SOLGAS). Reflective markers and force platform data were collected and ankle, knee and hip joint angles, moments and powers were calculated using inverse dynamic model for both legs. Characteristic peaks of averaged kinematic and kinetic patterns were compared among all four experimental conditions in one-way ANOVA. In the left leg SOL contracture mainly influenced the ankle angle trajectory, while GAS and SOLGAS contractures influenced the ankle and knee angle trajectories. GAS and SOLGAS contractures significantly increased ankle moment during midstance as compared to SOL contracture and NW. All three toe-walking experimental conditions exhibited significant power absorption in the ankle during loading response, which was absent in the NW condition, while during preswing significant decrease in power absorption as compared to NW was seen. In the knee joint SOL contracture diminished, GAS contracture increased while SOLGAS contracture approximately halved knee extensor moment during midstance as compared to NW. All three toe-walking experimental conditions decreased hip range of motion, hip flexor moment and power requirements during stance phase. Main difference in the right leg kinematic and kinetic patterns was seen in the knee moment trajectory, where significant increase in the knee extensor moment took place in terminal stance for GAS and SOLGAS experimental conditions as compared to SOL and NW. The kinetic trajectories under SOL and GAS experimental conditions were qualitatively compared to two selected clinical cases showing considerable similarity. This implies that distinct differences in kinetics between SOL, GAS and SOLGAS experimental conditions, as described in this paper, may be clinically relevant in determining the relative contribution of soleus and gastrocnemius muscles contractures to toe-walking in particular pathological gait.

The Vulpius procedure for correction of equinus deformity in patients with hemiplegia

Spasm or contracture of gastrocnemius causes an equinus deformity of the ankle in both cerebral palsy and hemiplegia. Its release is therefore required in the treatment of those patients who do not respond to conservative measures. The Vulpius procedure is a simple and effective method for the release of gastrocnemius and is particularly indicated when long periods of immobilisation of the foot and ankle are not desirable. We have used this procedure with good results to correct an equinus deformity in 230 adults with a cerebrovascular accident and various associated medical conditions. It is not only effective in cerebral palsy, but should be considered at an early stage in all adult patients with deformity of the ankle and foot in whom spasm of gastrocnemius is the major cause.

The vulpius procedure for correction of equinus deformity in patients with hemiplegia.

Spasm or contracture of gastrocnemius causes an equinus deformity of the ankle in both cerebral palsy and hemiplegia. Its release is therefore required in the treatment of those patients who do not respond to conservative measures. The Vulpius procedure is a simple and effective method for the release of gastrocnemius and is particularly indicated when long periods of immobilisation of the foot and ankle are not desirable. We have used this procedure with good results to correct an equinus deformity in 230 adults with a cerebrovascular accident and various associated medical conditions. It is not only effective in cerebral palsy, but should be considered at an early stage in all adult patients with deformity of the ankle and foot in whom spasm of gastrocnemius is the major cause.

The reliability of a new device designed to assess gastrocnemius contracture.

Gastrocnemius contracture may be a significant cause of many foot disorders. Gastrocnemius tension can be estimated clinically by measuring maximum ankle dorsiflexion during full knee extension. Such measurements, when made with currently available goniometric devices, are subject to high levels of intra- and inter-observer variability. We have designed a device to more consistently measure ankle dorsiflexion, using three dimensional tracking sensors on the leg and foot. The applied dorsiflexion torque is kept constant by a computer, and the computer also monitors hindfoot position to maintain a neutrally aligned foot during testing. Repeated measurements on 26 feet were taken to determine the consistency of the device. The correlation coefficient for the measurements was 0.96, indicating very low intra-observer variability. The standard deviation of the repeated measures was 2 degrees. Based on the 95% confidence interval, the device can be considered accurate to within 4 degrees. Given this accuracy, this instrument could be used to assess gastrocnemius tension, its role in foot pathology, and the effectiveness of surgical lengthening. Compared to other currently available measuring devices, this instrument is the most reliable in estimating ankle dorsiflexion, since it is capable of controlling hindfoot position and applied dorsiflexion torque, and it can be easily constructed by other laboratories.

ISOLATED GASTROCNEMIUS TIGHTNESS

Contracture of the gastrocnemius-soleus complex has well-documented deleterious effects on lower-limb function in spastic or neurologically impaired individuals. There is scarce literature, however, on the existence of isolated gastrocnemius contracture or its impact in otherwise normal patients. We hypothesized that an inability to dorsiflex the ankle due to equinus contracture leads to increased pain in the forefoot and/or midfoot and therefore a population with such pain will have less maximum ankle dorsiflexion than controls. We further postulated that the difference would be present whether the knee was extended or flexed. This investigation was a prospective comparison of maximal ankle dorsiflexion, as a proxy for gastrocnemius tension, in response to a load applied to the undersurface of the foot in two healthy age, weight, and sex-matched groups. The patient group comprised thirty-four consecutive patients with a diagnosis of metatarsalgia or related midfoot and/or forefoot symptoms. The control group consisted of thirty-four individuals without foot or ankle symptoms. The participants were clinically examined for gastrocnemius and soleus contracture and were subsequently assessed for tightness with use of a specially designed electrogoniometer. Measurements were made both with the knee extended (the gastrocnemius under tension) and with the knee flexed (the gastrocnemius relaxed). With the knee fully extended, the average maximal ankle dorsiflexion was 4.5 degrees in the patient group and 13.1 degrees in the control group (p < 0.001). With the knee flexed 90 degrees, the average was 17.9 degrees in the patient group and 22.3 degrees in the control population (p = 0.09). When gastrocnemius contracture was defined as dorsiflexion of < or = 5 degrees during knee extension, it was identified in 65% of the patients compared with 24% of the control population. However, when gastrocnemius contracture was defined as dorsiflexion of < or = 10 degrees, it was present in 88% and 44%, respectively. When gastrocnemius-soleus contracture was defined as dorsiflexion of < or = 10 degrees with the knee in 90 degrees of flexion, it was identified in 29% of the patient group and 15% of the control group. On the average, patients with forefoot and/or midfoot symptoms had less maximum ankle dorsiflexion with the knee extended than did a control population without foot or ankle symptoms. When the knee was flexed 90 degrees to relax the gastrocnemius, this difference was no longer present. These findings support the existence of isolated gastrocnemius contracture in the development of forefoot and/or midfoot pathology in otherwise healthy people. These data may have implications for preventative and therapeutic care of patients with chronic foot problems.

The effect on ankle dorsiflexion of gastrocnemius recession.

Gastrocnemius equinus contracture has been suggested as an etiologic factor in mechanical diseases of the foot and ankle and in ulcer formation in the foot. The purpose of this study is to assess the correction in ankle dorsiflexion that can be achieved with a gastrocnemius recession. An isolated gastrocnemius release (Strayer procedure) was performed on 26 legs, in 20 consecutive patients, for clinically significant gastrocnemius equinus contracture. Ankle dorsiflexion was assessed using a validated electrogoniometer. Ankle dorsiflexion was recorded with the knee straight and with the knee bent. Measurements were recorded preoperatively, and immediately postoperatively. Measurements at an average of 55.0 days postsurgery (range, 37 to 128 days) were performed on 20 legs (15 patients). Average preoperative ankle dorsiflexion with the knee straight was 5.1 degrees. Average preoperative ankle dorsiflexion with the knee bent was 22.8 degrees. Immediately following surgery the average ankle dorsiflexion with the knee straight was 23.2 degrees. The average correction was 18.1 degrees and this increase was significant (p < 0.0001.) In the 15 patients (20 legs) available for follow-up, the increase in ankle dorsiflexion with the knee straight was maintained (average: 24.9 degrees). Patients with gastrocnemius contracture who underwent an isolated gastrocnemius release increased their ankle dorsiflexion (knee straight) by an average of 18.1 degrees with postoperative ankle dorsiflexion (knee straight) being equivalent (23.2 and 22.8 degrees) to preoperative ankle dorsiflexion (knee bent). This correction appears to be maintained (23.2 vs. 24.9 degrees) at short-term follow-up.

THE MYONEURAL EFFECTS OF LITHIUM CHLORIDE ON THE NERVE–MUSCLE PREPARATIONS OF RATS. ROLE OF ADENOSINE TRIPHOSPHATE-SENSITIVE POTASSIUM CHANNELS

The potential effects of lithium chloride on the neuromuscular transmission and muscular contraction were studied using in vitro and in vivo nerve–muscle preparations of rats. Addition of lithium chloride to the fluid bathing the isolated rat diaphragm produced a concentration-dependent inhibition of diaphragmatic contractions elicited by either indirect or direct electrical stimulation. The threshold concentrations were 1 mmol l−1and 3 mmol l−1, respectively. Similarly, the intravenous administration of lithium chloride as bolus injections, produced a dose-dependent progressive inhibition of the indirectly- and directly-induced gastrocnemius muscle contractions during the 2-h period of investigation. The indirectly-induced contractions were much more sensitive to the inhibitory effect of lithium chloride than directly-induced contractions. Also, lithium chloride was found to be capable of enhancing the paralysis of the indirectly stimulated rat diaphragm in vitro and gastrocnemius muscle in vivo induced by either pipecuronium or succinylcholine. The combination of lithium chloride and pipecuronium led to a synergistic inhibition of the neuromuscular transmission, while the combination of lithium chloride and succinylcholine led to additive inhibition. Pretreatment with lithium chloride at the threshold concentrations enhanced the inhibitory effects of verapamil on diaphragmatic contractions elicited either indirectly or directly. The inhibitory effects of verapamil on the indirectly- and directly-induced rat gastrocnemius muscle contractions were potentiated by lithium chloride administration to rats. Glibenclamide was found to be capable of inhibiting the relaxant effects of lithium chloride on the indirectly- and directly-elicited contractions of rat diaphragm in vitro and rat gastrocnemius muscle in vivo, in a concentration- and dose-dependent manner, respectively. Doubling the concentration of magnesium in the bathing fluid potentiated the inhibitory effects of the threshold concentrations of lithium chloride on the diaphragmatic contractions induced either indirectly or directly. Pretreatment with 4-aminopyridine or barium chloride inhibited the relaxant effects of lithium chloride on the indirectly- and directly-elicited diaphragmatic contractions. The inhibitory effects of diazoxide on the indirectly-evoked contractions of rat diaphragm in vitro and rat gastrocnemius muscle in vivo were potentiated by lithium chloride. Pretreatment with glibenclamide inhibited markedly the combined effects of lithium chloride and diazoxide on the contractions of the diaphragm and gastrocnemius muscles induced indirectly. Additionally, the intravenous administration of lithium chloride into rats as bolus injections produced a dose-dependent progressive increase in plasma potassium level and a dose-dependent progressive decrease in the intracellular levels of adenosine triphosphate in the sciatic nerve and gastrocnemius muscle. It is concluded that lithium chloride, via activation of adenosine triphosphate- sensitive potassium channels, acts presynaptically to inhibit the neuromuscular transmission and acts at the muscle membrane to inhibit the muscular contraction.

Mechanism of action of honey bee (Apis mellifera L.) venom on different types of muscles.

1. The effect of crude honeybee (Apis mellifera) venom on the skeletal, smooth as well as cardiac muscles were studied in this investigation. 2. Perfusion of gastrocnemius-sciatic nerve preparation of frogs with 1 microgram/ml venom solution has weakened the mechanical contraction of the muscle without recovery. Blocking of nicotinic receptors with 3 micrograms/ml flaxedil before bee venom application sustained normal contraction of gastrocnemius muscle. 3. The electrical activity of duodenum rabbits was recorded before and after the application of 1 microgram/ml venom solution. The venom has depressed the amplitude of the muscle contraction after 15 min pretreatment with atropine nearly abolished the depressor effect of the venom on smooth muscle. 4. In concentrations from 0.5-2 micrograms/ml, bee venom caused decrease of heart rate of isolated perfused toad heart. This bradycardia was accompanied by elongation in the P-R interval. A gradual and progressive increase in the R-wave amplitude reflected a positive inotropism of the venom. Application of 5 micrograms/ml verapamil, a calcium channels blocking agent, abolished the noticed effect of the venom. 5. Marked electrocardiographic changes were produced within minutes of the venom application on the isolated perfused hearts, like marked injury current (elevation or depression of the S-T segment), atrioventricular conduction disturbances and sinus arrhythmias. Atropine and nicotine could decrease the toxic effect of the venom on the myocardium. 6. Results of the present work lead to the suggestion that bee venom is mediated through the peripheral cholinergic neurotransmitter system. General neurotoxicity of an inhibitory nature involving the autonomic as well as neuromuscular system are established as a result of the venom, meanwhile a direct effect on the myocardium membrane stabilization has been suggested.

THE MYONEURAL EFFECTS OF PROPOFOL EMULSION (DIPRIVAN) ON THE NERVE-MUSCLE PREPARATIONS OF RATS

The potential effects of propofol emulsion (Diprivan) on the neuromuscular transmission and muscular contraction were studied usingin vitroandin vivonerve-muscle preparations of rats. The contractions of the isolated rat diaphragm elicited by either indirect or direct electrical stimulation were inhibited by propofol emulsion at threshold concentrations of 42 and 112 μmol l−1, respectively. Similarly, the gastrocnemius muscle contractions induced by either indirect or direct electrical stimulationin vivowere inhibited by propofol emulsion administration as a bolus injection of 2.5 mg kg−1intravenously, followed by intravenous infusion of 150 μg kg−1min−1for 1 h into rats. The inhibitory effects of propofol in both preparations were greater with indirect rather than direct stimulation. Propofol emulsion was found to be capable of enhancing the paralysis of the indirectly stimulated rat diaphragmin vitroand gastrocnemius musclein vivoinduced by either pipecuronium or succinylcholine. The combination of propofol and pipecuronium led to a synergistic inhibition of the neuromuscular transmission, while the combination of propofol and succinylcholine led to additive inhibition. Pretreatment with propofol emulsion at these threshold concentrations markedly inhibited the stimulant effects of aminophylline and digoxin on the indirectly and directly induced diaphragmatic contractions. Also, the enhancement effects of aminophylline on the indirectly and directly and of digoxin on indirectly induced rat gastrocnemius muscle contractions were markedly inhibited by propofol emulsion administration to rats. Pretreatment with propofol emulsion at the threshold concentrations enhanced the inhibitory effects of verapamil on diaphragmatic contractions elicited either indirectly or directly and enhanced the inhibitory effect of adenosine on the contractions elicited indirectly. Similarly, the inhibitory effects of verapamil on the indirectly and directly and of adenosine on indirectly induced rat gastrocnemius muscle contractions were markedly potentiated by propofol emulsion administration to rats. In addition, doubling the concentration of calcium in the bathing fluid produced no change in the inhibitory effects of propofol emulsion on either indirectly or directly elicited diaphragmatic contractions, while doubling the concentration of external magnesium potentiated the propofol effects. Pretreatment with 4-aminopyridine suppressed the inhibitory effects of propofol emulsion on diaphragmatic contractions elicited either indirectly or directly. These results suggest that propofol acts presynaptically to inhibit the neuromuscular transmission and acts at the muscle membrane to inhibit the muscular contraction.

Effects of cutaneous afferent input on fatigue-induced changes in fusimotor activity of decerebrate cats

Interaction of cutaneous and small-diameter, primarily fatigue-induced, muscle afferent inputs on fusimotor neurons has been studied in decerebrate cats. Spike discharges of fusimotor neurons to medial gastrocnemius were recorded from filaments dissected free from this muscle nerve. Non-noxious mechanical stimuli (10 Hz, 2 mm vibration) were applied to the skin area on the lateral side of the heel, innervated by sural nerve, during long-lasting (250 s) fatiguing contraction of lateral gastrocnemius and soleus muscles, elicited by electrical stimulation (40 Hz, 1.3×motor threshold) of the muscle nerves. In 15 units (58%) the pattern of responses to muscle contraction and/or fatigue (initial transient, and late long-lasting increase in firing rate, respectively) was preserved in the presence of skin vibration which, by itself, provoked either a slight increase or no changes in fusimotor discharge rate. Pattern of the response to skin vibration prevailed in the presence of muscle contraction and fatigue only if the vibration by itself induced marked increase in fusimotor discharge rate (three units). In the remaining eight units the responses to both stimuli applied simultaneously were dissimilar in pattern to the response to either stimulus applied alone: the initial, tension-related, increase in firing rate was prolonged, while the late, fatigue-induced one was attenuated and its post-contraction part almost abolished. Possible mechanisms and functional role of interaction between cutaneous and muscle afferent inflows are discussed.

Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. I. Motor-unit recruitment.

1. The recruitment order of 64 pairs of motor units, comprising 21 type-identified units, was studied during centrally evoked muscle contractions of the cat medial gastrocnemius (MG) muscle in an unanesthetized, high decerebrate preparation. Motor units were functionally isolated within the MG nerve by intra-axonal (or intramyelin) penetration with conventional glass microelectrodes. 2. Graded stimulation of the mesencephalic locomotor region (MLR) was used to evoke smoothly graded contractions, which under favorable conditions was estimated to reach 40% of maximum tetanic tension of the MG muscle. With this method of activation, 100% of slow twitch (type S) units, 95% of fast twitch, fatigue-resistant (type FR) units, 86% of fast twitch, fatigue-intermediate (type FI) units, and 49% of fast twitch, fatigable (type FF) units studied were recruited. 3. Motoneuron size as estimated by axonal conduction velocity (CV) was correlated with muscle-unit size as estimated by maximum tetanic tension (Po). Although the correlation between these properties was significant among type S and FR units, no significant correlation was found for these properties among type FI and FF units. 4. Motor-unit recruitment was ordered by physiological type (S > F, 100% of pairs; S > FR > FI > FF, 93% of pairs). Although none of the motor-unit properties studied predicted recruitment order perfectly, motor-unit recruitment was found to proceed by increasing Po (89% of pairs), decreasing contraction time (79% of pairs), decreasing fatigue index (80% of pairs), and increasing CV (76% of pairs). These percentages were significantly different from random (i.e., 50%). Statistically, all four motor-unit properties were equivalent in predicting recruitment order. These results are similar to those reported by other investigators for motor-unit recruitment order evoked from other supraspinal centers, as well as from peripheral sites. 5. When, however, motor-unit recruitment within pairs of motor units containing two fast-twitch (type F) units was examined, Po was a significantly better predictor of recruitment order than CV (85% vs. 52% of pairs). One explanation for this observation is that the correlation between Po and CV is high among type S, type FR units, and possibly among the lower-tension type FF units, but not among the remaining higher-tension type FF units. 6. The reproducibility of recruitment order in multiple contractions was investigated in 16 motor-unit pairs. Recruitment order was found to be variable in only three motor-unit pairs, all of which contained units of similar physiological type and recruitment threshold. 7. Analysis of recruitment order by pair-wise testing confirms the general conclusion reached in human studies that the muscle force level at recruitment for a motor unit is highly correlated with its strength. As an additional confirmation, the whole-muscle force level at recruitment for 41 units was measured in a series of contractions in which the rate of rise of muscle tension was limited to rates < 1,000 g/s. For these contractions, a significant correlation was found between muscle tension at recruitment and motor-unit Po.

Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. II. Motoneuron firing-rate modulation.

1. The aim of this study was to examine the nature of motoneuron firing-rate modulation in type-identified motor units during smoothly graded contractions of the cat medial gastrocnemius (MG) muscle evoked by stimulation of the mesencephalic locomotor region (MLR). Motoneuron discharge patterns, firing rates, and the extent of firing-rate modulation in individual units were studied, as was the extent of concomitant changes in firing rates within pairs of simultaneously active units. 2. In 21 pairs of simultaneously active motor units, studied during 41 evoked contractions, the motoneurons' discharge rates and patterns were measured by processing the cells' recorded action potentials through windowing devices and storing their timing in computer memory. Once recruited, most motoneurons increased their firing rates over a limited range of increasing muscle tension and then maintained a fairly constant firing rate as muscle force continued to rise. Most motoneurons also decreased their firing rates over a slightly larger, but still limited, range of declining muscle force before they were derecruited. Although this was the most common discharge pattern recorded, several other interesting patterns were also seen. 3. The mean firing rate for slow twitch (type S) motor units (27.8 imp/s, 5,092 activations) was found to be significantly different from the mean firing rate for fast twitch (type F) motor units (48.4 imp/s, 11,272 activations; Student's t-test, P < 0.001). There was no significant difference between the mean firing rates of fast twitch, fatigue-resistant (type FR) and fast twitch, fatigable (type FF) motor units. When the relationship between motoneuron firing rate and whole-muscle force was analyzed, it was noted that, in general, smaller, lower threshold motor units began firing at lower rates and reached lower peak firing rates than did larger, higher threshold motor units. These results confirm both earlier experimental observations and predictions made by other investigators on the basis of computer simulations of the cat MG motor pool, but are in contrast to motor-unit discharge behavior recorded in some human motor-unit studies. 4. The extent of concomitant changes in firing rate within pairs of simultaneously active motor units was examined to estimate the extent of simultaneous motoneuron firing-rate modulation across the motoneuron pool. A smoothed (5 point sliding average) version of the two motoneurons' instantaneous firing rates was plotted against each other, and the slope and statistical significance of the relationship was determined. In 16 motor-unit pairs, the slope of the motoneurons' firing-rate relationship was significantly distinct from 0. Parallel firing-rate modulation (< 10-fold difference in firing rate change reflected by a slope of > 0.1) was noted only in pairs containing motor units of like physiological type and then only if they were of similar recruitment threshold. 5. Other investigators have demonstrated that changes in a motoneuron's "steady-state" firing rate predictably reflect changes in the amount of effective synaptic current that cell is receiving. The finding in the present study of limited parallel firing-rate modulation between simultaneously active motoneurons would suggest that changes in the synaptic drive to the various motoneurons of the pool is unevenly distributed. This finding, in addition to the findings of orderly motor-unit recruitment and the relationship between motor-unit recruitment threshold and motoneuron firing rate, cannot be adequately accommodated for by the existing models of the synaptic organization in motoneuron pools. Therefore a new model of the synaptic organization within the motoneuron pool has been proposed.

Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. III. Muscle-unit force modulation.

1. The aim of this study was to examine the extent of muscle-unit force modulation due to motoneuron firing-rate variation in type-identified motor units of the cat medial gastrocnemius (MG) muscle, and to investigate the contribution of muscle-unit force modulation to whole-muscle force regulation. The motoneuron discharge patterns recorded from 8 pairs of motor units during 12 smoothly graded muscle contractions evoked by stimulation of the mesencephalic locomotor region (MLR) were used to reactivate those units in isolation to estimate what their force profiles would have been like during the evoked whole-muscle contractions. 2. For most motor units, muscle-unit force modulation was similar to motoneuron firing-rate modulation, in that muscle-unit force increased over a limited range (120-600 g) of increasing whole-muscle tension and was then maintained at a near maximal (> 70%) output level as muscle force continued to rise. Most muscle units also decreased their force outputs over a slightly larger range of declining whole-muscle force before relaxing. This second finding was best explained by the counterclockwise hysteresis recorded in the motor units' frequency-tension (f-t) relationships. 3. In those instances when whole-muscle force fluctuated just above the recruitment threshold of a motor unit, a substantial percentage (10-25%) of the change in whole-muscle force could be accounted for by force modulation in that motor unit alone. This finding suggested that few motor units in the pool were simultaneously simultaneously undergoing force modulation. To evaluate this possibility, the extent of parallel muscle-unit force modulation within the 8 pairs of simultaneously active motor units was evaluated. As with parallel motoneuron firing-rate modulation, the extent of parallel muscle-unit force modulation was limited to unit pairs of the same physiological type and recruitment threshold. In several instances, pairs of motor units displayed parallel motoneuron firing-rate modulation but did not show parallel muscle-unit force modulation because of the nature of the motor units' f-t relationships. 4. The limited extent of parallel muscle-unit force modulation seen in these experiments implies that the major strategy for force modulation in the cat MG muscle, involving contractions estimated to reach 30-40% of maximum, may be motor-unit recruitment rather than motor-unit firing-rate variation with resulting force modulation. Given, however, that the majority of motor units are already recruited at these output levels (< 40%), it is proposed that motor-unit firing-rate variation with resulting force modulation may take over as the major muscle force modulating strategy at higher output levels.