Muscle temperature, contractile speed, and motoneuron firing rates during human voluntary contractions.

A study was made of motoneuron firing rates and mechanical contractile parameters during maximum voluntary contraction of human hand muscles. A comparison of muscles that had been fatigued after a 60-s maximum voluntary contraction (MVC) with muscles that were cooled by approximately 5 degrees C showed that the contractile properties, in particular the rates of contraction and relaxation, were similarly affected in both conditions. In contrast, the motoneuron firing rate was affected differently by the two treatments. In the case of the fatigued muscles the motoneuron firing rate was reduced by 36%, as was expected from previous studies, but in the case of the cooled muscles, there was no significant change in the motoneuron firing rate. We conclude that the reflex reduction in the motoneuron firing rate seen in the fatigued muscle is not triggered directly by a change in the mechanical properties of the muscle.

Impulse propagation and muscle activation in long maximal voluntary contractions.

With fatigue, force generation may be limited by several factors, including impaired impulse transmission and/or reduced motor drive. In 5-min isometric maximal voluntary contraction, no decline was seen in the peak amplitude of the tibialis anterior compound muscle mass action potential (M wave) either during or immediately after the voluntary effort, provided maximal nerve stimulation was retained. For first dorsal interosseous (FDI) muscle, M wave amplitudes declined by 19.4 +/- 1.6% during the first 2 min but did not change significantly thereafter, despite the continued force reduction (up to 94% in 5 min for both muscles). The duration of the FDI M waves increased (greater than 30%), suggesting that the small decline in amplitude was the result of increased dispersion between the responses of different motor units. Some subjects kept FDI maximally activated throughout, but when they used tibialis anterior, twitch occlusion and tetanic muscle stimulation showed that most subjects were usually only able to do so for the first 60 s and thereafter only during brief "extra efforts." Thus force loss during isometric voluntary contractions sustained at the highest intensities results mainly from failure of processes within the muscle fibers.

Motor drive and metabolic responses during repeated submaximal contractions in humans.

Contractile failure during various types of exercise has been attributed to intramuscular metabolic changes. We examined the temporal changes in force-generating capacity and metabolic state during intermittent isometric contractions in humans. One-legged quadriceps contractions at 30% maximum voluntary contraction (MVC) were executed for 6 s, with 4 s of rest between. The decrease in force-generating capacity was tested from brief MVC's and short bursts of 50-Hz stimulation applied at 5-min intervals. After 1 min of exercise, the MVC force declined linearly and in parallel to the 50-Hz stimulation force, indicating that the contractile failure was due to intramuscular processes. After 30 min of exercise the MVC force had declined by approximately 40% compared with the value obtained after 1 min. In separate experiments the same contraction protocol was followed, but two-legged contractions were used. Muscle biopsies taken after 5, 15, and 30 min of exercise showed only minor changes in the concentrations of glycogen, lactate, creatine phosphate (CrP), and ATP. However, at exhaustion, defined as loss of ability to sustain the target force, the concentrations of CrP and glycogen were reduced by 73 and 32%, and muscle lactate concentration had increased to 4.8 mmol/kg wet wt. Thus the gradual decline in force-generating capacity was not due to lactacidosis or lack of substrates for ATP resynthesis and must have resulted from excitation/contraction coupling failure, whereas exhaustion was closely related to phosphagen depletion, without significant lactacidosis.

The use of deep duplex scanning to predict hemodynamically significant aortoiliac stenoses.

Hemodynamic assessment of aortoiliac occlusive disease is necessary for successful arterial reconstruction of the aorta and legs. Various methods have been proposed and "pull-through" intra-arterial pressures are the "gold standard." Deep Doppler duplex imaging was supplemented with real-time spectral analysis and velocity measurements in 29 cases. Twenty-three of these patients needed arteriography. One hundred sixty-six (166) arterial segments extending from the proximal aorta to the common femoral arteries were independently graded on duplex scans and arteriograms. For severe occlusive disease, duplex scanning is highly accurate (sensitivity 82%, specificity 93%). Velocity measurements were useful in determining the hemodynamic significance of stenoses. Peak systolic velocities in stenoses were measured with a duplex scanner. The pressure gradient calculated with the modified Bernoulli equation (delta P = 4Vmax2) correlated well with the gradients measured during arteriography (r = 0.9, n = 11). These noninvasive velocity measurements and Bernoulli calculations alert arteriographers to obtain special views of suspected areas and suggest the need for "pull-through" pressures and possible balloon angioplasty. In addition, these noninvasive measurements are useful to follow up patients who have mild to moderate aortoiliac disease and after angioplasty.

Evidence for a fatigue-induced reflex inhibition of motoneuron firing rates.

1. In previous studies on the adductor pollicis and biceps brachii muscles we suggested that motoneuron firing rates are inhibited by a reflex from the muscle during fatigue, since: the firing rates decline during a sustained maximal voluntary contraction (MVC); recovery of MVC firing rates is prevented if the fatigued state of the muscle is preserved for 3 min by local occlusion of its blood supply; and full recovery occurs during this time once the blood supply to the peripheral muscle is restored. These findings were confirmed in the present study for quadriceps contractions. 2. These results do not necessarily imply an inhibitory reflex. The lower firing rates recorded from the muscle fibers during an MVC following 3 min of postfatigue ischemia may have been caused by either reduced subject effort (decreased muscle activation by the CNS) or impaired peripheral impulse transmission under these conditions. The present experiments, carried out on the quadriceps and adductor pollicis muscles, were designed to test this alternative explanation. 3. For both muscles, MVC contractions were sustained for 40 s with a blood pressure cuff inflated to 200 mmHg. This was followed by 3 min ischemic rest and a second 20-s MVC before cuff release. Three minutes after the blood supply to the muscle was restored a third 20-s MVC was made. Single shocks were delivered to the muscle throughout to record twitches from the relaxed muscle (Tr) before and after each MVC, and any twitches super-imposed on the voluntary contractions (Ts). The degree to which the muscle could be activated by voluntary effort was assessed from the ratio [1 - Ts/Tr]. For adductor pollicis, changes in the amplitude of the evoked M-waves were also measured. 4. Spike frequencies were only recorded during quadriceps experiments. These declined by 30% during the initial 40-s MVC. No recovery was seen in the second MVC following 3 min ischemic rest, but full recovery occurred within 3 min of cuff release. 5. Failure to retain full muscle activation was frequently seen in all three MVCs. However, for many well-motivated subjects twitch occlusion showed no reduction in the degree to which either the adductor pollicis or quadriceps muscles could be activated voluntarily during the MVC executed after 3 min of ischemic rest compared with that performed 3 min after the blood supply had been restored.(ABSTRACT TRUNCATED AT 400 WORDS)

Distal arterial reconstruction: patency and limb salvage in diabetics.

Controversy regarding efficacy and durability of distal bypass grafting of the diabetic patient exists. A 22-year-long series of 259 vascular procedures with 100% follow-up (57% in diabetic individuals) is examined to compare these results with those of non-diabetic patients. Extensive review of predisposing factors, operative indications, preoperative medical evaluation, as well as techniques and peculiarities of angiography is rendered. Peripheral arterial case mix between the two groups is examined. The high utilization rate of solely venous conduits (94%) in diabetics compared with 76% in nondiabetics, along with anastomosis site selection, provides the most reasonable explanation for the successful outcome in both groups. Modifications in revascularization techniques contributing to successful outcome are presented. In the diabetic patients, both the cumulative graft patency rate (63%) and the limb salvage rate (77%) at 6 years were superior to those of the non-diabetics (52% and 65%, respectively). Diabetes mellitus does not predispose the person requiring revascularization for limb salvage to a lesser likelihood of success.

Duplex B-mode imaging for the diagnosis of deep venous thrombosis.

Real-time B-mode venous imaging has numerous advantages for the diagnosis of acute deep venous thrombosis (DVT). During the 11 months ending Feb 1, 1986, we examined 431 patients for possible acute DVT using a 5-MHz hand-held continuous wave Doppler stethoscope and a duplex real-time B-mode imager. Clots were seen in 86 patients in multiple views and cross sections. Normal veins completely collapsed with probe pressure on the skin. Blood flow was seen and heard, and abnormal flow was detected. There were no false-negatives (100% sensitivity). Early in the study, two false-positives occurred (78% specificity), but these errors will not recur. These tests are accurate, noninvasive, and inexpensive. They differentiate acute from chronic thrombosis and are repeatable. Duplex imaging may become the "gold standard" for the diagnosis of DVT.

Contractile properties of the human diaphragm in vivo.

The mechanical properties of the human diaphragm have been studied at fractional residual capacity in normal seated subjects with closed glottis. The transdiaphragmatic pressure (Pdi) developed in response to single shocks or to trains of stimuli at increasing frequency was approximately 3 times greater during bilateral than unilateral stimulation. During unilateral phrenic nerve stimulation the Pdi twitches increased as the interval (0-200 ms) of a preceding conditioning stimulus to the contralateral phrenic nerve was decreased suggesting that the two hemidiaphragms are mechanically coupled in series. The contraction time and half-relaxation time of single bilateral twitches as well as the Pdi-frequency relationship (5-35 Hz) during bilateral tetanic stimulation indicate that the contractile properties of the human diaphragm are intermediate between those of fast- and slow-twitch muscle fibers. The results suggest that the contractile properties of the human diaphragm are well illustrated by single bilateral twitches recorded from the relaxed muscle, but that the responses to unilateral stimulation are misleading due to distortion by abnormal changes in the muscle geometry.

Fatigue of intermittent submaximal voluntary contractions: central and peripheral factors.

Central and peripheral factors were studied in fatigue of submaximal intermittent isometric contractions of the human quadriceps and soleus muscles. Subjects made repeated 6 s, 50% maximal voluntary contractions (MVC) followed by 4 s rest until the limit of endurance (Tlim). Periodically, a fatigue test was performed. This included a brief MVC, either a single shock or 8 pulses at 50 Hz during a rest period and a shock superimposed on a target force voluntary contraction. At Tlim, the MVC force had declined by 50%, usually in parallel with the force from stimulation at 50 Hz. The twitches superimposed on the target forces declined more rapidly, disappearing entirely at Tlim. In similar experiments on adductor pollicis, no reduction of the evoked M wave was seen. The results suggest that, during fatigue of quadriceps and adductor pollicis induced by this protocol, no central fatigue was apparent, but some was seen in soleus. Thus the reduced force-generating capacity could result mainly or entirely from failure of the muscle contractile apparatus.

Comparison of initial limb salvage in 98 consecutive patients with either reversed autogenous or in situ vein bypass graft procedures.

A retrospective review of 98 consecutive patients undergoing femoropopliteal or distal bypass procedures was conducted to determine whether in situ bypass grafting offers statistically significant initial limb salvage over reversed autogenous techniques. Over a 40 month period, 98 consecutive patients received either in situ or reversed autogenous vein grafts to effect limb salvage. The groups were similar in incidences of diabetes and previous myocardial infarctions, as well as in site of distal anastomosis (beneath the tibial peroneal trunk in more than 80 percent). The in situ vein graft group had an overall limb salvage rate of 92 percent with an 88 percent cumulative patency rate at 4 to 18 month follow-up, whereas the reversed autogenous vein graft group had a limb salvage rate of 86 percent with a 79 percent cumulative patency rate at up to 18 months. Results after 30 days showed 47 patients had improvement and 3 patients (6 percent) had died in the in situ vein graft group. In the reversed autogenous vein graft group, 44 patients improved, 4 did not improve and required amputations, and 2 (1 percent) died. Our study supports the use of in situ vein bypass grafting for limb salvage.

Surgical implications of fibrinolytic therapy.

Intraarterial fibrinolytic therapy was used in 37 cases (34 patients) of severe peripheral ischemia. Nineteen patients (56 percent) required surgical intervention (5 amputations and 14 successful reconstructive procedures). Twenty-four patients (71 percent) were significantly improved (average ankle-to-arm index 0.84), whereas only 5 patients (15 percent) lost their limbs. Five patients were angiographically unchanged with no or slight improvement in the ankle-to-arm index (0.22 to 0.32) and were discharged on anticoagulant therapy. One death and two cerebrovascular accidents occurred. The usefulness of intraarterial fibrinolytic therapy needs to be evaluated within the total realm of vascular surgery. It offers options for therapy where previously none existed. Some situations might be treated equally well with either intraarterial fibrinolytic therapy or surgery. Finally, surgery might be required to maintain initial successful results with intraarterial fibrinolytic therapy or to rescue intraarterial fibrinolytic therapy failures in striving to achieve superior results in limb salvage.

Changes in muscle contractile properties and neural control during human muscular fatigue.

The factors limiting force production and exercise endurance time have been briefly described, together with some of the changes occurring at various sites within the muscle and central nervous system. Evidence is presented that, in fatigue of sustained maximal voluntary contractions (MVC) executed by well-motivated subjects, the reduction in force generating capacity need not be due to a decline in central nervous system (CNS) motor drive or to failing neuromuscular transmission, but can be attributed solely to contractile failure of the muscles involved. However, despite this conclusion, both the integrated electromyogram (EMG) and the mean firing rate of individual motor units do decline progressively during sustained MVC. This, however, does not necessarily result in loss of force since the parallel slowing of muscle contractile speed reduces tetanic fusion frequency. It is suggested that the range of motoneuron firing rates elicited by voluntary effort is regulated and limited for each muscle to the minimum required for maximum force generation, thus preventing neuromuscular transmission failure and optimizing motor control. Such a CNS regulating mechanism would probably require some reflex feedback from the muscle.

Linear and non-linear surface EMG/force relationships in human muscles. An anatomical/functional argument for the existence of both.

The EMG/force relations of various human muscles were examined over the full isometric force range using uniform experimental methods. The muscles chosen reflected varying fiber composition, fiber distribution and force generating patterns (recruitment/frequency coding). Our results generally suggest linear relations for muscles of near uniform fiber composition. Non-linear relations were obtained from muscles of mixed fiber composition. The non-linearity persisted despite variations in recording configuration, electrode placement and limb position. These were suggested by others to account for the lack of linearity in the basic EMG/force relation. The roles of uneven fiber distribution, motor unit potential amplitudes and possible supratetanic motor unit driving are discussed within the context of muscle force generating mechanisms as possible explanations for non-linear EMG/force relations.

Motor-unit discharge rates in maximal voluntary contractions of three human muscles.

Single motor-unit firing rates have been recorded during maximal voluntary contractions using tungsten microelectrodes. Over 300 units from four subjects were sampled from each of three muscles. These were the biceps brachii, adductor pollicis, and soleus, chosen because of known differences in their fiber-type composition and contractile properties. In all cases the contraction maximality was assured by delivering single supramaximal shocks during the voluntary contractions. All motor units were deemed to have already been fully activated if no additional force resulted. Thus for each muscle, the firing rates elicited by a maximal voluntary effort are sufficient to generate a fully fused tetanus in each motor unit. For the biceps brachii and adductor pollicis muscles, the mean firing rates (+/- SD) were 31.1 +/- 10.1 and 29.9 +/- 8.6 Hz, respectively, while for soleus they were only 10.7 +/- 2.9 HZ. For each muscle the firing rates distribution covered approximately a four-fold range about the mean value. The mean firing rates for each muscle varied roughly in proportion to their respective twitch contraction and half relaxation times. These contractile time measurements for both biceps brachii and adductor pollicis agreed well with the mean values reported for human fast-twitch motor units, while those for soleus fell in the range observed for human slow-twitch units. An argument is presented that suggests that, in response to voluntary effort, the range of discharge rates of each motor-unit pool is limited to those only just sufficient to produce maximum force in each motor unit. This suggestion is based on the relationship between the range of motor-unit firing frequencies observed during maximum voluntary contractions, their range of contraction times, and the stimulation frequencies required for maximum force generation. The implications of this hypothesis for motor control are discussed.

Changes in motoneurone firing rates during sustained maximal voluntary contractions.

Tungsten micro-electrodes have been used to record the electrical activity of single motor units in the human adductor pollicis during maximal voluntary contractions. The potentials were characteristic of those from single muscle fibres. In brief maximal contractions, the firing rates of over 200 motor units were obtained from five normal subjects. Four subjects had a similar range (mean 26.4 +/- 6.5 Hz) while the fifth was slightly higher (35 +/- 7.4 Hz). When maximal voluntary force was sustained for 40-120 s, there was a progressive decline in the range and mean rate of motor-unit discharge. In the first 60 s, mean rates fell from about 27 Hz to 15 Hz. There was some evidence to suggest that those units with the highest initial frequencies changed rate most rapidly. It is suggested that this decline in motor unit discharge rates is not responsible for force loss, but that it may enable effective modulation of voluntary strength by rate coding to continue during fatigue.

Contractile speed and EMG changes during fatigue of sustained maximal voluntary contractions.

Measurements were made from the human adductor pollicis muscle of force, contractile speed, and electromyographic activity (EMG) before, during, and after maximal isometric voluntary contractions sustained for 60 s. The use of brief test periods of maximal nerve stimulation with single shocks or trains of shocks enabled various muscle mechanical properties to be studied throughout each contraction. Electrical activity was measured after rectification and smoothing of the surface potentials and also by counting the total number of potentials per unit time from a population of motor units using fine wire intramuscular electrodes. During a 60-s maximal voluntary contraction, the force fell by 30-50%. Throughout the experiment the voluntary force matched that produced by supramaximal tetanic nerve stimulation. This indicated that, with sufficient practice, full muscle activation could be maintained by voluntary effort. However, the amplitude of the smoothed, rectifed EMG and the rate of spike counts declined. Since no evidence for neuromuscular block was found, the decline in EMG and spike counts was attributed to a progressive reduction of the neural drive from the central nervous system, despite maintained maximum effort. After the prolonged voluntary contractions twitch duration was prolonged, mainly as a result of slowing in relaxation rate. Twitch summation in unfused tetani increased. Both the maximum rate of relaxation and the time course of force decay declined by 50-70%. Similar changes were seen in both voluntary contractions and in test periods of stimulation. The percentage change in muscle contractile speed measured by these parameters approximately equaled the percentage change in the surface EMG measured simultaneously. It is concluded that 1) during a 60-s sustained maximal voluntary contraction there is a progressive slowing of contraction speed such that the excitation rate required to give maximal force generation is reduced, 2) the simultaneous decline in EMG may be due to a continuous reduction in motoneuron discharge rate, and 3) the EMG decline may not necessarily contribute to force loss.

The absence of neuromuscular transmission failure in sustained maximal voluntary contractions.

1. Muscle mass action potentials (M waves) were evoked by supramaximal single shocks to the ulnar nerve given at 5-10 s intervals throughout sustained isometric maximal voluntary contractions (m.v.c.) of the adductor pollicis and first dorsal interosseous muscles. Both muscles were fatigued simultaneously. Recordings were made from the muscle surface and also intramuscularly. 2. During a maximal contraction lasting for 60 s there was 30-50% loss of force. No decline was observed in intramuscularly recorded M wave amplitude, while the areas of the total and half M wave forms increased due to a slowing in conduction velocity. The area measured over a fixed time period declined. No evidence was obtained that these M wave potentials were contaminated by electrical activity arising in adjacent muscles. The size of the single unit potentials appeared to remain unaltered during maximal voluntary activity. 3. We conclude that neuromuscular block is not a cause of force loss during this type of fatiguing voluntary contraction.