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PubMed1980

The relationship of Motor-unit activation to isokinetic muscular contraction at different contractile velocities.

This investigation was designed to examine 1) the relationship between motor-unit activation (as recorded by integrated EMG) and speed of contraction and 2) the relationship between mechanical work, power output, peak torque, average torque, and both velocity of movement and integrated electromyographic recordings in the elbow flexor muscles. A series of isokinetic contractions of the elbow flexor muscles was performed by six normal subjects over a range of contractile velocities. Integrated electromyographic discharge and mechanical torque were recorded simultaneously. The results of an analysis of variance, corrected for repeated measures, indicated that both torque and motor-unit electrical activity decreased as contractile velocity increased. The relationship between torque and integrated electromyographic activity was linear and highly significant (r = .95 and r = .93). Implications of a neural interpretation of the in vivo torqu-velocity relationship in muscle are discussed.

Electromyography↗
PubMed1987

Relative dynamic muscular endurance of patients with neuromuscular disorders and of healthy matched control subjects.

This study was undertaken to determine whether the relative muscular endurance of patients with paresis secondary to neuromuscular disorders was different from that of comparable healthy persons. Using an isokinetic dynamometer, I measured the peak knee extension torque of five patients with neuromuscular disorders and of five matched controls. Torque was measured during an initial and 40 subsequent maximal knee extensions at 180 degrees/sec. Gravity-corrected peak torques of the initial repetition and every fifth subsequent repetition were compared between groups, as were the percentages of decrease in torque (calculated relative to the initial torque) using two-way analysis of variance procedures. The results of these procedures demonstrated that the healthy subjects were significantly stronger than the patients with paresis (p less than .01) but that the patients had significantly better relative muscular endurance (ie, smaller percentages of decrease in torque) than the healthy subjects (p less than .05). These findings suggest that, for the patients tested, the relative muscular endurance of patients with paresis is not compromised. This finding does not preclude patients with paresis being unable to complete multiple repetitions of specific functional tasks requiring some critical level of muscular strength.

Adult↗
PubMed1994

Use of electrical stimulation to enhance recovery of quadriceps femoris muscle force production in patients following anterior cruciate ligament reconstruction.

BACKGROUND AND PURPOSE: Electrical stimulation has been shown to be effective in aiding the recovery of quadriceps femoris muscle force production after anterior cruciate ligament reconstruction. The actual dosage of stimulation (training intensity) has not been well described. The purpose of this investigation was to establish a dose-response curve for electrical stimulation regimens designed to improve quadriceps femoris muscle recovery in patients after anterior cruciate ligament reconstruction. SUBJECTS AND METHODS: We analyzed data from a subsample (n = 52) of patients receiving electrical stimulation (N = 110) who were involved in a large, multicenter randomized clinical trial investigating treatment strategies designed to enhance quadriceps femoris muscle recovery. Fifty-two subjects (40 male, 12 female), with an age range of 15 to 43 years (mean = 25, SD = 7), participated in 4 weeks of quadriceps femoris muscle training using either portable, battery-powered home stimulators or console stimulators designed for clinical use. Training intensities were monitored by logging the electrically elicited knee extension torque and expressing this torque as a percentage of the uninvolved quadriceps femoris muscles' maximal voluntary contraction force. After the 4 weeks of training, isometric muscle torque was assessed and a dose-response curve was generated. The relationship between training intensity and quadriceps femoris muscle torque was assessed with Pearson Product-Moment Correlation Coefficients. RESULTS: A significant, linear correlation was found between training intensity and quadriceps femoris muscle torque. Subjects training with console, clinical generators trained at higher intensities than those training with portable, battery-operated generators; such training resulted in higher quadriceps femoris muscle torque. CONCLUSION AND DISCUSSION: These results support the use of high-intensity electrical stimulation and do not support the use of low-intensity or battery-powered stimulators when the goal is recovery of quadriceps femoris muscle force production in the early phases of rehabilitation after anterior cruciate ligament surgery. [Snyder-Mackler L, Delitto A, Stralka SW, Bailey SL. Use of electrical stimulation to enhance recovery of quadriceps femoris muscle force production in patients following anterior cruciate ligament reconstruction.

Adolescent↗
PubMedDate not supplied

The etiology of loosened connections in hospital-grade plugs.

The loosening of terminal connections over time in wired, hospital-grade plugs has been reported. This study was performed to determine the extent of the problem and the cause(s) of the loosening connections. Of 45 hospitals responding to the survey, 15 reported a total of 25 melted plugs. Terminal torques were measured on the plugs of ninety-nine randoMly selected medical devices; 21% of the inspected torques measured zero in-lbs. Static mechanical tests were performed on the plug terminals of five manufacturers. A maximum decrease in torque of 18% was noted over a 6-month period. The combined affects of current cycling and cold flow on the terminal torques were also noted to be about 43% over a six-month period. Loosening torques appear to be more frequently associated with appliances requiring high currents and those that are frequently inserted and withdrawn from receptacles. It is concluded that a serious problem exists with loosened terminals in hospital-grade plugs. Until a design change can correct this screw loosening, maintenance groups have a responsibility to assure adequately low wire-to-terminal resistances. A semi-annual torquing of the plug connections of critical function medical devices should be considered.

California↗
PubMed2002

State of the art review: techniques to avoid pin loosening and infection in external fixation.

UNLABELLED: The purpose of this manuscript is to review techniques of optimizing the interface between the bone and pin in external fixation to minimize pin loosening and infection. CONCLUSIONS: Among the different techniques to improve the bone-pin interface in external fixation, coating the pins with hydroxyapatite proved to be the most effective. In a highly loaded animal study, three pin types were compared. Type A remained uncoated, type B was coated with hydroxyapatite, and Type C was coated with titanium. Radiographic rarefaction of the bone pin tract was lower in type B pins. Extraction torque was thirteen times higher in type B pins compared to type A and two times higher compared to type C pins. Extraction torque was significantly lower compared to the corresponding insertion torque in both types A and C. In contrast, in the hydroxyapatite coated pins there was no difference between extraction and insertion torque. At sixty times magnification, bone pin contact of type B and C pins was significantly higher than type A. At 10,000 times magnification direct bone pin contact was found only in type B pins. In a clinical study the pin insertion and extraction torque forces were measured in a study of seventy-six external fixation pins in nineteen patients treated with hemicallotasis for osteoarthritis of the medial side of the knee. The patients were randomized to be treated with either standard tapered pins or tapered pins coated with hydroxyapatite. Extraction torque of the hydroxyapatite coated pins was higher than the standard ones in both cancellous and cortical bone. These studies show that in hydroxyapatite coated pins there is no deterioration of the bone-pin interface strength and there is optimal bone-pin contact. Among the various pin types coated with hydroxyapatite, the best results were obtained with the tapered pins.

Animals↗
PubMed2000

Motor response of the leg muscles produced by position-selective stimulation of spinal nerve roots.

OBJECTIVE: To define and measure motor responses of the leg muscles in the ankle associated with position-selective and tetanic stimulation of spinal nerve roots L3-S1. METHODS: Sixteen lumbosacral spinal nerve roots in 14 subjects were stimulated intraoperatively after surgical exposure and decompression for a herniated disc. Each contact of a spiral cuff multielectrode was wrapped around the root and used to excite a spatially defined population of axons beneath the electrode. The motor response from each stimulated position was evaluated in terms of three-dimensional vector torque in the ankle. RESULTS: Each position at which the stimulating electrode was placed around the root exhibited the same vector torque qualitatively, but at different thresholds. The root was most excitable ventrally. The S1 roots responded with a uniform three-dimensional torque pattern: plantar flexion plus lateral leg and foot rotation plus inversion. All L5 roots responded by plantar flexion. Dorsiflexion torque was possible only with stimulation of the L3 and L4 roots. Eversion was not possible with stimulation of the S1 roots or with most of the L5 roots. CONCLUSION: Position-selective stimulation of the extrathecal spinal nerve roots influences the threshold of the biomechanical response, the torque recruitment dynamics, and the magnitude of three-dimensional vector torque. Selective activation of some leg muscles or agonist muscle groups with stimulation of a single nerve root could not be achieved owing to the low spatial selectivity of the stimulation design and/or the low muscle specificity of motor fascicles in the root. Direct extrathecal stimulation of spinal nerve roots has some hypothetical advantages over stimulation of other sites along the peripheral nerves, owing to their unique anatomy, and may contribute to functional electrical stimulation of the lower extremities. Further investigation with a more selective multielectrode configuration and the use of multiple root stimulation is suggested.

Adult↗
PubMed1985

Torsional fatigue of the lumbar intervertebral joints.

The effect of cyclic torsional loads on the behavior of intact lumbar intervertebral joints was investigated. A total of 24 specimens, divided in two groups, were tested. In the first group, a specimen was subjected to a specified constant cyclic torque, and the resulting change in angular displacement with time was recorded. The specimens in the second group were subjected to a specified constant angular displacement and the resultant torque recorded. The results of the constant torque tests showed that fatigue failure was primarily a function of the averaged initial angular displacement and secondarily of the applied torque. The critical threshold values appear to be an initial angular displacement of +/- 1.5 degrees. All specimens, displaying an averaged initial angular displacement of less than +/- 1.5 degrees, did not fail in fatigue, while the others did. However, the specimens with angular displacements of less than +/- 1.5 degrees had lower applied torques (11.3 and 22.6 Nm), while specimens with higher applied torques (33.9 and 45.2 Nm) all exceeded the +/- 1.5 degrees threshold and failed in fatigue. Failure locations occurred in such diverse regions as end-plates, facets, laminae, capsular ligaments, etc. All specimens exhibited a synovial fluid discharge from the apophyseal joint capsule sometime during testing. Post-test examinations of all the cartilage surfaces showed fibrillation, whether or not the intervertebral joint failed.

Adult↗
PubMed1992

The effect of injury on rotational coupling at the lumbosacral joint. A biomechanical investigation.

The lumbosacral joint is frequently indicated as a source of low-back pain, a cause of which may be abnormal patterns of vertebral motions. The goal of this study was to describe the influence of injury on the coupled motions of the L5-S1 joint in a human cadaveric model. Nine whole lumbosacral spine specimens were studied under the application of flexion, extension, left/right axial torque and right/left lateral bending pure moments. Injuries to the posterior ligaments, intervertebral disc, and articular facets at L5-S1 were produced, and the motion at L5-S1 was determined after each sequential injury. No significant coupled rotations were observed under flexion or extension moments. Under axial torque, lateral rotation at L5-S1 occurred to the same side as the applied torque and increased significantly only after injury to the intervertebral disc. Also coupled to axial torque was flexion rotation in the intact specimen, which became extension rotation after facetectomy. Under lateral bending moments, coupled axial rotation was to the opposite side of the applied moment and increased significantly only after removal of the facets of L5. Based on these results, it was concluded that intervertebral disc most resisted the coupled motion of lateral rotation under the application of axial torque, whereas the articular facets most resisted the coupled axial rotation under the application of lateral bending at the lumbosacral joint. Also, the facets were the structures that produced the flexion rotation of L5 on S1 under axial torque loading.

Back Pain↗
PubMed1998

Moments and forces during pedicle screw insertion. In vitro and in vivo measurements.

STUDY DESIGN: Moments and forces during pedicle screw insertion were measured in vivo and in vitro and were correlated to several parameters of the screw-bone interface. OBJECTIVES: To compare the in vitro and in vivo screw insertion loads and to relate these measurements to bone mineral density, pedicle size, and other screw parameters (material, diameter). SUMMARY OF BACKGROUND DATA: The in vitro screw insertion torque has been correlated to the screw pullout forces and the number of cycles to ultimate interface failure. However, there are no comparable in vivo data. METHODS: One hundred three pedicle screws were included in the study, 43 in vivo and 60 in vitro. Duel-energy x-ray absorptiometry boen mineral density data were available for 20 in vivo and 32 in vitro specimens. A custom-made sterilizable six-axis load cell was integrated into a torque wrench, enabling the recording of the applied moments and forces during screw insertion. Statistical analysis was performed to detect differences and correlations. RESULTS: The mean in vivo insertion torque (1.29 Nm) was significantly greater than the in vitro value (0.67 Nm). The linear correlation between insertion torque and bone mineral density was significant for the in vitro data but not for the in vivo data. No correlation was observed between insertion torque and pedicle diameter. Two patterns of torque were observed during the insertion process. CONCLUSIONS: There is a significant difference between the insertion loads measured in vivo and those measured in vitro. Additional research is needed to verify whether this method provides an indication of screw fixation quality.

Absorptiometry, Photon↗
PubMed1999

Test-retest reliabilty of isokinetic hip adductor and flexor muscle strength.

OBJECTIVE: To determine the test-retest reliability of eccentric hip flexor and adductor peak torque and endurance using a Cybex Norm isokinetic dynamometer and to evaluate muscle soreness after exercise as a result of the testing procedure. DESIGN: Prospective test-retest reliability study. PARTICIPANTS: A total of 19 male volunteers (21-43 years old). INTERVENTION: Tests included hip flexor and adductor peak torque strength measured at 60 degrees/s (concentric and eccentric) and endurance measurements (total work done over 20 continuous concentric/eccentric contractions at 150 degrees/s). Subjects were tested on three occasions at 1-week intervals. MAIN OUTCOME MEASURES: Repeated measures analysis of variance was performed for all dependent variables including the subject's dominant and nondominant side among the three test sessions. Intraclass correlation coefficients (ICCs) were calculated to compare the data between sessions. In addition, simple linear regression analysis was completed to determine the relationship between session measurements for all measures with ICC > 0.75. RESULTS: The main findings of this study are that eccentric peak hip adductor torque was reliably measured on a Cybex Norm isokinetic dynamometer using a continuous eccentric/ concentric cycle protocol at a speed of 60 degrees/s (ICC > 0.84). Peak concentric adductor torque and peak eccentric and concentric hip flexor torque were less reliable using the same protocol (ICC < 0.67). Endurance measurements, using total work done over 20 repetitions, were not reliable for hip flexor and adductor muscle groups using a continuous eccentric/concentric cycle protocol at a speed of 150 degrees/s (ICC < 0.55). Posttest muscle soreness was found to be of significant concern with such a protocol. CONCLUSION: The results from this test-retest reliability study confirm the reliability of eccentric peak hip adductor torque measurements for use clinically and in future research. In addition, the need to assess posttest muscle soreness after isokinetic muscle testing is confirmed.

Adult↗
PubMed1989

Tetanic responses of electrically stimulated paralyzed muscle at varying interpulse intervals.

The influence of stimulus interpulse interval (IPI) on torque output during electrically-evoked contractions was investigated for the knee extensor muscles of paralyzed subjects. The parameters measured were the rise time, magnitude, and relaxation time of the contraction at stimulus IPI's ranging from 62 to 7 ms. Torque output increased as IPI's were decreased from 62 to 15 ms. Peak torques were recorded at IPI's of 12-15 ms; IPI's less than these resulted in an insignificant loss of torque. Rise times decreased as IPI's were decreased. Relaxation time generally increased as IPI's were decreased with the longest relaxation times occurring with stimulation at an IPI of 12 ms. To demonstrate the influence of IPI on muscle fatigue, the effect of prolonged stimulation at short (12 ms) and long (50 ms) IPI's was also compared. After 30 s of stimulation with an IPI of 12 ms, mean torque had declined to 5 +/- 3 percent and after 30 s of stimulation with an IPI of 50 ms, mean torque had declined to 82 +/- 4 percent of the initial value. Knowledge of how stimulus IPI influences the response of paralyzed muscle to electrical stimulation may assist in the development of rehabilitation devices which utilize these technologies.

Electric Stimulation Therapy↗
PubMed1987

The effects of beta-blockade on electrically stimulated contraction in fatigued human triceps surae muscle.

The contractile characteristics of the triceps surae muscle group were examined before and after repeated isometric contractions in two groups of eight healthy young males. Single twitches and trains of stimuli at 10, 20, 50 and 100 Hz were delivered to the muscle using supramaximal voltages. Subjects were treated with beta-blockade (2 X 80 mg oral propranolol, beta-b) or matched placebo in a double-blind crossover design. Four different exercise conditions were studied: (I) maximal voluntary contraction (MVC); (II) MVC during circulatory occlusion; (III) electrical stimulation at 20 Hz using 50% of voltage required for maximal torque production; and (IV) electrical stimulation with occlusion. Each contraction was for 5 s with 5 s recovery. Total duration of exercise was 10 min for non-occluded contractions and to a 50% decline in torque output with occlusion. At rest prior to exercise, maximal voluntary contraction was significantly reduced (5.7%) by beta-b during 40 observations in 16 subjects. Following exercise without occlusion (I and III), the reduction in torque output of the muscle at 10 and 20 Hz stimulation was generally greater during beta-b than placebo. This low frequency fatigue was longer-lasting with beta-b. The shorter lasting reduction in torque at 50 and 100 Hz was generally not different between beta-b placebo. After exercise with occlusion (II and IV), the torque output at all stimulation frequencies was reduced to a similar extent in both placebo and beta-b at most comparison points. Twitch responses after exercise with occlusion showed decreases in peak tension and time to peak tension and a lengthening of one-half relaxation time in both placebo and beta-b. It was concluded that the greater reduction in torque output of the triceps surae muscle group at low frequencies during beta-b was probably a consequence of a reduction in blood flow relative to the placebo treatment. This relative low frequency fatigue could be responsible for the increased perception of effort in patients exercising during beta-blocker therapy.

Adult↗
PubMed1989

Does post-tetanic potentiation compensate for low frequency fatigue?

The purpose of this study was to examine the effect of post-tetanic potentiation on low frequency fatigue in adult human quadriceps muscle. Sixteen subjects (10 male and six female) performed three 60 s sets of knee extension exercise in order to induce low frequency fatigue (reduction in torque output at 10 and 20 Hz). The potentiating stimulus (a 10 s maximal voluntary contraction) induced a 58% increase in twitch tension (Pt) during the pre-fatigue state. Immediately following the fatiguing exercise, torque (X +/- SE, Nm) at 10 and 20 Hz (submaximal transcutaneous stimulation, 50 microsecond pulses) decreased (P less than 0.05) from 54.8 +/- 5.8 and 94.9 +/- 9.6 to 40.3 +/- 6.1 and 77.0 +/- 11, respectively. Although potentiation at this time increased Pt from 40.9 +/- 4.0 to 54.8 +/- 3.7 (P less than 0.05), torque at 10 and 20 Hz was unaffected. At 60, 120 and 240 min post-contraction, torque at 10 and 20 Hz remained depressed. Following potentiation, which increased twitch tensions to between 64 and 75%, torque at 10 Hz was increased (P less than 0.05) at 60 min (36.3 +/- 4.1 vs. 50.7 +/- 6.2), 120 min (40.8 +/- 6.3 vs. 56.5 +/- 8.9) and 240 min (42.0 +/- 4.7 vs. 57.5 +/- 8.3) of recovery. Similar effects were also noted at 20 Hz. These findings indicate that post-tetanic potentiation can overcome the low frequency fatigue during the post-contraction period and restore torque to pre-exercise levels.

Adult↗
PubMed1997

Determinants of musculoskeletal flexibility: viscoelastic properties, cross-sectional area, EMG and stretch tolerance.

Cross-sectional area, stiffness, viscoelastic stress relaxation, stretch tolerance and EMG activity of the human hamstring muscle group were examined in endurance-trained athletes with varying flexibility. Subjects were defined as tight (n = 10) or normal (n = 8) based on a clinical toe-touch test. Cross-sectional area was computed from magnetic resonance imagining (MRI) images. Torque (Nm) offered by the hamstring muscle group, electromyographic (EMG) activity, knee joint angle and velocity were continuously monitored during two standardized stretch protocols. Protocol 1 consisted of a slow stretch at 0.087 rad/s (dynamic phase) to a pre-determined final angle followed by a 90-s static phase. In the dynamic phase final angle and stiffness was lower in tight (28.0+/-2.9 Nm/rad) than normal subjects (54.9+/-6.5 Nm/rad), P<0.01. In the static phase tight subjects had lower peak (15.4+/-1.8 Nm) and final torque (10.8+/-1.6 Nm) than normal subjects (31.6+/-4.1 Nm, 24.1+/-3.7 Nm, respectively)(P<0.01), but torque decline was similar. Protocol 2 consisted of a slow stretch to the point of pain and here tight subjects reached a lower maximal angle, torque, stiffness and energy than normal subjects (P<0.01). On the other hand, stiffness was greater in tight subjects in the common range (P<0.01). Cross-sectional area of the hamstring muscles and EMG activity during the stretch did not differ between the groups. However, lateral hamstring cross-sectional area was positively related to mid-range stiffness (P<0.05), but inversely related to final stiffness, peak torque and the toe-touch test (P<0.01). Final angle and peak torque in protocol 1 combined to improve the predictability of the toe-touch test (R2=0.77, P<0.001). These data show that the toe-touch test is largely a measure of hamstring flexibility. Further, subjects with a restricted joint range of movement on a clinical toe-touch test have stiffer hamstring muscles and a lower stretch tolerance.

Biomechanical Phenomena↗
PubMed1999

Anterior tibial translation during eccentric, isokinetic quadriceps work in healthy subjects.

The effect of increasing isokinetic, eccentric quadriceps torques on sagittal translation of the tibia was examined in six healthy volunteers and compared to the translation at 20 degrees of knee flexion during a drawer test with 90 N force. The tibial translation increased in a linear fashion with a mean of 0.5 mm per 20% torque increase. In 20 degrees of knee flexion, 10% of eccentric quadriceps peak torque consumed 80% of the anterior tibial translation induced by the 90 N Lachman test while eccentric quadriceps peak torque utilized 100% of the translation at the same test. The in vivo relation between muscle force and tibial translation is of importance in the treatment of patients with injury to the cruciate ligaments. The results indicate that an already low eccentric quadriceps torque causes a tibial translation that reaches the limit of the passive knee joint displacement where strain is assumed to develop in the anterior cruciate ligament. Already low eccentric quadriceps torque levels may therefore be harmful during rehabilitation after anterior cruciate ligament surgery.

Adult↗
PubMed1991

Responses to eccentric and concentric resistance training in females and males.

The adaptive responses to 12 weeks of accommodated resistance training were compared in females (n = 11) and males (n = 11). They performed four to five sets of six maximum bilateral coupled concentric and eccentric quadriceps muscle actions at 1.05 rad s-1 three times per week. Uni- and bilateral concentric and eccentric peak torque at different angular velocities (0.52, 1.57 and 2.62 rad s-1), three repetition maximum half-squat and vertical jump height were measured before and after training. Both groups displayed marked increases (P less than 0.05) in concentric and eccentric peak torque at all angular velocities. The relative increases (P less than 0.05) in unilateral concentric (26 vs. 26%) and eccentric (28 vs. 36%) peak torque across speeds were similar in females and males. The corresponding increases (P less than 0.05) in bilateral concentric and eccentric peak torque across speeds were 20 vs. 28% and 24 vs. 39% respectively. The three repetition maximum half-squat (20 vs. 25%) and vertical jump height (10 vs. 8%) increased (P less than 0.05) equally in females and males. These results suggest that the overall increases in concentric and eccentric peak torque and functional strength, in response to short-term accommodated resistance training, occur at a rate that is independent of sex. The torque-velocity relationship, however, appears to change in males suggesting a relatively greater enhancement of maximum voluntary force in the slow-speed, high-force region.

Adult↗
PubMed1984

Corticomotoneuronal cells contribute to long-latency stretch reflexes in the rhesus monkey.

To test the hypothesis that a transcortical reflex contributes to the stretch-evoked long-latency electromyographic (e.m.g.) response we documented the responses of identified corticomotoneuronal (c.m.) cells and their target muscles to perturbations of active wrist movements. Macaque monkeys performed ramp-and-hold wrist movements against elastic loads, alternating between flexion and extension zones; brief (25 ms) torque pulses were intermittently applied during the hold period. C.m. cells were identified by a clear post-spike facilitation in spike-triggered averages of forelimb muscle e.m.g. activity. Activity of c.m. cells and twelve wrist and digit flexor and extensor muscles was recorded during: (a) active ramp-and-hold wrist movements, (b) passive ramp-and-hold wrist movements, and (c) torque perturbations applied during the hold phase of active flexion and extension which either lengthened or shortened the c.m. cell's target muscles. Muscle-lengthening perturbations evoked a reproducible pattern of average e.m.g. activity in the stretched muscles, consisting of two peaks: the first response (M1) had an onset latency of 11.2 +/- 2.1 ms (mean +/- S.D.), and the second (M2) began at 27.9 +/- 5.1 ms. Torque perturbations which shortened the active muscles also evoked a characteristic e.m.g. response consisting of an initial cessation of activity at 13.5 +/- 3.4 ms followed by a peak beginning at 33.9 +/- 3.0 ms. The responses of twenty-one c.m. cells which facilitated wrist muscles were documented with torque pulse perturbations applied during active muscle contraction. Twenty of twenty-one c.m. cells responded at short latency (23.4 +/- 8.8 ms) to torque perturbations which stretched their target muscles. For each c.m. cell-target muscle pair, transcortical loop time was calculated as the sum of the onset latency of the c.m. cell's response to lengthening perturbations (afferent time) and the onset latency of post-spike facilitation (efferent time). The mean transcortical loop time was 30.4 +/- 10.2 ms, comparable to the mean onset latency of the M2 peak (27.9 +/- 5.1). The duration of a c.m. cell's response to torque perturbations provides a further measure of the extent of its potential contribution to the M2 muscle response. In all cases but two, the c.m. cell response, delayed by the latency of the post-spike facilitation, overlapped the M2 e.m.g. peak.

Action Potentials↗
PubMed1987

Muscle fibre conduction velocity in motor units of the human anterior tibial muscle: a new size principle parameter.

1. Twitch contractions were elicited in human anterior tibial muscle by intramuscular microstimulation of single motor axons with a bipolar needle electrode. The population of stimulated motor units studied was fairly representative for the muscle. 2. The conduction velocity of the fibres in the motor unit was calculated as the ratio between the electrode separation (15 mm) in a tripolar array of surface electrodes and the conduction delay of the motor unit potential along the electrode array. The motor unit conduction velocity ranged from 2.6 to 5.3 m/s with a mean of 3.7 m/s. 3. The contractile properties of the motor units were obtained by averaging the torque developed around the ankle joint. Twitch torques ranged from less than 10(-3) to 16 x 10(-3) N m, with a mean of 5.7 x 10(-3) N m. The twitch torque of the whole anterior tibial muscle was approximately 5 N m. Rise times were 47-80 m/s with a mean of 61 m/s, and half-relaxation times were 40-78 ms with a mean of 60 ms. 4. The mechanical properties of individual motor units were highly correlated (rise time and twitch torque: r = -0.81; rise time and half-relaxation time: r = 0.75; twitch torque and half-relaxation time: r = -0.81). 5. The motor unit conduction velocity was highly correlated to twitch torque (r = 0.87), rise time (r = -0.75) and half-relaxation time (r = -0.66). This indicates that the motor unit conduction velocity can be included in the family of interrelated 'size principle parameters'.

Axons↗