Subject's Muscle Research Articles

Relationships Between Muscle Power Output Using the Stretch-Shortening Cycle and Eccentric Maximum Strength

This study aimed to examine the relationships between muscle power output using the stretch-shortening cycle (SSC) and eccentric maximum strength under elbow flexion. Eighteen young adult males pulled up a constant light load (2 kg) by ballistic elbow flexion under the following two preliminary conditions: 1) the static relaxed muscle state (SR condition), and 2) using the SSC with countermovement (SSC condition).Muscle power was determined from the product of the pulling velocity and the load mass by a power measurement instrument that adopted the weight-loading method. We assumed the pulling velocity to be the subject's muscle power parameters as a matter of convenience, because we used a constant load. The following two parameters were selected in reference to a previous study: 1) peak velocity (m x s(-1)) (peak power) and 2) 0.1-second velocity during concentric contraction (m x s(-1)) (initial power). Eccentric maximum strength by elbow flexion was measured by a handheld dynamometer.Initial power produced in the SSC condition was significantly larger than that in the SR condition. Eccentric maximum strength showed a significant and high correlation (r = 0.70) with peak power in the SSC condition but not in the SR condition. Eccentric maximum strength showed insignificant correlations with initial power in both conditions. In conclusion, it was suggested that eccentric maximum strength is associated with peak power in the SSC condition, but the contribution of the eccentric maximum strength to the SSC potentiation (initial power) may be low.

Predicting Ankle Joint Moments Using A Hybrid EMG-driven Model

To study sport medicine injuries and their effects on human motion, accurate biomechanical models are important. For these models to characterize the response to injury, they must take into account the differences in the ways injured people move their limbs and activate their muscles. We have created a biomechanical model of the ankle that uses subject specific muscle activation as an input and predicts the corresponding joint moment (plantar-flexion/dorsiflexion). PURPOSE To demonstrate the ability of our model to accurately predict joint moments given electromyograms (EMGs) and joint position data. METHODS Three types of data were collected during isokinetic and gait trials: (1) EMG from the tibialis anterior, medial gastrocnemius, lateral gastrocnemius, and soleus; (2) joint position, (3) and reaction forces (from the ground or dynamometer). Ankle joint moment was calculated in two ways: (1) forward dynamics using EMG and joint position data, and (2) inverse dynamics using joint position and reaction force data. The joint moment determined from the inverse dynamics calculation was used to calibrate the forward dynamic estimation of the moment. Adjustable parameters in the forward dynamics model were optimized to produce a best fit. Once calibrated, only the forward dynamics model was used to predict the joint moments from novel trials. RESULTS Preliminary data were collected from three test subjects. A comparison between the predicted and the measured ankle joint moment was performed; root mean squared error was approximately 10%, and R-squared values were of 0.95, 0.95, 0.87 for each subject respectively. CONCLUSION The curve shape was very closely matched in the prediction of concentric tasks, but the largest errors were observed during eccentric tasks. Further adjustment and refinement of the model parameters should correct this. One of the major strengths of the model is that is allows estimation of muscle forces (in contrast to inverse dynamics models) and relies on a subject's actual muscle activation values (in contrast to optimization approaches). Supported by NIH grant R01 HD38582.

Predicting Ankle Joint Moments Using A Hybrid EMG-driven Model

To study sport medicine injuries and their effects on human motion, accurate biomechanical models are important. For these models to characterize the response to injury, they must take into account the differences in the ways injured people move their limbs and activate their muscles. We have created a biomechanical model of the ankle that uses subject specific muscle activation as an input and predicts the corresponding joint moment (plantar-flexion/dorsiflexion). PURPOSE To demonstrate the ability of our model to accurately predict joint moments given electromyograms (EMGs) and joint position data. METHODS Three types of data were collected during isokinetic and gait trials: (1) EMG from the tibialis anterior, medial gastrocnemius, lateral gastrocnemius, and soleus; (2) joint position, (3) and reaction forces (from the ground or dynamometer). Ankle joint moment was calculated in two ways: (1) forward dynamics using EMG and joint position data, and (2) inverse dynamics using joint position and reaction force data. The joint moment determined from the inverse dynamics calculation was used to calibrate the forward dynamic estimation of the moment. Adjustable parameters in the forward dynamics model were optimized to produce a best fit. Once calibrated, only the forward dynamics model was used to predict the joint moments from novel trials. RESULTS Preliminary data were collected from three test subjects. A comparison between the predicted and the measured ankle joint moment was performed; root mean squared error was approximately 10%, and R-squared values were of 0.95, 0.95, 0.87 for each subject respectively. CONCLUSION The curve shape was very closely matched in the prediction of concentric tasks, but the largest errors were observed during eccentric tasks. Further adjustment and refinement of the model parameters should correct this. One of the major strengths of the model is that is allows estimation of muscle forces (in contrast to inverse dynamics models) and relies on a subject's actual muscle activation values (in contrast to optimization approaches). Supported by NIH grant R01 HD38582.

Maintenance of whole muscle strength and size following resistance training in older men.

Following a progressive resistance training (PRT) program of 3 days per week, we sought to examine how effective a resistance training maintenance program of 1 day per week would be to preserve muscle strength and size in older men. Each subject's whole muscle strength (1 repetition maximum, or 1RM) and whole muscle size (determined by computed tomography scan) were measured before (T1) and after (T2) 12 weeks of PRT and again following 6 months (T3) of training (TR) or detraining (DT). During the 12-week PRT, older men (N = 10; age 70 +/- 4 years) trained their knee extensors 3 days per week at 80% of their 1RM. The maintenance program consisted of older men (n = 5; TR; 75 +/- 1 years) who completed 3 sets of 10 repetitions at 80% of their 1RM 1 day per week (this was equivalent to a single training session that was performed 3 days per week during the 12-week PRT). The other group of older men (n = 5; DT; 69 +/- 1 years) resumed their normal lifestyle (no regular physical activity) following the 12-week PRT. From T1 to T2, muscle strength increased (p <.05) 45% (66 +/- 10 to 94 +/- 10 kg) in the TR group and 53% (50 +/- 6 to 74 +/- 7 kg) in the DT group. From T2 to T3, whole muscle strength of the TR group was unchanged (96 +/- 11 kg), whereas strength decreased (p <.05) in the DT group by 11% (66 +/- 6 kg). Muscle size demonstrated a similar pattern with a 7% increase (p <.05) in both groups from T1 to T2. No change in muscle size was found in the TR group from T2 to T3, whereas the DT group had a 5% reduction (p <.05). These data indicate that resistance training 1 day per week was sufficient to maintain muscle strength and size in these older men following a 12-week PRT program. Furthermore, the men who resumed their normal lifestyle (no regular physical activity) experienced significant losses in muscle strength and size.

The response of muscle interstitial prostaglandin E2(PGE2), prostacyclin I2(PGI2) and thromboxane A2(TXA2) levels during incremental dynamic exercise in humans determined by in vivo microdialysis

The microdialysis in vivo technique allows the isolation, purification and quantitative determination of bioactive molecules with low molecular weight (<20.000 Da) from interstitial fluid (IF) of the muscles. PGE2and PGI2are vasodilator local hormones, while the TXA2is a vasoconstrictor. PGI2and TXA2are unstable and convert to stable products 6-keto-PGF1aand TXB2, respectively. The purpose of this study was to evaluate the response of PGE2, PGI2and TXA2in the IF of human muscle (vastus lateralis) during dynamic exercise with a cycle ergometer. In this study two microdialysis probes were inserted with CMA-60 microdialysis catheters into the vastus lateralis muscle of the right leg of eight healthy volunteers aged 24.1±2.1 years, height 177.5±1.5 cm and body weight 78.1±2.4 kg. After insertion the microdialysis probes perfused at a rate of 3.0 μl/min with Ringer acetate solution. The dialysate fluid was collected a) during the 30′ rest period, b) during the 30′ exercise period at 100 watts, c) during the 30′ exercise period at 150 watts and d) during the 30′ rest period after exercise. Our measurements (by the RIA method) showed that the levels of PGE2and 6-keto-PGF1ain the I.F. of the vastus lateralis muscle increased significantly, while there was a significant decrease in TXB2during exercise. The changes in the above biomolecules were increased proportionately with the strain of the subject's muscle. Conclusion: Dynamic exercise of the muscles produces a local increase of the vasodilators PGE2and PGI2while the vasoconstrictor TXA2is reduced in the IF of the muscles. This is further evidence that exercise induces propitious biochemical changes. Furthermore, the muscle production of arachidonic acid metabolites during exercise depends on the intensity of the exercise.

The effect of endurance exercise on muscle force generating capacity of the lower limbs

The purpose of this study was to investigate the recovery of muscle force generating capacity (FGC) of the lower limbs following a session of cycle exercise (CE). Fourteen male cyclists (mean +/- SD age 25 +/- 4 yrs and VO2max 65.8 +/- 5 ml x kg(-1)min(-1)) performed tests assessing lower limb muscle FGC at rest (pre-test), as well as 6 and 24 hrs following CE performed on a mechanically-braked cycle ergometer. The CE consisted of 30 min at a workload corresponding to the lactate (Dmax) threshold (+/-15 W), and four 60 s rides at 120% VO2max with one min rest between each ride. At the completion of the CE a 6 or 24 hr recovery period was initiated, after which, each subject's muscle FGC was measured. The analysis of lower limb muscle FGC included, (1) 6 s all-out cycle test; (2) a maximal isokinetic leg extension at 60, 120 and 180 degrees x s(-1); and (3) a maximal concentric squat jump. Statistical analysis showed that compared to pre-test levels, a significant reduction in both isokinetic peak torque at 60 degrees x s(-1) and isoinertial maximum force occurred after 6 hrs of recovery. Although not significant, reductions also occurred at 6 hrs of recovery in isokinetic peak torque at 120 and 180 degrees x s(-1), as well as maximum rate of force development (RFD) during the squat jumps. No significant differences were observed between isokinetic peak torque, maximum force or RFD pre-test and following the 24 hr recovery period, indicating these tests had returned to normal by this time. No significant differences were found between peak power (PP) during the 6 s cycle test, pre-test and following either 6 or 24 hrs of recovery. These findings confirm earlier research that maximal voluntary strength is reduced for at least 6 hours following exhaustive dynamic exercise. The reduction in muscle FGC should be considered when resistance training is scheduled after endurance exercise.

Effects of cold stimulation of human skin on motor unit activity.

This study examined the recruitment threshold of motor units (MUs) and the effects of a maintained cold sensation that mediates the cold reflex activation of the cutaneous receptors, produced by cooling skin with cold air. The changes of the threshold force of the motor units in the human biceps muscle were measured during a slow ramp contraction (2.52 +/- 0.13 kg/s) for 3 s, while the skin temperature on the subject's muscle was continuously reduced using a cooling box fixed at -10 degrees C. The temperature range of the skin on the activated muscle was about 33-25 degrees C. In the same trials, MU activities were sampled during two controlled 30% voluntary isometric ramp contractions (VIRCs) with changes in the skin temperature to 31, 29, 27, and 25 degrees C. Data on recruitment behavior were obtained for 51 MUs in 5 subjects. A total of 13/51 MUs were newly recruited by the decrease of threshold force with the skin temperature at 25 degrees C. Also, 38/51 MUs appeared among a ramp contraction at each temperature from control (Tc) to 25 degrees C. The threshold forces of 7/38 MUs were not affected by cold stimulation to the skin, but other MUs were more affected by the stimulation. The threshold forces of 12/38 MUs were decreased and those of 19/38 MUs were increased with the reduction in skin temperature. These data suggest that either an increase or no change of the threshold force affects the low-amplitude and low-threshold-force MUs (LT-MUs), and a decrease in the threshold force affects the high-amplitude and high-threshold-force MUs (HT-MUs). The threshold force of the MUs at Tc vs four cooling temperatures was correlated in all subjects. The slope (coefficient) of the different regression line, however, clearly decreased with the reduction in skin temperature except for 27-25 degrees C (p > 0.05). Therefore, these results suggest that some recruitment thresholds might change among the HT-MUs that have synaptic connections with the peripheral afferent, because the cutaneous afferent input does not contribute to the same extent at all MUs.

Motor unit territory in relation to tendons in the human masseter muscle

In the human masseter, motor-unit (MU) territories are reportedly focal and it is possible that they are confined to the muscle's internal aponeuroses. However, previous electromyographic (EMG) investigations of MU sizes have not correlated them with the muscle's internal architecture. In this study, 162 single-MU territories were assessed by scanning EMG recordings throughout the masseter muscles of 4 subjects. The needle-electrode scans were stereotactically located with magnetic resonance imaging and an optical system capable of tracking the needle movement in three dimensions. Mediolateral territorial dimensions were then displayed graphically within muscle reconstructions. The mean territorial width was 3.7 +/- 2.3 mm and varied between 0.4 and 13.1 mm. The widths were comparable in size to those of previous reports, and were related to the subject's muscle size. Most MU territories were confined, and lay between tendons, although 10% of the units clearly extended across at least one tendon. The dispersion of most territories within discrete tendon-bounded compartments in the masseter provides an anatomical basis for selective activation of the muscle. However, it is also possible that this arrangement provides a flexible means for ensuring tendon stiffness and mechanical adaptation of the multipennate masseter during growth and development, whether or not the muscle is activated selectively.

Step-tracking movements of the wrist. III. Influence of changes in load on patterns of muscle activity

Human subjects performed step-tracking movements of the wrist in the radial direction. Movement amplitude, external load, and accuracy instructions were varied. We used surface electrodes to record muscle activity from an agonist, extensor carpi radialis longus, and an antagonist, extensor carpi ulnaris. When subjects performed movements "as fast as possible" that were opposed by different external loads, we observed two distinct patterns of modulation of the agonist burst. In one pattern, termed pulse-height modulation, the force of the agonist muscle was graded by varying the peak amplitude of a short-duration agonist burst. This pattern occurred when subjects performed movements of different amplitudes with a lightweight manipulandum. In the other pattern, termed pulse-width modulation, the force of the agonist muscle was graded by varying the duration of an agonist burst of nearly maximal amplitude. When the agonist burst was prolonged, the onset of antagonist activity was delayed. This pattern occurred when subjects performed movements of different amplitudes that were opposed by elastic or viscoelastic loads applied to a heavy manipulandum. The strongest subject exhibited more pulse-height modulation and less pulse-width modulation of the agonist burst than other subjects. Conversely, the weakest subject displayed more pulse-width modulation of the agonist burst than other subjects. These observations indicate that the force requirements of a task, relative to the force generating capacity of a subject's agonist muscle(s), have a significant influence on the pattern of agonist modulation. In a second experiment using three nonhuman primates, we observed that agonist bursts in wrist flexor and extensor muscles exhibited strikingly different patterns of modulation. For wrist flexion, agonist bursts in wrist flexors were brief and displayed pulse-height modulation when movement amplitude was varied. For wrist extension, agonist bursts in wrist extensors were prolonged and displayed largely pulse-width modulation when movement amplitude was varied. We suggest that the distinct patterns of modulation observed in the wrist muscles of monkeys were due to differences in the strength of wrist flexors and extensors, rather than to alterations in movement strategy. In a third experiment, we instructed human subjects to be "accurate" when they made step-tracking movements. When subjects performed movements with a lightweight manipulandum, most displayed short-duration agonist bursts that were pulse-height modulated. When subjects performed "accurate" movements that were opposed by elastic loads, they displayed pulse-width modulation of a small-amplitude agonist burst.(ABSTRACT TRUNCATED AT 400 WORDS)

Peak Torque Occurrence in the Range of Motion During Isokinetic Extension and Flexion of the Knee

The purpose of this study was to 1) assess the knee angles of hamstring and quadriceps peak torques at slow (60 degrees.sec-1) and moderate (180 degrees.sec-1) isokinetic velocities in healthy adult males (N = 143) and females (N = 106), 2) determine if velocity has an effect on these peak torque angles, and 3) determine if subject's age, sex, and muscle strength have an effect on the results. The Cybex II dynamometer was used to record the measurements. At 60 degrees.sec-1, the mean peak torque for the hamstrings occurred at 33 degrees for men and 37 degrees for women (p < 0.001) (0 degree = full extension). At 180 degrees.sec-1, the corresponding angles were 40 and 44 degrees (p < 0.01). In both groups the increase was 7 degrees (p < 0.001). At 60 degrees.sec-1, the peak torque angle of the quadriceps was 54 degrees in both groups. At 180 degrees.sec-1, the angle occurred significantly later in men (11 degrees later) and in women (10 degrees later) (p < 0.001 for both). Subject's age (18 to 40 years) did not affect the results. In female hamstrings, the knee angle of peak torque had a highly significant (p < 0.001) negative correlation to muscle strength; that is, in female hamstrings, the peak torque angles occurred later in the range of motion when muscle strength decreased. This possibly explained the difference observed between men and women in the peak torque angles of hamstrings. In conclusion, people involved in isokinetic testing of muscular performance should be aware that the peak torque occurs later in the range of motion with increasing angular velocity, especially when testing weak muscle groups. In high angular velocities this may become a problem since the limb may pass the optimal joint position for muscular performance, and the recorded peak torque may not represent the subject's maximal torque capacity.

Isokinetics in the assessment of rehabilitation: a case report

The purpose of this study was to quantify the rehabilitation of an English First Division soccer player by using the Cybex II Isokinetic system. The subject had received extensive injuries to the right knee during play. Isokinetic measurement permitted the objective assessment of the initial injury and the subsequent quantification of the effectiveness of the rehabilitation programme. The subject was required to repeat the same test protocol on three occasions 12, 20, and 27 weeks post-operatively. At the initial assessment bilateral comparative data provided the physiotherapist with objective rehabilitation targets based on the subject's own muscle function profile. Comparative data was also made available by testing an uninjured ‘control’ subject. In addition to conventional physiotherapeutic techniques, two different isokinetic devices were used in the subject's daily rehabilitative regime. On the final assessment, 27 weeks post-operatively, the subject showed a 344% increase in the peak torque of the quadriceps, and a 302% increase in the peak torque of the hamstrings. Range of movement increased from 40° to 106° and the bilateral deficit was reduced from 70% to 26% in the quadriceps, and from 52% to 16% in the hamstrings.

Suction applied to a muscle biopsy maximizes sample size

A method for increasing the size of a percutaneous needle biopsy specimen of skeletal muscle is described. Suction (700 TORR) is applied to the inner bore of the biopsy needle after the needle has been inserted into the subject's muscle. The suction pulls the surrounding muscle tissue into the needle, thus insuring the taking of a larger piece (X = 78.5 mg). In most cases, this technique will eliminate the need for repeated biopsies because of inadequate muscle sample size and enhance the validity of subsequent analysis procedures.