ABSTRACT
Disused rat hindlimb caused by sciatic denervation is characterized by osteopenia accompanying alterations in trabecular bone architecture. We studied the effects of short-term denervation followed by reinnervation on the 2-dimensional architecture of trabecular bone using a unilateral sciatic nerve freezing model rat of temporary disuse. Male Fischer-344 rats aged 11-weeks underwent unilateral hind-limb denervation by either sciatic neurectomy (SN) or nerve freezing (NF) by contact with a stainless steel rod cooled in liquid nitrogen, while control rats were sham-operated. Right and left tibiae of denervated and control rats were obtained at 0, 1, 2, 3, 4 and 5 weeks after surgery. Histomorphometric analyses were performed on longitudinal sections of proximal tibial metaphyseal secondary spongiosa. Sciatic denervation by SN or NF resulted in a marked loss of trabecular bone, mostly within first 2 weeks after denervation. Trabecular bone area decreased and gradually recovered with the breaking point at 3 weeks, returning to approximately 55% of basal-control levels (at 0 weeks) by 5 weeks after NF. Both the thickness and length of trabecular bone were significantly decreased after denervation. Trabecular thickness at 5 weeks after NF was significantly greater compared to that at 3 weeks after NF and at 5 weeks after SN, while decreased trabecular length after NF did not during the experimental period. These findings suggest that 1) sciatic nerve freezing results in marked loss of trabecular bone, mostly within the first 2 weeks after surgery ; 2) temporary denervation and subsequent reinnervation reversibly affects trabecular bone architecture, particularly trabecular thickness.
ABSTRACT
Several studies have described "Alternate activity", in which individual muscles of the synergistic muscle group alternate between high activity and silent periods, and rotate in a complementary pattern to maintain constant torque, during sustained low-level contractions. The purpose of this study was to investigate the physiological property of alternate activity among the synergists. Eight human subjects performed static contractions at an ankle joint angle of 110° plantar flexion at 10% maximal voluntary contraction (MVC) for 120 min. Simultaneously, a surface electromyogram (EMG) was recorded of the triceps surae muscles. Doppler ultrasound techniques were used to measure blood velocity and the arterial diameter of the popliteal artery during exercise. Maximal torque at MVC and mean EMG (mEMG) level decreased significantly after sustained isometric low-level contractions, but there was no significant difference in the mEMG/maximal force ratio of triceps surae muscles, which is thought to indicate peripheral fatigue between pre- and post-exercise. Throughout the "Alternate activity", arterial diameter and pulse rate did not change, but blood flow significantly increased (p<0.001) depending on an increase in blood velocity. These results suggest that "Alternate activity" among the triceps surae muscles may be one mechanism that functions either to decrease or to postpone peripheral fatigue during sustained low-level contraction.
ABSTRACT
Disused rat hindlimb caused by sciatic neurectomy is characterized by skeletal muscle atrophy and bone loss accompanying alterations in trabecular bone architecture. We studied the time course responses of the two-dimensional architecture of trabecular bone and the relationships between bone loss and muscle atrophy using a unilateral sciatic nerve denervation model of disuse atrophy.<BR>Fischer-344 male rats aged 11 weeks were denervated. The right and left tibiae, soleus and extensor digitorum longus muscle of denervated and control rats were obtained at 1, 3, 7, 10 days and 2, 3, 4, 5, 6, 7, 8, 10 weeks after surgery. Histomorphometric analyses were performed on longitu-dinal sections of proximal tibial metaphyseal secondary spongiosa.<BR>Marked trabecular bone loss and muscle weight loss were observed mainly at 7-10 days and 1-3 days after denervation, respectively. Time courses of the percent decrease in trabecular width and length from 0 day (i, e., thinning rate; TR and fragmentation rate; FR) were not matched at 7-10 days and 5-6 weeks after denervation. The scatterplot for TR; FR showed that the FR changed stepwise with the first threshold between 12-25% of TR.
ABSTRACT
A study was conducted to examine activity patterns of surface electromyograms (EMGs) in the triceps surae muscles (medial gastrocnemius, MG ; lateral gastrocnemius, LG ; soleus, SOL) during isometric contraction in plantar flexion (60% MVC, 20% MVC) after immersion in water at three different temperatures. Seven healthy male subjects were immersed in water at 2-3°C (ICE), 19-21°C (MID), and 40-42°C (HOT) . The results of the study are summarized as follows :<BR>1) In the MG and LG, there was a slight increase in the rate of integrated electromyograms (IEMGs) in the MG (the main agonist muscle) . However, the rate of increase in the LG, which is the synergistic muscle, was much greater than that in the MG when contraction was sustained at 60% MVC. Therefore, the activity pattern suggests that muscle activity in the LG compensates for that in the MG.<BR>2) The IEMGs of the SQL with sustained contraction, increased significantly after immersion in HOT and MID, but the IEMGs decreased after immersion in ICE. These results suggest that the recruitment threshold in slow-type motor units should increase during sustained isometric contrac-tion in ICE.<BR>As illustrated above, the activity of the LG increased to compensate for that of MG. In the SQL, muscle activity with sustained contraction decreased in ICE. These results suggest that control mechanisms of the central nervous system might play an important role in the associated movement of the triceps surae muscles.
ABSTRACT
Effects of various tetanic stimulation frequencies (50 Hz, 100 Hz and 200 Hz) on the time course of twitch potentiation were examined in the adduction of human pollicis. Isometric twitch tension curve and surface electromyogram were recorded immediately after (1 sec) the 10 sec-tetanus and at intervals of lOsec (up to 300sec) . The stimulating methods were by nerve and direct stimulations in order to examine the role of nerve on the post-tetanic potentiation. The results were summarized as follows :<BR>1) The greatest potentiation was observed immediately after the tetanus. The potentiation decreased rapidly in the period of 60 sec after the tetanus. Then, the potentiation continued to decrease at the low frequency (50 Hz) ; however, it tended to level off at the high frequencies (100 Hz and 200 Hz) .<BR>2) The time course of twitch potentiation by the nerve stimulation was larger than that by the direct stimulation. This result suggests that nerve has a factor which enhances potentiation. The duration of the potentiation varied according to the stimulation frequencies by the nerve stimulation. It seems that this phenomenon was strongly influenced by the nerve.<BR>3) The degree of increase in twitch tension tended to depend greatly on the peak rate of force development. This result suggests that the increase in twitch tension was closely related to the intensity of the active state.<BR>4) Time course of time to peak tension relatively corresponded to the half relaxation time. It seems possible for the duration of the active state to be related to calcium uptake at the sarcoplasmic reticulum.<BR>5) The coupling efficiency by the direct stimulation tended to be smaller than that by the nerve stimulation after 10 sec. This result suggests that the stimulating nerve possesses a factor which prolongs the period of potentiation.
ABSTRACT
In order to examine muscle activities, surface electromyo-grams (EMGs) were recorded from the triceps surae muscles (lateral gastrocnemius: LG, medial gastrocnemius: MG, soleus: SOL) during prolonged isometric contractions at 20°of ankle joint angle under constant load (10% MVC) . The results were summarized as follows;<BR>1) EMG recordings were obtained which showed complementary activity in each muscle in the time course. Changes in activity was found between whole muscles in the synergist of triceps surae muscles.<BR>2) Muscle activity most frequently took the form of MG+SOL, followed by MG acting alone and SOL acting alone. Ta/Ts value (the ratio of total activity period and total silent period on EMG activity) showed 4.0 in MG and SOL. In contrast, it showed 0.3 in the LG.<BR>3) In the latter half of all exercise times, frequent changes in activity were shown, and Ta/Ts value increased in the gastrocnemius muscle and decreased in the soleus muscle. These results indicate that frequent changes in activity in the latter half were caused by the reduc tion in Ts in the gastrocnemius muscle, and the reduction in Ta in the soleus muscle.<BR>4) Mean integrated EMGs increased significantly in the middle and latter phases in each burst activity, but those in the latter phase did not increase compared to those in the middle phase. These results suggest that the rotation of activity between muscles may have occured before the remarkable increase of EMG activity with fatigue.
ABSTRACT
Surface electromyograms (EMGs) were recorded from the triceps surae muscles in order to examine muscle activities during concentric and eccentric ankle joint movements at various angular velocities under constant load (5 and 10%MVC) . The results were summarized as follows ;<BR>1) At slow angular velocity (6 deg/s) of ankle joint movement, EMG amplitude tended to increase at a larger ankle joint angle in the gastrocnemius muscle, and at a smaller angle in the soleus muscle.<BR>2) Following an increase in angular velocities (6→30→60 deg/s), peak values of integrated EMGs were significantly increased (7-15%) in the medial gastrocnemius muscle. However, these peak values were significantly decreased in the soleus muscle (22-49%) . These results suggest selective recruitment of motor units depending on angular velocity.<BR>3) Ankle joint angle at the peak integrated EMGs was significantly increased following an increase in angular velocity (6→30→60 deg/s) in the soleus muscle. This suggests that the angle at the recruitment of motor units may be dependent on angular velocity in the soleus muscle.