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PubMed1994

Effects of isoproterenol on the mechanical activity of isolated papillary muscles and perfused rat hearts in various calcium concentrations.

The aim of the study was to examine, in papillary muscles and in a whole heart preparation, the effects of isoproterenol on the myocardial mechanical activity at different extracellular Ca2+ concentrations. Papillary muscles from left ventricles, contracting isometrically, and rat hearts perfused by the Langendorff technique developing isovolumetric pressure at a fixed rate (200 bpm) and diastolic pressure of 5 mmHg were studied at different Ca2+ concentrations for analysis of the effects of increasing doses of isoproterenol. Papillary muscles were treated with isoproterenol (0.5 to 8 ng ml-1) at four extracellular Ca2+ concentrations (0.25, 0.5, 1.25 and 2.5 mM) and Langendorff perfused hearts were stimulated by isoproterenol (0.05 ng ml-1 to 0.8 ng ml-1), also at four extracellular Ca2+ concentrations (0.5, 1.25, 2.5 and 3.75 mM). Both papillary muscles and perfused hearts showed that force and isovolumetric systolic pressure increase in response to isoproterenol at low Ca2+ concentrations. As Ca2+ concentration is increased, isoproterenol's positive inotropic effect subsides. However, papillary muscle isometric contractions showed a similar time to peak tension decrease in response to isoproterenol at all external Ca2+ concentrations used. The results suggest that the positive inotropic response to isoproterenol, in isolated preparations, changes as a function of extracellular Ca2+ concentration decreasing as external Ca2+ increases. Time to peak tension reduction reinforces the idea that this small positive inotropic response to isoproterenol of the rat myocardium, at the physiological Ca2+ concentration, is due to the Ca2+ saturation of the mechanical activity.

Animals↗

PubMed2004

Estimation of the muscle fibre semi-length under varying joint positions on the basis of non-invasively extracted motor unit action potentials.

Changes in muscle fibre length and surface electrode position with respect to the muscle fibres affect the amplitude and frequency characteristics of surface electromyography (SEMG) in different ways. Knowledge of changes in muscle fibre length would help towards a better interpretation of the signals. The possibility of estimating the length through SEMG during voluntary contractions was checked in this study. The fibres' semi-length was estimated from the product of the conduction velocity and conduction time during which the wave of excitation propagated from the end-plate region to the ends of the fibres. Short (10 s), moderate (30% of maximum voluntary contraction) isometric contractions were performed by 10 subjects at different elbow joint angles (80-140 degrees in steps of 20 degrees ). Monopolar signals were detected non-invasively, using a two-dimensional electrode array. High spatial resolution EMG and a decomposition technique were utilised to extract single motor unit activities for triggered averaging and to estimate conduction velocity. A significant increase with joint angle was found in conduction time and estimated fibre semi-length. Changes in conduction velocity with joint angle were found to be not significant. The methodology described allows the relative changes in fibres' semi-length to be estimated from SEMG data.

Action Potentials↗

PubMed1997

Regulation of Na(+)-K+ pump activity in contracting rat muscle.

1. In rat soleus muscle, high frequency electrical stimulation produced a rapid increase in intracellular Na+ (Na+i) content. This was considerably larger in muscles contracting without developing tension than in muscles contracting isometrically. During subsequent rest a net extrusion of Na+ took place at rates which, depending on the frequency and duration of stimulation, approached the maximum transport capacity of the Na(+)-K+ pumps present in the muscle. 2. In isometrically contracting muscles, the net extrusion of Na+ continued for up to 10 min after stimulation, reducing Na+i to values 30% below the resting level (P < 0.001). This undershoot in Na+i, seen in both soleus and extensor digitorum longus muscles, could be maintained for up to 30 min and was blocked by ouabain or cooling to 0 degree C. 3. The undershoot in Na+i could be elicited by direct stimulation as well as by tubocurarine-suppressible stimulation via the motor endplate. It could not be attributed to a decrease in Na+ influx, to effects of noradrenaline or calcitonin gene-related peptide released from nerve endings, to an increase in extracellular K+ or the formation of nitric oxide. 4. The results indicate that excitation rapidly activates the Na(+)-K+ pump, partly via a change in its transport characteristics and partly via an increase in intracellular Na+ concentration. This activation allows an approximately 20-fold increase in the rate of Na+ efflux to take place within 10 s. 5. The excitation-induced activation of the Na(+)-K+ pump may represent a feed-forward mechanism that protects the Na(+)-K+ gradients and the membrane potential in working muscle. Contrary to previous assumptions, the Na(+)-K+ pump seems to play a dynamic role in maintenance of excitability during contractile activity.

Albuterol↗

PubMed2003

Comparison between tests of fatigue and force for trunk flexion.

STUDY DESIGN: A quasi-experimental study was conducted. OBJECTIVE: To compare intraindividual differences between trunk flexion isometric force, fatigue during isometric contraction (isometric fatigue), and fatigue during sit-ups (dynamic fatigue). SUMMARY OF BACKGROUND DATA: Trunk flexion force and trunk flexion fatigue commonly are used to assess trunk flexion fitness. The use of one test is appropriate only if the tests are highly correlated. METHODS: This study measured force and either isometric fatigue (time to task failure, n = 79) or dynamic fatigue (number of sit-ups in 2 minutes, n = 73) in self-selected, healthy secondary school (I-III) subjects. In 15 subjects, both isometric and dynamic fatigue were measured. RESULTS: After control was used for upper body mass, the R2(adj) was 0.34 between isometric force and isometric fatigue, 0.31 between isometric force and dynamic fatigue was, and 0.27 between dynamic fatigue and isometric fatigue. Including gender in the model did not affect the results. CONCLUSIONS: The lack of a high correlation between trunk flexion isometric fatigue, dynamic fatigue, and isometric force suggests that health care professionals should be careful to select the test that answers their particular needs or question.

Abdomen↗

PubMed1969

The mechanical properties of cat soleus muscle during controlled lengthening and shortening movements.

1. By supplying pulses to different subdivisions of the ventral nerve roots in rotation, it was possible to obtain smooth contractions of cat soleus with low rates of stimulation.2. After contracting isometrically the muscle was subjected to constant velocity lengthening or shortening movements.3. During shortening the tension always fell below the isometric value. The fall in tension was usually greatest when low rates of stimulation were used.4. The effect of lengthening on tension depended on the rate of stimulation. At high rates of stimulation the tension during lengthening always rose above the isometric tension. At lower rates of stimulation (5-15 pulses/sec) the tension rose at the beginning of an extension, but decreased later in the movement to a level that was often less than the isometric tension corresponding to that muscle length. At these stimulus rates the tension during isometric contraction was usually higher than during a sustained movement in either direction.5. At low rates of stimulation longitudinal vibratory movements of more than 0.1 mm also reduced the tension far below the isometric value, whereas the reduction was quite slight when the rate of stimulation was high.6. The isometric tension during smooth contractions at low stimulus rates was remarkable in the following respects: it developed rather slowly, it was higher than the tension during or immediately after movements, and it was only slowly regained after movement had ceased.7. The results are discussed in relation to the sliding filament theory of muscle contraction, which, with certain assumptions, provides an explanation for many of the findings.

Animals↗

PubMed2003

Intrinsic properties and reflex compensation in reinnervated triceps surae muscles of the cat: effect of movement history.

Effects of prior motion on ramp stretch responses of reflexive and areflexive muscles were measured in decerebrate cats. Soleus and gastrocnemius muscles were rendered areflexive by reinnervation a minimum of 9 mo before the terminal experiments. The introduction of a shortening phase prior to the ramp stretch increased the normalized initial stiffness of muscles and decreased the tendency to yield of the reinnervated muscles as compared with the case in which muscles contracted isometrically prior to stretch. Yielding was compensated by reflex action for all amplitudes of prior shortening in soleus and gastrocnemius muscles. The comparison of responses of untreated and reinnervated muscles indicated that the contribution of reflex action progressively declined with the amplitude of prior shortening as the extent of yielding diminished. In soleus muscle, during a variable delay period of isometric contraction interposed between shortening and lengthening force generation, initial stiffness and yielding returned to levels seen with isometric contractile history. However, these attributes recovered at different rates, suggesting that distinct processes are responsible for initial stiffness and yielding. Yielding was compensated for by reflex action regardless of the length of the interposed delay or of the amplitude of the prior shortening. These and previous findings indicate that the stretch reflex regulates muscular stiffness for a wide range of conditions. This regulation apparently arises from complementary mechanical properties of intrafusal and extrafusal muscle.

Animals↗

PubMed1987

Contractile activity of neonatal platelets.

Platelet contractile activity was evaluated by observation of tension development during isometric contraction of platelet-fibrin clots. Cylindrical clots were made with platelet-rich plasma obtained from cord blood or from adult controls. These clots were allowed to contract isometrically at 37 degrees C while attached to a transducer to record tension development. The rate of tension development was dependent on platelet concentration but was equivalent for neonatal and adult platelet clots. Although abnormalities in neonatal platelet aggregation and secretion have been well documented the platelet functions required for clot contraction such as fibrin binding and actin-myosin interaction appear to be intact in neonatal platelets.

Adult↗

PubMed1987

Differential effects of reoxygenation on intracellular calcium and isometric tension.

We used the bioluminescent Ca2+ indicator, aequorin to record intracellular calcium transients during reoxygenation of hypoxic ferret ventricular muscle in order to determine whether alterations in the amplitude and time course of isometric contraction are mediated by changes in [Ca2+]i. Papillary muscles less than or equal to 1 mm in diameter were removed from the hearts of male ferrets and perfused with a bicarbonate-buffered physiologic salt solution at 30 degrees C. Muscles were stimulated to contract isometrically at 0.33 Hz and were loaded with aequorin by a chemical procedure. Hypoxia was induced by changing the gas mixture bubbling the perfusate to 95% N2, 5% CO2; reoxygenation was accomplished by switching the gas mixture to 95% O2, 5% CO2. Hypoxia produced a decrease in peak Ca2+ and tension that was reversed by reoxygenation. However, the effects on tension of changes in oxygenation were greater than expected from the degree of change in [Ca2+]i. The time courses of the Ca2+ transient and isometric twitch moved in opposite directions and were respectively prolonged/abbreviated by hypoxia and abbreviated/prolonged by reoxygenation. These results indicate that changes in the amplitude and time course of the isometric twitch induced by hypoxia and reoxygenation cannot be attributed alone to changes in intracellular Ca2+ availability and are caused in part by a significant decrease in the calcium sensitivity of the contractile apparatus.

Aequorin↗

PubMed1995

Intramuscular pressures for monitoring different tasks and muscle conditions.

Intramuscular fluid pressure (IMP) can easily be measured in man and animals. It follows the law of Laplace which means that it is determined by the tension of the muscle fibers, the recording depth and by fiber geometry (fiber curvature or pennation angle). Thick, bulging muscles create high IMPs (up to 1000 mmHg) and force transmission to tendons becomes inefficient. High resting or postexercise IMPs are indicative of a compartment syndrome due to muscle swelling within a low-compliance osseofascial boundary. IMP increases linearly with force (torque) independent of the mode or speed of contraction (isometric, eccentric, concentric). IMP is also a much better predictor of muscle force than the EMG signal. During prolonged low-force isometric contractions, cyclic variations in IMP are seen. Since IMP influences muscle blood flow through the muscle pump, autoregulating vascular elements, and compression of the intramuscular vasculature, alterations in IMP have important implications for muscle function.

Electromyography↗

PubMed2002

Physiologic increases in extracellular sodium salt enhance coronary vasoconstriction and Ca2+ entry.

High dietary sodium salt has been suggested to increase the risk of coronary vasospasm and coronary artery disease. However, whether high-sodium salt directly affects the mechanisms of coronary artery contraction is unclear. This study investigated whether physiologic and supraphysiologic increases in extracellular concentrations of sodium chloride ([NaCl]e) enhance the Ca2+ handling mechanisms of coronary smooth muscle contraction. Isometric contraction and 45Ca2+ influx were measured in endothelium-denuded porcine coronary artery strips incubated in Krebs solution (2.5 mM Ca2+) containing increasing [NaCl]e (120, 121, 123, 126, 130, 140, and 150 mM). Increasing [NaCl]e for 30 min did not increase the resting coronary tone or 45Ca2+ influx. 5-Hydroxytryptamine (5-HT) caused concentration-dependent increases in contraction and 45Ca2+ influx. Preincubation of coronary strips in increasing [NaCl]e for 30 min did not change the median effective dose of 5-HT. However, the magnitude of the 5-HT contraction and 45Ca2+ influx was significantly increased at 121-126 mM [NaCl]e. Preincubation with 2,4-dichlorobenzamil (10-5 M), inhibitor of the Na+/Ca2+ exchanger, or KB-R7943 (10-5 M), selective inhibitor of the reverse mode of the Na+/Ca2+ exchanger, abolished the increases in 5-HT contraction and 45Ca2+ influx at 121-126 mM [NaCl]e. Preincubation in Krebs solution containing 120 mM NaCl plus 1-6 mM LiCl or N-methyl-d-glucamine did not increase 5-HT contraction or 45Ca2+ influx. Higher [NaCl]e (140-150 mM) increased 5-HT-induced 45Ca2+ influx but inhibited 5-HT contraction. 5-HT (10-5 M)- and caffeine (25 mM)-induced contraction in Ca2+-free (2 mM EGTA) solution, a measure of Ca2+ release from the intracellular stores, was not affected by small increases in [NaCl]e (121-126 mM) but was inhibited at higher [NaCl]e (130-150 mM). Thus increases in [NaCl]e within the physiologic range enhance coronary smooth muscle contraction to 5-HT by a mechanism possibly involving Ca2+ entry via the reverse mode of the Na+/Ca2+ exchanger, but not Ca2+ release from the intracellular stores. The reduction of coronary contraction with supraphysiologic [NaCl]e in both Ca2+-containing and Ca2+-free Krebs could be related to excessive increases in ionic strength and may mask significant coronary vasoconstrictor effects of physiologic increases in [NaCl]e.

Animals↗

PubMed2004

Elastase promotes aortic dilation by inhibiting Ca2+ influx into vascular smooth muscle.

Abdominal aortic aneurysm (AAA) is a common vascular disease with, as of yet, unclear mechanism. Increased elastase activity and elastin degradation in the aorta are consistent findings in human AAA. Also, elastase perfusion of the aorta promotes aortic dilation in animal models of AAA. Although elastase-induced degradation of extracellular matrix proteins and the ensuing inflammation of the aortic wall have been implicated as possible causes of the aortic dilation in AAA, little is known regarding the effects of elastase on the mechanisms of aortic smooth muscle contraction. The purpose of this study was to test the hypothesis that elastase promotes aortic dilation by inhibiting the Ca2+ mobilization mechanisms of smooth muscle contraction. Isometric contraction and 45Ca2+ influx were measured in aortic strips isolated from male Sprague-Dawley rats non-treated or treated with elastase. Initial experiments suggested that elastase alone caused matrix degradation. To avoid potential degradation of the extracellular matrix proteins by elastase, the same experiments were repeated in the presence of saturating concentrations of elastin (10 mg/ml). In normal Krebs (2.5 mM Ca2+), phenylephrine (Phe, 10(-5) M) caused contraction of the aortic strips that was significantly inhibited by elastase. The elastase-induced inhibition of Phe contraction was concentration- and time-dependent. At 5 U/ml elastase, the inhibition of Phe contraction was rapid in onset (2.4 +/- 0.3 minutes) and complete in 32 +/- 4 minutes. The inhibitory effects of elastase on Phe contraction were partially reversible. In Ca2+-free (2 mM EGTA) Krebs, Phe caused a small contraction that was not inhibited by elastase, suggesting that elastase does not inhibit Ca2+ release from the intracellular stores. Membrane depolarization by 96 mM KCl, which stimulates Ca2+ entry from the extracellular space, caused a contraction that was inhibited by elastase in a time-dependent and reversible fashion. The reversible inhibitory effects of elastase, particularly in the presence of saturating concentrations of elastin, suggest that they are not due to dissolution of the extracellular matrix or permanent damage to the smooth muscle contractile proteins. Elastase also caused significant inhibition of Phe- and KCl-induced 45Ca2+ influx. These data suggest that elastase promotes aortic relaxation by inhibiting the Ca2+ entry mechanism of vascular smooth muscle contraction, and thus further explain the role of increased elastase activity during the early development of AAA.

Animals↗

PubMed1969

Isotonic lengthening and shortening movements of cat soleus muscle.

1. By supplying pulses to different subdivisions of the ventral nerve roots in rotation it was possible to obtain smooth contractions of cat soleus with low rates of stimulation.2. After contracting isometrically the muscle was subjected to ;step' changes in tension after which it lengthened or shortened isotonically.3. Isotonic lengthening movements usually began relatively slowly but proceeded with increasing velocity; this acceleration was most conspicuous when low rates of stimulation were used.4. At low rates of stimulation the isotonic lengthening movement often continued beyond the length at which the muscle could have generated that tension in an isometric contraction. The muscle then shortened slowly back toward that length.5. Isotonic shortening movements began relatively rapidly, but as shortening continued the movement became slower, and often had an irregular oscillatory course.6. The isotonic movements are discussed in relation to the sliding filament theory of muscle contraction.7. The compliance of the series elastic elements was calculated from the relative amplitudes of the ;step' changes in tension and length. The stiffness of this component increased with increasing muscle tension.

Animals↗

PubMed1996

Effect of gradual reduction in O2 delivery on intracellular homeostasis in contracting skeletal muscle.

This study was designed to investigate 1) whether a protocol employing a gradual reduction in O2 availability to submaximally contracting muscle results in relatively minor disturbances in intracellular homeostasis and 2) the interaction between tissue oxygenation and the proposed regulators of muscle respiration, metabolism, and force production. O2 delivery to isolated submaximally contracting [isometric contractions at 3 Hz; approximately 50% of peak O2 uptake (VO2)] in situ canine gastrocnemius (n = 6) was manipulated by decreasing arterial PO2 (hypoxemia; H) or muscle blood flow (ischemia; I) during three separate periods in each muscle [control (C), H, or I; each separated by 45 min of rest]. O2 delivery was reduced gradually in small steps every 3 min by H or I during two of the contraction periods (6 steps for a total of 21 min; O2 delivery reduced by 67% by the end of 21 min), whereas C was at normal O2 delivery for a 15-min period. Muscle VO2 was maintained at control levels for the first two O2 delivery reduction steps for the H and I conditions and then fell proportionally with O2 delivery to approximately 35% of the initial value by the end of the 21-min contraction period. Muscle force development generally fell in parallel with VO2. There was no significant changes from the values obtained during C contractions in intracellular concentrations of ATP, phosphocreatine, NH3, calculated free ADP, lactate, and redox state ratios as the O2 delivery was reduced, even with the severe decline in VO2 and developed force. These results demonstrated that when O2 availability was reduced gradually to contracting skeletal muscle, 1) developed force (ATP utilization) was reduced through a tight coupling with aerobic ATP supply, such that there was little additional disruption of intracellular homeostasis, and 2) there was an apparent dissociation of some of the proposed regulators of cell respiration and force development from the control of these processes.

Animals↗

PubMed1985

Isometric muscle contraction in endocrine myopathies.

Isometric contraction of the adductor pollicis muscle of the thumb was evoked by stimulating the ulnar nerve in 20 normal adults and 58 patients with endocrine disorders. We evaluated mechanical latency, electromechanical latency, contraction time, half relaxation time, isometric twitch force, compound muscle action potential, nerve conduction velocity, and relative refractory period. Mechanical characteristics were altered in diseases of the pituitary, thyroid, and parathyroid glands. The close relationship between thyroid hormone and contractile properties discriminated between hypo- and hyperthyroidism.

Adrenal Gland Diseases↗

PubMed1995

Myosin light chain kinase-regulated endothelial cell contraction: the relationship between isometric tension, actin polymerization, and myosin phosphorylation.

The phosphorylation of regulatory myosin light chains by the Ca2+/calmodulin-dependent enzyme myosin light chain kinase (MLCK) has been shown to be essential and sufficient for initiation of endothelial cell retraction in saponin permeabilized monolayers (Wysolmerski, R. B. and D. Lagunoff. 1990. Proc. Natl. Acad. Sci. USA. 87:16-20). We now report the effects of thrombin stimulation on human umbilical vein endothelial cell (HUVE) actin, myosin II and the functional correlate of the activated actomyosin based contractile system, isometric tension development. Using a newly designed isometric tension apparatus, we recorded quantitative changes in isometric tension from paired monolayers. Thrombin stimulation results in a rapid sustained isometric contraction that increases 2- to 2.5-fold within 5 min and remains elevated for at least 60 min. The phosphorylatable myosin light chains from HUVE were found to exist as two isoforms, differing in their molecular weights and isoelectric points. Resting isometric tension is associated with a basal phosphorylation of 0.54 mol PO4/mol myosin light chain. After thrombin treatment, phosphorylation rapidly increases to 1.61 mol PO4/mol myosin light chain within 60 s and remains elevated for the duration of the experiment. Myosin light chain phosphorylation precedes the development of isometric tension and maximal phosphorylation is maintained during the sustained phase of isometric contraction. Tryptic phosphopeptide maps from both control and thrombin-stimulated cultures resolve both monophosphorylated Ser-19 and diphosphorylated Ser-19/Thr-18 peptides indicative of MLCK activation. Changes in the polymerization of actin and association of myosin II correlate temporally with the phosphorylation of myosin II and development of isometric tension. Activation results in a 57% increase in F-actin content within 90 s and 90% of the soluble myosin II associates with the reorganizing F-actin. Furthermore, the disposition of actin and myosin II undergoes striking reorganization. F-actin initially forms a fine network of filaments that fills the cytoplasm and then reorganizes into prominent stress fibers. Myosin II rapidly forms discrete aggregates associated with the actin network and by 2.5 min assumes a distinct periodic distribution along the stress fibers.

Actins↗

PubMedDate not supplied

[Somatosensory evoked potentials during long-term isometric muscle contraction].

The effect of sustained isometric contraction on somatosensory evoked potentials (SEPs) was studied. An attenuation of early SEP components N20--P30, P30--N35) was observed during the last minute of the endurance time. The late components (P45--N55, N55--P100) showed a significant increase of the amplitude during the first minute of the contraction. The amplitude of N35--P45 did not change during voluntary contraction, although it was decreased immediately after the contraction. An increase of the integrated EMG of forearm flexors was observed in the last minute of the endurance time. The maximal voluntary contraction was decreased immediately after the sustained contraction. The observed changes in SEPs could be attributed to possible changes in sensory information and motor command due to motor task.

Adult↗

PubMed1993

Neural modulation of bovine mesenteric lymph node contraction.

Spontaneous isometric contractions and field-evoked responses were recorded in vitro from strips of bovine mesenteric lymph node capsule. Field stimulation (0.3 ms pulses, 60 V nominal, 2 min trains) at frequencies > or = 2 Hz evoked a frequency-dependent increase in baseline tension which was maximum at frequencies > or = 16 Hz. Evoked contractions were significantly reduced (P = 0.02) by the neurotoxin tetrodotoxin (10(-6) M) although they were unaffected by the alpha-adrenoceptor antagonists phentolamine, rauwolscine and prazosin (3 x 10(-6) M). Similarly, responses were unaffected by the beta-adrenoceptor antagonist propranolol (10(-6) M), the cholinergic antagonist atropine (10(-6) M) or the uptake blocker cocaine (10(-6) M). Field-evoked contraction was also unaffected by a 30 min exposure to alpha,beta-methylene ATP (10(-6) M). The results suggest that bovine lymph node capsular smooth muscle is innervated by excitatory nerves which are non-adrenergic, non-cholinergic and non-purinergic in nature.

Adenosine Triphosphate↗

PubMed1997

Strength, endurance, and work capacity after muscle strengthening exercise in postpolio subjects.

OBJECTIVE: To determine whether a 12-week home quadriceps muscle strengthening exercise program would increase muscle strength, isometric endurance, and tension time index (TTI) in postpolio syndrome subjects without adversely affecting the surviving motor units or the muscle. DESIGN: A longitudinal study to investigate the effect of a 12-week exercise program on neuromuscular function and electromyographic variables. SETTING: Neuromuscular laboratory of a university hospital. SUBJECTS: Seven subjects were recruited from a cohort of 12 subjects who had participated in a previous exercise study. All subjects had greater than antigravity strength of the quadriceps. Upon completion of a postpolio questionnaire, all acknowledged common postpolio syndrome symptoms such as new fatigue, pain, and weakness; 6 of the 7 acknowledged new strength decline. INTERVENTION: On Mondays and Thursdays subjects performed three sets of four maximal isometric contractions of the quadriceps held for 5 seconds each. On Tuesdays and Fridays subjects performed three sets of 12 dynamic knee extension exercises with ankle weights. MAIN OUTCOME MEASURES: Neuromuscular variables of the quadriceps muscles were measured at the beginning and completion of the exercise program and included: isokinetic peak torque (ISOKPT, at 60 degrees/sec angular velocity) and total work performed of four contractions (ISOKTW), isometric peak torque (MVC), endurance (EDUR, time subject could hold isometric contraction at 40% of the initial MVC), isometric tension time index (TTI, product of endurance time and torque at 40% of MVC), and initial and final ankle weight (WGT, kg) lifted. Electromyographic variables included: fiber density (FD), jitter (MCD), and blocking (BLK) from single fiber assessment and median macro amplitude (MACRO). Serum creatine kinase (CK) was also measured initially and at 4-week intervals throughout the study. RESULTS: The following variables significantly (p < .05) increased: WGT by 47%, ISOKPT, 15%, ISOKTW, 15%; MVC, 36%; EDUR, 21%; TTI, 18%. The following variables did not significantly (p > .05) change: FD, MCD, BLK, MACRO, and CK. CONCLUSIONS: This home exercise program significantly increased strength, endurance, and TTI without apparently adversely affecting the motor units or the muscle, as the EMG and CK variables did not change.

Adult↗