In small mammals, muscles with shorter twitch contraction times and a predominance of fast-twitch, type II fibers exhibit greater posttetanic twitch force potentiation than muscles with longer twitch contraction times and a predominance of slow-twitch, type I fibers. In humans, the correlation between potentiation and fiber-type distribution has not been found consistently. In the present study, postactivation potentiation (PAP) was induced in the knee extensors of 20 young men by a 10-s maximum voluntary isometric contraction (MVC). Maximal twitch contractions of the knee extensors were evoked before and after the MVC. A negative correlation (r = -0. 73, P < 0.001) was found between PAP and pre-MVC twitch time to peak torque (TPT). The four men with the highest (HPAP, 104 +/- 11%) and lowest (LPAP, 43 +/- 7%) PAP values (P < 0.0001) underwent needle biopsies of vastus lateralis. HPAP had a greater percentage of type II fibers (72 +/- 9 vs. 39 +/- 7%, P < 0.001) and shorter pre-MVC twitch TPT (61 +/- 12 vs. 86 +/- 7 ms, P < 0.05) than LPAP. These data indicate that, similar to the muscles of small mammals, human muscles with shorter twitch contraction times and a higher percentage of type II fibers exhibit greater PAP.

It is concluded that fibre type distribution influences potentiation and fatigue of the twitch, and potentiation of the M-wave during fatiguing exercise.

Our laboratory has recently demonstrated that low-frequency electrical stimulation (ES) of quadriceps muscles alone significantly enhanced glucose disposal rate (GDR) during euglycemic clamp (Hamada T, Sasaki H, Hayashi T, Moritani T, and Nakao K. J Appl Physiol 94: 2107-2112, 2003). The present study is further follow-up to examine the acute metabolic effects of ES to lower extremities compared with voluntary cycle exercise (VE) at identical intensity. In eight male subjects lying in the supine position, both lower leg (tibialis anterior and triceps surae) and thigh (quadriceps and hamstrings) muscles were sequentially stimulated to cocontract in an isometric manner at 20 Hz with a 1-s on-off duty cycle for 20 min. Despite small elevation of oxygen uptake by 7.3 +/- 0.3 ml x kg(-1) x min(-1) during ES, the blood lactate concentration was significantly increased by 3.2 +/- 0.3 mmol/l in initial period (5 min) after the onset of the ES (P < 0.01), whereas VE showed no such changes at identical oxygen uptake (7.5 +/- 0.3 ml x kg(-1) x min(-1)). ES also induced enhanced whole body carbohydrate oxidation as shown by the significantly higher respiratory gas exchange ratio than with VE (P < 0.01). These data indicated increased anaerobic glycolysis by ES. Furthermore, whole body glucose uptake determined by GDR during euglycemic clamp demonstrated a significant increase during and after the cessation of ES for at least 90 min (P < 0.01). This post-ES effect was significantly greater than that of the post-VE period (P < 0.01). These results suggest that ES can substantially enhance energy consumption, carbohydrate oxidation, and whole body glucose uptake at low intensity of exercise. Percutaneous ES may become a therapeutic utility to enhance glucose metabolism in humans.

No abstract

Caffeine (1,3,7-trimethylxanthine) has been implicated in the regulation of glucose and lipid metabolism including actions such as insulin-independent glucose transport, glucose transporter 4 expression, and fatty acid utilization in skeletal muscle. These effects are similar to the exercise-induced and 5'adenosine monophosphate-activated protein kinase (AMPK)-mediated metabolic changes in skeletal muscle, suggesting that caffeine is involved in the regulation of muscle metabolism through AMPK activation. We explored whether caffeine acts on skeletal muscle to stimulate AMPK. Incubation of rat epitrochlearis and soleus muscles with Krebs buffer containing caffeine (> or =3 mmol/L, > or =15 minutes) increased the phosphorylation of AMPKalpha Thr(172), an essential step for full kinase activation, and acetyl-coenzyme A carboxylase Ser(79), a downstream target of AMPK, in dose- and time-dependent manners. Analysis of isoform-specific AMPK activity revealed that both AMPKalpha1 and alpha2 activities increased significantly. This enzyme activation was associated with a reduction in phosphocreatine content and an increased rate of 3-O-methyl-d-glucose transport activity in the absence of insulin. These results suggest that caffeine has similar actions to exercise by acutely stimulating skeletal muscle AMPK activity and insulin-independent glucose transport with a reduction of the intracellular energy status.

. Increased submaximal insulin-stimulated glucose uptake in mouse skeletal muscle after treadmill exercise. J Appl Physiol 101: 1368 -1376, 2006. First published June 29, 2006 doi:10.1152/japplphysiol.00416.2006The primary purpose of this study was to determine the effect of prior exercise on insulin-stimulated glucose uptake with physiological insulin in isolated muscles of mice. Male C57BL/6 mice completed a 60-min treadmill exercise protocol or were sedentary. Paired epitrochlearis, soleus, and extensor digitorum longus (EDL) muscles were incubated with [ 3 H]-2-deoxyglucose without or with insulin (60 U/ml) to measure glucose uptake. Insulin-stimulated glucose uptake for paired muscles was calculated by subtracting glucose uptake without insulin from glucose uptake with insulin. Muscles from other mice were assessed for glycogen and AMPK Thr 172 phosphorylation. Exercised vs. sedentary mice had decreased glycogen in epitrochlearis (48%, P Ͻ 0.001), soleus (51%, P Ͻ 0.001), and EDL (41%, P Ͻ 0.01) and increased AMPK Thr 172 phosphorylation (P Ͻ 0.05) in epitrochlearis (1.7-fold), soleus (2.0-fold), and EDL (1.4-fold). Insulin-independent glucose uptake was increased 30 min postexercise vs. sedentary in the epitrochlearis (1.2-fold, P Ͻ 0.001), soleus (1.4-fold, P Ͻ 0.05), and EDL (1.3-fold, P Ͻ 0.01). Insulinstimulated glucose uptake was increased (P Ͻ 0.05) ϳ85 min after exercise in the epitrochlearis (sedentary: 0.266 Ϯ 0.045 mol⅐g Ϫ1 ⅐15 min sedentary. These results demonstrate enhanced submaximal insulinstimulated glucose uptake in the epitrochlearis and soleus of mice 85 min postexercise and suggest that it will be feasible to probe the mechanism of enhanced postexercise insulin sensitivity by using genetically modified mice. glucose transport; exertion; insulin sensitivity; Akt; AMP-activated protein kinaseA SINGLE EXERCISE BOUT HAS several effects on skeletal muscle glucose transport, including an increase in insulin-independent glucose transport during exercise. This effect begins to reverse relatively rapidly after completion of exercise, with most of the effect lost within 1-4 h postexercise in isolated rat skeletal muscle (42,47). A second, more persistent effect, found in rats (5,7,8,10,24,35,42) and humans (36,43,46), is improved muscle glucose uptake with a submaximally effective, physiological insulin dose. This effect has often been observed at ϳ1-4 h postexercise, but under some circumstances the enhanced glucose uptake with submaximal insulin can persist as long as 48 h after completion of the exercise (8, 33). Prior exercise can also enhance glucose transport with supraphysiological, maximally effective insulin concentrations in rat and human skeletal muscle (5,8,42). The effects of physiological and supraphysiological insulin do not always coincide, suggesting that they may be regulated by distinct cellular events (5, 8).The availability of genetically modified animals has opened new opportunities for understanding the mechanisms that underlie physiological processes, including enhanced i...

Our results suggest that of the two α isoforms of AMPK, AMPKα1 is predominantly activated by caffeine via an energy-independent mechanism and that the activation of AMPKα1 increases glucose transport and ACC phosphorylation in skeletal muscle.

. Enhancement of whole body glucose uptake during and after human skeletal muscle low-frequency electrical stimulation. J Appl Physiol 94: 2107-2112, 2003. First published January 31, 2003 10.1152/japplphysiol.00486.2002There is considerable evidence to suggest that electrical stimulation (ES) activates glucose uptake in rodent skeletal muscle. It is, however, unknown whether ES can lead to similar metabolic enhancement in humans. We employed low-frequency ES through surface electrodes placed over motor points of quadriceps femoris muscles. In male subjects lying in the supine position, the highest oxygen uptake was obtained by a stimulation pattern with 0.2-ms biphasic square pulses at 20 Hz and a 1-s on-off duty cycle. Oxygen uptake was increased by approximately twofold throughout the 20-min stimulation period and returned to baseline immediately after stimulation. Concurrent elevation of the respiratory exchange ratio and blood lactate concentration indicated anaerobic glycogen breakdown and utilization during ES. Whole body glucose uptake determined by the glucose disposal rate during euglycemic clamp was acutely increased by 2.5 mg ⅐ kg Ϫ1 ⅐ min Ϫ1 in response to ES and, moreover, remained elevated by 3-4 mg ⅐ kg Ϫ1 ⅐ min Ϫ1 for at least 90 min after cessation of stimulation. Thus the stimulatory effect of ES on whole body glucose uptake persisted not only during, but also after, stimulation. Low-frequency ES may become a useful therapeutic approach to activate energy and glucose metabolism in humans. glucose transport; euglycemic clamp; exercise; insulin sensitivity; oxygen uptake PHYSICAL EXERCISE HAS PROFOUND effects on energy and fuel metabolism in contracting skeletal muscle. It is well established that exercise can directly activate glucose uptake in skeletal muscle by inducing translocation of GLUT-4 glucose transporters to the cell surface via an insulin-independent mechanism (contraction-stimulated glucose transport) (10,11,31,37,49). This phenomenon is considered responsible for the acute effect of exercise on glucose transport, with the majority of glucose being taken up by contracting skeletal muscle. In fact, contraction-stimulated GLUT-4 translocation is not impaired in insulin-resistant conditions such as Type 2 diabetes and obesity (25). Furthermore, the period after exercise is also characterized by a substantial increase in insulin sensitivity that leads to insulin-dependent GLUT-4 translocation and glucose transport as a local phenomenon restricted to exercised muscles (9, 13, 39, 40, 48a). Thus these insulin-independent and -dependent mechanisms of exercise have been widely utilized to prevent individuals from developing glucose intolerance and to improve glycemic control in patients with Type 2 diabetes.Clinically, electrical stimulation (ES) of muscle is useful as a modality of assisting muscle contraction for those who have difficulties in performing voluntary exercise. The use of ES has been traditionally employed for muscle strengthening, maintenance of muscle mass and strength, a...