Nonalcoholic fatty liver is frequently observed in obese individuals, yet the factors that predict its development and progression to liver disease are poorly understood. We proposed that proton magnetic resonance spectroscopy (

During their reproductive years the hormone levels in women fluctuate due to the menstrual cycle. The four hormonal markers of the menstrual cycle (oestrogen, progesterone, follicle stimulating hormone (FSH) and luteinising hormone (LH)) change continuously throughout the cycle. These fluctuations in female steroid hormones affect the autonomic nervous system and metabolic functions (Florini, 1987). Therefore certain physiological parameters and athletic performance could change along with the menstrual cycle phases (Becker et al. 1982). However, the influence of the menstrual cycle phase on exercise performance, particularly muscle strength, is unclear. Sarwar et al. (1996) tested skeletal muscle strength, relaxation rate and fatiguability of the quadriceps during the menstrual cycle. They found no changes in these parameters for women taking oral contraceptives. For women not taking oral contraceptives, however, the quadriceps were stronger, more fatiguable and had a longer relaxation time at mid-cycle (day 12-18). Phillips et al. (1996) reported a higher adductor pollicis strength during the follicular phase than during the luteal phase, with a rapid decrease in strength around ovulation. They suggested that oestrogen has a strengthening action on skeletal muscle, although the underlying mechanism is not clear. Greeves et al. (1999), however, reported the highest quadriceps strength during the mid-luteal phase and found a positive relationship between strength and progesterone concentration. Several other studies have found no changes in skeletal muscle strength over the menstrual cycle (DiBrezzo et al. 1991;Quadango et al. 1991;Lebrun et al. 1995;Gür, 1997).The main problem in the measurement of maximum voluntary strength is ensuring that the contraction truly reflects the maximum force-generating capacity of the muscle. Even well-motivated subjects may not always reach full neural activation of their muscles (Rutherford et al. 1986). The extent of neural activation can be evaluated by applying a superimposed electrical stimulus to the muscle during the performance of a maximal voluntary contraction (MVC). When comparing strength over a period of time, such as in menstrual cycle research, it is especially important to ensure maximal neural activation during each test.A further problem encountered in research on the influence of the menstrual cycle on physical performance is the timing of the testing. It is difficult to predict the exact phases of the menstrual cycle and the concurrent reproductive hormone concentrations. Counting days 1. The influence of the different phases of the menstrual cycle on skeletal muscle contractile characteristics was studied in 19 regularly menstruating women. Muscle function was measured when (i) oestrogen and progesterone concentrations were low (menstruation), (ii) oestrogen was elevated and progesterone was low (late follicular phase), and (iii) oestrogen and progesterone were both elevated (luteal phase).2. Maximal isometric quadriceps strength, fatiguability and electrica...

The effects of sprint training on muscle metabolism and ion regulation during intense exercise remain controversial. We employed a rigorous methodological approach, contrasting these responses during exercise to exhaustion and during identical work before and after training. Seven untrained men undertook 7 wk of sprint training. Subjects cycled to exhaustion at 130% pretraining peak oxygen uptake before (PreExh) and after training (PostExh), as well as performing another posttraining test identical to PreExh (PostMatch). Biopsies were taken at rest and immediately postexercise. After training in PostMatch, muscle and plasma lactate (Lac(-)) and H(+) concentrations, anaerobic ATP production rate, glycogen and ATP degradation, IMP accumulation, and peak plasma K(+) and norepinephrine concentrations were reduced (P<0.05). In PostExh, time to exhaustion was 21% greater than PreExh (P<0.001); however, muscle Lac(-) accumulation was unchanged; muscle H(+) concentration, ATP degradation, IMP accumulation, and anaerobic ATP production rate were reduced; and plasma Lac(-), norepinephrine, and H(+) concentrations were higher (P<0.05). Sprint training resulted in reduced anaerobic ATP generation during intense exercise, suggesting that aerobic metabolism was enhanced, which may allow increased time to fatigue.

This study examined the effects of prolonged exercise on human quadriceps muscle contractile function and homogenate sarcoplasmic reticulum Ca2+ uptake and Ca2+-adenosinetriphosphatase activity. Ten untrained men cycled at 75 +/- 2% (SE) peak oxygen consumption until exhaustion. Biopsies were taken from the right vastus lateralis muscle at rest, exhaustion, and 20 and 60 min postexercise. Peak tension and half relaxation time of the left quadriceps muscle were measured during electrically evoked twitch and tetanic contractions and a maximal voluntary isometric contraction at rest, exhaustion, and 10, 20, and 60 min postexercise. At exhaustion, homogenate Ca2+ uptake and Ca2+ adenosinetriphosphatase activity were reduced by 17 +/- 4 and 21 +/- 5%, respectively, and remained depressed after 60 min recovery (P

This study investigated the adaptations of skeletal muscle sarcoplasmic reticulum (SR) Ca2+ uptake, relaxation, and fiber types in young (YW) and elderly women (EW) to high-resistance training. Seventeen YW (18-32 yr) and 11 EW (64-79 yr) were assessed for 1) electrically evoked relaxation time and rate of the quadriceps femoris; and 2) maximal rates of SR Ca2+ uptake and Ca2+-ATPase activity and relative fiber-type areas, analyzed from muscle biopsies of the vastus lateralis. EW had significantly slower relaxation rates and times, decreased SR Ca2+ uptake and Ca2+-ATPase activity, and a larger relative type I fiber area than did YW. A subgroup of 9 young (YWT) and 10 elderly women (EWT) performed 12 wk of high-resistance training (8 repetition maximum) of the quadriceps and underwent identical testing procedures pre- and posttraining. EWT significantly increased their SR Ca2+ uptake and Ca2+-ATPase activity in response to training but showed no alterations in speed of relaxation or relative fiber-type areas. In YWT none of the variables was altered after resistance training. These findings suggest that 1) a reduced SR Ca2+ uptake in skeletal muscle of elderly women was partially reversed with resistance training and 2) SR Ca2+ uptake in the vastus lateralis was not the rate-limiting mechanism for the slowing of relaxation measured from electrically evoked quadriceps muscle of elderly women.