Per A. Tesch

EMG frequency spectrum, muscle structure, and fatigue during dynamic contractions in man.

SummaryFatigue of the vastus lateralis muscle was studied in healthy well-conditioned students, who differed considerably regarding their muscle fibre type distribution. Muscle force decline during repeated maximum voluntary knee extensions at a constant angular velocity (180‡×s−1 or π rad×s−1), using isokinetic equipment, was taken as the criterion for the degree of fatigue. In an attempt to study quantitative as well as qualitative changes in the EMG pattern, integrated EMG (IEMG) and the frequency of the mean power (MPF), computed from the power spectral density function (PSDF), were analysed. It was found that individuals with muscles made up of a high proportion of fast twitch (FT) muscle fibres demonstrated higher peak knee extension torque, and a greater susceptibility to fatigue than did individuals with muscles mainly composed of slow twitch (ST) muscle fibres. An IEMG decline (p<0.01) was demonstrated during 100 contractions in individuals rich in FT fibres. Only a slight, but not significant, reduction in IEMG occurred in individuals with a high percentage of ST fibres. Concomitantly, MPF decreased (p<0.001) in individuals with a high percentage of FT fibres, while their opposites demonstrated only a slight decrease (non-significant). It is suggested that muscle contraction failure might also be related to qualitative changes in the motor unit recruitment pattern, and that these changes occur more rapidly in muscles composed of a high proportion of FT muscle fibres than in muscles composed of a high proportion of ST fibres.

Quadriceps EMG/force relationship in knee extension and leg press.

PURPOSE This study compared the relationship between surface electromyographic (EMG) activity and isometric force of m. quadriceps femoris (QF) in the single-joint knee extension (KE) and the multi-joint leg press (LP) exercises. METHODS Nine healthy men performed unilateral actions at a knee angle of 90 degrees at 20, 40, 60, 80, and 100% of maximal voluntary contraction (MVC). EMG was measured from m. vastus lateralis (VL), m. vastus medialis (VM), m. rectus femoris (RF), and m. biceps femoris (BF). RESULTS There were no differences in maximum EMG activity of individual muscles between KE and LP. The QF EMG/force relationship was nonlinear in each exercise modality. VL showed no deviation from linearity in neither exercise, whereas VM and RF did. BF activity increased linearly with increased loads. CONCLUSIONS The EMG/force relationship of all quadricep muscles studied appears to be similar in isometric multi-joint LP and single-joint KE actions at a knee angle of 90 degrees. This would indicate the strategy of reciprocal force increment among muscles involved is comparable in the two models. Furthermore, these data suggest a nonuniform recruitment pattern among the three superficial QF muscles and surface EMG recordings from VL to be most reliable in predicting force output.

Knee extensor and plantar flexor muscle size and function following 90 days of bed rest with or without resistance exercise

Skeletal muscle atrophy and strength loss induced by short-term simulated spaceflight are offset or attenuated by resistance exercise (RE). This study compared the effects of plantar flexor and knee extensor RE on muscle size and function in 17 healthy men (aged 26–41years) subjected to 90 days 6° head-down-tilt bed rest with (BRE; n=8) or without (BR; n=9) RE. The RE program consisted of coupled maximal concentric and eccentric actions in the supine squat (4 sets of 7 repetitions) and calf press (4×14) every third day employing a gravity-independent flywheel ergometer (FW). Prior to, and following bed rest, muscle volume was assessed using magnetic resonance imaging. Similarly, muscle strength and power and surface electromyographic (EMG) activity were determined during maximal actions using FW or isokinetic dynamometry. In BR, knee extensor and plantar flexor muscle volume decreased (P<0.05) 18% and 29%, respectively. Torque or force and power decreased (P<0.05) 31–60% (knee extension) and 37–56% (plantar flexion) while knee extensor and plantar flexor EMG activity decreased 31–38% and 28–35%, respectively following BR. Muscle atrophy in BRE was prevented (P>0.05; knee extensors) or attenuated (−15%; plantar flexors). BRE maintained task-specific force, power and EMG activity. The decrease in non-task-specific torque was less (P<0.05) than in BR. The present data imply that the triceps surae and quadriceps muscles show different responsiveness to long-term bed rest with or without resistance exercise. The results also suggest that designing in-flight resistance exercise protocols for space travellers is complex and must extend beyond preserving muscle only.

Skeletal muscle adaptations consequent to long-term heavy resistance exercise.

Heavy resistance training is associated with increased body weight, lean body mass, and muscle cross-sectional area. The increased muscle cross-sectional area is mainly brought about by hypertrophy of individual muscle fibers. There is a greater increase in the area of fast twitch fibers compared to slow twitch fibers. In addition, long-term heavy resistance training may produce fiber proliferation. Mitochondrial volume density decreases in proportion to muscle hypertrophy in response to training. Typically, no capillary neoformation occurs during strength training. Therefore, capillary density decreases consequent to heavy resistance training. It appears, though, that bodybuilders, relying on a high repetition training system, in contrast to Olympic weight- and power lifters, display a small increase in number of capillaries per fiber. Enzyme activities, reflecting oxidative potential; decrease during long-term heavy resistance training, resulting in muscle hypertrophy. Although glycogen storage capacity is enhanced in strength trained athletes, enzyme activities reflecting anaerobic metabolism do not increase in response to heavy resistance exercise.

Human single muscle fibre function with 84 day bed-rest and resistance exercise

Muscle biopsies were obtained from the vastus lateralis before and after 84 days of bed‐rest from six control (BR) and six resistance‐exercised (BRE) men to examine slow‐ and fast‐twitch muscle fibre contractile function. BR did not exercise during bed‐rest and had a 17 and 40% decrease in whole muscle size and function, respectively. The BRE group performed four sets of seven maximal concentric and eccentric supine squats 2–3 days per week (every third day) that maintained whole muscle strength and size. Slow (MHC I) and fast (MHC IIa) muscle fibres were studied at 15°C for diameter, peak force (Po), contractile velocity (Vo) and force–power parameters. SDS‐PAGE was performed on each single fibre after the functional experiments to determine MHC isoform composition. MHC I and IIa BR fibres were, respectively, 15 and 8% smaller, 46 and 25% weaker (Po), 21 and 6% slower (Vo), and 54 and 24% less powerful after bed‐rest (P < 0.05). BR MHC I and IIa Po and power normalized to cell size were lower (P < 0.05). BRE MHC I fibres showed no change in size or Vo after bed‐rest; however, Po was 19% lower (P < 0.05), resulting in 20 and 30% declines (P < 0.05) in normalized Po and power, respectively. BRE MHC IIa fibres showed no change in size, Po and power after bed‐rest, while Vo was elevated 13% (P < 0.05). BRE MHC IIa normalized Po and power were 10 and 15% lower (P < 0.05), respectively. MHC isoform composition shifted away from MHC I fibres, resulting in an increase (P < 0.05) in MHC I/IIa (BR and BRE) and MHC IIa/IIx (BR only) fibres. These data show that the contractile function of the MHC I fibres was more affected by bed‐rest and less influenced by the resistance exercise protocol than the MHC IIa fibres. Considering the large differences in power of human MHC I and IIa muscle fibres (5‐ to 6‐fold), the maintenance of whole muscle function with the resistance exercise programme is probably explained by (1) the maintenance of MHC IIa power and (2) the shift from slow to fast (MHC I → MHC I/IIa) in single fibre MHC isoform composition.

Muscle metabolism during intense, heavy-resistance exercise

SummaryThe objective of this study was to examine the muscle metabolic changes occurring during intense and prolonged, heavy-resistance exercise. Muscle biopsies were obtained from the vastus lateralis of 9 strength trained athletes before and 30 s after an exercise regimen comprising 5 sets each of front squats, back squats, leg presses and knee extensions using barbell or variable resistance machines. Each set was executed until muscle failure, which occurred within 6–12 muscle contractions. The exercise: rest ratio was approximately 1∶2 and the total performance time was 30 min. Concentrations of adenosine triphosphate (ATP), creatine phosphate (CP), creatine, glycogen, glucose, glucose-6-phosphate (G-6-P), α-glycerophosphate (α-G-P) and lactate were determined on freeze-dried tissue samples using fluorometric assays. Blood samples were analyzed for lactate and glucose. The exercise produced significant reductions in ATP (p<0.01) and CP (p<0.001), while α-G-P more than doubled (p<0.05), glucose increased tenfold (p<0.001) and G-6-P fourfold (p<0.001). Muscle lactate concentration at cessation of exercise averaged 17.3 mmol · kg−1 w.w. Glycogen concentration decreased (p<0.001) from 160 to 118 mmol · kg−1 w. w. It is concluded that high intensity, heavy resistance exercise is associated with a high rate of energy utilization through phosphagen breakdown and activation of glycogenolysis.

Muscle strength and fiber composition in athletes and sedentary men.

Members of Swedish national teams in track and field events (sprinting and jumping), downhill skiing, race walking, orienteering, and a group of sedentary men were studied to examine the relationship between muslce fiber characteristics in needle biopsy samples form m. vastus lateralis and muscle strength measured as peak torque during isokinectic knee extensions. In comparison with the sedentary group the following differences were found: a) percentage fast twitch fibers was lower in the endurance athetes, b) fast to slow twitch muscle fiber area ratio was higher in the track athletes, c)track athletes and downhill skier attained higher peak torque values at all angular velocities examined. The track athletes had, however, higher torque values at the fastest angular velocity as compared to the downhill skiers, whereas there was no differnce under isometric conditions. The proportion of fast twitch fibers was related to torque produced, especially at high motion velocity. The training also appeared to affect the force-velocity relationship.

Muscle hypertrophy in bodybuilders

SummaryMuscle biopsy samples were obtained from m. vastus lateralis and m. deltoideus of three high caliber bodybuilders. Tissue specimens were analysed with respect to relative distribution of fast twitch (FT) and slow twitch (ST) fiber types and different indices of fiber area. In comparison to a reference group of competitive power/weight-lifters the following tendencies were observed: the percentage of FT fibers was less, mean fiber area was smaller and selective FT fiber hypertrophy was not evident. Values for fiber type composition and fiber size were more similar to values reported for physical education students and non-strength trained individuals. The results suggest that weight training induced muscle hypertrophy may be regulated by different mechanisms depending upon the volume and intensity of exercise.

Dynamic neck strength training effect on pain and function

This study examined whether neck resistance training could increase strength and reduce pain in workers with a high prevalence of neck disorders. Middle-aged women workers (n = 17) exercised twice weekly for 8 weeks. Each session (12min) consisted of three sets of 12 repetitions of resisted rotation, flexion, and extension muscles using hydraulic dampers. Resistance was set individually and progressively increased every second week. Angle-specific maximal isometric rotator, extensor, and flexor torque was measured before and after training. Torque during rotation, flexion and extension increased (p < .05) on average 35%, 27%, and 19%, respectively, after training. Perceived pain, assessed using a four-graded scale, was reduced (p < .05) after training. It is suggested that 12 minutes of specific neck strength training twice weekly for 8 weeks increases strength of rotator, extensor, and flexor muscles of the neck. Also all individuals who had pain reported reduced perceived neck pain after training.

Glycogen and triglyceride utilization in relation to muscle metabolic characteristics in men performing heavy-resistance exercise

SummaryNine bodybuilders performed heavy-resistance exercise activating the quadriceps femoris muscle. Intermittent 30-s exhaustive exercise bouts comprising 6–12 repetitions were interspersed with 60-s periods for 30 min. Venous blood samples were taken repeatedly during and after exercise for analyses of plasma free fatty acid (FFA) and glycerol concentration. Muscle biopsies were obtained from the vastus lateralis muscle before and after exercise and assayed for glycogen, glycerol-3-phosphate, lactate and triglyceride (TG) content. The activities of citrate synthase (CS), lactate dehydrogenase, hexokinase (HK), myokinase, creatine kinase and 3-hydroxyacyl-CoA dehydrogenase (HAD), were analysed. Histochemical staining procedures were used to assess fibre type composition, fibre area and capillary density. TG content before and after exercise averaged (SD) 23.9 (13.3) and 16.7 (6.4) mmol kg−1 dry wt. The basal triglyceride content varied sixfold among individuals and the higher the levels the greater was the change during exercise. The glycogen content decreased (P<0.001) from 690 (82) to 495 (95) mmol kg−1 dry wt. and lactate and glycerol-3-phosphate increased (P<0.001) to 79.5 (5.5) and 14.5 (7.3) mmol kg−1 dry wt., respectively, after exercise. The HK and HAD/CS content respectively correlated with glycogen or TG content at rest and with changes in these metabolites during exercise. FFA and glycerol concentrations increased slightly (P<0.001) during exercise. Lipolysis may, therefore, provide energy during heavy-resistance exercise of relatively short duration. Also, storage and utilization of intramuscular substrates appear to be influenced by the metabolic profile of muscle.