Efficacy of a gravity‐independent resistance exercise device as a countermeasure to muscle atrophy during 29‐day bed rest

Aim: This study determined changes in knee extensor and plantar flexor muscle volume during 29 days of bed rest with or without resistance exercise using a gravity‐independent flywheel ergometer. Methods: Seventeen men (26–41 years) were subjected to 29 days of bed rest with (n = 8) or without (n = 9) resistance exercise; Supine Squat (SS) and Calf Press (CP) performed every third day. Quadriceps and triceps surae muscle volume was determined before and after bed rest and force and power were measured during training. Prior to these interventions, reproducibility of this device for training and testing was assessed in 23 subjects who performed bilateral maximal concentric, eccentric and isometric (MVC) knee extensions and plantar flexions over repeated sessions with simultaneous measurements of force, power and electromyographic (EMG) activity. Results: Quadriceps and triceps surae muscle volume decreased (P < 0.05) 10 and 16%, respectively, after 29 days bed rest. Exercise maintained quadriceps volume and mitigated triceps surae atrophy. Thus, either muscle showed different response across subject groups (P < 0.05). Force and power output during training were either maintained (P > 0.05) or increased (P < 0.05). EMG amplitude in the training mode was similar (SS; P > 0.05) or greater (CP; P < 0.05) compared with that elicited during MVC. Peak force and power test‐retest coefficient of variation (CV) ranged 5–6% and 7–8% for SS and CP, respectively. Conclusion: The present data suggest that this resistance exercise paradigm counteracts quadriceps and abates the more substantial triceps surae muscle atrophy in bedridden subjects, and therefore should be an important asset to space travellers.     

Muscle hypertrophy following 5-week resistance training using a non-gravity-dependent exercise system

AIM: The efficacy of a mechanical, gravity-independent resistance exercise (RE) system to induce strength gains and muscle hypertrophy was validated. Designed for space crew in orbit, this technique offers resistance during coupled concentric and eccentric actions by utilizing the inertia of a rotating flywheel(s), set in motion by the trainee. METHODS: Ten middle-aged (30-53 years) men and women performed four sets of seven maximal, unilateral (left limb) knee extensions two or three times weekly for 5 weeks. Knee extensor force and electromyographic (EMG) activity of the three superficial quadriceps muscles were measured before and after this intervention. In addition, with the use of magnetic resonance imaging (MRI), volume of individual knee extensor and ankle plantar flexor muscles was assessed. RESULTS: Over the 12 training sessions, the average concentric (CON) and eccentric (ECC) force generated during exercise increased by 11% (P < 0.05). Likewise, maximal isometric strength (maximal voluntary contraction, MVC) at 90 and 120 degrees knee angle increased by (P < 0.05) 11 and 12% respectively, after training. Neither individual quadriceps muscle showed a change (P > 0.05) in maximal integrated EMG (iEMG) activity. Quadriceps muscle volume increased by 6.1% (P < 0.05). Although the magnitude of response varied, all individual quadriceps muscles showed increased (P < 0.05) volume after training. As expected, ankle plantar flexor volume of the trained limb was unchanged (P > 0.05). Likewise, MVC, CON and ECC force, iEMG and knee extensor and plantar flexor muscle volume were unaltered (P > 0.05) in the right, non-trained limb. CONCLUSION: The results of this study show that the present RE regimen produces marked muscle hypertrophy and important increases in maximal voluntary strength and appears equally effective as RE paradigms using gravity-dependent weights, in this regard.     

Influence of eccentric actions on skeletal muscle adaptations to resistance training

Three different training regimens were performed to study the influence of eccentric muscle actions on skeletal muscle adaptive responses to heavy resistance exercise. Middle-aged males performed the leg press and leg extension exercises two days each week. The resistance was selected to induce failure within six to twelve repetitions of each set. Group CON/ECC (n= 8) performed coupled concentric and eccentric actions while group CON (n= 8) used concentric actions only. They did four or five sets of each exercise. Group CON/CON (n= 10) performed twice as many sets with only concentric actions. Eight subjects did not train and served as controls. Tissue samples were obtained from m. vastus lateralis using the biopsy technique before and after 19 weeks of training, and after four weeks of detraining. Histochemical analyses were performed to assess fibre type composition, fibre area and capillarization. Training increased (P < 0.05) Type IIA and decreased (P < 0.05) Type IIB fibre percentage. Only group CON/ECC increased Type I area (14%, P < 0.05). Type II area increased (P < 0.05) 32 and 27%, respectively, in groups CON/ECC and CON/CON, but not in group CON. Mean fibre area increased (P < 0.05) 25 and 20% in groups CON/ECC and CON/CON, respectively. Capillaries per fibre increased (P < 0.05) equally for Type I and Type II fibres. Capillaries per fibre area for both fibre types, however, increased (P <0.05) only in groups CON and CON/CON. The changes in fibre type composition and capillary frequency were manifest after detraining. At this time only group CON/ECC showed mean fibre hypertrophy, while capillary density was elevated in groups CON/CON and CON. This study suggests that optimal muscle hypertrophy in response to resistance exercise is not attained unless eccentric muscle actions are performed. The data also show that heavy resistance exercise may produce muscle fibre transformation and capillary neoformation.     

Relationship between muscle fibre composition,glucose transporter protein 4 and exercise training: possible consequences in non‐insulin‐dependent diabetes mellitus

Skeletal muscle is composed of different fibre types, which differ in contractile as well as in metabolic properties. The myosin molecule, which exists in several different isoforms, is of major importance in determining the contractile properties of the muscle cell. The plasticity of skeletal muscle is reflected in this tissue’s adaptability to changes in the functional demand. In both rats and humans, a decrease in activity level will in most cases change the muscle fibre composition towards faster myosin isoforms and an increase in activity level (such as seen with exercise training) will induce an increase in slower myosin isoforms. The glucose transporter protein 4 (GLUT4), which is the major insulin regulatable glucose transporter in mammalian skeletal muscle, is found in larger amounts in slow muscle fibres compared with fast muscle fibres. An increase in activity level will increase the GLUT4 protein expression and a decrease in activity level will in most cases decrease GLUT4. Thus, there seems to be some kind of relationship between the muscle fibre type and GLUT4. However, the main factor regulating both the GLUT4 protein expression and the muscle fibre composition seems to be the activity level of the muscle fibre. Patients suffering from non‐insulin‐dependent diabetes mellitus (NIDDM) are insulin resistant in their skeletal muscles but are generally normal when it comes to skeletal muscle fibre composition and the GLUT4 protein expression. There is good evidence that exercise training beneficially impacts on insulin sensitivity in healthy individuals and in patients with type II diabetes. An increase in the GLUT4 protein expression in skeletal muscle may at least partly explain this effect of training.