Muscle strength and bone mass have shown positive adaptations in response to resistive exercise training. Resistive exercise has been shown to be an effective means of increasing muscle strength and mass in ambulatory subjects as well as maintaining muscle strength, mass, and endurance in bed rest. Resistive exercise also has been shown to be effective at increasing BMD in ambulatory subjects and maintaining BMD during bed rest. Resistive exercise usually consists of two discrete actions, concentric (muscle shortening) and eccentric (muscle lengthening). During space flight, past exercise countermeasure hardware has not provided an equal 1:1 ratio of eccentric to concentric load. Eccentric contractions may be important in maintaining muscle mass and strength during space flight, as well as play a role in increasing BMD.
To date, resistance exercise devices used during space flight (i.e. interim Resistive Exercise Device [iRED]) have provided minimal to moderate levels of eccentric loading compared with traditional free weight exercise. The purpose of this study was to determine the relative effectiveness of training with different levels of eccentric to concentric resistance ratios to induce changes in muscle strength, lean tissue mass, BMD, and markers of bone metabolism. Further, these results may aid in the definition of requirements for future exercise hardware to support long duration stays aboard the International Space Station (ISS), on the moon, or in transit to, stays on, and return from Mars. These results also may influence pre-, in- and postflight exercise prescriptions as well as define effective exercise prescriptions for bed rest.
Subjects participated in a 12-week study comprised of three weeks of pre-testing, eight weeks of training, and one week of post-testing utilizing the supine leg press and supine heel raise exercises. Training was conducted and supervised by an experienced strength training coach. Subjects were grouped according to their pre-training of one-repetition maximum (1RM) for leg press and randomly assigned to one of five groups. Training groups were:
1. 0% eccentric, 100% concentric (concentric-only training)
2. 33% eccentric, 100% concentric
3. 66% eccentric, 100% concentric
4. 100% eccentric, 100% concentric
5. 138% eccentric, 100% concentric
Training was performed using an Agaton Fitness System® (Agaton Fitness AB, Boden, Sweden.) The system was modified to include an eccentric under load capability in addition to traditional weight training and eccentric overload. In the conventional training mode (100% Group), the subject raised and lowered the weight on the left shaft only. When the machine was placed in the eccentric overload mode (138% Group), a motor located inside the tower raises the shaft on the right side of the tower to the upper most position, while the shaft on the left side of the tower remained at the bottom. As the subject rose up, they lifted the load placed on the left shaft (concentric motion). Once at the top of the range of motion the subject lowered the load (eccentric motion). As the subject lowered the load, a clutch in the tower engaged and locked the shafts on the left and right side together. Therefore, when the subject began to lower himself, he would lower the combined weight of both the left and right shafts. In the eccentric underload mode (0%, 33%, and 66% Groups), both shafts remained at the bottom of the tower. As the subject raised up, the clutch locked both the right and left shafts together causing the subject to lift the combined load of both shafts. When the subject reached the top of the range of motion, the motor engaged and lower the weight on the right shaft to the start position at the base of the tower. The subject then lowered the weight on the left shaft.
There was a main effect of training on L1, L2, L3, L4, total lumbar, and greater trochanter BMD, but there were no differences between groups. Eights weeks of lower body resistive exercise increased greater trochanter and lumbar BMD. Inability to detect group differences may have been influenced by a potentially osteogenic vibration associated with device operation in the 0, 33, and 66% groups.
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