Effects of lower limb unloading on skeletal muscle mass and function in humans

The reliability, validity, and stability of a measure of physical activity in the elderly

OBJECTIVE

To assess the reliability, validity, and stability of an accelerometer-based monitor of physical activity in an elderly population.

DESIGN

Six studies were conducted to assess the Tritrac, a newly available accelerometer capable of storing three-dimensional activity data collected in minute epochs. After initial bench testing, the waist-worn devices were assessed for test-retest reliability during sitting and treadmill walking at 1mph and 2mph. Validity testing examined the Tritrac's ability to discriminate between different levels of physical activity and also compared the Tritrac with an established wrist-worn acclerometer, the Actigraph. Stability of the measure was examined over a period of 3 to 7 days, and in a subset of subjects this measurement was performed for a second week.

SETTING AND SUBJECTS

Residents of a VA Nursing Home (n = 40; mean age = 76), participants in an in-home nonaerobic exercise program (n = 36; mean age = 77), and community participants in an aerobic exercise program (n = 10; mean age = 71).

RESULTS

Intraclass correlation coefficients (ICC) for both the bench testing and test-retest reliability were .97. The device discriminated among subjects independently categorized as sedentary, moderately active, or active (F = 49.4, p = .0001) and between specific activities of varying intensity (F = 114.5, p = .0001). Tritrac and Actigraph measurement comparison showed a correlation of r = .77, p = .0001. Stability of the measure was demonstrated by ICCs = .81 and .78 for mean activity values and proportion of time spent in sedentary activity, respectively. No significant differences were observed when comparing activity measured for two separate weeks.

CONCLUSION

The Tritrac is a reliable and valid instrument producing activity measurement that was stable over time in the elderly population we studied. During our longitudinal testing, 20% of the subjects did not comply with wearing the device; this noncompliance issue must be considered in any use of the Tritrac.

Vulnerability to dysfunction and muscle injury after unloading

OBJECTIVE

To test whether unloading increases vulnerability to eccentric exercise-induced dysfunction and muscle injury.

DESIGN

Before-after trial.

SETTING

General community.

PATIENTS OR OTHER PARTICIPANTS

Two women and 5 men (73 +/- 3kg [mean +/- SE]) who were active college students but were not trained in lower body resistance exercise volunteered.

INTERVENTION

Five weeks of unilateral lower limb suspension (ULLS), which has been shown to decrease strength and size of the unloaded, left, but not load-bearing, right quadriceps femoris muscle group (QF) by 20% and 14%, respectively; performance of 10 sets of ten eccentric actions with each QF immediately after the ULLS strength tests with a load equivalent to 65% of the post-ULLS eccentric 1-repetition maximum.

MAIN OUTCOME MEASURE(S)

Concentric and eccentric 1-repetition maximum for the left, unloaded and the right, load-bearing QF measured immediately after ULLS and 1,4,7,9, and 11 days later; cross-sectional area and spin-spin relaxation time (T2) of each QF as determined by magnetic resonance imaging and measured the last day of ULLS and 3 days later.

RESULTS

The mean load used for eccentric exercise was 23 +/- 2 and 30 +/- 3kg for the left, unloaded and right, load-bearing QF, respectively. The concentric and eccentric 1-repetition maximum for the unloaded and already weakened left QF was further decreased by 18% (p = .000) and 27% (p = .000), respectively, 1 day after eccentric exercise. Strength did not return to post-ULLS levels until 7 days of recovery. The right, load-bearing QF showed a 4% decrease (p = .002) in the eccentric 1-repetition maximum 1 day after eccentric exercise. The left, unloaded QF showed an increase in T2 (p = .002) in 18% of its cross-sectional area 3 days after the eccentric exercise, thus indicating muscle injury. The right, load-bearing QF showed no elevation in T2 (p = .280).

CONCLUSION

Unloading increases vulnerability to eccentric exercise-induced dysfunction and muscle injury, even at relatively light loads.

Effects of detraining following short term resistance training on eccentric and concentric muscle strength

Healthy males were examined before and after 12 weeks of accommodated resistance training (three week-1) and after 12 weeks of detraining. Training consisted of four to five sets of six coupled maximum voluntary bilateral concentric and eccentric (Grp ECCON; n = 10) or 12 concentric (Grp CON; n = 8) quadriceps muscle actions. Concentric and eccentric peak torque at various constant angular velocities and three repetition maximum half-squat and vertical jump height were measured. Grp ECCON showed greater (P less than 0.05) overall increase in peak torque after training and detraining than Grp CON. Thus, concentric peak torque (0.52 rad s-1) increased more (P less than 0.05) over the experimental period in Grp ECCON and increases in eccentric peak torque were preserved in Grp ECCON only. Increases in peak torque in response to training were greater (P less than 0.05) at 0.52 than at 2.62 rad s-1. Alterations in the torque-velocity patterns induced by training remained after detraining in Grp ECCON but not in Grp CON. The retained increases (P less than 0.05) in half-squat were 12 and 18% in Grps CON and ECCON, respectively. Neither group showed increased vertical jump height after detraining. This study showed greater preservation of concentric and eccentric peak torque after detraining following coupled concentric and eccentric than concentric resistance training. Only the former regime induced a change in the shape of torque-velocity curves that was manifest after detraining.(ABSTRACT TRUNCATED AT 250 WORDS)