Abstract
As muscles get smaller with age, decreases in muscle function can follow and lead to a reduction in physical activity which then leads to more atrophy, creating a vicious cycle that continues as the person ages. Additionally, an important factor contributing to loss of function and reduced mobility in the elderly is skeletal muscle fatigue, which is a poorly understood and under-appreciated consequence of aging. Mechanisms of muscle fatigue are not clearly understood in the elderly population and accordingly, exercise regimes are not optimized to maximize both muscle and physical performance in this expanding population. PURPOSE: To determine the response of the quadriceps femoris (QF) of older women to fatiguing protocols utilizing either maximum voluntary- or neuromuscular electrical stimulation-induced (NMES) contractions. METHODS: Seventeen older females participated in the study (64 ± 4.2 years, 168 ± 5.9 cm, 76 ± 13 kgs). Participants performed a series of maximum voluntary isometric contractions (MVIC) of the right and left QF. Central activation ratios (CAR) were calculated using the burst superimposition technique during MVIC. Fatigue protocols consisted of voluntary- and NMES-induced fatigue protocols. The voluntary fatigue protocol was conducted on the right leg and consisted of a series of MVICs, each contraction was 5 seconds in duration with 5 seconds rest between contractions for 5 minutes (30 total contractions). CAR was calculated during the 1st, 15th, and 30th contraction. The NMES-induced fatigue protocol (50 Hz, 450µsec biphasic pulses) was conducted on the left leg, 5 seconds on and 5 seconds off for 5 minutes (30 total contractions). RESULTS: There was greater muscle fatigue during the NMES-induced fatigue protocol compared to the voluntary protocol (40 ± 12 % vs. 24 ± 16 %, p<0.01). CAR was 0.95 ± 0.03 during MVIC testing. CAR was not different between the 1st, 15th, and 30th contractions during the voluntary fatigue protocol (0.90 ±.06, 0.91 ±.07, and 0.91 ±.08, respectively). CONCLUSIONS: Differences in muscle recruitment strategies between voluntary- and NMES-induced contractions likely contribute to higher levels of muscle fatigue with the NMES protocol. Older women had a difficult time with activation when repeated maximal efforts were required. Supported by NIH Grant R01AG027084
Published Version
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