Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, C57
Superior skeletal muscle fatigue resistance in old compared with young adult humans during sustained but not during intermittent contractions
J. S. McPhee1, T. M. Maden-Wilkinson1, H. Degens1, D. A. Jones1
1. Institute for Biomedical Research into Human Movement and Health, Manchester Metropolitan University, Manchester, United Kingdom.
Ageing skeletal muscle is characterised by loss of strength and power, and a slowing of the contractile properties (Deschenes, 2004). We hypothesised that the slower contractile properties would confer enhanced fatigue resistance in old compared with young adults. To investigate this, 12 old (O: 69 +/- 1 yrs) and 12 young (Y: 24 +/- 1 yrs) adults completed maximum voluntary isometric contractions (MVC) of the knee extensors using the twitch interpolation technique to assess voluntary activation. Electrically-stimulated contractions (ES) were used to determine the force-frequency relationship, and rates of contraction and relaxation were determined from twitches. Fatigue resistance was measured in two ways: in one leg it was measured as the time to task-failure during a sustained voluntary contraction at 50% MVC, and ES doublets were superimposed every 10 sec to provide an objective distinction between central and peripheral components of fatigue. On the other leg, sequences of ES contractions (30 Hz, 50% duty cycle: 1 sec on, 1 sec off) were applied to the knee extensors and the percentage of force remaining after 2 min was calculated (FI). Data are presented as mean +/- SEM, compared by Students’ t-tests and ANOVA. The MVC was 43% lower in O compared with Y (P<0.001), but levels of voluntary activation during MVCs were similar. A left-shift of the force frequency relation, slower maximum rate of contraction and slower relaxation time (all P <0.05) were observed in older adults. O sustained the isometric contraction for 31% longer than Y (O= 101 +/- 6; Y = 77 +/- 5; P<0.01) and in both groups task failure was almost entirely due to peripheral rather than central fatigue. However, there was no significant difference between O and Y in fatigue elicited by the intermittent ES fatigue test (O = 55% +/- 3; Y = 58% +/- 2). In Y, there was a strong correlation between the Sustained and FI tests (r = 0.67; P<0.01), but around half of the older adults did not conform to the same linear relationship as Y, resulting in a loss of the correlation between the two types of fatigue test in O (Figure 1). These results show that, on average, knee extensor contractile properties are slower in older adults compared with young. The older adults showed superior fatigue resistance during sustained contractions (primarily ischaemic), but not during conditions that more closely resemble in vivo intermittent contraction dynamics of normal locomotion. Failure to translate slower contractile properties to superior fatigue resistance during intermittent contractions could indicate differences in, e.g. rates of ATP use and production in older people. There was, however, large interindividual variability in results, with some older muscles behaving like muscles of young adults. The reasons for this are not known.
Where applicable, experiments conform with Society ethical requirements