Abstract
Aging is related to a variety of changes at the muscular level. It seems that the age-related changes in motor unit activation are muscle- and intensity dependent. The purpose of this study was to examine the motor unit discharge rate (MUDR) in both isometric and dynamic contractions of the aging soleus muscle. Eight elderly males participated in the study. The subjects performed isometric and dynamic plantar flexions while seated in an ankle dynamometer. The force levels studied were 10, 20, 40, 60, 80 and 100% of the isometric (ISO) maximal voluntary contractions (MVC) in ISO and 10, 20 and 40% in concentric (CON) and eccentric (ECC) contractions. Soleus intramuscular EMG was recorded with bipolar fine-wire electrodes and decomposed to individual trains of motor unit discharges. In ISO the MUDR increased with each force level from 40 to 100% MVC. In dynamic contractions the descriptive analysis showed a higher MUDR in CON compared to ISO or ECC. The difficulties of recording single motor units in dynamic contractions, especially in the elderly is discussed.
Highlights
It is well known that ageing is related to changes at the muscular level, leading to a decline in motor performance
The current results show that in ISO trials motor unit discharge rate (MUDR) was significantly lower in the elderly in most measured force levels (10, 40, 60 and 80% maximal voluntary contractions (MVC))
The observed age-difference in MUDR confirms the results of our previous measurements in isometric conditions (Kallio et al, 2012, 2013), as well as in submaximal dynamic (Kallio et al, 2010) contractions matched by relative surface EMG instead of force
Summary
It is well known that ageing is related to changes at the muscular level, leading to a decline in motor performance. It seems that such changes in motor unit activation are muscleand intensity dependent. The soleus motor unit discharge rate (MUDR) was found to be lower in the elderly only at low forcelevels, and for other muscles no age related decrease in MUDR was found in studies by Howard et al (1988) and Galganski et al (1993). Age-related differences in motor unit discharge behavior have mainly been investigated in isometric contractions. A large number of studies in young subjects have shown that neuromuscular control of isometric and dynamic contractions differ in many ways, including MUDR (Tax et al, 1989; Howell et al, 1995; Søgaard et al, 1996, 1998; Kossev and Christova, 1998; Del Valle and Thomas, 2005; Pasquet et al, 2006; Altenburg et al, 2009; Kallio et al, 2013) and discharge patterns (Søgaard, 1995; Søgaard et al, 1998) and double discharges (Søgaard et al, 2001)
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