Abstract
We examined activation patterns of the gastrocnemius medialis (GM), gastrocnemius lateralis (GL), soleus (SO), and tibialis anterior (TA) muscles in eight older (58.4 ± 3.3 years) and seven young (23.1 ± 2.9 years) participants, before and after 14 days of horizontal bed rest. Visual feedback on the exerted muscle torque was provided to the participants. The discharge patterns of individual motor units (MUs) were studied in three repetitions of isometric plantar flexion at 30 and 60% of Maximum Voluntary Contraction (MVC), before, and 1 day after the 14-day bed rest, respectively. In the GL and GM muscles, the older participants demonstrated higher MU discharge rates than the young, regardless of the contraction level, both before and after the bed rest. In the TA and SO muscles, the differences between the older and young participants were less consistent. Detailed analysis revealed person-specific changes in the MU discharge rates after the bed rest. To quantify the coactivation patterns we calculated the correlation coefficients between the cumulative spike trains of identified MUs from each muscle, and measured the root mean square difference of the correlation coefficients between the trials of the same session (intra-session variability) and between different sessions (inter-session variability) in each participant (intra-person comparison) and across participants (inter-person comparison). In the intra-person comparison, the inter-session variability was higher than the intra-session variability, either before or after the bed rest. At 60% MVC torque, the young demonstrated higher inter-person variability of coactivation than the older participants, but this variability decreased significantly after the bed rest. In older participants, inter-person variability was consistently lower at 60% than at 30% MVC torque. In young participants, inter-person variability became lower at 60% than at 30% MVC torque only after the bed rest. Precaution is required when analyzing the MU discharge and coactivation patterns, as individual persons demonstrate individual adaptations to aging or bed rest.
Highlights
Several studies demonstrated the decrease of motor unit (MU) discharge rates with age in individual muscles such as first dorsal interosseous (Erim et al, 1999; Vaillancourt et al, 2003), tibialis anterior (TA) (Connelly et al, 1999), soleus (SO) (Kallio et al, 2012), and vastus lateralis (Watanabe et al, 2016), suggesting that age-related loss of muscle strength is related to MU discharge properties
Before the bed rest the number of identified MUs was higher in older than in young participants for the SO muscle at 30% Maximum Voluntary Contraction (MVC) torque, and for the TA and SO muscles at 60% MVC torque
We demonstrated that the changes of MU discharge rates and muscle coactivation patterns during submaximal isometric plantar flexion after the bed rest are person-specific, and depend on the level of plantar flexion
Summary
Several studies demonstrated the decrease of motor unit (MU) discharge rates with age in individual muscles such as first dorsal interosseous (Erim et al, 1999; Vaillancourt et al, 2003), tibialis anterior (TA) (Connelly et al, 1999), soleus (SO) (Kallio et al, 2012), and vastus lateralis (Watanabe et al, 2016), suggesting that age-related loss of muscle strength is related to MU discharge properties. In the past few years, person- and condition-specific coactivation patterns of triceps surae muscle were studied extensively (Hug et al, 2019; Aeles et al, 2021), demonstrating that the gastrocnemius medialis (GM), gastrocnemius lateralis (GL), SO and TA muscles exhibit person-specific coactivation patterns, and support person discrimination across measuring sessions. These findings led to the establishment of the concept of person-specific muscle coactivation signatures (Hug et al, 2019)
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