Triceps surae contractile properties and firing rates in the soleus of young and old men

Abstract

Mean maximal motor unit firing rates (MUFRs) of the human soleus are lower (5-20 Hz) than other limb muscles (20-50 Hz) during brief sustained contractions. With healthy adult aging, maximal MUFRs are 20-40% lower and twitch contractile speed of lower limb muscles are 10-40% slower compared with young adults. However, it is unknown whether the inherently low maximal MUFRs for the soleus are further reduced with aging in association with age-related slowing in contractile properties. The purpose of the present study was to compare the changes in triceps surae contractile properties and MUFRs of the soleus throughout a variety of contraction intensities in six old ( approximately 75 yr old) and six young ( approximately 24 yr old) men. Neuromuscular measures were collected from the soleus and triceps surae during repeated sessions (2-6 sessions). Populations of single MUFR trains were recorded from the soleus with tungsten microelectrodes during separate sustained 6- to 10-s isometric contractions of varying intensities [25%, 50%, 75%, and 100% maximal voluntary isometric contraction (MVC)]. The old men had weaker triceps surae strength (MVC; 35% lower) and slower contractile properties (contraction duration; 20% longer) than the young men. However, there was no difference in average MUFRs of the soleus at 75% and 100% MVC ( approximately 14.5 Hz and approximately 16.5 Hz, respectively). At 25% and 50% MVC, average rates were 10% and 20% lower in the old men compared with young, respectively. Despite a significant slowing in triceps surae contraction duration, there was no age-related change in MUFRs recorded at high contractile intensities in the soleus. Thus the relationship between the whole muscle contractile properties and MUFRs found in other muscle groups may not exist between the triceps surae and soleus and may be muscle dependent.

The altered vestibular-evoked myogenic and whole-body postural responses in old men during standing

“…With healthy adult aging, whole muscle contractile properties are slower (Dalton et al, 2009, motor unit firing rates are lower (Dalton et al, 2009(Dalton et al, , 2010Rubinstein and Kamen, 2005) and inherent motor neuron properties are altered (Kalmar et al, 2009;Piotrkiewicz et al, 2007), but these age-related declines cannot be generalized to all muscles and their constitutive motor neurons (Dalton et al, 2008(Dalton et al, , 2009Deschenes et al, 2010;Ishihara et al, 1987;Moran et al, 2005). Further, high-frequency sound and vibrotactile detection thresholds are increased with adult aging (Wells et al, 2003;Willott, 1984).…”

In-vivo function of human plantar intrinsic foot muscles

“…The peak discharge rates reported in our study are at the lower end of the range of peak discharge rates (between 10 to 25Hz) described in other lower limb muscles (Connelly et al, 1999;Roos et al, 1999;Dalton et al, 2009) during isometric contractions at similar contraction intensities. Our observed low peak discharge rates may be related to the postural function of this muscle , as some postural muscles are known to discharge at relatively low rates Dalton et al, 2009) for sustained periods in order to resist gravitational forces and maintain upright posture (Sherrington, 1915).…”

“…Similarly to other postural muscles (Macefield et al, 2000;Dalton et al, 2009) motor units in AH displayed low peak discharge rates and were resistant to fatigue. The anatomical configuration of a low number of motor units, relative to PCSA that are also fatigue resistant, may allow this muscle to generate and sustain moderate to large amounts of force for prolonged periods of time, in order to provide postural support for the foot.…”

“…Our observed low peak discharge rates may be related to the postural function of this muscle , as some postural muscles are known to discharge at relatively low rates Dalton et al, 2009) for sustained periods in order to resist gravitational forces and maintain upright posture (Sherrington, 1915).…”

“…The utilisation of recruitment and rate coding strategies varies greatly between muscles. Postural muscles, such as soleus, are active for sustained periods maintaining upright posture (Sherrington, 1915) generally display relatively low peak discharge rates (10-25 Hz) (Bellemare et al, 1983;Kuchinad et al, 2004;Dalton et al, 2009) and are reliant on recruitment to generate and maintain force across their full range of force production . Whereas muscles requiring precise control of movement, such as hand muscles, tend to have complete recruitment achieved at relatively low force levels (≈ 30 -50% of maximal voluntary contraction (MVC), thereafter relying on rate coding to generate and maintain force Thomas, 2006).…”