Motor unit recruitment as reflected by muscle fibre glycogen loss in a prosimian (bushbaby) after running and jumping

SUMMARY Motor units are generally considered to follow a set, orderly pattern of recruitment within each muscle with activation occurring in the slowest through to the fastest units. A growing body of evidence, however, suggests that recruitment patterns may not always follow such an orderly sequence. Here we investigate whether motor unit recruitment patterns vary within and between the ankle extensor muscles of the rat running at 40 cm s-1 on a level treadmill. In the past it has been difficult to quantify motor unit recruitment patterns during locomotion; however, recent application of wavelet analysis techniques has made such detailed analysis of motor unit recruitment possible. Here we present methods for quantifying the interplay of fast and slow motor unit recruitment based on their myoelectric signals. Myoelectric data were collected from soleus, plantaris and medial gastrocnemius muscles representing populations of slow, mixed and fast fibres, respectively, and providing a good opportunity to relate myoelectric frequency content to motor unit recruitment patterns. Following wavelet transformation, principal component analysis quantified signal intensity and relative frequency content. Significant differences in signal frequency content occurred between different time points within a stride (P<0.001). We optimised high- and low-frequency wavelets to the major signals from the fast and slow motor units. The goodness-of-fit of the optimised wavelets to the signal intensity was high for all three muscles (r2>0.98). The low-frequency band had a significantly better fit to signals from the soleus muscle (P<0.001), while the high-frequency band had a significantly better fit to the medial gastrocnemius(P<0.001).

Section: Task-specific Recruitment Of Motor Unitsmentioning

confidence: 64%

Section: Introductionmentioning

confidence: 99%

Variations in motor unit recruitment patterns occur within and between muscles in the running rat (Rattus norvegicus)

Hodson-Tole

Wakeling

2007

“…Preferential recruitment of faster muscle fibres during rapid contractions has been shown in various animal studies, including humans (Gillespie et al, 1974;Gollnick et al, 1974;Grimby and Hannerz, 1977;Hoffer et al, 1981;Citterio and Agostoni, 1984;Wakeling et al, 2006;Hodson-Tole and Wakeling, 2008a;Lee et al, 2013). It has been suggested that complete inhibition of slow muscle fibres in a muscle with mixed fibre types would provide resistance to whole muscle shortening and therefore be detrimental to power production for very fast contractions (Josephson and Edman, 1988).…”

Section: Discussionmentioning

confidence: 99%

“…However, the size principle of motor unit recruitment (Henneman et al, 1965a;Henneman et al, 1965b) dictates that slower fibres, with longer relaxation times, are deactivated last. While the size principle adequately describes many recruitment strategies, alternative strategies may also exist (Gillespie et al, 1974;Gollnick et al, 1974;Grimby and Hannerz, 1977;Hoffer et al, 1981;Citterio and Agostoni, 1984;Wakeling et al, 2006;Hodson-Tole and Wakeling, 2008a) when the demands exceed the abilities of the slowest muscle fibres.…”

Section: Introductionmentioning

confidence: 99%

Early deactivation of slower muscle fibres at high movement frequencies

Blake

Wakeling

2014

Animals produce rapid movements using fast cyclical muscle contractions. These types of movements are better suited to faster muscle fibres within muscles of mixed fibre types as they can shorten at faster velocities and achieve higher activation-deactivation rates than their slower counterparts. Preferential recruitment of faster muscle fibres has previously been shown during high velocity contractions. Additionally, muscle deactivation takes longer than activation and therefore may pose a limitation to fast cyclical contractions. It has been speculated that slower fibres may be deactivated before faster fibres to accommodate their longer deactivation time. This study aimed to test whether shifts in muscle fibre recruitment occur with derecruitment of slow fibres before faster fibres at high cycle frequencies. Electromyographic (EMG) signals were collected from the medial gastrocnemius at an extreme range of cycle frequencies and workloads. Wavelets were used to resolve the EMG signals into time and frequency space and the primary sources of variability within the EMG frequency spectra were identified through principal component analysis. Early derecruitment of slower fibres was evident at the end of muscle excitation at higher cycle frequencies, as determined by reduced low-frequency EMG content, and additional slower fibre recruitment was present at the highest cycle frequency. The duration of muscle excitation reached a minimum of about 150 ms and did not change for the three highest cycle frequencies, suggesting a duration limit for the medial gastrocnemius. This study provides further evidence of modifications of muscle fibre recruitment strategies to meet the mechanical demands of movement.

“…However, evidence has emerged across a range of species that motor units may be preferentially recruited (e.g. Gillespie et al, 1974;Grimby et al, 1981;Nardone et al, 1989;Sokoloff and Cope, 1996;Wakeling et al, 2006;HodsonTole and Wakeling, 2007;Hodson-Tole and Wakeling, 2008b;Hodson-Tole and Wakeling, 2010). For example, there is evidence that faster motor units may be recruited, independent of slower units, during rapid locomotor tasks.…”

Section: Introductionmentioning

confidence: 99%

Recruitment of faster motor units is associated with greater rates of fascicle strain and rapid changes in muscle force during locomotion

Lee

Miara

Arnold

et al. 2012

SUMMARYAnimals modulate the power output needed for different locomotor tasks by changing muscle forces and fascicle strain rates. To generate the necessary forces, appropriate motor units must be recruited. Faster motor units have faster activation-deactivation rates than slower motor units, and they contract at higher strain rates; therefore, recruitment of faster motor units may be advantageous for tasks that involve rapid movements or high rates of work. This study identified motor unit recruitment patterns in the gastrocnemii muscles of goats and examined whether faster motor units are recruited when locomotor speed is increased. The study also examined whether locomotor tasks that elicit faster (or slower) motor units are associated with increased (or decreased) in vivo tendon forces, force rise and relaxation rates, fascicle strains and/or strain rates. Electromyography (EMG), sonomicrometry and muscle-tendon force data were collected from the lateral and medial gastrocnemius muscles of goats during level walking, trotting and galloping and during inclined walking and trotting. EMG signals were analyzed using wavelet and principal component analyses to quantify changes in the EMG frequency spectra across the different locomotor conditions. Fascicle strain and strain rate were calculated from the sonomicrometric data, and force rise and relaxation rates were determined from the tendon force data. The results of this study showed that faster motor units were recruited as goats increased their locomotor speeds from level walking to galloping. Slow inclined walking elicited EMG intensities similar to those of fast level galloping but different EMG frequency spectra, indicating that recruitment of the different motor unit types depended, in part, on characteristics of the task. For the locomotor tasks and muscles analyzed here, recruitment patterns were generally associated with in vivo fascicle strain rates, EMG intensity and tendon force. Together, these data provide new evidence that changes in motor unit recruitment have an underlying mechanical basis, at least for certain locomotor tasks.