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

Hill-type models are ubiquitous in the field of biomechanics, providing estimates of a muscle's force as a function of its activation state and its assumed force-length and force-velocity properties. However, despite their routine use, the accuracy with which Hill-type models predict the forces generated by muscles during submaximal, dynamic tasks remains largely unknown. This study compared human gastrocnemius forces predicted by Hill-type models with the forces estimated from ultrasound-based measures of tendon length changes and stiffness during cycling, over a range of loads and cadences. We tested both a traditional model, with one contractile element, and a differential model, with two contractile elements that accounted for independent contributions of slow and fast muscle fibres. Both models were driven by subject-specific, ultrasoundbased measures of fascicle lengths, velocities and pennation angles and by activation patterns of slow and fast muscle fibres derived from surface electromyographic recordings. The models predicted, on average, 54% of the time-varying gastrocnemius forces estimated from the ultrasound-based methods. However, differences between predicted and estimated forces were smaller under low speed-high activation conditions, with models able to predict nearly 80% of the gastrocnemius force over a complete pedal cycle. Additionally, the predictions from the Hill-type muscle models tested here showed that a similar pattern of force production could be achieved for most conditions with and without accounting for the independent contributions of different muscle fibre types.

Section: Muscle Activation Patternsmentioning

confidence: 99%

Comparison of human gastrocnemius forces predicted by Hill-type muscle models and estimated from ultrasound images

Dick

Biewener

Wakeling

2017

“…A filter bank of 24 wavelets (0≤k≤23) was used to decompose the EMG signals into intensities as a function of time and frequency. To exclude lowfrequency noise, the first four wavelet domains were excluded from further analysis (≤70Hz) such that the data were analyzed for wavelets 3 to 23 (Hodson-Tole and Wakeling, 2007). Thus, the frequency band of 70-1857Hz is presented in this analysis.…”

Section: Analysis Of Emg Datamentioning

confidence: 99%

“…Hodson-Tole and Wakeling, 2008b;Lichtwark et al, 2007;Gillis et al, 2001). However, few studies have measured muscle force and strain together directly during differing locomotor tasks, and even fewer have examined whether such measures are associated with the recruitment of faster or slower motor units (Hodson-Tole and Wakeling, 2007;Wakeling et al, 2006).…”

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.

“…The recently developed wavelet and principal components analysis of intramuscular myoelectric signals has linked lower and higher frequencies in the signal to the preferential use of slower or slower and faster motor unit populations Wakeling and Syme, 2002;Hodson-Tole and Wakeling, 2007;Lee et al, 2011). This analysis may provide new insight into the involvement of motor unit populations with different properties within and across muscles during specific dynamic motor tasks.…”

Section: Introductionmentioning

confidence: 99%

“…The conduction velocity of muscle fibres is determined, in part, by the relative rates of membrane depolarisation and hyperpolarisation, a property that varies between muscle fibre types (Adrian and Peachey, 1965;Wallinga-De Jonge et al, 1985). As the frequency of motor unit firing is related to conduction velocity under specific conditions, there is the potential for time-frequency representations of the myoelectric signal to indicate recruitment of different fibre-type populations in response to different motor demands (von Tscharner, 2000;Hodson-Tole and Wakeling, 2007).…”

Section: Introductionmentioning

confidence: 99%

Task dependent activity of motor unit populations in feline ankle extensor muscles

Hodson-Tole

Pantall

Maas

et al. 2012

SUMMARYUnderstanding the functional significance of the morphological diversity of mammalian skeletal muscles is limited by technical difficulties of estimating the contribution of motor units with different properties to unconstrained motor behaviours. Recently developed wavelet and principal components analysis of intramuscular myoelectric signals has linked signals with lower and higher frequency contents to the use of slower and faster motor unit populations. In this study we estimated the relative contributions of lower and higher frequency signals of cat ankle extensors (soleus, medial and lateral gastrocnemii, plantaris) during level, downslope and upslope walking and the paw-shake response. This was done using the first two myoelectric signal principal components (PCI, PCII), explaining over 90% of the signal, and an angle , a function of PCI/PCII, indicating the relative contribution of slower and faster motor unit populations. Mean myoelectric frequencies in all walking conditions were lowest for slow soleus (234Hz) and highest for fast gastrocnemii (307 and 330Hz) muscles. Motor unit populations within and across the studied muscles that demonstrated lower myoelectric frequency (suggesting slower populations) were recruited during tasks and movement phases with lower mechanical demands on the ankle extensors -during downslope and level walking and in early walking stance and paw-shake phases. With increasing mechanical demands (upslope walking, mid-phase of paw-shake cycles), motor unit populations generating higher frequency signals (suggesting faster populations) contributed progressively more. We conclude that the myoelectric frequency contents within and between feline ankle extensors vary across studied motor behaviours, with patterns that are generally consistent with muscle fibre-type composition.