Motor unit firing behavior during prolonged 50% MVC dorsiflexion contractions in young and older adults

Christie, Anita; Kamen, Gary

doi:10.1016/j.jelekin.2008.03.005

Cited by 34 publications

(24 citation statements)

References 58 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…have recorded motor unit discharge rate during sustained contractions performed by older adults and have either indicated no reduction in discharge rate for both young and old adults during a sustained submaximal contraction (Christie and Kamen 2009) or a similar decline in discharge rate for young and old adults during intermittent contractions (Rubinstein and Kamen 2005). In agreement with previous findings, the mean discharge rate in the current study did not change from the first 20% to the last 20% epoch of the nearly 60-s contraction (P ϭ 0.091).…”

Section: Discharge Characteristics Of Motor Units In Old Adultsmentioning

confidence: 99%

Discharge Characteristics of Biceps Brachii Motor Units at Recruitment When Older Adults Sustained an Isometric Contraction

Pascoe

Holmes

Enoka

2011

Journal of Neurophysiology

View full text Add to dashboard Cite

Pascoe MA, Holmes MR, Enoka RM. Discharge characteristics of biceps brachii motor units at recruitment when older adults sustained an isometric contraction. J Neurophysiol 105: 571-581, 2011. First published December 15, 2010 doi:10.1152/jn.00841.2010. The purpose of this study was to compare the discharge characteristics of motor units recruited during an isometric contraction that was sustained with the elbow flexor muscles by older adults at target forces that were less than the recruitment threshold force of each isolated motor unit. The discharge times of 27 single motor units were recorded from the biceps brachii in 11 old adults (78.8 Ϯ 5.9 yr). The target force was set at either a relatively small (6.6 Ϯ 3.7% maximum) or large (11.4 Ϯ 4.5% maximum) difference below the recruitment threshold force and the contraction was sustained until the motor unit was recruited and discharged action potentials for about 60 s. The time to recruitment was longer for the large target-force difference (P ϭ 0.001). At recruitment, the motor units discharged repetitively for both target-force differences, which contrasts with data from young adults when motor units discharged intermittently at recruitment for the large difference between recruitment threshold force and target force. The coefficient of variation (CV) for the first five interspike intervals (ISIs) increased from the small (18.7 Ϯ 7.9) to large difference (35.0 Ϯ 10.2%, P ϭ 0.008) for the young adults, but did not differ for the two target force differences for the old adults (26.3 Ϯ 14.7 to 24.0 Ϯ 13.1%, P ϭ 0.610). When analyzed across the discharge duration, the average CV for the ISI decreased similarly for the two target-force differences (P ϭ 0.618) in old adults. These findings contrast with those of young adults and indicate that the integration of synaptic input during sustained contractions differs between young and old adults.

show abstract

Section: Discharge Characteristics Of Motor Units In Old Adultsmentioning

confidence: 99%

Discharge Characteristics of Biceps Brachii Motor Units at Recruitment When Older Adults Sustained an Isometric Contraction

Pascoe

Holmes

Enoka

2011

Journal of Neurophysiology

View full text Add to dashboard Cite

show abstract

“…During the final 25% of the task we observed a decrease in motor unit discharge rate in 11 of the 13 motor units. This anomalous decrease in SMU discharge rate observed concurrently with an increase in central drive has been observed previously in upper Mottram, 2004;Riley et al, 2008a) and lower (Kuchinad et al, 2004;Christie & Kamen, 2009;Dalton et al, 2010) limb muscles and is possibly due to reduced spinal motoneurone responsiveness, which impairs the ability to integrate increased cortical input (McNeil et al, 2011a;. Reduced spinal motoneurone responsiveness may occur as a result of intrinsic motoneurone adaptation (due to repetitive discharge) (Kernell & Monster, 1982;McNeil et al, 2011b) and reflex inhibition by Group III and IV afferents (Rotto & Kaufman, 1988).…”

Section: Peak Discharge Ratesupporting

confidence: 85%

“…The target force was set at 1.5 times the recruitment threshold for the target unit. Participants were strongly encouraged to maintain the target force until failure, which was defined as when the force recording dropped by more than 5% of the target force trace for a period of more than 5 seconds and could not subsequently return to the target force level (Christie & Kamen, 2009;Vila-Cha et al, 2010). In a number of the fatigue trials additional motor units could be identified and discriminated from the beginning of the fatigue task.…”

Section: Fatigue Protocolmentioning

confidence: 99%

“…If the contraction force is below the upper limit of motor unit recruitment, an increase in excitatory drive will generally incite recruitment of new motor units in order to help maintain the required force. Despite an increase in central drive to the motoneurone pool, motor unit discharge behaviour can vary considerably during sustained submaximal contractions, with studies reporting a decrease (Garland et al, 1997;Mottram, 2004;Riley et al, 2008b;Dalton et al, 2010), an increase (Griffin et al, 2001;Kuchinad et al, 2004) and also no change in discharge rate (Christie & Kamen, 2009;Pascoe et al, 2011). It has been suggested that the reported inconsistencies between studies in discharge behaviour that occur during sustained submaximal contractions are due to varying interactions between cortical input and spinal motoneurone responsiveness (Kernell & Monster, 1982;McNeil et al, 2011a), as well as the nature of the task (ie.…”

Section: Introductionmentioning

confidence: 99%

“…The co-efficient of variation (CV) of inter-spike interval (ISI) is a relative measure of motor unit discharge variability and provides an insight to the interplay between increased central drive and spinal motoneurone inhibition that occurs during sustained contractions (Calvin & Stevens, 1968). Motor unit discharge variability significantly hinders force output and steadiness (Enoka et al, 2002;Tracy et al, 2005; and has been shown to increase under conditions of muscle fatigue (Christie & Kamen, 2009). …”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

In-vivo function of human plantar intrinsic foot muscles

Kelly¹

View full text Add to dashboard Cite

This thesis investigates the in-vivo function of the plantar intrinsic foot muscles. Though much speculation has been made of the function of these muscles, scant detail exists pertaining to their function. This thesis provides a novel description of the function of these muscles in providing active support for the longitudinal arch (LA) during postural tasks and locomotion. Furthermore, the following chapters provide evidence of an active mechanism to stiffen the LA, primarily provided by the graded activation of these muscles in response to increasing load. This mechanism may have important implications for how energy is stored and released within the foot. Chapter one provides a general overview of the existing literature pertaining to the function of these muscles. Chapters two, three, four and five contain the individual manuscripts from each experiment performed as part of this thesis. Chapter six provides a summary of the findings from the thesis and some general suggestions for the direction of future research in this field.Chapter two investigates the role of the plantar intrinsic foot muscles in providing postural support for the foot during quiet standing. Intra-muscular electromyographic (EMG) activity was recorded from abductor hallucis (AH), flexor digitorum brevis (FDB) and quadratus plantae (QP) while participants performed two balance tasks of graded difficulty. Each task was performed while standing on a force plate, allowing appraisal of any relationship between loading, postural sway and intrinsic foot muscle activity. Intrinsic foot muscle activation increased in response to postural demand, with these muscles displaying highly correlated inter-muscular activation patterns in response to medial postural sway. Contrary to previous thoughts, these muscles are clearly important in postural control and are recruited in a highly co-ordinated manner to stabilise the foot and maintain balance, particularly during single leg stance, in the medio-lateral direction.The purpose of Chapter three was to investigate if the neurophysiological properties of the largest intrinsic foot muscle (abductor halluces) are matched to its suggested postural function. A highly selective, quadrifilar arrangement of fine wire EMG electrodes was employed to describe the discharge properties of AH motor units during ramp and hold isometric contractions, as well as during a submaximal, constant force, fatiguing contraction. Abductor hallucis motor units displayed small rate coding ranges, relatively low peak discharge rates and were largely resistant to fatigue. This muscle is comparatively fatigue resistant and appears to rely predominantly on recruitment to generate force, optimizing the use of slow twitch, fatigue resistant fibres to generate moderate to large amounts of force for sustained periods of time. These properties appear well matched to AH's postural function that involves providing stabilisation of the LA during weight-bearing tasks. 3Chapter four examined the potential for the intrinsic foot muscles ...

show abstract

Motor Unit

Heckman

Enoka

2012

Comprehensive Physiology

409

449

View full text Add to dashboard Cite

Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.

show abstract

Motor unit firing behavior during prolonged 50% MVC dorsiflexion contractions in young and older adults

Cited by 34 publications

References 58 publications

Discharge Characteristics of Biceps Brachii Motor Units at Recruitment When Older Adults Sustained an Isometric Contraction

Discharge Characteristics of Biceps Brachii Motor Units at Recruitment When Older Adults Sustained an Isometric Contraction

In-vivo function of human plantar intrinsic foot muscles

Motor Unit

Contact Info

Product

Resources

About