Regional Variability of Stretch Reflex Amplitude in the Cat Medial Gastrocnemius Muscle During a Postural Task

1997 ◽  
Vol 78 (2) ◽  
pp. 1150-1154 ◽  
Author(s):  
Janice J. Eng ◽  
J. A. Hoffer
Keyword(s):  
Stretch Reflex ◽  
Gastrocnemius Muscle ◽  
Muscle Length ◽  
Medial Gastrocnemius ◽  
Regional Variability ◽  
Reflex Amplitude ◽  
Local Reflex ◽  
Whole Muscle ◽  
Postural Task ◽  
Medial Gastrocnemius Muscle

Eng, Janice J. and J. A. Hoffer. Regional variability of stretch reflex amplitude in the cat medial gastrocnemius muscle during a postural task. J. Neurophysiol. 78: 1150–1154, 1997. The relationship between local fibre stretch velocity (mechanical input) and the corresponding local reflex electromyographic (EMG) amplitude (a measure of the neural output) was assessed to determine the contribution of muscle spindle feedback in postural control. We hypothesized that traditionally measured input variables (e.g., the velocity of an external movement or whole muscle velocity) may not accurately represent the mechanical input to the muscle spindles, especially when the background forces are small. Three cats were trained to stand on pedestals while ankle rotations were applied to the left hindlimb. EMG and fiber movement in both proximal and distal regions of the muscle were recorded in addition to muscle length and tendon force. We found that local muscle velocity was correlated poorly with whole muscle velocity, demonstrating that internal and external muscle movements are often dissimilar, particularly during tasks that involve modest levels of muscle activation. Local EMG reflex amplitudes were correlated well with the corresponding local fiber stretch velocities ( R values ranging from 0.5 to 0.8) but not with muscle stretch velocity. The lack of crossed correlations between fiber stretch velocities and reflex EMG amplitudes measured in proximal versus distal regions of the muscle suggests the presence of a local reflex component. It is concluded that changes in local muscle fiber length represent the mechanical input to spindles better than changes in the total muscle length. Additionally, spindles have a specific role in the reflex activation of nearby muscle fibers.

scholarly journals Increased Ankle Plantar Flexor Stiffness Is Associated With Reduced Mechanical Response to Stretch in Adults With CP

2021 ◽  
Vol 9 ◽  
Author(s):  
Jakob Lorentzen ◽  
Rasmus Feld Frisk ◽  
Jens Bo Nielsen ◽  
Lee Barber
Keyword(s):  
Ankle Joint ◽  
Stretch Reflex ◽  
Gastrocnemius Muscle ◽  
Length Change ◽  
Medial Gastrocnemius ◽  
Reflex Response ◽  
Plantar Flexor ◽  
Flexor Muscle ◽  
Muscle Fascicles ◽  
Medial Gastrocnemius Muscle

Hyperexcitable stretch reflexes are often not present despite of other signs of spasticity in people with brain lesion. Here we looked for evidence that increased resistance to length change of the plantar flexor muscle-fascicles may contribute to a reduction in the stretch reflex response in adults with cerebral palsy (CP). A total of 17 neurologically intact (NI) adults (mean age 36.1; 12 female) and 13 ambulant adults with CP (7 unilateral; mean age 33.1; 5 female) participated in the study. Subjects were seated in a chair with the examined foot attached to a foot plate, which could be moved by a computer-controlled electromotor. An ultrasound probe was placed over the medial aspect of the leg to measure the length of medial gastrocnemius muscle fascicles. Slow (7 deg/s) and fast (200 deg/s) stretches with amplitude 6 deg of the plantar flexors were applied over an ankle range of 70 deg at 10 deg intervals between 60 and 130 deg plantarflexion. It was checked by EMG electrodes that the slow stretches were sufficiently slow not to elicit any activity and that the fast stretches were sufficiently quick to elicit a maximal stretch reflex in both groups. The torque elicited by the stretches was measured together with changes in the length of medial gastrocnemius muscle fascicles. Muscle fascicles increased significantly in length with increasing dorsiflexion position in both populations (p < 0.001), but the fascicles were shorter in the CP population at all positions. Slow stretches elicited significantly larger torque and significantly smaller length change of muscle fascicles as the ankle joint position was moved more towards dorsiflexion in CP than in NI (p < 0.001). Fast stretches elicited larger torque responses at ankle joint positions of 80–100 deg in the NI than in the CP group (p < 0.01). A significant negative correlation was observed between the torque response and muscle fascicle length change to slow stretch in CP (p < 0.05), but not in NI. These findings support that increased passive resistance of the ankle plantar flexor muscle-tendon unit and development of contractures may conceal stretch reflex response in adults with CP. We argue that this should be taken into account in the neurological examination of spasticity.

scholarly journals The Mechanics of the Medial Gastrocnemius Muscle in the Freely Hopping Wallaby (Thylogale Billardierii)

1989 ◽  
Vol 147 (1) ◽  
pp. 439-456 ◽  
Author(s):  
R. I. GRIFFITHS
Keyword(s):  
Energy Storage ◽  
High Speed ◽  
Gastrocnemius Muscle ◽  
Mechanical Impedance ◽  
Medial Gastrocnemius ◽  
Elastic Recoil ◽  
Work Done ◽  
Whole Muscle ◽  
Constant Impedance ◽  
Medial Gastrocnemius Muscle

Muscle force, electromyogram and length were monitored in the medial head of the gastrocnemius (MG) muscle in freely hopping wallabies (Thylogale billardierii Desmarest). During take-off hops from rest, MG muscle developed force with an isometric contraction. For constant-speed hops, force was produced in MG muscle during rapid stretch. The muscle resisted this stretch with a constant impedance that was independent of hopping speed. The rate of stretch of the muscle during high-speed hopping was as high as 1 ms−1 (5–6 muscle lengths per second) at the onset of stretch and slowed to no stretch at the peak of force. Since the mechanical impedance was constant while the stretch velocity changed, there was no significant viscosity present in the muscle. The tendon stretched by 3.2% at 7kmh−1 hopping and by 4.4% at 18kmh−1 hopping. Elastic energy storage in the tendons increased with hopping speed but the percentage of total work done by elastic recoil of the whole muscle did not increase at higher hopping speeds. The significance of the muscle stretch is in producing high forces rapidly and, in addition, there is considerable energy storage in the tend

scholarly journals Effect of synchronization of firings of different motor unit types on the force variability in a model of the rat medial gastrocnemius muscle

2020 ◽  
Author(s):  
Rositsa Raikova ◽  
Vessela Krasteva ◽  
Piotr Krutki ◽  
Hanna Drzymała-Celichowska ◽  
Katarzyna Kryściak ◽  
...  
Keyword(s):  
Root Mean Square ◽  
Gastrocnemius Muscle ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Force Variability ◽  
Mean Square ◽  
Synchronization Index ◽  
The Mean ◽  
Whole Muscle ◽  
Medial Gastrocnemius Muscle

AbstractOscillations of muscle force, observed as physiological tremors, rely upon the synchronized firings of active motor units (MUs). This study aimed to investigate the effects of synchronizing the firings of three types of MUs on force development using a mathematical model of the rat medial gastrocnemius muscle. The model was designed based on the actual proportion and physiological properties of MUs and motoneurons innervating the muscle. The isometric muscle and MU forces were simulated by a model predicting non-synchronized firing of a pool of 57 MUs (including eight slow, 23 fast resistant to fatigue, and 26 fast fatigable) to ascertain a maximum excitatory signal when all MUs were recruited into the contraction. The mean firing frequency of each MU depended upon the twitch contraction time, whereas the recruitment order was determined according to increasing forces (the size principle). The synchronization of firings of individual MUs was simulated using four different modes and inducing the synchronization of firings within three time windows (± 2, ± 4, and ± 6 ms) for four different combinations of MUs. The synchronization was estimated using two parameters, the correlation coefficient and the cross-interval synchronization index. The four scenarios of synchronization increased the values of the root-mean-square, range, and maximum force in correlation with the increase of the time window. Greater synchronization index values resulted in higher root-mean-square, range, and maximum of force outcomes for all MU types as well as for the whole muscle output; however, the mean spectral frequency of the forces decreased, whereas the mean force remained nearly unchanged. The range of variability and the root-mean-square of forces were higher for fast MUs than for slow MUs; meanwhile, the relative values of these parameters were highest for slow MUs, indicating their important contribution to muscle tremor, especially during weak contractions.Author summaryThe synchronization of firings of motor units (MUs), the smallest functional elements of skeletal muscle increases fluctuations in muscle force, known as physiological tremor, which can disturb high-precision movements. In this study, we adopted a recently proposed muscle model consisting of MUs of three different types (fast fatigable, fast resistant to fatigue, and slow) to study four different scenarios of MU synchronization during a steady level of excitatory input to motoneurons. The discharge patterns were synchronized between pairs of MUs by shifting in time individual pulses, which occurred within a short time interval, and a degree of synchronization was then estimated. The increased synchronization index resulted in increased force variability for all MU types as well as for the whole muscle output; however, the mean force levels remained nearly unchanged, whereas the frequencies of the force oscillations were decreased. The absolute range of force variability was higher for fast than for slow MUs, indicating their dominant influence on muscle tremor at strong contractions, but the highest relative increase in force variability was observed for synchronized slow MUs, indicating their significant contribution to tremor during weak contractions, in which only slow MUs are active.

Damage to different motor units from active lengthening of the medial gastrocnemius muscle of the cat

2002 ◽  
Vol 92 (3) ◽  
pp. 1104-1110 ◽  
Author(s):  
C. L. Brockett ◽  
D. L. Morgan ◽  
J. E. Gregory ◽  
U. Proske
Keyword(s):  
Gastrocnemius Muscle ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Optimum Length ◽  
Time To Peak ◽  
Slow Twitch ◽  
Fast Twitch ◽  
Whole Muscle ◽  
Medial Gastrocnemius Muscle ◽  
Anesthetized Cat

Slow-twitch motor units in the medial gastrocnemius muscle of the anesthetized cat were found to have an average optimum length for active tension that was 0.8 ± 0.5 (SE) mm longer than the whole muscle optimum. For fast-twitch units (time to peak < 50 ms), the average optimum was 1.3 ± 0.3 mm shorter than the whole muscle optimum. After the muscle had been subjected to 10 stretches while maximally activated, beginning at the whole muscle optimum length, the optimum lengths of the 27 fast-twitch motor units shifted significantly further in the direction of longer muscle lengths (mean 4.3 ± 0.3 mm) than for the eight slow-twitch units (2.1 ± 0.4 mm). A shift in the muscle's length-tension relation was interpreted as being due to sarcomere disruption. Statistical analysis showed that a motor unit's optimum length for a contraction, relative to the whole muscle optimum, was a better indicator of the unit's susceptibility to damage from active lengthenings than was motor unit type.

The effect of long-term immobilization on the motor unit population of the cat medial gastrocnemius muscle

Neuroscience ◽  
1981 ◽  
Vol 6 (4) ◽  
pp. 725-739 ◽  
Author(s):  
R.F. Mayer ◽  
R.E. Burke ◽  
J. Toop ◽  
J.A. Hodgson ◽  
K. Kanda ◽  
...  
Keyword(s):  
Motor Unit ◽  
Gastrocnemius Muscle ◽  
Medial Gastrocnemius ◽  
Medial Gastrocnemius Muscle
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