1. We studied quiet stance investigating strategies for maintaining balance. Normal subjects stood with natural stance and with feet together, with eyes open or closed. Kinematic, kinetic and EMG data were evaluated and cross-correlated. 2. Cross-correlation analysis revealed a high, positive, zero-phased correlation between anteroposterior motions of the centre of gravity (COG) and centre of pressure (COP), head and COG, and between linear motions of the shoulder and knee in both sagittal and frontal planes. There was a moderate, negative, zero-phased correlation between the anteroposterior motion of COP and ankle angular motion. 3. Narrow stance width increased ankle angular motion, hip angular motion, mediolateral sway of the COG, and the correlation between linear motions of the shoulder and knee in the frontal plane. Correlations between COG and COP and linear motions of the shoulder and knee in the sagittal plane were decreased. The correlation between the hip angular sway in the sagittal and frontal planes was dependent on interaction between support and vision. 4. Low, significant positive correlations with time lags of the maximum of cross-correlation of 250-300 ms were found between the EMG activity of the lateral gastrocnemius muscle and anteroposterior motions of the COG and COP during normal stance. Narrow stance width decreased both correlations whereas absence of vision increased the correlation with COP. 5. Ankle mechanisms dominate during normal stance especially in the sagittal plane. Narrow stance width decreased the role of the ankle and increased the role of hip mechanisms in the sagittal plane, while in the frontal plane both increased. 6. The modulation pattern of the lateral gastrocnemius muscle suggests a central program of control of the ankle joint stiffness working to predict the loading pattern.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-7793.1999.915ad.x

Frequency analysis of stretch reflex and its main subsystems in triceps surae muscles of the cat.

Feedforward ankle strategy of balance during quiet stance in adults

Regulation of soleus muscle stiffness in premammillary cats: intrinsic and reflex components.

1. The electrical activity of single motor units has been recorded from the first dorsal interosseus muscle of normal human subjects during voluntary, isometric contractions, together with the force generated by the muscle.2. By averaging the force correlated with the impulses from a single motor unit, the contraction time and twitch tension generated by that motor unit could be measured. When the rate of discharge was limited, either voluntarily or by automatic selection of intervals for analysis, the time for the tension to decline to half its maximum value (half-relaxation time) could also be measured for some motor units.3. Under our experimental conditions the trains of impulses from different motor units in most subjects were generated quite independently as tested by (a) measuring the correlation between activity in single units and that in the whole muscle as recorded by the surface electromyogram (e.m.g.), (b) measuring the cross-correlations between pairs of single units and (c) comparing the tension generated by stimulating single motor units with the average tension correlated in time with voluntary activity of single units in the same location.4. In one normal subject evidence of synchronization between separate motor units was obtained. Cross-correlation studies suggested that the cause of the synchronization was the presence of substantial common excitation received by the various motor units in the muscle.5. The frequency response for the contractions of single motor units was well fitted by that for a linear, second-order system with nearly critical damping. However, when stimulation of a few motor units was superimposed on a voluntary contraction, underdamped (oscillatory) responses were seen which were probably of reflex origin.6. The significance of these results in relation to the normal postural tremor in hand muscles is discussed.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1973.sp010087

The contractile properties of human motor units during voluntary isometric contractions

The mammalian muscle spindle and its central control.

Study Design. Lifting dynamics and electromyographic graphic activity were evaluated using a biomechanical model of spinal equilibrium and stability to assess cost-benefit effects of antagonistic muscle cocontraction on the risk of stability failure. Objectives. To evaluate whether increased biomechanical stability associated with antagonistic cocontraction was capable of stabilizing the related increase in spinal load. Summary of Background Data. Antagonistic cocontraction contributes to improved spinal stability and increased spinal compression. For cocontraction to be considered beneficial, stability must increase more than spinal load. Otherwise, it may be possible for cocontraction to generate spinal loads that cannot be stabilized. Methods. A biomechanical model was developed to compute spinal load and stability from measured electro-myography and motion dynamics. As 10 healthy men performed sagittal lifting tasks, trunk motion, reaction loads, and electromyographic activities of eight trunk muscles were recorded. Spinal load and stability were evaluated as a function of cocontraction and trunk flexion angle. Stability was quantified in terms of the maximum spinal load the system could stabilize. Results. Cocontraction was associated with a 12% to 18% increase in spinal compression and a 34% to 64% increase in stability. Spinal load and stability increased with trunk flexion. Conclusions. Despite increases in spinal load that had to be stabilized, the margin between stability and spinal compression increased significantly with cocontraction. Antagonistic cocontraction was found to be most beneficial at low trunk moments typically observed in upright postures. Similarly, empirically measured antagonistic cocontraction was recruited less in high-moment conditions and more in low-moment conditions.

Frequency analysis of stretch reflex and its main subsystems in triceps surae muscles of the cat.

Citations

Feedforward ankle strategy of balance during quiet stance in adults

Regulation of soleus muscle stiffness in premammillary cats: intrinsic and reflex components.

The contractile properties of human motor units during voluntary isometric contractions

The mammalian muscle spindle and its central control.

Cost-benefit of muscle cocontraction in protecting against spinal instability.

Related Papers (5)

The sensitivity of muscle spindle afferents to small sinusoidal changes of length

The mechanical properties of cat soleus muscle during controlled lengthening and shortening movements

Mammalian muscle receptors and their central actions

Peripheral control of movement

The basis of motor control