Muscle activation dynamics

There are two dynamic effects that must be accounted for to accurately model the physiological control mechanism. First, the change in peripheral resistance is delayed. This represents a delay from the time that the flow changes to when the vascular smooth muscle activity actually initiates the process of correcting the flow. This time delay in the change of peripheral resistance is denoted tq. To account for this effect, the change in peripheral resistance determined from the steady value was written as ARs(t — tq). 

Shue, G., Crago, P.E., and Chizeck, H.J., Muscle-joint models incorporating activation dynamics, moment-angle and moment-velocity properties, IEEE Trans. Biomed. Eng. BME-42 212-223, 1995. 

Indications of muscle activity, that is, voltage potentials produced by contracting muscles, known as dynamic electromyography (EMG)... 

Modeling Activation Dynamics. As noted is Sec. 6.5.1 ( Neural Excitation of Muscle ), muscle cannot activate or relax instantaneously. The delay between excitation and activation (or the development of muscle force) is due mainly to the time taken for calcium pumped out of the sarcoplasmic reticulum to travel down the T-tubule system and bind to troponin (Ebashi and Endo, 1968). This delay is often modeled as a first-order process (Zajac and Gordon 1989 Pandy et al., 1992) ... 

For the actuator shown in Fig. 6.21, musculotendon dynamics is described by a single, nonlinear, differential equation that relates musculotendon force (F "), musculotendon length (P"). musculotendon shortening velocity (v "), and muscle activation (n") to the time rale of change in musculotendon force ... 

Equations (6.1) and (6.7) can be combined to form a model of the musculoskeletal system in which the inputs are the muscle activation histories (a) and the outputs are the body motions (q, 4, q) (Fig. 6.22). Measurements of muscle EMG and body motions can be used to calculate the time histories of the musculotendinous forces during movement (Hof et al., 1987 Buchanan et al., 1993). Alternatively, the goal of the motor task can be modeled and used, together with dynamic optimization theory, to calculate the pattern of muscle activations needed for optimal performance of the task (Hatze, 1976 Pandy et al., 1990 Raasch et al., 1997 Pandy, 2001). Thus, one reason why the forward-dynamics method is potentially more powerful for evaluating musculotendinous forces than... 

A better approach involves parameterizing the input muscle activations (or controls) and converting the dynamic optimization problem into a parameter optimization problem (Pandy et al., 1992). The procedure is as follows. First, an initial guess is assumed for the control variables a. The system dynamical equations [Eq. (6.7) and (6.1)] are then integrated forward in time to evaluate the cost function in Eq. (6.14). Derivatives of the cost function a constraints are then calculated and... 

Forces play an integral role in the dynamic behavior of all human mechanics. In terms of human movement, forces can be defined as intrinsic or extrinsic. For example, a couple about a particular joint will involve the intrinsic muscle and frictional forces as well as any extrinsic loads sustained by the system. If the influence of intrinsic muscle activity within the system is to be considered, the location of the insertion points for each muscle must be determined to properly solve the equations of motion. 

Muscle contraction dynamics include the mechanical properties of muscle tissues and tendons, which are expressed as force-length and force-velocity relations. The activation dynamics include the voluntary and nonvoluntary (reflex) excitation signal and motor unit recruitment level in the muscle. It is well known that regardless of fatigue, the generated torque in each joint is dependent on muscle activation levels (MALs) and joint angle when in a stationary position. This was first observed by Tnman et al. 

Souza GM, Baker LL, Powers CM. Electromyographic activity of selected trunk muscles during dynamic spine stabilization exercises. Arch Phys Med Rehabil. 2001 82 1551-7... 

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