Experimental findings of non invasive in-vivo monitoring are essential to study the diversity and evolution of
musculoskeletal kinematics. In this paper, results obtained from the uni-axial monitoring of the quasi-static
dynamics of the biceps muscle-belly are reported. Monitoring of the belly diameter is based on a custom
developed ultrasonic caliper combined with the synchronously recorded applied external force and joint angle
variations detected with a custom build ultrasonic force sensor and a resistive angle decoder respectively. The
monitored muscle action includes the processes of active muscle contraction and relaxation in a closed path
starting with an initial isotonic contraction followed by eccentric (spring like) stretching. The technology
applied here allows for observations of those processes and registration of their paths in the length-forceangle
parameter space. That way of presentation reveals that at some conditions the closed-loop human cycles
follow in close approximation characteristic lines of well identifiable elementary processes. The presentation
of these processes in the length-force parameters space allows for discussion of the mechanical energy
expenditure during different muscle actions. Comparative studies of identical closed-loop muscle actions and
the joint angle-force-length relationships of the muscle-tendon complex are presented. This synchronous
monitoring also allows quantifying the joint torques and positions with high accuracy for living person.
KEYWORDS: Ultrasonics, Sensors, In vivo imaging, Spatial resolution, Data modeling, Data acquisition, Energy transfer, Tissues, Temporal resolution, Performance modeling
The force-length relation is one of the most important mechanical properties of skeletal muscular tissue. Due to the
rather limited availability of non-invasive methods suitable to quantify the in-vivo biomechanical properties of activated
human muscles and connected tendons, the quantification of the bio-mechanical properties is difficult. The measurement
principle applied here is based on the detection of the dynamics of the muscle under observation by an ultrasonic caliper
and monitoring of the externally present forces by a synchronously operated ultrasonic force sensor. The developed
monitoring scheme is exemplified for gradual increasing voluntary isometric contraction (MVIC) of the gastrocnemius
muscle up to maximum contraction, with the force sensor restricting the flexion of the joint. The temporal resolution for
the monitoring is 0.01 s, relating to a monitoring rate of 100 Hz and is achieved with a spatial resolution concerning the
observed lateral extension of the muscle of 0.01 mm. The employed low power, economic and non-intrusive detection
scheme and respective instrumentation have the demonstrated potential to quantify the in-vivo hysteretic behavior of the
observed force-length relation for MVIC of the human gastrocnemius muscle for the first time. The purpose of this study
was to determine in-vivo the force-length relations for the human gastrocnemius and biceps muscles noninvasively by
suitable experimental techniques with high temporal and spatial resolution concerning monitoring of the biomechanical
relevant parameters involved in the dynamics of activated muscle. The data is collected and analyzed to derive
quantitative information on force-length relations, essential for the analysis of muscle performance and interpretation by
musculoskeletal models. The involved technologies are demonstrated and the respective results are presented and
discussed.
A non-intrusive novel detection scheme has been implemented to detect the lateral muscle extension, force of the
skeletal muscle and the motor action potential (EMG) synchronously. This allows the comparison of muscle dynamics
and EMG signals as a basis for modeling and further studies to determine which architectural parameters are most
sensitive to changes in muscle activity. For this purpose the transmission time for ultrasonic chirp signal in the frequency
range of 100 kHz to 2.5 MHz passing through the muscle under observation and respective motor action potentials are
recorded synchronously to monitor and quantify biomechanical parameters related to muscle performance. Additionally
an ultrasonic force sensor has been employed for monitoring. Ultrasonic traducers are placed on the skin to monitor
muscle expansion. Surface electrodes are placed suitably to pick up the potential for activation of the monitored muscle.
Isometric contraction of the monitored muscle is ensured by restricting the joint motion with the ultrasonic force sensor.
Synchronous monitoring was initiated by a software activated audio beep starting at zero time of the subsequent data
acquisition interval. Computer controlled electronics are used to generate and detect the ultrasonic signals and monitor
the EMG signals. Custom developed software and data analysis is employed to analyze and quantify the monitored data.
Reaction time, nerve conduction speed, latent period between the on-set of EMG signals and muscle response, degree of
muscle activation and muscle fatigue development, rate of energy expenditure and motor neuron recruitment rate in
isometric contraction, and other relevant parameters relating to muscle performance have been quantified with high
spatial and temporal resolution.
Skeletal muscle is a classic example of a biological soft matter . At both macro and microscopic levels, skeletal muscle
is exquisitely oriented for force generation and movement. In addition to the dynamics of contracting and relaxing
muscle which can be monitored with ultrasound, variations in the muscle force are also expected to be monitored. To
observe such force and sideways expansion variations synchronously for the skeletal muscle a novel detection scheme
has been developed. As already introduced for the detection of sideways expansion variations of the muscle, ultrasonic
transducers are mounted sideways on opposing positions of the monitored muscle. To detect variations of the muscle
force, angle of pull of the monitored muscle has been restricted by the mechanical pull of the sonic force sensor. Under
this condition, any variation in the time-of-flight (TOF) of the transmitted ultrasonic signals can be introduced by the
variation of the path length between the transducers. The observed variations of the TOF are compared to the signals
obtained by ultrasound monitoring for the muscle dynamics. The general behavior of the muscle dynamics and muscle
force shows almost an identical concept. Since muscle force also relates the psychological boosting-up effects, the
influence of boosting-up on muscle force and muscle dynamics can also be quantified form this study. Length-tension or
force-length and force-velocity relationship can also be derived quantitatively with such monitoring.
Beside of changes in the shape of contracting and relaxing muscle, which can be monitored with ultrasound, also
changes in the velocity of ultrasound are expected. To observe such changes with high resolution for the gastrocnemius
muscle of athletes a novel detection scheme has been developed. As already introduced for the detection of sideways
expansion of the muscle, ultrasonic transducers are mounted sideways on opposing positions of the skin. To detect
variations of the speed of sound, the expansion of the muscle is suppressed by mechanical clamping. Under this
condition, any variation in the time-of-flight of ultrasonic signals can only be introduced by a variation of the speed of
sound along the path of the ultrasound transit signal. The observed rather small variations of the speed of sound are
compared to the signals obtained by ultrasound monitoring for the extension and contraction observed for free sideways
motion (unclamped muscle). Opposite to the general behavior of a free muscle the clamped muscle shows a diminishing
time-of-flight under contraction relating to an increase in the sound velocity. Since clamping also reduces effects of
inertia, the influence of inertia on muscle dynamics can be illustrated by comparison of measurements on clamped and
free muscle.
Ultrasonic monitoring allowing the evaluation of the performance of muscles under training has been developed. The
monitoring scheme is suitable to determine muscle movement and is based on the measurement of the transit time of
longitudinally polarized ultrasound propagating across the observed muscle. Variations of the length of the muscle lead
to variations of the lateral extension since the volume of the muscle is conserved. The corresponding variations of the
observed time-of-flight result dominantly from the variation of the path length. This allows the time-resolved detection
of the movement of the muscles in the path of the ultrasonic beam. In this way not only the degree of contraction or
relaxation, but also the speed of these processes can be quantitatively monitored. The muscle thickness has been
determined with a resolution of ± 0.02 mm corresponding to about ± 0.2 % of the thickness of the relaxed muscle. This
resolution is already in the range of unavoidable uncertainties caused by the surface structure of the individual muscles.
Similarly, the already obtained resolution in time corresponds to a fraction 1/750 of the time of the fastest known human
muscle movement of 7.5 ms, observed for the full contraction of the eye lid muscle. The time of flight is measured along
a line between two electro-acoustic transducers positioned on the skin on opposite sides of the monitored muscle. The
transducers can be placed at any desired position but should be positioned such, that no bones or intestines are
obstructing the path between them. The time-of-flight from which all other data is derived is observed with the aid of a
computer-controlled arbitrary function generator and a synchronized transient recorder. Even in the demonstrated
developmental state the equipment is already rather compact (lap-top size) and can be battery operated.
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