Multi-joint Muscle Research Articles

Lumbar spine maximum efforts and muscle recruitment patterns predicted by a model with multijoint muscles and joints with stiffness

The transmission of load through the lumbar spine was analyzed in a model of the five lumbar vertebrae, the sacrum/pelvis and the thorax, and 66 symmetric pairs of multijoint muscles. The model was used to test the hypotheses that (1) the need to maintain equilibrium simultaneously at all vertebral levels precludes simultaneous maximum activation of synergistic muscles and (2) that the maximum loads which could be carried by the spine and the degree of muscle activation increases with increasing motion segment stiffness. Maximum moments applied to T12 were calculated for moments in three principal directions, subject to equilibrium at all six joints and to constraints on the maximum muscle stress and intervertebral displacements. A model with realistic motion segment stiffness predicted maximum efforts between 1.4 and 3.3 times greater than a model with ‘ball-and-socket’ joints, and in better agreement with published results from maximum effort experiments. The differences in maximal effort were greater than the moments transmitted through the joints. While muscle activation levels were greater, many synergistic muscles were still submaximally activated. Antagonistic muscles were recruited to maintain multijoint equilibrium. We concluded that (1) muscle activations permitted in single anatomic level analyses are generally not compatible with equilibrium at other levels; (2) the effect of moment transmission in the joints gives a more realistic representation of the lumbar spine.

Effects of replacing 2-joint muscles with energetically equivalent 1-joint muscles on cost-function values of non-linear optimization approaches

Static, non-linear optimization was used to calculate the forces in muscles and the corresponding cost function values for a two-dimensional musculoskeletal system containing both 1-joint and 2-joint muscles, and an energetically equivalent system containing just 1-joint muscles. Agonistic muscular activity and co-contraction of pairs of antagonistic muscles were present in the general solutions of a system containing multi-joint muscles; such patterns of activity cannot be observed in a system containing just 1-joint muscles. If cost function values are considered to be a measure of some type of physiological effectiveness (as suggested implicitly or explicitly in the literature), then a system containing 2-joint muscles will tend to be more effective than a system containing 1-joint muscles, in situations where the directions of the resultant joint moments are equal. In situations where the direction of the joint moments are opposite, the system containing just 1-joint muscles tends to be more cost effective than the model containing 2-joint muscles.

Mechanical energy management in cycling

Conservation of energy suggests that during cycling the constrained lower extremity is capable of delivering energy to the bicycle without expending energy to move the limbs. The purpose of this study was to characterize the management of mechanical energy during cycling and, specifically, to evaluate the potential for system energetic conservatism. Mechanical energy contributions derived from lower extremity energy sources were computed for 12 experienced male cyclists riding at five combinations of cadence and power output. The knee joint dominated (> 50%) in contributing to system energy and a moderate amount of energy was derived from hip joint reaction forces (> 6%). Energy generations and dissipations at the sources were sensitive to power output and, within the range of conditions studied, insensitive to cadence. Two energy models estimated mechanical energy expenditure under hypothetical single-joint and multijoint muscle operating conditions. When multijoint muscles were incorporated into the energy management analysis, a significant reduction in mechanical work relative to the single-joint muscle operation occurred. Energy savings associated with multijoint muscle energy transfers were enhanced at higher bicycle power levels, suggesting that conservation of mechanical energy is plausible given appropriate actions of two-joint muscles.

Predictions of antagonistic muscular activity using nonlinear optimization

Optimization theory is used more often than any other method to predict individual muscle forces in human movement. One of the limitations frequently associated with optimization algorithms based on efficiency criteria is that they are thought to not provide solutions containing antagonistic muscular forces; however, it is well known that such forces exist. Since analytical solutions of nonlinear optimization algorithms involving multi-degree-of-freedom models containing multijoint muscles are not available, antagonistic behavior in such models is not well understood. The purpose of this investigation was to study antagonistic behavior of muscles analytically, using a three-degree-of-freedom model containing six one-joint and four two-joint muscles. We found that there is a set of general solutions for a nonlinear optimal design based on a minimal cost stress function that requires antagonistic muscular force to reach the optimal solution. This result depends on a system description involving multijoint muscles and contradicts earlier claims made in the biomechanics, physiology, and motor learning literature that consider antagonistic muscular activities inefficient.

Biomechanics of Cycling and Factors Affecting Performance

Cycling performance in human powered vehicles is affected by the interaction of a number of variables, including environment, mechanical and human factors. Engineers have generally focused on the design and development of faster, more efficient human-powered vehicles based on minimising aerodynamic drag, neglecting the human component. On the other hand, kinesiologists have examined cycling performance from a human perspective, but have been constrained by the structure of a standard bicycle. Therefore, a gap exists between research in the various disciplines. To maximise/optimise cycling performance in human-powered vehicles requires a bridging of this gap through interdisciplinary research. Changes in different variables can affect the energy requirements of cycling. These variables include: (a) changes in body position, configuration, and orientation; (b) changes in seat to pedal distance; and (c) the interaction of workload, power output, and pedalling rate. Changes in these variables alter joint angles, muscle lengths, and muscle moment arm lengths, thus affecting the tension-length, force-velocity-power relationships of multi-joint muscles and the effectiveness of force production. This is ultimately manifested as a change in the energetics of cycling. A large number of factors affect cycling performance in human-powered vehicles and a gap still exists between cycling research in various disciplines. To bridge this gap, if not completely close it, requires cooperation between disciplines and further interdisciplinary research.

A method for the determination of the force-length relation of selected in-vivo human skeletal muscles.

In this paper a method is presented to determine force-length relations of in-vivo human skeletal muscles. The method is experimental and can be used for selected multi-joint muscles. It contains three basic assumptions: (a) the maximal, isometric force a muscle can exert is constant for a given muscle length, (b) antagonistic muscle activity for the experimental contractions is constant, and (c) resultant joint moments obtained during the experiments are produced by muscular forces exclusively. Experimentally determined force-length relations of intact in-vivo human skeletal muscles have not been determined yet. Application of this method will allow the comparison of actual force-length relations of selected human skeletal muscles to force-length relations used previously. Proposed mechanisms responsible for the force-length characteristics of a muscle, such as the cross-bridge theory, may be critically evaluated. Differences of force-length relations obtained under in vivo and in vitro conditions may be quantified.

Length and circumference measurements in one-joint and multijoint muscles in rabbits after immobilization.

In this article, we compared changes in the length and circumference of the soleus and the plantaris muscles in rabbits. The rabbits were assigned to a nonimmobilized external control group (n = 4), an experimental shortened-position group (n = 10), or an experimental lengthened-position group (n = 9). One hind limb of each animal in the experimental groups was immobilized in a cast for four weeks to put the muscles in either a shortened or lengthened position. The contralateral limb served as an internal control for the animals in the experimental groups. After the immobilization period, the limbs were fixed in situ, the muscles were dissected, and length and circumference measurements were taken. In both experimental conditions, the immobilized soleus muscles were shorter than their contralateral counterparts (p less than .05); the length of the plantaris muscles did not change. The immobilized muscles had decreased circumference values regardless of the immobilization angle. When compared with the control condition, the shortened soleus muscles had a greater decrease in circumference than did the plantaris muscles. More connective tissue was observed in the muscle bellies of the soleus after immobilization than in the plantaris. Similar significant changes (p less than .05) were found in the nonimmobilized limbs of the animals in the experimental groups. Specifically, the soleus muscles demonstrated length and circumference changes, whereas the plantaris muscles showed changes only in circumference. These findings suggest that when a weight-bearing limb is immobilized, adaptations in gross muscle length and circumference are to be expected in the involved and uninvolved limbs. An assessment of similar adaptations in humans should be a part of the evaluation during recovery.

Studies in the Mechanics of the Tetrapod Skeleton

Studies in the Mechanics of the Tetrapod Skeleton