The purpose of this study was to investigate, theoretically, to what extent muscle properties could contribute to recovery from perturbations during locomotion. Four models with different actuator properties were created: the FLVT model, which encompassed force-length (FL) and force-velocity (FV) characteristics of human muscles as well as muscle stimulation inputs as functions of time (T); the FLT model, which had muscles without force-velocity characteristics; the FVT model, which had muscles without specific force-length characteristics; and the MT model, which had no muscles but was driven by joint moments (M) as a function of time. Each model was exposed to static and dynamic perturbations and its response was examined. FLVT showed good resistance to both static and dynamic perturbations. FLT was resistant to static perturbation but could not counteract dynamic perturbation, whereas the opposite was found for FVT. MT could not counteract either of the perturbations. Based on the results of the simulations, skeletal muscle force-length-velocity properties, although interactively complex, contribute substantially to the dynamic stability of the musculoskeletal system.
Karin G.M. Gerritsen, Anton J. van den Bogert, Manuel Hulliger, and Ronald F. Zernicke
Susumu Yahagi, Zhen Ni, Makoto Takahashi, Yusaku Takeda, Toshio Tsuji, and Tatsuya Kasai
Using transcranial magnetic stimulation (TMS), differences in the excitability changes of motor evoked potentials (MEPs) between isometric (force task) and isotonic (movement task) muscle contractions in a distal (first dorsal interosseous; FDI) and a proximal (middle deltoid; MD) muscle were studied. In the FDI muscle, the active threshold of MEP recruitment was significantly lower in the isotonic than that in the isometric muscle contraction in spite of identical background EMG activity levels. Additionally, the dependence of the MEP amplitude on background EMG activity was significantly greater in the isotonic than in the isometric muscle contraction at low EMG activity levels, but the difference disappeared beyond middle EMG activity levels. In the MD muscle, the dependence of the MEP amplitude on background EMG activity was significantly greater in the isotonic than in the isometric muscle contraction, and further this dependence was kept at all muscle contraction levels. These results indicate that the dependence of the MEP amplitude on background EMG activity is modulated not only by the different muscle contraction modes (isotonic and isometric), but also by muscle properties (distal and proximal). Thus, the present findings suggest that the task-specific extra excitation in the proximal muscle is definitely produced corresponding to task differences (task-dependent subliminal fringe), which might be explained by the predominant frequency principle if applied to the proximal muscle. On the other hand, the lack of task-dependent extra excitation in the distal muscle is explained by the predominant recruitment principle for force grading in small hand muscles.
Samantha L. Winter and John H. Challis
For a physiologically realistic range of joint motion and therefore range of muscle fiber lengths, only part of the force-length curve can be used in vivo; i.e., the section of the force–length curve that is expressed can vary. The purpose of this study was to determine the expressed section of the force–length relationship of the gastrocnemius for humans. Fourteen male and fourteen female subjects aged 18–27 performed maximal isometric plantar flexions in a Biodex dynamometer. Plantar flexion moments were recorded at five ankle angles: −15°, 0°, 15°, 30°, and 40°, with negative angles defined as dorsiflexion. These measurements were repeated for four randomly ordered knee angles over two testing sessions 4 to 10 days apart. The algorithm of Herzog and ter Keurs (1988a) was used to reconstruct the force–length curves of the biarticular gastrocnemius. Twenty-four subjects operated over the ascending limb, three operated over the descending limb, and one operated over the plateau region. The variation found suggests that large subject groups should be used to determine the extent of normal in vivo variability in this muscle property. The possible source of the variability is discussed in terms of parameters typically used in muscle models.
Christopher J. Hasson, Richard E.A. van Emmerik, and Graham E. Caldwell
In this study, a comprehensive evaluation of static and dynamic balance abilities was performed in young and older adults and regression analysis was used to test whether age-related variations in individual ankle muscle mechanical properties could explain differences in balance performance. The mechanical properties included estimates of the maximal isometric force capability, force-length, force-velocity, and series elastic properties of the dorsiflexors and individual plantarflexor muscles (gastrocnemius and soleus). As expected, the older adults performed more poorly on most balance tasks. Muscular maximal isometric force, optimal fiber length, tendon slack length, and velocity-dependent force capabilities accounted for up to 60% of the age-related variation in performance on the static and dynamic balance tests. In general, the plantarflexors had a stronger predictive role than the dorsiflexors. Plantarflexor stiffness was strongly related to general balance performance, particularly in quiet stance; but this effect did not depend on age. Together, these results suggest that age-related differences in balance performance are explained in part by alterations in muscular mechanical properties.
Samantha L. Winter and John H. Challis
The muscle fiber force–length relationship has been explained in terms of the cross-bridge theory at the sarcomere level. In vivo, for a physiologically realistic range of joint motion, and therefore range of muscle fiber lengths, only part of the force–length curve may be used; that is, the section of the force– length curve expressed can vary. The purpose of this study was to assess the accuracy of a method for determining the expressed section of the force– length curve for biarticular muscles. A muscle model was used to simulate the triceps surae muscle group. Three model formulations were used so that the gastrocnemius operated over different portions of the force–length curve: the ascending limb, the plateau region, and the descending limb. Joint moment data were generated for a range of joint configurations and from this simulated data the region of the force– length relationship that the gastrocnemius muscle operated over was successfully reconstructed using the algorithm of Herzog and ter Keurs (1988a). Further simulations showed that the correct region of the force–length curve was accurately reconstructed even in the presence of random and systematic noise generated to reflect the effects of sampling errors, and incomplete muscle activation.
Markus Tilp, Simon Steib, Gudrun Schappacher-Tilp, and Walter Herzog
Force enhancement following muscle stretching and force depression following muscle shortening are well-accepted properties of skeletal muscle contraction. However, the factors contributing to force enhancement/depression remain a matter of debate. In addition to factors on the fiber or sarcomere level, fiber length and angle of pennation affect the force during voluntary isometric contractions in whole muscles. Therefore, we hypothesized that differences in fiber lengths and angles of pennation between force-enhanced/depressed and reference states may contribute to force enhancement/depression during voluntary contractions. The purpose of this study was to test this hypothesis. Twelve subjects participated in this study, and force enhancement/depression was measured in human tibialis anterior. Fiber lengths and angles of pennation were quantified using ultrasound imaging. Neither fiber lengths nor angles of pennation were found to differ between the isometric reference contractions and any of the force-enhanced or force-depressed conditions. Therefore, we rejected our hypothesis and concluded that differences in fiber lengths or angles of pennation do not contribute to the observed force enhancement/depression in human tibialis anterior, and speculate that this result is likely true for other muscles too.
Pedro L. Valenzuela, Guillermo Sánchez-Martínez, Elaia Torrontegi, Javier Vázquez-Carrión, Zigor Montalvo, and G. Gregory Haff
Purpose: To analyze the differences in the force–velocity (F–v) profile assessed under unconstrained (ie, using free weights) and constrained (ie, on a Smith machine) vertical jumps, as well as to determine the between-day reliability. Methods: A total of 23 trained participants (18  y) performed an incremental load squat jump test (with ∼35%, 45%, 60%, and 70% of the subjects’ body mass) on 2 different days using free weights and a Smith machine. Nine of these participants repeated the tests on 2 other days for an exploratory analysis of between-day reliability. F–v variables (ie, maximum theoretical force [F 0], velocity [v 0], and power, and the imbalance between the actual and the theoretically optimal F–v profile) were computed from jump height. Results: A poor agreement was observed between the F–v variables assessed under constrained and unconstrained conditions (intraclass correlation coefficient [ICC] < .50 for all). The height attained during each single jump performed under both constrained and unconstrained conditions showed an acceptable reliability (coefficient of variation < 10%, ICC > .70). The F–v variables computed under constrained conditions showed an overall good agreement (ICC = .75–.95 for all variables) and no significant differences between days (P > .05), but a high variability for v 0, the imbalance between the actual and the theoretically optimal F–v profile, and maximal theoretical power (coefficient of variation = 17.0%–27.4%). No between-day differences were observed for any F–v variable assessed under unconstrained conditions (P > .05), but all of the variables presented a low between-day reliability (coefficient of variation > 10% and ICC < .70 for all). Conclusions: F–v variables differed meaningfully when obtained from constrained and unconstrained loaded jumps, and most importantly seemed to present a low between-day reliability.
Tülay Çevik Saldıran, Emine Atıcı, Derya Azim Rezaei, Özgül Öztürk, Burcu Uslu, Burcu Ateş Özcan, and Begüm Okudan
evaluating the responses of the lower leg muscle tone and strength to SV-WBV were noticed. Our research question was how SV-WBV applications of different intensity will affect the muscle properties of the lower leg. According to our hypothesis, different doses of SV-WBV training would produce different
Lei Zhang, Andreas Straube, and Thomas Eggert
Unexpected small perturbations during reaching movements are normally compensated for automatically. Previous studies of such perturbations observed that the movement trajectory converges back to the preplanned end position. The question remains whether peripheral mechanisms formed by intrinsic muscle properties and stretch reflex are efficient for compensating for such perturbations. Even though this is suggested by a threshold position control model highlighting the role of peripheral mechanisms under central control in movement generation, it is neither developed nor extensively tested for this capability. The present study tests how this model can account for the compensation during single-joint fast reaching. Motor responses to transient, unpredictable, small perturbations at different movement phases were measured and compared with the model predictions. The results show good agreement concerning kinematic and dynamic responses. Simulations with altered mechanical parameters of the model suggest that reflexive responses are well tuned to the intrinsic muscle properties. We conclude that under central control, peripheral mechanisms cope efficiently with small transient perturbations.
Stan C.A.M. Gielen
EMG recordings are frequently used to obtain a better understanding in the coordination of movements. However, EMG activity is made up by the weighted summation of activity of many motor units with different contractile properties. Recent studies have revealed that different motor units contribute to muscle force in different motor tasks. The flexible recruitment of motor units with various contractile properties allows a flexible tuning of muscle properties, but also complicates the interpretation of EMG activity.