Zachary J. Domire

An induced energy analysis to determine the mechanism for performance enhancement as a result of arm swing during jumping

Arm swing during a jump can enhance performance. This study examined the mechanisms via which arm swing contributes to maximum vertical jump height. Two mechanisms have been proposed to explain how jump height can be increased by arm swing: production of additional energy or slow leg extension which may permit muscles to work on a more favorable region of the force-velocity curve. A simulation model of the vertical jump and an induced energy analysis were used to determine the contribution of each of these mechanisms. The results from the model suggest that both mechanisms for the role of arm swing in enhancing jump performance are at play. Arm swing did slow the hip extensors allowing for more force production from these muscles. The work done and the energy induced in the vertical direction by these muscles were greater in the jump simulated with arm swing. However, these increases were not sufficient to explain the entire improvement in jumping performance. The shoulder musculature generated a considerable amount of work and energy induced and is directly responsible for approximately one-third of the performance enhancement associated with arm swing.

An examination of possible quadriceps force at the time of anterior cruciate ligament injury during landing: A simulation study.

Anterior cruciate ligament (ACL) rupture is a common and traumatic injury. Although, identifying the mechanism of ACL injury has received considerable research attention, there are still many unanswered questions. One proposed mechanism asserts that the ACL is injured due to an aggressive quadriceps muscle contraction. However, recently it has been questioned if the magnitude of quadriceps force needed to tear the ACL is physiologically realistic under the conditions where injury occurs during landing (e.g. near full knee extension and within 50ms after impact). To answer this question, a simple simulation model was developed to examine the upper bounds of quadriceps force that can be developed under these conditions. The model included force-length, and force-velocity properties as well as activation dynamics. Model parameters were chosen to provide a high estimate for possible quadriceps force in a young healthy man. The effects of varying quadriceps pre-activation levels were also examined. When using realistic pre-activation levels, the simulated quadriceps force was less than half of what has been shown to cause ACL injury. Even when using maximum pre-activation, the quadriceps force still did not reach close to the level shown to cause injury. Therefore, we conclude that quadriceps force alone seems to be an unlikely mechanism for ACL injury.

Maximum height and minimum time vertical jumping

The performance criterion in maximum vertical jumping has typically been assumed to simply raise the center of mass as high as possible. In many sporting activities minimizing movement time during the jump is likely also critical to successful performance. The purpose of this study was to examine maximum height jumps performed while minimizing jump time. A direct dynamics model was used to examine squat jump performance, with dual performance criteria: maximize jump height and minimize jump time. The muscle model had activation dynamics, force-length, force-velocity properties, and a series of elastic component representing the tendon. The simulations were run in two modes. In Mode 1 the model was placed in a fixed initial position. In Mode 2 the simulation model selected the initial squat configuration as well as the sequence of muscle activations. The inclusion of time as a factor in Mode 1 simulations resulted in a small decrease in jump height and moderate time savings. The improvement in time was mostly accomplished by taking off from a less extended position. In Mode 2 simulations, more substantial time savings could be achieved by beginning the jump in a more upright posture. However, when time was weighted more heavily in these simulations, there was a more substantial reduction in jump height. Future work is needed to examine the implications for countermovement jumping and to examine the possibility of minimizing movement time as part of the control scheme even when the task is to jump maximally.

A critical examination of the maximum velocity of shortening used in simulation models of human movement

The maximum velocity of shortening of a muscle is an important parameter in musculoskeletal models. The most commonly used values are derived from animal studies; however, these values are well above the values that have been reported for human muscle. The purpose of this study was to examine the sensitivity of simulations of maximum vertical jumping performance to the parameters describing the force–velocity properties of muscle. Simulations performed with parameters derived from animal studies were similar to measured jump heights from previous experimental studies. While simulations performed with parameters derived from human muscle were much lower than previously measured jump heights. If current measurements of maximum shortening velocity in human muscle are correct, a compensating error must exist. Of the possible compensating errors that could produce this discrepancy, it was concluded that reduced muscle fibre excursion is the most likely candidate.

Heterogeneous fascicle behavior within the biceps femoris long head at different muscle activation levels

Magnetic resonance and ultrasound imaging have shown hamstring strain injuries occur most often in the biceps femoris long head (BFLH), and particularly in the proximal vs. distal region of this muscle. Animal research and musculoskeletal modeling (MSK) have detected heterogeneous fascicle behavior within muscle regions, and within fascicles. Understanding architectural behavior differences during muscle contractions may help to discern possible mechanisms behind proximal BFLH injuries. The purpose of our study was to assess the magnitude of shortening of the proximal and distal fascicles of the BFLH under a range of muscle activation levels under isometric conditions using ultrasound imaging (US). Thirteen healthy adults performed targeted sustained isometric contractions while US were taken of the entire BFLH. Measurements of fascicle lengths in both muscle regions were compared at 20%, 30%, 50%, and 67% MVIC. The results showed that while both regions shortened significantly with activation, the proximal fascicles were significantly longer, regardless of activation level (~38%), and shortened significantly more than the distal fascicles overall (~40%), and cumulatively at higher activation levels (30% and above). No significant strain differences were found between the two regions. These data suggest heterogeneous fascicle behavior exists in an absolute sense; however, differences in behavior are eliminated when normalized (strain). Coupled with MSK literature, the absence of regional fascicle strain differences in this study may indicate strain heterogeneity is not detectable at the whole fascicle level. Further knowledge of this commonly strained muscles regional behavior during dynamic movements could provide evidence of proximal hamstring strain predisposition.

The influence of an elastic tendon on the force producing capabilities of a muscle during dynamic movements

With increasing computer power, computer simulation of human movement has become a popular research tool. However, time to complete simulations can still be long even on powerful computers. One possibility for reducing simulation time, with models of musculo-skeletal system, is to simulate the muscle using a rigid tendon rather than the more realistic compliant tendon. This study examines the effect of tendon elasticity on muscle force output under different dynamic conditions. A single muscle, point mass model was used and simulations were performed varying the mass, the tendon length, the initial position, and the task. For simulations for relatively slow motion, as experienced for example in upper limb reaching motions or rising from a chair, tendon properties had little influence on muscle force, in contrast simulations of an explosive task similar to jumping or throwing tendon had a much larger effect.

Males and Females Respond Similarly to Walking With a Standardized, Heavy Load.

Females in the military sustain a higher incidence of lower extremity injuries compared to males. Previous investigations of gender differences during load carriage used loads normalized to body mass; as a result of anthropometric and strength differences between genders, this may partially normalize to strength, masking gender or size differences in response to load. We compared gait kinetics and kinematics between genders based on a standardized load, instead of loads relative to body mass. 11 males and 11 females walked at 1.5 m/s over level ground with a 22 kg rucksack using three load conditions: unloaded, low-back placement, and mid-back placement. We found a gender by load interaction for average trunk position (p < 0.05). Stride length decreased 1.3% in loaded vs. unloaded walking. Loaded walking increased knee extensor (65%) and ankle plantarflexor torque (23%, all p < 0.0001), but not hip extensor torque (p > 0.05) compared to unloaded walking. The lack of gender differences may indicate that females do not adapt gait mechanics to account for smaller stature and lesser absolute strength compared to males, which may contribute to the high injury rate in female military recruits.

Vertical Ground Reaction Forces for Given Human Standing Posture With Uneven Terrains: Prediction and Validation

Ground reaction forces (GRFs) on individual support vary with posture and motion for bipedal mechanisms or systems due to the redundancy in the system. In digital human modeling, specifically posture prediction, the GRFs are predicted, as they are unknown in a virtual environment. Traditionally, models in which the GRFs are predicted have been presented; however, they are always assumed to be on flat ground. Little work has been done to predict the GRFs on uneven or arbitrary terrain. This paper presents a generic method to calculate the vertical GRFs for given standing postures with uneven terrain. The vertical GRFs are predicted based on the generalized forces (torque in revolute joints; force in prismatic joints) calculated using the recursive Lagrangian formulation and a 3-D zero moment point. Motion capture experiments were used to obtain postures for common standing reaching tasks. Force plates were employed to record GRF information for each task. Experimental postures were reconstructed, and the GRF prediction algorithm was used to predict the associated vertical GRFs for each task. Experimental and predicted vertical GRFs are compared to validate the prediction model. The prediction method proved to be valid, with an overall error of 6%.

Assessing the accuracy of subject-specific, muscle-model parameters determined by optimizing to match isometric strength

Abstract Biomechanical models are sensitive to the choice of model parameters. Therefore, determination of accurate subject specific model parameters is important. One approach to generate these parameters is to optimize the values such that the model output will match experimentally measured strength curves. This approach is attractive as it is inexpensive and should provide an excellent match to experimentally measured strength. However, given the problem of muscle redundancy, it is not clear that this approach generates accurate individual muscle forces. The purpose of this investigation is to evaluate this approach using simulated data to enable a direct comparison. It is hypothesized that the optimization approach will be able to recreate accurate muscle model parameters when information from measurable parameters is given. A model of isometric knee extension was developed to simulate a strength curve across a range of knee angles. In order to realistically recreate experimentally measured strength, random noise was added to the modeled strength. Parameters were solved for using a genetic search algorithm. When noise was added to the measurements the strength curve was reasonably recreated. However, the individual muscle model parameters and force curves were far less accurate. Based upon this examination, it is clear that very different sets of model parameters can recreate similar strength curves. Therefore, experimental variation in strength measurements has a significant influence on the results. Given the difficulty in accurately recreating individual muscle parameters, it may be more appropriate to perform simulations with lumped actuators representing similar muscles.

The Relationships Between Muscle Force Steadiness and Visual Steadiness in Young and Old Adults

Since vision is used in studies of muscle force control, reduced muscle force control might be related to reduced visual ability. We investigated relationships between steadiness in eye movements and quadriceps muscle torque (a surrogate for force) during isometric contractions of constant and varying torques. Nineteen young adults with an average age of 20.7 years and 18 old adults with an average age of 71.6 years performed three vision tasks, three vision and torque tasks at 40% maximal voluntary contraction (MVC), and three vision and torque tasks at 54 nm. Age groups had identical torque steadiness (CV) in 40%-MVC and 54-nm conditions (p > .05). Old had similar vertical (p > .05) but decreased horizontal visual steadiness (SD) (p < .05) compared with young. Correlations between visual steadiness and muscle torque steadiness failed to show a significant relationship (p > .05). We were unable to identify a substantial relationship between muscle torque steadiness and eye movement, as a component of visual steadiness, and conclude that reduced visual steadiness does not contribute to reduced muscle torque steadiness.