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J. Ty Hopkins and Jason Adolph

Context:

The Omnikinetic closed-chain dynamometer calculates bilateral ankle, knee, and hip moments during a semirecumbent stepping motion against a predetermined load.

Objective:

To establish intersession reliability of several kinetic variables during different load and speed conditions over a 4-wk period.

Subjects:

19 healthy, physically active adults (age 21 ± 2 y, height 174.7 ± 10.5 cm, mass 74.0 ± 17.2 kg).

Measurement:

Peak measurements were recorded from the dominant leg for joint torque, power, root-mean-square (RMS) power, and knee shear.

Results:

Intersession reliability ranges across conditions were ankle torque .422−.947, ankle power .336−.877, ankle RMS .671−.893, knee torque .816−.918, knee power .823−.927, knee RMS .855−.943, knee shear .832−.936, hip torque .436−.752, hip power .691−.918, and hip RMS .448−.895.

Conclusions:

Moderate to strong reliability was observed in most measurements of the ankle and knee, whereas the hip showed increased variability.

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Yuji Kobayashi, Junjiro Kubo, Takeo Matsubayashi, Akifumi Matsuo, Kando Kobayashi and Naokata Ishii

The aims of the study were to investigate the differences in kinematics and kinetics between the dominant and nondominant leg during single-leg jumps without arm swing, and to determine the relationship between bilateral asymmetry in isokinetic knee strength and the single-leg jump. Isokinetic knee strength and single-leg jump kinematics and kinetics were measured in 11 male participants. The bilateral asymmetry index was calculated for each parameter. For isokinetic knee strength, there were no significant differences between the dominant and nondominant legs. Significant correlations were observed for the bilateral asymmetry index for isokinetic knee strength at 180 degrees per second and the bilateral asymmetry indexes for maximum flexion angle and the mean knee joint torque during the single-leg jumps. In conclusion, the findings of the current study suggest an association between knee strength imbalances and the joint angle, as well as the torque produced in single-leg jumps, although no relationship between knee strength and jump height was observed.

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Luis F. Áragón-Vargas and M. Melissa Gross

The purpose of this study was to examine the changes in both the coordination patterns of segmental actions and the dynamics of vertical jumping that accompany changes in vertical jump performance (VJP) occurring from trial to trial in single subjects. Ground reaction forces and video data were analyzed for 50 maximal vertical jumps for 8 subjects. It was possible to predict VJP from whole-body or even segmental kinematics and kinetics in spite of the small jump performance variability. Best whole-body models included peak and average mechanical power, propulsion time, and peak negative impulse. Best segmental models included coordination variables and a few joint torques and powers. Contrary to expectations, VJP was lower for trials with a proximal-to-distal sequence of joint reversals.

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Michael E. Feltner

Fastball pitches of eight collegiate baseball pitchers were filmed using the Direct Linear Transformation (DLT) method of three-dimensional (3D) cinematography. Coordinate data were obtained, and the model developed by Feltner and Dapena (1989) was used to fractionate the 3D angular acceleration of the upper arm and distal segment (the forearm, the hand, and prior to release, the baseball) of the throwing arm into terms associated with the joint torques exerted on the segments and the kinematic variables used to define the motions of the segments. The findings indicated that the extreme external rotation of the upper arm during the pitch was due mainly to the sequential actions of the horizontal adduction and abduction muscles at the shoulder. The study also found that the rapid elbow extension prior to ball release was due primarily to the counterclockwise angular velocity of the upper arm and trunk (viewed from above) that occurred during the pitch, and not to the elbow extensor muscles.

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Paul DeVita and Tibor Hortobagyi

Functional knee braces used during rehabilitation from injury and surgery to the anterior cruciate ligament (ACL) have been reported to provide a strain-shielding effect on the ACL in healthy people while standing, reduce quadriceps electromyoraphy in ACL-deficient individuals, and alter joint torque patterns in people with ACL reconstruction during walking. These results led to the hypothesis that functional knee braces protect a reconstructed ACL during dynamic activity by reducing the anterior shear load applied to the knee. This hypothesis was tested by investigating the effects of a functional knee brace on lower extremity muscle forces and the anteroposterior shear force at the knee joint during the stance phase of walking in people with ACL reconstruction. Ground reactions and sagittal plane video were recorded from 9 ACL-reconstructed individuals as they walked with and without a functional knee brace, and from 10 healthy people without the functional knee brace. Inverse dynamics were used to calculate the net joint torques in the lower extremity during the stance phase. Hamstrings, quadriceps, and gastrocnemius muscle and knee anteroposterior shear force were then predicted with a sagittal-plane mathematical model. Compared to healthy individuals, those with ACL reconstruction walked with 78% more hamstrings impulse and 19% less quadriceps impulse (both p < .05). The functional knee brace produced an additional 43% increase in hamstrings impulse and an additional 13% decrease in quadriceps impulse in the ACL group. Peak anterior knee shear force and anterior impulse were 41% lower and 16% lower in ACL vs. healthy individuals, respectively. The functional knee brace further reduced the peak knee shear force and impulse 28% and 19%, respectively, in the ACL group. It was concluded that a functional knee brace protects a reconstructed ACL during walking by altering muscle forces and reducing the anterior shear force applied to the knee joint.

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Ge Wu and Weifeng Zhao

This study examined the role of feedback from cutaneous mechanoreceptors in the stability of human upright posture. A two-link, one degree of freedom, inverted pendulum model was constructed for the human body with ankle joint torque proportional to the delayed outputs from muscle receptors, joint receptors, and cutaneous mechanoreceptors in the foot. Theoretical analysis and numerical simulations indicated that the use of mechanoreceptive information reduced the frequency range and the maximum peak-peak value of the dynamic response of the system. However, without the use of muscle receptors, the mechanoreceptive feedback could not stabilize the system. In addition, body movement of human subjects was measured when their balanced upright posture was disturbed by a transient, forward/backward movement of a supporting platform. The loss of or change in cutaneous mechanoreceptive sense in their feet was induced by (a) having healthy subjects stand on a soft surface and (b) testing neuropathic patients with loss of vibratory sensation in their feet. The results showed significant increases in frequency range and maximum peak-peak value of ankle rotation and velocity for subjects standing on a soft (vs. hard) surface and for neuropathic patients (vs. age- and gender-matched healthy subjects).

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Tom G. Welter and Maarten F. Bobbert

We have investigated, in fast movements, the hypothesis that bi-articular muscles are preferentially selected to control me direction of force exerted on the environment, while mono-articular muscles are selected to control both this exerted force direction as well as the movement direction. Fourteen subjects performed ballistic arm movements involving shoulder and elbow rotations in the horizontal plane, either with or without an external force applied at the wrist. Joint torques required to counteract the external force were in the same order of magnitude as those required to overcome the inertial load during movements. EMG was recorded from mono- and bi-articular flexors and extensors of me elbow and shoulder. Signals were rectified and integrated (IREMG) over 100 ms following the first detected activity. MANOVA revealed mat, contrary to the hypothesis, IREMG of bi-articular muscles varied with movement direction just as that of the mono-articular muscles. It was concluded that the present data do not support me hypothesis mentioned above. A second finding was that movement effects on IREMG were much stronger than external force effects. This could not be explained using Hill's force-velocity relationship. It may be an indication that in the initiation of fast movements, IREMG is not only tuned to movement dynamics and muscle contractile properties, but also to me dynamics of the build up of an active state of the muscle.

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Sean P. Flanagan, Kara M. Kessans and George J. Salem

Context:

Information regarding how the mechanical demand differs with variants of the step exercise may be used by clinicians to more appropriately prescribe lower-extremity exercise.

Objective:

To quantify the joint torque contributions of the lower extremity during three different step exercises: forward step-up (FS), lateral step-up (LS), and step-down (SD).

Design:

An experiment with a repeated measures design.

Setting:

Biomechanics laboratory.

Participants:

18 healthy subjects (9 men, 9 women, age 25.67 ± 4.23 years, height 1.73 ± 0.10 meters, mass 72.73 ± 10.67 kilograms).

Intervention:

Participants performed three sets of three repetitions of each exercise while instrumented for biomechanical analysis.

Main Outcome Measure:

Mechanical effort of the hip, knee, and ankle of both limbs during each exercise.

Results:

The greatest contribution from the hip was required during the FS, while the contribution from the knee was required during the SD. The greatest contribution from the ankle was required during the LS and SD.

Conclusion:

Choice of step exercise results in different distributions of mechanical demand across the lower extremities.

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Michael J. Hiley and Maurice R. Yeadon

The undersomersault, or felge, to handstand on parallel bars has become an important skill in Men’s Artistic Gymnastics as it forms the basis of many complex variations. To receive no deductions from the judges, the undersomersault must be performed without demonstrating the use of strength to achieve the final handstand position. Two male gymnasts each performed nine undersomersaults from handstand to handstand while data were recorded using an automatic motion capture system. The highest and lowest scoring trials of each gymnast, as determined by four international judges, were chosen for further analysis. Three optimization criteria were used to generate undersomersault technique during the swing phase of the skill using a computer simulation model: minimization of peak joint torques, minimization of horizontal velocity before release, and maximization of angular momentum. The techniques used by both gymnasts could be explained using the second optimization criterion which facilitated further skill development. The first optimization criterion generated a technique advocated for beginners where strength might be expected to be a limiting factor. The third optimization criterion resulted in a different type of undersomersault movement of greater difficulty according to the FIG Code of Points.

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Stephanie E. Forrester and Matthew T.G. Pain

This study aimed to identify areas of reduced surface EMG amplitude and changed frequency across the phase space of a maximal dynamic knee extension task. The hypotheses were that (1) amplitude would be lower for eccentric contractions compared with concentric contractions and unaffected by fiber length and (2) mean frequency would also be lower for eccentric contractions and unaffected by fiber length. Joint torque and EMG signals from the vasti and rectus femoris were recorded for eight athletic subjects performing maximum knee extensions at 13 preset crank velocities spanning ±300°⋅s−1. The instantaneous amplitude and mean frequency were calculated using the continuous wavelet transform time–frequency method, and the fiber dynamics were determined using a muscle model of the knee extensions. The results indicated that (1) only for the rectus femoris were amplitudes significantly lower for eccentric contractions (p = .019) and, for the vasti, amplitudes during eccentric contractions were less than maximal but this was also the case for concentric contractions due to a significant reduction in amplitude toward knee extension (p = .023), and (2) mean frequency increased significantly with decreasing fiber length for all knee extensors and contraction velocities (p = .029). Using time–frequency processing of the EMG signals and a muscle model allowed the simultaneous assessment of fiber length, velocity, and EMG.