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The pennated arrangement of muscle fibers has important implications for muscle function in vivo, but complex arrangement of muscle fascicles in whole muscle raises the question whether the arrangement of fascicles produce variations in pennation angle throughout muscle. The purpose of this study was to describe the variability in pennation angle observed throughout the first dorsal interosseous (FDI) muscle using magnetic resonance imaging (MRI). Two cadaveric muscles were scanned in a 14.1 tesla MRI unit. Muscles were divided into slices and pennation angle was measured in the same representative location throughout the muscle in each slice for the medial-lateral and anterior posterior-image planes. Data showed large nonuniform variation in pennation angles throughout the muscles. For example, for cadaver 2, pennation angle in 287 planes along the medial-lateral axis ranged from 3.2° to 22.6°, while for the anterior-posterior axis, in 237 planes it ranged from 3.1° to 24.5°. The nonnormal distribution of pennation angles along each axis suggests a more complex distribution of fascicles than is assumed when a single pennation angle is used to represent an entire muscle. This distribution indicates that a single pennation angle may not accurately describe the arrangement of muscle fascicles in whole muscle.

The purpose of this research was to determine the functions of the gluteus maximus, biceps femoris, semitendinosus, rectus femoris, vastus lateralis, soleus, gastrocnemius, and tibialis anterior muscles about their associated joints during full (deep-knee) squats. Muscle function was determined from joint kinematics, inverse dynamics, electromyography, and muscle length changes. The subjects were six experienced, male weight lifters. Analyses revealed that the prime movers during ascent were the monoarticular gluteus maximus and vasti muscles (as exemplified by vastus lateralis) and to a lesser extent the soleus muscles. The biarticular muscles functioned mainly as stabilizers of the ankle, knee, and hip joints by working eccentrically to control descent or transferring energy among the segments during ascent. During the ascent phase, the hip extensor moments of force produced the largest powers followed by the ankle plantar flexors and then the knee extensors. The hip and knee extensors provided the initial bursts of power during ascent with the ankle extensors and especially a second burst from the hip extensors adding power during the latter half of the ascent.

The purpose of this investigation was to determine the relationship between isometric measures of muscular function at two different joint angles and dynamic performance. Thirteen experienced weight trainers performed two isometric tests in a bench press position, at elbow angles of 90 and 120°. Performance was assessed by a one repetition maximum (1-RM) bench press and a series of upper body bench press throws at loads of 15, 30, and 60% of the 1-RM load. The results clearly show that changing the joint angle from 120 to 90° improved the relationship between most of the tests and performance by more than 100%, possibly due to differences in motor unit recruitment patterns and differing muscle mechanics (e.g., length-tension), at varying joint angles. It was suggested that the best angle at which to assess isometric function may be the joint angle at which peak force is developed in the performance of interest.

In an effort to seek further understanding of lower limb muscle function in the rowing movement, an electromyographic analysis was undertaken of rowers rowing on a Gjessing ergometer. A strain gauged transducer was inserted in the ergometer linkage between handle and flywheel to detect pulling force. Electrodes were placed on the following lower limb muscles: gluteus maximus, biceps femoris, rectus femoris, vastus lateralis, gastrocnemius, and tibialis anterior. Linear envelope electromyograms from each muscle and the force signals were sampled synchronously at 50 Hz. The results indicated that all six muscles were active from catch to finish of the drive phase. Biceps femoris, gluteus maximus, gastrocnemius, and vastus lateralis all began their activity at or just prior to catch and reached maximal excitation near peak force of the stroke. Rectus femoris and tibialis anterior activity began prior to the catch and reached maximal excitation subsequent to peak force. The coactivation of the five leg muscles, of which four were biarticular, included potentially antagonistic actions that would cancel each other’s effects. Clearly, however, other explanations must be considered. Both gastrocnemius and biceps femoris have been shown to act as knee extensors and may do so in the case of the rowing action. Furthermore, rectus femoris may act with unchanging length as a knee extensor by functioning as a rigid link between the pelvis and tibia. In this manner, energy created by the hip extensors is transferred across the knee joint via the isometrically contracting rectus femoris muscle.

The understanding of individual muscle impairments that affect swing phase in stroke gait will lead to better rehabilitation strategies for this population. We used induced acceleration analysis to evaluate the potential each muscle has to accelerate the hip and knee joints of the swing limb, using kinematics from three stroke subjects and five healthy subjects. To determine the influence of altered limb position on muscle function, we augmented hip extension by 10° in swing phase for all subjects. We found that in early swing, healthy subjects had greater potential to accelerate the knee into flexion than stroke subjects, whereas stroke subjects had greater potential to accelerate the hip into flexion. Perturbing the hip flexion angle into greater extension increased the potential of biarticular muscles to flex the knee in swing phase. The potential of muscles to improve swing phase dynamics depends on the initial posture of the limb and highlights the importance of subject-specific evaluations in the design of appropriate therapeutic interventions.

Muscle inhibition (MI) in human knee extensors increases with increasing maximal voluntary force as a function of knee angle. It was speculated that this angle-dependent MI was modulated by force-dependent feedback, likely Golgi tendon organ pathways. Such angle-dependent MI is of clinical and theoretical importance. The purpose of this study was to determine MI in human elbow flexors for maximal voluntary contractions. Muscle inhibition, elbow flexor force, and electromyographic (EMG) activity were measured in 31 volunteers at elbow angles between 30º and 120º. MI and elbow flexor EMG were the same at all elbow angles. Maximal isometric forces were greatest at the 70º angle, and never fell below 70% of the peak force at any of the tested angles. From these results it is concluded that force-dependent modulation of MI did not occur in the elbow flexors, possibly because maximal isometric force remained relatively close (within 30%) to the peak force. In contrast, force-dependent modulation of MI occurred in the knee extensors at the most extended angles, when the average knee extensor force had dropped to 50% or less of the maximal knee extensor force. It is likely that human maximal voluntary contractions are not associated with a given activation. Rather, activation appears to be modulated by force-dependent feedback at force levels below 70% of the absolute peak force, which manifests itself in a change of MI that parallels the level of maximal isometric force in voluntary contractions.

The mechanics of moving along a curved path suggest that runners must change their body positions and thus adjust their lower extremity function as they accomplish a track turn. The purpose of the present study was to investigate the changes in the kinetics and kinematics of the lower extremity as runners proceed around the turn of a 400-m track (radius 31.5 m). Five skilled runners served as subjects in the study and were required to perform 10 trials in three conditions, running at 6.31 m/s plus or minus 5% (4:15 min/mile pace). The right and left limbs on a track turn and the right limb on the straightaway were evaluated using ground reaction force data and kinematic data from high-speed film. Statistical analysis of the 18 ground reaction force variables and 4 kinematic variables suggested that the right and left limbs at the midpoint of the track turn were asymmetrical and that most of the differences occurred in the first portion of the footfall Significant differences were found in the touchdown angle, maximum pronation angle, all mediolateral variables, and in the vertical variables describing the collision phase of the footfall (p < .05). The data suggest that the etiologies of injuries to the right and left lower extremity differ, with right foot injuries being of the impact type and left leg injuries being of the overpronation type.

The geometric method combines a volume and a density function to estimate body segment parameters and has the best opportunity for developing the most accurate models. In the trunk, there are many different tissues that greatly differ in density (e.g., bone versus lung). Thus, the density function for the trunk must be particularly sensitive to capture this diversity, such that accurate inertial estimates are possible. Three different models were used to test this hypothesis by estimating trunk inertial parameters of 25 female and 24 male college-aged participants. The outcome of this study indicates that the inertial estimates for the upper and lower trunk are most sensitive to the volume function and not very sensitive to the density function. Although it appears that the uniform density function has a greater influence on inertial estimates in the lower trunk region than in the upper trunk region, this is likely due to the (overestimated) density value used. When geometric models are used to estimate body segment parameters, care must be taken in choosing a model that can accurately estimate segment volumes. Researchers wanting to develop accurate geometric models should focus on the volume function, especially in unique populations (e.g., pregnant or obese individuals).