The purpose of this investigation was to determine, for a planar, multijoint throwing skill, if the interactions of segment energetics change over the course of practice. Eighteen men threw a weighted ball with their dominant arm at a target while the motion was restrained to a horizontal plane. From video data and body segment inertia! estimations, the energy transferred by the net joint force and the mechanical work attributed to the net joint moment were calculated for selected practice trials. Performance scores showed an expected improvement over trial blocks. An energetics analysis indicated that, for the throw, the mechanical work generated by muscle and transferred through muscle (i.e., via the net joint moment) across the elbow joint and the energy transferred by the net joint force across the wrist joint increased early in practice; however, no changes were observed in the relative contributions made by these components. The results indicated that, although performance increased significantly, the movement strategy used by subjects was intact throughout practice.
Kathy Liu and Gary D. Heise
Dynamic stability is often measured by time to stabilization (TTS), which is calculated from the dwindling fluctuations of ground reaction force (GRF) components over time. Common protocols of dynamic stability research have involved forward or vertical jumps, neglecting different jump-landing directions. Therefore, the purpose of the present investigation was to examine the influence of different jump-landing directions on TTS. Twenty healthy participants (9 male, 11 female; age = 28 ± 4 y; body mass = 73.3 ± 21.5 kg; body height = 173.4 ± 10.5 cm) completed the Multi-Directional Dynamic Stability Protocol hopping tasks from four different directions—forward, lateral, medial, and backward—landing single-legged onto the force plate. TTS was calculated for each component of the GRF (ap = anterior-posterior; ml = medial-lateral; v = vertical) and was based on a sequential averaging technique. All TTS measures showed a statistically significant main effect for jump-landing direction. TTSml showed significantly longer times for landings from the medial and lateral directions (medial: 4.10 ± 0.21 s, lateral: 4.24 ± 0.15 s, forward: 1.48 ± 0.59 s, backward: 1.42 ± 0.37 s), whereas TTSap showed significantly longer times for landings from the forward and backward directions (forward: 4.53 ± 0.17 s, backward: 4.34 0.35 s, medial: 1.18 ± 0.49 s, lateral: 1.11 ± 0.43 s). TTSv showed a significantly shorter time for the forward direction compared with all other landing directions (forward: 2.62 ± 0.31 s, backward: 2.82 ± 0.29 s, medial: 2.91 ± 0.31 s, lateral: 2.86 ± 0.32 s). Based on these results, multiple jump-landing directions should be considered when assessing dynamic stability.
Eadric Bressel and Gary D. Heise
The purpose of this study was to compare muscle activity, kinematic, and oxygen consumption characteristics between forward and reverse arm cranking. Twenty able-bodied men performed 5-min exercise bouts of forward and reverse arm cranking while electromyographic (EMG), kinematic, and oxygen consumption data were collected. EMG activity of biceps brachii, triceps brachii, deltoid, and infraspinatus muscles were recorded and analyzed to reflect on-time durations and amplitudes for each half-cycle (first 180° and second 180° of crank cycle). Kinematic data were quantified from digitization of video images, and oxygen consumption was calculated from expired gases. Dependent measures were analyzed with a MANOVA and follow-up univariate procedures; alpha was set at .01. The biceps brachii, deltoid, and infraspinatus muscles displayed greater on-time durations and amplitudes for select half-cycles of reverse arm cranking compared to forward arm cranking (p < 0.01). Peak wrist flexion was 9% less in reverse arm cranking (p < 0.01), and oxygen consumption values did not differ between conditions (p = 0.25). Although there were no differences in oxygen consumption and only minor differences kinematically, reverse arm cranking requires greater muscle activity from the biceps brachii, deltoid, and infraspinatus muscles. These results may allow clinicians to more effectively choose an arm cranking direction that either minimizes or maximizes upper extremity muscle activity depending on the treatment objectives.
Philip E. Martin and Gary D. Heise
Archery instructors believe that force distribution (FD) between the hand and bow grip can have a considerable effect on arrow flight, but there is no empirical support for this speculation. This study examined FD on the bow grip in experienced archers and explored the possible relationships between FD, performance, and fatigue. FD was quantified for 15 experienced archers (8 highly skilled [HS] and 7 less skilled [LS]) using 15 unobtrusive force sensors as each archer completed 72 shots. Arrow position relative to the target center, estimated net moments and moment arms about vertical and horizontal axes through the grip, and shot-to-shot variability in the estimated moments and moment arms were computed for three blocks of six shots. Results demonstrated that (a) estimated moments and moment arms were not consistently related to observed vertical or horizontal deviations in arrow position, (b) there were no systematic differences in FD between HS and LS archers, (c) fatigue had no quantifiable effect on FD, and (d) HS archers displayed less shot-to-shot variability in vertical FD than LS archers, but similar variability horizontally. Results did not support the above-noted common belief of archery instructors.
Eadric Bressel, Gary D. Heise and Greg Bachman
The purpose of this study was to determine how muscle activity and oxygen consumption are influenced by reverse pedaling (RP) compared to forward pedaling (FP). Seventeen physically active males performed FP and RP at an external workrate of 157 W (80 rpm) while EMG data were collected from five muscles: rectus femoris (RF), biceps femoris (BF), gastrocnemius (GN), tibialis anterior (TA), and vastus medialis (VM). Oxygen consumption (V̇O2 L·min-1) data were collected. On-time durations and EMG amplitudes were quantified for each half-cycle (first 180° and second 180° of crank angle). V̇O2 was similar between pedaling conditions while muscles RF and BF exhibited phasic shifts in response to RP with no amplitude change. VM showed an increase and GN displayed a decrease in EMG amplitude from FP to RP. The phasic shifts in muscle activation seen in RP, particularly in RF and BF, may alter the sequence of the knee extensor–hip extensor joint moments during the first half-cycle of pedaling.
Kevin D. Dames, Jeremy D. Smith and Gary D. Heise
Gait data are commonly presented as an average of many trials or as an average across participants. Discrete data points (eg, maxima or minima) are identified and used as dependent variables in subsequent statistical analyses. However, the approach used for obtaining average data from multiple trials is inconsistent and unclear in the biomechanics literature. This study compared the statistical outcomes of averaging peaks from multiple trials versus identifying a single peak from an average profile. A series of paired-samples t tests were used to determine whether there were differences in average dependent variables from these 2 methods. Identifying a peak value from the average profile resulted in significantly smaller magnitudes of dependent variables than when peaks from multiple trials were averaged. Disagreement between the 2 methods was due to temporal differences in trial peak locations. Sine curves generated in MATLAB confirmed this misrepresentation of trial peaks in the average profile when a phase shift was introduced. Based on these results, averaging individual trial peaks represents the actual data better than choosing a peak from an average trial profile.