Janet S. Dufek

Contributions of lower extremity joints to energy dissipation during landings.

PURPOSE The purpose of the study was to investigate changes in lower extremity joint energy absorption for different landing heights and landing techniques. METHODS Nine healthy, active male subjects volunteered to perform step-off landings from three different heights (0.32 m, 2.5 m(-s); 0.62 m, 3.5 m(-s); and 1.03 m, 4.5 m(-s)) using three different landing techniques (soft, SFL; normal, NML; and stiff landing, STL). Each subject initially performed five NML trials at 0.62 m to serve as a baseline condition and subsequently executed five trials in each of the nine test conditions (3 heights x 3 techniques). RESULTS The results demonstrated general increases in peak ground reaction forces, peak joint moments, and powers with increases in landing height and stiffness. The mean eccentric work was 0.52, 0.74, and 0.87 J x kg(-1) by the ankle muscles, and 0.94, 1.31, and 2.15 J x kg(-1) by the hip extensors, at 0.32, 0.62, and 1.03 m, respectively. The average eccentric work performed by the knee extensors was 1.21, 1.63, and 2.26 J x kg(-1) for the same three heights. CONCLUSIONS The knee joint extensors were consistent contributors to energy dissipation. The ankle plantarflexors contributed more in the STL landings, whereas the hip extensors were greater contributors during the SFL landings. Also a shift from ankle to hip strategy was observed as landing height increased.

The evaluation and prediction of impact forces during landings

The investigation of impact force attenuation during landings may help identify performance strategies. The purpose of the study was to evaluate the effects of height (three), distance (three), and technique (three) on impact forces during landings. Three male volunteer subjects were filmed while performing three right foot landings onto a force platform for each combination of height, distance, and technique for a total of 81 trials per subject. Between- and within-subject three-way ANOVAs and three regression models (mechanical, biomechanical, refined biomechanical) were computed on the dependent variables of first (F1) and second (F2) maximum vertical force. Results of the between-subject ANOVAs indicated significant (P less than 0.05) height, distance, and technique main effects for F1 and a height x technique interaction for F2. The within-subject ANOVA results identified unique models for each of the three subjects. The biomechanical regression model exhibited the best predictions of F1 and F2 for S1 (81.0 and 72.0% explained variance, respectively), while the refined biomechanical model accounted for 83.4, 81.3, 80.9, and 88.0% of the F1 and F2 variances for S2 and S3, respectively. In conclusion, the within-subject results identified unique individual landing strategies that were masked by the group analyses suggesting that caution be exercised in using between-subject analysis techniques.

Increased jump height and reduced EMG activity with an external focus

Jump height is increased when performers are given external focus instructions, relative to an internal focus or no focus instructions (Wulf & Dufek, 2009; Wulf, Zachry, Granados, & Dufek, 2007). The purpose of present study was to examine possible underlying neurophysiological mechanisms of this effect by using electromyography (EMG). Participants performed a vertical jump-and-reach task under two conditions in a counterbalanced order: external focus (i.e., focus on the rungs of the measurement device) and internal focus (i.e., focus on the fingers with which the rungs were to be touched). EMG activity of various muscles (anterior tibialis, biceps femoris, vastus lateralis, rectus femoris, gastrocnemius) was measured during jumps. Jump height was greater with an external compared to an internal focus. While there were no differences in muscle onset times between attentional focus conditions, EMG activity was generally lower with an external focus. These results suggest that neuromuscular coordination is enhanced by an external focus of attention. The present findings add to the evidence that an external focus facilitates the production of effective and efficient movement patterns.

The effect of trial size on statistical power.

Many research studies produce results that falsely support a null hypothesis due to a lack of statistical power. The purpose of this research was to demonstrate selected relationships between single subject (SS) and group analyses and the importance of data reliability (trial size) on results. A computer model was developed and used in conjunction with Monte Carlo procedures to study the effects of sample size (subjects and trials), within- and between-subject variability, and subject performance strategies on selected statistical evaluation procedures. The inherent advantages of the approach are control and replication. Selected results are presented in this paper. Group analyses on subjects using similar performance strategies identified 10, 5, and 3 trials for sample sizes of 5, 10, and 20, respectively, as necessary to achieve statistical power values greater than 90% for effect sizes equal to one standard deviation of the condition distribution. SS analyses produced results exhibiting considerably less power than the group results for corresponding trial sizes, indicating how much more difficult it is to detect significant differences using a SS design. These results should be of concern to all investigators especially when interpreting nonsignificant findings.

Increased jump height with an external focus due to enhanced lower extremity joint kinetics.

ABSTRACT Individuals jump higher when they adopt an external focus of attention, relative to an internal focus or no focus of attention (G. Wulf, T. Zachry, C. Granados, & J. S. Dufek, 2007). In the present study, the authors determined the underlying cause of this effect. Participants performed a vertical jump-and-reach task for (a) an external focus condition (i.e., participants focused on the rungs of a Vertec [Perform Better, Cranston, RI] measurement device that they touched) and (b) an internal focus condition (i.e., participants focused on the finger with which they touched the rungs). Participants’ jump height, center-of-mass displacement, jump impulse, and lower extremity joint moments were greater with an external focus compared with an internal focus. These results suggest that participants jump higher by producing greater forces when they adopt an external focus. This finding adds to evidence that an external focus facilitates the production of effective and efficient movement patterns.

Characteristics of shock attenuation during fatigued running

The aim of this study was to examine shock attenuation before and after completing a maximal effort graded exercise test while running on a treadmill. Ten individuals ran before and after a maximal graded exercise test with running speed controlled between conditions. Transfer functions were calculated using surface-mounted accelerometers to represent shock attenuation. An accelerometer was mounted on the distal aspect of the tibia and another on the anterior aspect of the forehead. Ten strides were analysed in each condition for all participants. Paired t-tests were used to compare each dependent variable (shock attenuation, stride length, rate of oxygen consumption) between conditions (running before vs after the exercise test). Oxygen consumption was 16% greater when running after the graded exercise test (47.9±5.0 ml · kg–1 · min–1; mean±s) than when running before it (41.1±2.7 ml · kg–1 · min–1) (P <0.05). Stride length was similar during running before (2.71±0.15 m) and after (2.75±0.17 m) the graded exercise test (P >0.05). Shock attenuation was, on average, 12% lower during running after (–9.8±2.6 dB) than before (–11.3±2.7 dB) the graded exercise test (P <0.05). We conclude that less shock was attenuated during fatigued than non-fatigued running and that only subtle changes in stride length were made while fatigued.

Effects of injury proneness and task difficulty on joint kinetic variability

PURPOSE The purpose was to investigate the effects of lower extremity overuse injury proneness and landing height on the variability of selected joint moment characteristics. METHODS Ten subjects from each of two groups (healthy and injury prone) performed 10 landings from a platform (heights: 50, 100, and 200% of maximum vertical jump). Force platform (1000 Hz) and video (200 Hz) information were collected and used to compute ankle, knee, and hip joint moment values during impact (0-100 ms post contact). Moment peak, time to peak, and impulse values were used to determine variability across 10 trials for each subject and height. MANOVAs and univariate follow-up tests were used to assess variability differences between groups and among heights. RESULTS Results revealed ankle moment variability differences (P < or = 0.05; Tukey) between groups for peak and time to peak variables at the 100% and 50% heights, respectively. The injury-prone group exhibited greater variability for the peak variable, whereas the healthy group showed greater variability for the temporal variable. Groups also differed on the impulse variables (P < or = 0.02; MANOVA), but follow-up tests failed to determine specific joints or directions of differences. Both groups exhibited tendencies for greater variability (P < or = 0.05; Tukey) with increases in height up to 100% MVJ and decreases in variability with further height increases. CONCLUSIONS Results suggest that groups differed in joint moment variability, possibly indicating a relationship between variability and overuse injuries, although the variables and directions of differences were inconsistent. For some variables, variability increased and then decreased with height increases for both groups, suggesting a range of heights within which the neuromuscular system adapted.

Evaluation of time-series data sets using the Pearson product-moment correlation coefficient.

The Pearson product-moment correlation has been used by researchers to compare time series data sets to assess the temporal similarities. Computer generated data, vertical ground reaction force (VGRF) data and hybrid data (constructed by combining features of computer generated and VGRF data) were used to investigate the influence of timing and amplitude differences on the effectiveness of this technique. Under a specific set of conditions the correlation coefficient is a valid and reliable indicator of temporal similarity. Deviations from these conditions, however, result in interactive effects between timing and amplitude components with subsequent reductions in the value of the coefficient. Although GRF data were evaluated, the results apply equally to other types of curves as well. The correlation coefficient is easy to use and can be used to evaluate the entire curve as opposed to discrete data points. Its usefulness is jeopardized, however, since it can be influenced by timing and amplitude differences as well as the characteristics of the curves being analyzed. A high coefficient is always indicative of temporal similarity but a lesser value does not guarantee a lack of temporal similarity.

Bilateral performance symmetry during drop landing: a kinetic analysis.

The assumption that lower extremity function is bilaterally symmetrical is prevalent throughout the biomechanics literature. The unilateral development of many overuse and acute injuries may suggest that this assumption is inaccurate. Analyses of bilateral function report conflicting results and may be partially attributable to the types of movement activities utilized and to research methodology. The purpose of this study was to evaluate the concurrent bilateral performance of the lower extremities during an apparently symmetrical movement task. Concurrent left/right side vertical ground reaction forces (VGRF) and lower extremity joint moments (JM) were obtained for 10 subjects performing 25 voluntary hanging drop landings (60 cm) on each of 3 consecutive days. Bilateral variability (BV) and systematic bilateral asymmetry (BA) were calculated for each trial. Bilateral variability was less for VGRF variables (12.8%) than JM variables (25.3%). Bilateral asymmetry was identified more often among VGRF variables (52.5%) than JM variables (16.7%). The magnitude of identified BA was greater than either the group BV (25%) or BA (110%) and indicates that bilateral differences are biomechanically meaningful.

The effects of sample size and variability on the correlation coefficient

The purpose of the study was to investigate the effects of variability as a function of sample size on the Pearson product-moment correlation coefficient (PCC) under the assumption of a perfect relationship between two variables. The effects of sample size (subjects/trials) and variability on the PCC were demonstrated using a computer model. The model was also used to evaluate selected examples taken from the literature. The results indicated that variability in excess of 10% of the range for each variable resulted in a mean reduction of the shared variance by 50% or greater. Although sample size did not affect the mean PCC, it did have a dramatic effect on extreme percentile values producing unreliable results. These results indicate that a small PCC value can be an artifact of variability. It is suggested, therefore, that one should be cautious when stating conclusions regarding the relationship between two variables without having knowledge of the associated variabilities.