Movement variability response to change in the rate of hopping.

Movement variability is often considered undesirable, but growing evidence demonstrates positive aspects of variability. During unipedal hopping, control of limb stiffness and limb length are paramount. PURPOSE: The purpose of this study was to compare two methods of measuring movement variability that provide information at the task level, and their capacities to illuminate the neuromotor control system's response to change in hopping rate. METHODS: The typical task-level movement variability measure of the standard deviation of vertical limb length was compared to uncontrolled manifold analysis. We examined the relationship between change scores in deviation from spring-mass model-type behavior and these two variability measures for the shift from typical (2.3 Hz) to slow (1.7 Hz) hopping. RESULTS: The change scores for deviation from spring-mass model-type behavior and vertical limb length standard deviation demonstrated no correlation ( p = 0.784, R = 0.051). In contrast, the change scores for deviation from spring-mass model-type behavior and the uncontrolled manifold analysis measure demonstrated a moderate correlation ( p = 0.004, R = 0.502). CONCLUSIONS: Uncontrolled manifold analysis considers not just variability in the sense of error, but illustrates how the neuromotor control system distributes movement variability into performance-irrelevant and performance-destabilizing subspaces. As such, this type of analysis may be more effective at illuminating global control aspects of movement variability than the typical variability measure of limb length standard deviation.

Changing one's focus of attention alters the structure of movement variability.

Substantial evidence supports the beneficial effect of an external (vs. internal) focus of attention on task performance during goal-directed movements. Counter-intuitively, an external focus has also been shown to increase joint-level movement variability. OBJECTIVE: To determine whether shifting attentional focus can alter the structure of movement variability, thereby offering a probable mechanistic explanation for how adopting an external focus of attention might confer its benefits. METHODS: Thirty-five healthy adults (age 23-55) performed unipedal hopping under three different attentional foci: natural (no directed focus), internal focus, and external focus. Uncontrolled manifold analysis was used to examine the structure of movement variability with respect to stabilization of leg orientation and vertical leg length during hopping. Takeoff/landing event bin and stance phase integrals of performance-irrelevant and performance-destabilizing variability were compared across focus conditions. RESULTS: Accuracy of hopping in place improved with both external and internal foci compared to the natural condition (.004 ≤ p ≤ .035). External focus, to a greater degree than internal focus, destabilized leg orientation at takeoff and landing compared to the natural condition (.001 ≤ p ≤ .038). External focus increased - but internal focus decreased - leg length stabilization throughout stance compared to the natural condition (p < .001). CONCLUSION: External focus was superior to internal and natural focus conditions in terms of increasing flexibility within the system to orient the leg differently at takeoff and landing to compensate for unintentional drift during hopping. An external focus increased leg length stabilization in stance by preferentially increasing the subset of variability that explores multiple successful performance options. These results provide an understanding of the mechanism underlying external focus benefits - improving movement variability/coordination.

Toe Flexor Strength, Flexibility and Function and Flexor Hallucis Longus Tendon Morphology in Dancers and Non-Dancers.

Tendinopathy of the flexor hallucis longus (FHL), colloquially referred to as "dancer's tendinitis," is a common condition in dancers and attributed to high demand on this muscle in positions of extreme ankle plantarflexion and metatarsophalangeal (MTP)) flexion and extension. Despite such a high prevalence, there has been little research into preventative or nonsurgical interventions. As a means to identify potential targets for prevention and intervention, this study aimed to characterize toe flexors in dancers by measuring strength, flexibility, function, and FHL tendon morphology. Dancers (n=25) were compared to non-dancers (n=25) in toe flexor isometric strength, first MTP joint range of motion, foot longitudinal arch flexibility, balance ability, endurance during modified heel raises without use of the toes, and FHL tendon thickness, cross-sectional area, and peak spatial frequency. Significant differences were found in functional first MTP joint extension (dancers 101.95°, non-dancers 91.15°, p<0.001), balance ability during single-leg stance on the toes (dancers 11.43 s, non-dancers 5.90 s, p=0.013), and during modified heel raises (dancers 22.20 reps, non-dancers 28.80 reps, p=0.001). Findings indicate that dancers rely on toe flexors more than non-dancers to complete balance and heel raise tasks. Efficacy of using this modified heel raise task with the toes off the edge of a block as a means to train larger plantarflexors and as a nonsurgical intervention should be studied in the future. Improving interventions for FHL tendinopathy will be impactful for dancers, in whom this condition is highly prevalent.

Ground reaction forces and knee mechanics in the weight acceptance phase of a dance leap take-off and landing.

Aesthetic constraints allow dancers fewer technique modifications than other athletes to negotiate the demands of leaping. We examined vertical ground reaction force and knee mechanics during a saut de chat performed by healthy dancers. It was hypothesized that vertical ground reaction force during landing would exceed that of take-off, resulting in greater knee extensor moments and greater knee angular stiffness. Twelve dancers (six males, six females; age 18.9 ± 1.2 years, mass 59.2 ± 9.5 kg, height 1.68 ± 0.08 m, dance training 8.9 ± 5.1 years) with no history of low back pain or lower extremity pathology participated in the study. Saut de chat data were captured using an eight-camera Vicon system and AMTI force platforms. Peak ground reaction force was 26% greater during the landing phase, but did not result in increased peak knee extensor moments. Taking into account the 67% greater knee angular displacement during landing, this resulted in less knee angular stiffness during landing. In conclusion, landing was accomplished with less knee angular stiffness despite the greater peak ground reaction force. A link between decreased joint angular stiffness and increased soft tissue injury risk has been proposed elsewhere; therefore, landing from a saut de chat may be more injurious to the knee soft tissue than take-off.