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Hoffman, J. R., Liebermann, D., & Gusis, A. (1997). Relationship of leg strength and power to ground reaction forces in both experienced and novice jump trained personnel. Aviat Space Environ Med, 68(8), 710–714.
Abstract: METHODS: There were 14 male soldiers who participated in this study examining the relationship of leg strength and power on landing performance. Subjects were separated into two groups. The first group (E, n = 7) were parachute training instructors and highly experienced in parachute jumping. The second group of subjects (N, n = 7) had no prior parachute training experience and were considered novice jumpers. All subjects were tested for one-repetition maximum (1 RM) squat strength and maximal jump power. Ground reaction forces (GRF) and the time to peak force (TPF) at landing were measured from jumps at four different heights (95 cm, 120 cm, 145 cm, and 170 cm). All jumps were performed from a customized jump platform onto a force plate. RESULTS: No differences were seen between E and N in either IRM squat strength or in MJP. In addition, no differences were seen between the groups for time to peak force at any jump height. However, significantly greater GRF were observed in E compared to N. Moderate to high correlations between maximal jump power and GRF (r values ranging from 0.62-0.93) were observed in E. Although maximal jump power and the TPF was significantly correlated (r = -0.89) at only 120 cm for E, it was interesting to note that the correlations between MJP and the time to peak force in E were all negative and that the correlations between these variables in N were all positive. CONCLUSIONS: These results suggest that experienced parachutists may use a different landing strategy than novice jumpers. This difference may be reflected by differences in GRF generated during impact and a more efficient utilization of muscle power during the impact phase of the landing.
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Biess, A., Flash, T., & Liebermann, D. G. (2011). Riemannian geometric approach to human arm dynamics, movement optimization, and invariance. Phys Rev E Stat Nonlin Soft Matter Phys, 83(3 Pt 1), 031927.
Abstract: We present a generally covariant formulation of human arm dynamics and optimization principles in Riemannian configuration space. We extend the one-parameter family of mean-squared-derivative (MSD) cost functionals from Euclidean to Riemannian space, and we show that they are mathematically identical to the corresponding dynamic costs when formulated in a Riemannian space equipped with the kinetic energy metric. In particular, we derive the equivalence of the minimum-jerk and minimum-torque change models in this metric space. Solutions of the one-parameter family of MSD variational problems in Riemannian space are given by (reparameterized) geodesic paths, which correspond to movements with least muscular effort. Finally, movement invariants are derived from symmetries of the Riemannian manifold. We argue that the geometrical structure imposed on the arm's configuration space may provide insights into the emerging properties of the movements generated by the motor system.
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Roijezon, U., Djupsjobacka, M., Bjorklund, M., Hager-Ross, C., Grip, H., & Liebermann, D. G. (2010). Kinematics of fast cervical rotations in persons with chronic neck pain: a cross-sectional and reliability study. BMC Musculoskelet Disord, 11, 222.
Abstract: BACKGROUND: Assessment of sensorimotor function is useful for classification and treatment evaluation of neck pain disorders. Several studies have investigated various aspects of cervical motor functions. Most of these have involved slow or self-paced movements, while few have investigated fast cervical movements. Moreover, the reliability of assessment of fast cervical axial rotation has, to our knowledge, not been evaluated before. METHODS: Cervical kinematics was assessed during fast axial head rotations in 118 women with chronic nonspecific neck pain (NS) and compared to 49 healthy controls (CON). The relationship between cervical kinematics and symptoms, self-rated functioning and fear of movement was evaluated in the NS group. A sub-sample of 16 NS and 16 CON was re-tested after one week to assess the reliability of kinematic variables. Six cervical kinematic variables were calculated: peak speed, range of movement, conjunct movements and three variables related to the shape of the speed profile. RESULTS: Together, peak speed and conjunct movements had a sensitivity of 76% and a specificity of 78% in discriminating between NS and CON, of which the major part could be attributed to peak speed (NS: 226 +/- 88 degrees /s and CON: 348 +/- 92 degrees /s, p < 0.01). Peak speed was slower in NS compared to healthy controls and even slower in NS with comorbidity of low-back pain. Associations were found between reduced peak speed and self-rated difficulties with running, performing head movements, car driving, sleeping and pain. Peak speed showed reasonably high reliability, while the reliability for conjunct movements was poor. CONCLUSIONS: Peak speed of fast cervical axial rotations is reduced in people with chronic neck pain, and even further reduced in subjects with concomitant low back pain. Fast cervical rotation test seems to be a reliable and valid tool for assessment of neck pain disorders on group level, while a rather large between subject variation and overlap between groups calls for caution in the interpretation of individual assessments.
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Krasovsky, T., Berman, S., & Liebermann, D. G. (2010). Kinematic features of continuous hand reaching movements under simple and complex rhythmical constraints. J Electromyogr Kinesiol, 20(4), 636–641.
Abstract: BACKGROUND: Auditory cues are known to alter movement kinematics in healthy people as well as in people with neurological conditions (e.g., Parkinson's disease or stroke). Pacing movement to rhythmical constraints is known to change both the spatial and temporal features of movement. However, the effect of complexity of pacing on the spatial and temporal kinematic properties is still poorly understood. The current study investigated spatial and temporal aspects of movement (path and speed, respectively) and their integration while subjects followed simple isochronous or complex non-isochronous rhythmical constraints. Spatiotemporal decoupling was expected under the latter constraint. METHODS: Ten subjects performed point-to-point hand movements towards visual targets on the surface of a hemisphere, while following continuous auditory cues of different pace and meter. The spatial and temporal properties of movement were compared to geodesic paths and unimodal bell-shaped speed profiles, respectively. Multiple two-way RM-ANOVAs (pace [1-2 Hz] x meter [duple-triple]) were performed on the different kinematic variables calculated to assess hand deviations from the model data (p< or = 0.05). RESULTS: As expected, increasing pace resulted in straighter hand paths and smoother speed profiles. Meter, however, affected only the path (shorter and straighter under triple) without significantly changing speed. Such an effect was observed at the slow pace only. CONCLUSIONS: Under simple rhythmic cues, an increase in pace causes spontaneous adjustments in spatial features (straighter hand paths) while preserving temporal ones (maximally-smoothed hand speeds). Complex rhythmical cues in contrast perturb spatiotemporal coupling and challenge movement control. These results may have important practical implications in motor rehabilitation.
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Melzer, I., Krasovsky, T., Oddsson, L. I. E., & Liebermann, D. G. (2010). Age-related differences in lower-limb force-time relation during the push-off in rapid voluntary stepping. Clin Biomech (Bristol, Avon), 25(10), 989–994.
Abstract: BACKGROUND: This study investigated the force-time relationship during the push-off stage of a rapid voluntary step in young and older healthy adults, to study the assumption that when balance is lost a quick step may preserve stability. The ability to achieve peak propulsive force within a short time is critical for the performance of such a quick powerful step. We hypothesized that older adults would achieve peak force and power in significantly longer times compared to young people, particularly during the push-off preparatory phase. METHODS: Fifteen young and 15 older volunteers performed rapid forward steps while standing on a force platform. Absolute anteroposterior and body weight normalized vertical forces during the push-off in the preparation and swing phases were used to determine time to peak and peak force, and step power. Two-way analyses of variance ('Group' [young-older] by 'Phase' [preparation-swing]) were used to assess our hypothesis (P </= 0.05). FINDINGS: Older people exerted lower peak forces (anteroposterior and vertical) than young adults, but not necessarily lower peak power. More significantly, they showed a longer time to peak force, particularly in the vertical direction during the preparation phase. INTERPRETATIONS: Older adults generate propulsive forces slowly and reach lower magnitudes, mainly during step preparation. The time to achieve a peak force and power, rather than its actual magnitude, may account for failures in quickly performing a preventive action. Such delay may be associated with the inability to react and recruit muscles quickly. Thus, training elderly to step fast in response to relevant cues may be beneficial in the prevention of falls.
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