Arthur Spaepen

Hierarchical control of different elbow-wrist coordination patterns

Abstract The present paper focused on the role of mechanical factors arising from the multijoint structure of the musculoskeletal system and their use in the control of different patterns of cyclical elbow-wrist movements. Across five levels of cycling frequency (from 0.45 Hz up to 3.05 Hz), three movement patterns were analyzed: (1) unidirectional, including rotations at the elbow and wrist in the same direction; (2) bidirectional, with rotation at the joints in opposite directions, and (3) free-wrist pattern, which is characterized by alternating flexions and extensions at the elbow with the wrist relaxed. Angular position of both joints and electromyographic activity of biceps, triceps, the wrist flexor, and the wrist extensor were recorded. It was demonstrated that control at the elbow was principally different from control at the wrist. Elbow control in all three patterns was similar to that typically observed during single-joint movements: elbow accelerations-decelerations resulted from alternating activity of the elbow flexor and extensor and were largely independent of wrist motion at all frequency plateaus. The elbow muscles were responsible not only for the elbow movement, but also for the generation of interactive torques that played an important role in wrist control. There were two types of interactive torques exerted at the wrist: inertial torque arising from elbow motion and restraining torque arising from physical limits imposed on wrist rotation. These interactive torques were the primary source of wrist motion, whereas the main function of wrist-muscle activity was to intervene with the interactive effects and to adjust the wrist movement to comply with the required coordination pattern. The unidirectional pattern was more in agreement with interactive effects than the bidirectional pattern, thus causing their differential difficulty at moderate cycle frequencies. When cycling frequency was further increased, both the unidirectional and bidirectional movements lost their individual features and acquired features of the free-wrist pattern. The deterioration of the controlled patterns at high cycling frequencies suggests a crucial role for proprioceptive information in wrist control. These results are suppportive of a hierachical organization of control with respect to elbow-wrist coordination, during which the functions of control at the elbow and wrist are principally different: the elbow muscles generate movement of the whole linkage and the wrist muscles produce corrections of the movement necessary to fulfill the task.

Influence of Mental Stress on Heart Rate and Heart Rate Variability

Stress is a huge problem in today’s society. Being able to measure stress, therefore, may help to address this problem. Although stress has a psychological origin, it affects several physiological processes in the human body: increased muscle tension in the neck, change in concentration of several hormones and a change in heart rate (HR) and heart rate variability (HRV). The brain innervates the heart by means of stimuli via the Autonomic Nervous System (ANS), which is divided into sympathetic and parasympathetic branches. The sympathetic activity leads to an increase in HR (e.g. during sports exercise), while parasympathetic activity induces a lower HR (e.g. during sleep). The two circuits are constantly interacting and this interaction is reflected in HRV. HRV, therefore, provides a measure to express the activity of the ANS, and may consequently provide a measure for stress. We therefore explored measures of HR and HRV with an imposed stressful situation. We recorded changes in HR and HRV in a group of 28 subjects at rest, and with a mental stressor. The results suggest that HR and HRV change with a mental task. HR and HRV recordings may have the potential, therefore, to measure stress levels and guide preventive measures to reduce stress related illnesses.

Evaluation of stroke performance in tennis.

In the present studies, the Leuven Tennis Performance Test (LTPT), a newly developed test procedure to measure stroke performance in match-like conditions in elite tennis players, was evaluated as to its value for research purposes. The LTPT is enacted on a regular tennis court. It consists of first and second services, and of returning balls projected by a machine to target zones indicated by a lighted sign. Neutral, defensive, and offensive tactical situations are elicited by appropriately programming the machine. Stroke quality is determined from simultaneous measurements of error rate, ball velocity, and precision of ball placement. A velocity/precision (VP) an a velocity/precision/error (VPE) index are also calculated. The validity and sensitivity of the LTPT were determined by verifying whether LTPT scores reflect minor differences in tennis ranking on the one hand and the effects of fatigue on the other hand. Compared with lower ranked players, higher ones made fewer errors (P < 0.05). In addition, stroke velocity was higher (P < 0.05), and lateral stroke precision, VP, and VPE scores were better (P < 0.05) in the latter. Furthermore, fatigue induced by a prolonged tennis load increased (P < 0.05) error rate and decreased (P < 0.05) stroke velocity and the VP and VPE indices. It is concluded that the LTPT is an accurate, reliable, and valid instrument for the evaluation of stroke quality in high-level tennis players.

Evaluation of the effect of backpack load and position during standing and walking using biomechanical, physiological and subjective measures

Recommendations on backpack loading advice restricting the load to 10% of body weight and carrying the load high on the spine. The effects of increasing load (0%–5%–10%–15% of body weight) and changing the placement of the load on the spine, thoracic vs. lumbar placement, during standing and gait were analysed in 20 college-aged students by studying physiological, biomechanical and subjective data. Significant changes were: (1) increased thorax flexion; (2) reduced activity of M. erector spinae vs. increased activation of abdominals; (3) increased heart rate and Borg scores for the heaviest loads. A trend towards increased spinal flexion, reduced pelvic anteversion and rectus abdominis muscle activity was observed for the lumbar placement. The subjective scores indicate a preference for the lumbar placement. These findings suggest that carrying loads of 10% of body weight and above should be avoided, since these loads induce significant changes in electromyography, kinematics and subjective scores. Conclusions on the benefits of the thoracic placement for backpack loads could not be drawn based on the parameter set studied.

Posture, muscle activity and muscle fatigue in prolonged VDT work at different screen height settings

With the increasing use of video display terminals (VDTs), there is growing concern over the corresponding increase in the number of health problems reported. Although much research has focused on identifying the optimal screen height, there is to date no consensus. This study aimed to investigate the effect of prolonged (89 min) VDT work at four different screen heights on head-neck posture, muscle activity and the development of muscle fatigue. The results show that lowering screen height, starting from 15 cm above the baseline (i.e. top of the screen level with eye height while sitting), decreased the ear-eye angle, increased the viewing angle, increased the viewing angle relative to the ear-eye line, and increased the muscle activity of the neck extensor muscles. There were also some significant time effects on postural angles and muscle activity. In this study there were only rare occurrences of muscle fatigue, defined as a simultaneous increase in EMG amplitude and a shift of the EMG power spectrum to lower frequencies. Muscle activity increased significantly in some muscles and for certain screen heights.

Subject-specific hip geometry and hip joint centre location affects calculated contact forces at the hip during gait

Hip loading affects the development of hip osteoarthritis, bone remodelling and osseointegration of implants. In this study, we analyzed the effect of subject-specific modelling of hip geometry and hip joint centre (HJC) location on the quantification of hip joint moments, muscle moments and hip contact forces during gait, using musculoskeletal modelling, inverse dynamic analysis and static optimization. For 10 subjects, hip joint moments, muscle moments and hip loading in terms of magnitude and orientation were quantified using three different model types, each including a different amount of subject-specific detail: (1) a generic scaled musculoskeletal model, (2) a generic scaled musculoskeletal model with subject-specific hip geometry (femoral anteversion, neck-length and neck-shaft angle) and (3) a generic scaled musculoskeletal model with subject-specific hip geometry including HJC location. Subject-specific geometry and HJC location were derived from CT. Significant differences were found between the three model types in HJC location, hip flexion-extension moment and inclination angle of the total contact force in the frontal plane. No model agreement was found between the three model types for the calculation of contact forces in terms of magnitude and orientations, and muscle moments. Therefore, we suggest that personalized models with individualized hip joint geometry and HJC location should be used for the quantification of hip loading. For biomechanical analyses aiming to understand modified hip joint loading, and planning hip surgery in patients with osteoarthritis, the amount of subject-specific detail, related to bone geometry and joint centre location in the musculoskeletal models used, needs to be considered.

Personalized MR-based musculoskeletal models compared to rescaled generic models in the presence of increased femoral anteversion: effect on hip moment arm lengths.

Advanced biomechanical analysis of muscle function during gait relies on the use of a musculoskeletal model. In clinical practice, personalization of the model is usually limited to rescaling a generic model to approximate the patients anthropometry, even in the presence of bony deformities, as in the case of cerebral palsy (CP). However, the current state of the art in biomechanics allows highly detailed subject-specific models to be built based on magnetic resonance (MR) images. We hypothesized that moment arm length (MAL) calculations from MR-based models would be more accurate than those from rescaled generic musculoskeletal models. Our study compared hip muscle MAL estimated by (1) a personalized model based on full-leg MR scans and (2) a rescaled generic model of both lower limbs in six children presenting with increased femoral anteversion. Personalized MR-based models were created using a custom-built workflow. Rescaled generic models were created based on three-dimensional positions of anatomical markers measured during a standing trial. For all 12 lower limb models, the hip flexion, adduction and rotation MAL of 13 major muscles were analyzed over a physiological range of hip motion using Software for interactive musculoskeletal modelling (SIMM) (Motion Analysis Corporation, USA). Our results showed that rescaled generic models, which do not take into account the subjects femoral geometry, overestimate MAL for hip flexion, extension, adduction, abduction and external rotation, but underestimate MAL for hip internal rotation. The differences in MAL introduced by taking the aberrant femoral geometry into account in the MR-based model were consistent with major gait characteristics presented in CP patients.

Subject-specific hip geometry affects predicted hip joint contact forces during gait

Hip loading affects bone remodeling and implant fixation. In this study, we have analyzed the effect of subject-specific modeling of hip geometry on muscle activation patterns and hip contact forces during gait, using musculoskeletal modeling, inverse dynamic analysis and static optimization. We first used sensitivity analysis to analyze the effect of isolated changes in femoral neck-length (NL) and neck-shaft angle (NSA) on calculated muscle activations and hip contact force during the stance phase of gait. A deformable generic musculoskeletal model was adjusted incrementally to adopt a physiological range of NL and NSA. In a second similar analysis, we adjusted hip geometry to the measurements from digitized radiographs of 20 subjects with primary hip osteoarthrosis. Finally, we studied the effect of hip abductor weakness on muscle activation patterns and hip contact force. This analysis showed that differences in NL (41-74 mm) and NSA (113-140 degrees ) affect the muscle activation of the hip abductors during stance phase and hence hip contact force by up to three times body weight. In conclusion, the results from both the sensitivity and subject-specific analysis showed that at the moment of peak contact force, altered NSA has only a minor effect on the loading configuration of the hip. Increased NL, however, results in an increase of the three hip contact-force components and a reduced vertical loading. The results of these analyses are essential to understand modified hip joint loading, and for planning hip surgery for patients with osteoarthrosis.

Validation of the wavelet spectral estimation technique in Biceps Brachii and Brachioradialis fatigue assessment during prolonged low-level static and dynamic contractions

An experiment was carried out to investigate the myoelectrical manifestations of fatigue of the Biceps Brachii and Brachioradialis muscles in low-level (15% MVC) prolonged isometric and dynamic contractions. The range of the joint angle was 70-110 degrees and the mean speed of flexion and extension was about 33.33 degrees /s (1.2 s for 40 degrees ). The use of Wavelet transform (IMNF) in weakly stationary dynamic SEMG signals was validated in comparison with the Fourier transform (MPF). The development of fatigue and its myoelectrical manifestations (increase in RMS and decrease in mean frequency) in dynamic contractions show no deviations from what is found in literature for both spectral estimation techniques. The benefit of Wavelets is its future use in non-stationary conditions. Lower IMNF slopes in dynamic compared to isometric contractions for Biceps Brachii might be an indication that wavelets reflect more the changes in muscle fiber propagation velocity. The results of the use of Wavelet transform in detecting frequency modulations in different movement phases of the dynamic tests show that in the eccentric phase a systematic shift towards lower frequencies occurs. It also reveals the great possibilities of phase separation using Wavelets with high resolution and low interaction.

Calculated moment-arm and muscle-tendon lengths during gait differ substantially using MR based versus rescaled generic lower-limb musculoskeletal models

Biomechanical analysis of gait relies on the use of lower-limb musculoskeletal models. Most models are based on a generic model which takes into account the subjects skeletal dimensions by isotropic or anisotropic rescaling. Alternatively, personalized models can be built based on information from magnetic resonance (MR) images. We have studied the effect of these approaches on muscle-tendon lengths (MTLs) and moment-arm lengths (MALs) for 16 major muscles of the lower limb of a normal adult during both normal and pathologic gait. For most muscles, the MTL and MAL calculated using the rescaled generic models showed high correlation values, but large offsets when compared to values calculated using personalized models. MTL and MAL differences with the personalized model are only slightly smaller for an anisotropic than for an isotropic rescaled model. Gait kinematics influenced the observed inter-model differences and correlations due to an altered range of joint angles in both gait patterns. In conclusion, both generic rescaling methods failed to accurately estimate absolute values for MTL and MAL calculated using the personalized model. However, the magnitude of MTL and MAL changes during normal and pathologic gait corresponded between all three models for most muscles. Since rescaling depends strongly on modelling assumptions and cannot fully take into account subject-specific musculoskeletal geometry, interpretation of MTL and MAL even in normal adult subjects requires extreme caution.