Human pelvis motions when walking and when riding a therapeutic horse.

A prevailing rationale for equine assisted therapies is that the motion of a horse can provide sensory stimulus and movement patterns that mimic those of natural human activities such as walking. The purpose of this study was to quantitatively measure and compare human pelvis motions when walking to those when riding a horse. Six able-bodied children (inexperienced riders, 8-12years old) participated in over-ground trials of self-paced walking and leader-paced riding on four different horses. Five kinematic measures were extracted from three-dimensional pelvis motion data: anteroposterior, superoinferior, and mediolateral translations, list angle about the anteroposterior axis, and twist angle about the superoinferior axis. There was generally as much or more variability in motion range observed between riding on the different horses as between riding and walking. Pelvis trajectories exhibited many similar features between walking and riding, including distorted lemniscate patterns in the transverse and frontal planes. In the sagittal plane the pelvis trajectory during walking exhibited a somewhat circular pattern whereas during riding it exhibited a more diagonal pattern. This study shows that riding on a horse can generate movement patterns in the human pelvis that emulate many, but not all, characteristics of those during natural walking.

Isometric shoulder girdle strength of healthy young adults.

BACKGROUND: Shoulder girdle muscles are important for stabilizing the scapula and orienting the glenoid for upper-extremity motion. However, data describing shoulder girdle strength and how it varies with position is lacking. METHODS: A series of experiments was conducted to measure isometric strength at three positions each for elevation, depression, protraction, and retraction of the shoulder girdle. Nineteen healthy men and women (ages 19 to 23 years) participated in the study. Subjects were seated in a custom apparatus and asked to push or pull with extended arms as forcefully as possible against force-sensing handles. Shoulder girdle elevation angle and protraction angle were recorded with a video system during the tests. FINDINGS: In each direction the force generated by the shoulder girdle varied significantly (P<0.05) and monotonically with position. The greatest forces in elevation (mean 1101N, SD 370N) and protraction (mean 1117N, SD 471N) occurred at the most depressed and retracted positions, respectively. Similarly, the greatest forces in depression (mean 810N, SD 274N) and retraction (mean 914N, SD 362N) occurred at the most elevated and protracted positions, respectively. Male subjects generated 38-81% greater force than female subjects, depending on direction. Shoulder girdle elevation and protraction strengths correlated significantly (P<0.01) with bodyweight (r>0.71) and with one-repetition maximum bench-press strength (r>0.83). INTERPRETATION: Functional tasks such as bench-press may be good indicators of shoulder girdle strength in some directions.

Estimation of musculotendon properties in the human upper limb.

The purpose of this study was to develop and apply a general method for estimating the architectural properties of human muscles in vivo. The method consists of a two-phase, nested optimization procedure in which the values of peak isometric force, optimal muscle-fiber length, and tendon slack length are calculated for each musculotendon actuator, knowing muscle volume and the minimum and maximum physiological lengths of the actuator. In phase I, the positions of the bones and the activation levels of the muscles are found by maximizing the isometric torque developed for each degree of freedom at each joint. In phase II, the architectural properties of each musculotendon actuator are found by matching the strength profile of the model to that measured for subjects. The method is used to estimate the architectural properties of 26 major muscle groups crossing the shoulder, elbow, and wrist. Wherever possible, the model calculations are compared against measurements obtained from anatomical studies reported in the literature. Architectural data obtained from our work should be useful to researchers interested in developing musculoskeletal models of the upper limb.

The Obstacle-Set Method for Representing Muscle Paths in Musculoskeletal Models.

A computational method is introduced for modeling the paths of muscles in the human body. The method is based on the premise that the resultant muscle force acts along the locus of the transverse cross-sectional centroids of the muscle. The path of the muscle is calculated by idealizing its centroid path as a frictionless elastic band, which moves freely over neighboring anatomical constraints such as bones and other muscles. The anatomical constraints, referred to as obstacles, are represented in the model by regular-shaped, rigid bodies such as spheres and cylinders. The obstacles, together with the muscle path, define an obstacle set. It is proposed that the path of any muscle can be modeled using one or more of the following four obstacle sets: single sphere, single cylinder, double cylinder, and sphere-capped cylinder. Assuming that the locus of the muscle centroids is known for an arbitrary joint configuration, the obstacle-set method can be used to calculate the path of the muscle for all other joint configurations. The obstacle-set method accounts not only for the interaction between a muscle and a neighboring anatomical constraint, but also for the way in which this interaction changes with joint configuration. Consequently, it is the only feasible method for representing the paths of muscles which cross joints with multiple degrees of freedom such as the deltoid at the shoulder.

A Kinematic Model of the Upper Limb Based on the Visible Human Project (VHP) Image Dataset.

A kinematic model of the arm was developed using high-resolution medical images obtained from the National Library of Medicine's Visible Human Project (VHP) dataset. The model includes seven joints and uses thirteen degrees of freedom to describe the relative movements of seven upper-extremity bones: the clavicle, scapula, humerus, ulna, radius, carpal bones, and hand. Two holonomic constraints were used to model the articulation between the scapula and the thorax. The kinematic structure of each joint was based on anatomical descriptions reported in the literature. The three joints comprising the shoulder girdle - the sternoclavicular joint, the acromioclavicular joint, and the glenohumeral joint - were each modeled as a three degree-of-freedom, ideal, ball-and-socket joint. The articulations at the elbow and wrist - humeroulnar flexion-extension, radioulnar pronation-supination, radiocarpal flexion-extension, and radiocarpal radial-ulnar deviation - were each modeled as a single degree-of-freedom, ideal, hinge joint. Locations of the joint centers and joint axes were derived by graphically inspecting the three-dimensional surfaces of the reconstructed bones. The relative positions of the bones were defined by fixing a reference frame to each bone; the position and orientation of each reference frame were based on the positions of anatomical landmarks and on the shapes of the reconstructed bone surfaces. Tables are provided which specify the positions and orientations of the joint axes and the bone-fixed reference frames for the model arm.