Sensitivity of Shoulder Musculoskeletal Model Predictions to Muscle-Tendon Properties.

OBJECTIVE: While the sensitivity of estimated muscle forces to muscle-tendon properties is well documented for the lower limbs, little is known about the shoulder and upper limbs. The purpose of this study was to assess the sensitivity of estimated shoulder muscle forces and scapulohumeral joint force to muscle-tendon properties. METHODS: One healthy male participant performed arm flexions and simulated throwing maneuvers. Kinematics were recorded using intra-cortical pins. Muscle forces were estimated using static optimization with the generic delft shoulder and elbow in OpenSim, and scapulohumeral joint forces were calculated from the estimated forces. Then, variations from -25% to +25% of the nominal values of the tendon slack length, the optimal fiber length, the maximal isometric force, and the pennation angle were applied to the musculoskeletal model to compute affected muscle forces and scapulohumeral joint force. RESULTS: The variations in muscle-tendon properties led to changes up to 9.6 N or 174% in the muscle nominal forces. The more sensitive muscles were those that produced the greatest force: the rotator cuff muscles and the prime movers specific to the task. Among the four muscle-tendon properties, the maximal isometric force and the optimal fiber length had the greatest influence on the muscle force variability. Glenohumeral force was slightly influenced by muscle-tendon properties (<8%). CONCLUSION: Generic models (i.e., those without personalization of muscle-tendon properties) can lead to misinterpretations of muscle force. Efforts should focus on the maximal isometric force and the optimal fiber length of the rotator cuff muscles and prime movers.

Short-term effects of integrated motor imagery practice on muscle activation and force performance.

The effect of motor imagery (MI) practice on isometric force development is well-documented. However, whether practicing MI during rest periods of physical training improves the forthcoming performance remains unexplored. We involved 18 athletes in a counterbalanced design including three physical training sessions scheduled over five consecutive days. Training involved 10 maximal isometric contractions against a force plate, with the elbow at 90°. During two sessions, we integrated MI practice (focusing on either muscle activation or relaxation) during the inter-trial rest periods. We measured muscle performance from force plate and electromyograms of the biceps brachii and anterior deltoideus. We continuously monitored electrodermal activity (EDA) to control sympathetic nervous system activity. MI of muscle activation resulted in higher isometric force as compared to both MI of muscle relaxation and passive recovery (respectively +2.1% and +3.5%). MI practice of muscle relaxation also outperformed the control condition (+1.9%). Increased activation of the biceps brachii was recorded under both MI practice conditions compared to control. Biceps brachii activation was similar between the two MI practice conditions, but electromyography revealed a marginal trend toward greater activation of the anterior deltoideus during MI practice of muscle activation. EDA and self-reports indicated that these effects were independent from physiological arousal and motivation. These results might account for priming effects of MI practice yielding to higher muscle activation and force performance. Present findings may be of interest for applications in sports training and neurologic rehabilitation.

The effects of seat height and foot placement on lumbar spine load during sit-to-stand tasks.

The purpose of this study was to determine the influence of seat height and foot position in the sagittal plane on L5-S1 joint load. Fourteen healthy male adults stood up from a chair with three different seat heights and positions of the feet in the sagittal plane. L5-S1 net joint torque, mechanical work, range of motion and electromyographic activity of the erector spinae muscle were measured. L5-S1 net joint work increased by about 50% from high to low seat position. The mean and peak L5-S1 net joint torques increased about 30% from foot-back to foot-neutral position. These results were reinforced by a greater integrated electromyography signal from the erector spinae from high to low seat position and from foot-back to foot-neutral position. A high chair and placement of the feet behind the knees may be advisable to lessen lumbar load during sit-to-stand movements.

Asymmetries in joint work during multi-joint movement after anterior cruciate ligament reconstruction: a pilot study.

After anterior cruciate ligament reconstruction (ACL-R), many studies have reported a deficit of performance on the injured leg during multi-joint tasks. However, the total mechanical joint work (WTotal ), parameter best related to the vertical displacement of the body mass center during vertical jumping, has not yet been studied. The aim of this research was to compare asymmetries between ACL-R subjects and healthy matched subjects, through the analysis of the kinematics and kinetics during a single-leg squat jump. Asymmetries are defined by the Limb Symmetry Index (LSI). A greater LSI was observed for WTotal in the ACL-R group than in the healthy group. There was no difference in LSI for knee joint work between the two groups, while the LSI for hip and ankle joint work was significantly larger in the ACL-R group. This was explained by greater LSI for the hip and ankle joint range of motion in the ACL-R group than in the healthy group. After ACL-R, patients exhibited greater asymmetries than healthy subjects during single-leg squat jump. Physiotherapists should focus on quality execution of multi-joint movement, especially on hip and ankle joints range of motion in order to reduce asymmetries and to improve vertical jumping performance.

Effect of arm swing on effective energy during vertical jumping: experimental and simulation study.

Arm swing helps to increase vertical jump height (VJH), in part by a greater hip joint muscle work. The force-velocity relationship has been put forward to explain the increase in hip joint work. Nevertheless, the efficacy ratio, muscle shortening length, and active state might be parameters that affect the effective energy and then VJH. The purpose of this study was to evaluate the influence of arm swing on effective energy in vertical jumping. Eight subjects performed maximal squat jumps with (SJ arm ) and without arm swing (SJ). A simulation model of the musculo-skeletal system was applied. For subjects and simulation, VJH was about 20% higher during SJ arm. In subjects, this was explained by the shoulder joint work (34%) and an increase of L5-S1 joint work (66%). In simulated jump, effective energy increase during SJ arm (+80.74 J) was related to an improvement of the total muscle work and not to the efficacy ratio. The increase in total muscle work was due to anterior deltoid work and to greater erector spinae, biceps femoris, and gluteus work. The greater muscle works were explained by a slower shortening velocity for all the muscles and by a greater shortening length and active state for the biceps femoris.