The author derived the exact analytical expression of the instantaneous joint power in exercises with single-joint, variable-resistance, selectorized strength-training equipment, taking into account all the relevant geometric, kinematic, and dynamic variables of both the movable equipment elements (resistance input lever, cam–pulley system, weight stack) and of the user’s exercising limb. A numerical algorithm was also designed to express, in the presence of a cam, the rectilinear kinematic variables of the weight stack as a function of the rotational kinematic variables of the resistance input lever, and vice versa. Given that information, one can measure the value of the instantaneous and mean joint power exclusively by means of a linear encoder placed on the weight stack or, alternatively, only by the use of an angular encoder placed on the rotational axis of the resistance lever. The results highlight that, for knee extension exercises with leg extension equipment, the real values of both instantaneous and mean joint power may differ by more than 50% in comparison with the values obtained by taking into account only the mass and velocity of the weight stack. These differences are notable not only in explosive exercises, but also whenever considerable joint velocities/accelerations occur within the range of motion.
We have developed a 2-D analytical biomechanical model for monoarticular open kinetic-chain exercises with lever selectorized equipment, and different relative placement between the joint center of rotation (J) and the center of rotation (C) of the resistance input lever (“off-center” exercises). All the relevant geometrical aspects of such exercises have been characterized: the change with the joint angle of the distance between the resistance pad (P) and J, and of the angle between CP and JP (i.e., the angle between the resistance input lever and the exercising limb). These changes may strongly affect the joint load and the muscle torque in inverse dynamic problems, given the joint kinematics and the mass of the selected weight stack. Therefore, the muscle torque, the shear and axial components of the joint load have been calculated analytically as a function of the relative positioning of C and J, and the length CP, in addition to the parameters that define the joint kinematics, the equipment mechanics, and the external load. From these results we have derived the optimal cam profiles for “off-center” exercises, as well as the geometrical “off-center” setting that minimizes the shear component of the tibiofemoral joint load in leg extension equipment.
A two-dimensional model has been developed to predict and explain the effects of the variation of muscle moment arms during dynamic exercises involving heavy external loads. The analytical dependence of the muscle moment arm on the joint angle and on the origin and insertion position was derived for an ideal uniaxial hinge joint, modeling the muscle as a cable following an idealized minimum distance path from the origin to insertion that wraps around the bony geometry. Analytical expressions for the ratios of muscular force and the joint restraining reaction components to the external load weight were deduced, for isokinetic and static exercises, as a function of joint angle, joint angular velocity, and the other geometric parameters defining the model. Therefore, external load weight, joint angular velocity, and constraints to joint range of motion may be adjusted reciprocally in order to control in advance the peak value of the components of the joint load during isokinetic exercises. A dynamic formulation of forearm flexion/extension was solved numerically under the condition of constant biceps force in order to highlight the key role played by the variation in muscle moment arm in preventing injury during lifting of external loads against gravity. For example, our analysis indicates that the mean and peak resultant joint loads decrease by 5% and 14%, respectively, as a result of the change in muscle moment arm that occurs over the range of motion.
Samuele Contemori and Andrea Biscarini
Context: Isolated infraspinatus atrophy (IIA) is a common condition among overhead activity athletes, which affects the hitting shoulder and is caused by suprascapular nerve injury. The loss of infraspinatus function could lead to altered activity of the glenohumeral and scapulothoracic (ST) muscles and compromise the optimal shoulder function. Objective: To assess the surface electromyographic (sEMG) activity patterns, relationships, and response latencies of relevant shoulder girdle muscles in professional volleyball players with IIA and in healthy control players. Design: Cross-sectional study. Setting: Research laboratory. Patients or Other Participants: Twenty-four male professional volleyball players (12 players with diagnosed IIA and 12 healthy players) recruited from local volleyball teams. Intervention(s): sEMG activity of anterior, middle, and posterior deltoid; upper, middle, and lower trapezius; and serratus anterior (SA) was recorded and evaluated during a movement of shoulder abduction in the scapular plane, monitored with an optoelectronic motion capture system. Main Outcome Measure(s): sEMG activity, relationships, and response latencies of the selected muscles were analyzed with analysis of variance models to highlight statistical differences within and between groups. Results: Athletes with IIA demonstrated significant higher deltoid and trapezius muscles activity and lower SA activity compared with the contralateral shoulder and healthy athletes. The shoulder with IIA also showed a higher activity ratio between the upper trapezius and other ST muscles in addition to anticipated activation of the upper trapezius and delayed activation of the SA, with regard to the onset of shoulder movement. Conclusions: This study highlighted altered shoulder muscle activity levels, ST muscles imbalances, and abnormal ST recruitment patterns in the hitting shoulder of professional volleyball players with IIA, secondary to suprascapular nerve neuropathy. Such shoulder girdle muscles’ impairments may compromise the optimal scapulohumeral rhythm and function, increasing the risk of acute and overuse shoulder injuries.
Andrea Biscarini, Samuele Contemori, and Giuditta Grolla
Context: Previous studies highlighted that exercises executed on unstable surfaces can yield important benefits to the function of the core musculature in rehabilitation settings, general conditioning settings, and athletic training when properly introduced within a periodized training schedule. No previous study has analyzed core-stability exercises executed in lying, quadruped, plank, and bridge positions on a whole-body wobble board (WWB) specifically designed to accommodate the exerciser’s entire body and promote whole-body instability. We have designed a WWB allowed to roll in a plane perpendicular to its longitudinal axis to promote proactive and reactive activation of the core muscles with a transverse or diagonal line of action, which provides trunk and pelvic stability with low spine compression forces. Purpose: To determine the effect of the use of this newly designed WWB by assessing differences in core-muscle activity during core-stability exercises performed on the ground, in a stable condition, and on the WWB. Design: Controlled laboratory study. Setting: Research laboratory. Patients or Other Participants: Eighteen participants recruited from fitness centers. Intervention(s): The electromyographic (EMG) activity of lumbopelvic and scapular muscles has been recorded during core-stability exercises executed on the WWB (unstable condition) and on ground (stable condition). Main Outcome Measure(s): Mean and peak EMG activity were compared between stable and unstable condition with paired t tests or Wilcoxon signed-rank tests. Results: Overall, exercises performed on the WWB yielded significantly higher EMG activity in the serratus anterior and anterolateral abdominal muscles compared with the same exercises executed on the ground. Conversely, for the bird dog exercise, lower-back muscle activity was significantly higher on the ground. Conclusions: Compared with the ground, core-stability exercises executed on WWB constitute a simple and effective strategy to increase the activity level of the core muscles that control transverse-plane lumbopelvic and trunk stability, avoiding the use of external overload.
Andrea Biscarini, Fabio M. Botti, and Vito E. Pettorossi
We developed a biomechanical model to determine the joint torques and loadings during squatting with a backward/forward-inclined Smith machine. The Smith squat allows a large variety of body positioning (trunk tilt, foot placement, combinations of joint angles) and easy control of weight distribution between forefoot and heel. These distinctive aspects of the exercise can be managed concurrently with the equipment inclination selected to unload specific joint structures while activating specific muscle groups. A backward (forward) equipment inclination decreases (increases) knee torque, and compressive tibiofemoral and patellofemoral forces, while enhances (depresses) hip and lumbosacral torques. For small knee flexion angles, the strain-force on the posterior cruciate ligament increases (decreases) with a backward (forward) equipment inclination, whereas for large knee flexion angles, this behavior is reversed. In the 0 to 60 degree range of knee flexion angles, loads on both cruciate ligaments may be simultaneously suppressed by a 30 degree backward equipment inclination and selecting, for each value of the knee angle, specific pairs of ankle and hip angles. The anterior cruciate ligament is safely maintained unloaded by squatting with backward equipment inclination and uniform/forward foot weight distribution. The conditions for the development of anterior cruciate ligament strain forces are clearly explained.
Samuele Contemori, Andrea Biscarini, Fabio M. Botti, Daniele Busti, Roberto Panichi, and Vito E. Pettorossi
Context : Isolated infraspinatus muscle atrophy (IIMA) affects only the hitting shoulder of overhead-activity athletes and is caused by suprascapular nerve neuropathy. No study has assessed the static and dynamic stability of the shoulder in overhead professional athletes with IIMA to reveal possible shoulder sensorimotor alterations. Objective: To assess the shoulder static stability, dynamic stability, and strength in professional volleyball players with IIMA and in healthy control players. Design: Cross-sectional study. Setting: Research lab. Patients or Other Participants: A total of 24 male professional volleyball players (12 players with diagnosed IIMA and 12 healthy players) recruited from local volleyball teams. Intervention(s): Static stability was evaluated with 2 independent force platforms, and dynamic stability was assessed with the “Upper Quarter Y Balance Test.” Main Outcome Measure(s): The static stability assessment was conducted in different support (single hand and both hands) and vision (open and closed eyes) conditions. Data from each test were analyzed with analysis of variance and paired t-test models to highlight statistical differences within and between groups. Results: In addition to reduced abduction and external rotation strength, athletes with IIMA consistently demonstrated significant less static (P < .001) and dynamic stability (P < .001), compared with the contralateral shoulder and with healthy athletes. Closed eyes condition significantly enhanced the static stability deficit of the shoulder with IIMA (P = .04 and P = .03 for both hand and single hand support, respectively) but had no effect on healthy contralateral and healthy players’ shoulders. Conclusions: This study highlights an impairment of the sensorimotor control system of the shoulder with IIMA, which likely results from both proprioceptive and strength deficits. This condition could yield subtle alteration in the functional use of the shoulder and predispose it to acute or overuse injuries. The results of this study may help athletic trainers and physical/physiotherapists to prevent shoulder injuries and create to specific proprioceptive and neuromuscular training programs.