It is always advantageous to describe the joint torque in the local (segmental) reference frame since the local components reflect the dominant muscle groups. Figure 1 shows several local reference frames fixed to the segments. Note that the vertical axis of the global frame is labeled as ZG, which means the conventional 3-D coordinate system is used here.
For consistency, the origins of the segmental reference frames are located at the proximal ends of the segments. The X, Y & Z axes of the segmental reference frame are aligned to the segment's mediolateral, anteroposterior & longitudinal axes, respectively. Table 1 shows the local joint torque components and the corresponding dominant muscle groups. As shown in the table, the sign-dominant muscle group relationship varies from one side to another. For example, a positive Tz for the right upperarm means a medial (internal) rotator-dominant torque while that for the left upperarm means a lateral (external) rotator-dominant torque. One must be cautious in assigning practical meanings to the components.
Table 1. Local Joint Torque Components vs. Dominant Muscle Groups
The 1st time-derivative of the joint angle provides additional information regarding the type of the muscle activation. If the 1st time-derivative of the joint angle shows the same sign to that of the torque component, it means the concentric activation of the muscle group. The opposite sign means the eccentric activation of the dominant muscle group.
Figure 2a & b (from Dillman, Fleisig & Andrews, 1993) show the changes in the shoulder joint angle during baseball pitching while Figure 3 (from Fleisig et al., 1995) shows the joint torque patterns at the shoulder during pitching. Table 2 summarizes the observations from Figures 2 and 3. As shown in the table, the deceleration phase is characterized by the eccentric activation of the adductors, concentric activation of the horizontal adductors followed by the eccentric activation of the horizontal abductors, and concentric activation of the internal rotators.
Table 2. Summary of the Muscle activation in Baseball Pitching
Interestingly, Fleisig et al. (1995) stated that "Andrews and Angelo (1988) found that most rotator cuff tears in throwers were... a consequence of tensile failure, as the muscles tried to resist distraction, horizontal adduction, and internal rotation at the shoulder during arm deceleration. Compression force (Fig. 2) and horizontal adduction torque (Fig. 3) were seen during arm deceleration in this study... thus [Andrews and Angelo's belief] is consistent with available biomechanical data."
Supraspinatus, the most commonly injured rotator cuff muscle, is responsible for shoulder external rotation and shoulder abduction (?). Unfortunately, the eccentric activation of the shoulder adductors and the concentric activation of the internal rotators during the deceleration phase does not really suggest the potential tensile failure of the Supraspinatus. The results of Fleisig et al. (1995) and Dillman et al. (1993) are actually contradictory to the Andrews & Angelo's belief and Andrews & Angelo's belief is not quite consistent with available biomechanical data. Fleisig et al. thus failed to accurately interpret their torque and joint angle data.
The joint torque is the net torque of all the muscle torques at a joint and doesn't reflect the co-contraction of the agonists and the antagonists. One may want to use additional measures such as EMG to analyze the muscle activation in depth.
As mentioned in Joint Torque and Net Joint Force, the net joint force is a hybrid of both the joint reaction force and the muscle force. Thus, it does not specifically reflect either the joint reaction force or the muscle force. It is simply the combined effect of the forces from the next segment attached through the joint and the muscle. Thus, one must exercise caution in interpreting the net joint force data. In any case, it should not be confused as the force acting at the joint between the two articulating bony structures. Unfortunately, wrong terminology is being used in some area of biomechanics. Here is an example.
Figure 4 shows the net joint force terminology currently being used in the baseball pitching studies. Interestingly, the term 'compression' is used for the longitudinal (proximal) force component while some directional terms are used for the other two components, such as anterior, posterior, superior, and inferior. Remember here that the net joint force is simply the combined effect of the joint reaction force and the muscle force from the shoulder to the upperarm. In other words, the force toward the shoulder shown in the figure is in fact a pulling force from the trunk. Since the trunk is dragging the upperarm, it is overall of tensile nature rather than compressive. It is unfortunately a bad choice of terminology. It is recommended to use a term which precisely describe the nature of the interaction between the upperarm and the trunk, pulling, to prevent unnecessary confusion.
Moreover, the term 'compressive' gives the illusion that the pulling force from the shoulder causes compression at the shoulder. A more in-depth analysis of the pulling force provides two possibilities. The muscle force is always tensile in nature and the joint reaction force can be either compressive or tensile depending on the situation. Here is why:
Case 1 happens when one predominantly relies on the muscles to produce acceleration of the distal body part (pitching arm). Case 2 occurs when one actively moves the proximal segment to cause the acceleration of the distal segments (dragging). Anyway, use of the term 'compression' for the pulling net joint force is inappropriate since it can actually cause either compression or tension in the shoulder. One serious problem may occur as a result of this confusion. People who are not quite familiar with the terminology will easily accept the 'compressive force' as an actual compression in the shoulder. Careful choosing of the terminology can prevent these problems.
In many cases, people are interested in the actual joint reaction force (J's in Figure 1) since this force is closely related to the injury mechanism. Sophisticated modeling & optimization methods such as the EMG-assisted optimization (Cholewicki & McGill, 1994) can be employed to assess the joint reaction force in the joint. The net joint force alone has very limited use and meaning.
Andrew, J.R., and Angelo, R.L. (1988). Shoulder arthroscopy for the throwing athlete. Tech Orthop 3, 75-81.
Cholewicki, J. and McGill, S.M. (1994). EMG assisted optimization: a hybrid approach for estimating muscle forces in an indeterminate biomechanical model. J. Biomechanics 27, 1287-1289.
Dillman, C.J., Fleisig, G.S., and Andrews, J.R. (1993). Biomechanics of pitching with emphasis upon shoulder kinematics. J Orthop Sports Phys Ther 18, 402-208.
Fleisig, G.S., Andrews, J.R., Dillman, C.J., and Escamilla, R.F. (1995). Kinetics of baseball pitching with implications about injury mechanism. American Journal of Sports Medicine 23, 233-239.
© Young-Hoo Kwon, 1998-