In comparison with the gold standard instrument, the measurement of shoulder muscle strength with HHD in healthy volunteers demonstrated moderate to excellent concurrent validity and accuracy to diagnose muscle imbalances. Such results are encouraging and suggest the use of HDD as a simple and objective to quantify the peak torque of the muscles in clinical practice.
As observed, HHD showed adequate diagnostic accuracy compared to ID, provided excellent sensitivity to diagnose muscle imbalances between contralateral muscle groups and moderate to excellent specificity to identify individuals without muscle deficit. The diagnosis agreement rate between the two strength assessment tools was considered to be moderate to excellent.
The prevalence of muscle imbalance observed in the sample investigated was relatively low, with a percentage lower than 25% for all movements of the assessed shoulder. Such prevalence has not been previously described in the literature. The number of studies that validated HHD using the ID is limited. When such validation is related to the upper limbs muscles, the evidence is even scarcer, being restricted to one plane movements21–24. Therefore, this is a pioneer study to investigate the concurrent validity for the main shoulder muscle groups in healthy individuals using the ID.
Magnusson et al24 in a study with 9 healthy individuals with a mean age similar to the present study found concurrent validation to be excellent r=0.86 for shoulder abductors, similar to the present results r=0.83. The difference between the measurement protocols is restricted to the fact that the study by Magnusson et al25 the measurement of HHD was performed in a seated position, while in the present study all measurements were performed in the supine position. Karabay et al13 in a study with 25 healthy individuals found concurrent validation of 0.76 for eccentric isotonic abduction of the shoulder with an angular velocity of 30°/s. Such value was lower than the validation found in the present study.
Johansson et al24 developed a study with 25 healthy individuals where Pearson's correlation between ID with angular velocity 30°/s and HHD were investigated. As observed in the study, coefficients were respectively 0.70 and 0.78, considering that ID was performed for eccentric muscle contraction of external rotation of the shoulder. The presented values were higher than the concurrent validation of 0.66 noticed in the present study. This difference may be related to the fact that in this study the eccentric isometric and non-isotonic contraction were assessed with an angular velocity of 60 °/s.
Anthropometric data were similar between studies, with both having females as the majority of their sample. A study developed by Chamorro et al12 with 24 healthy individuals, investigated the concurrent validity of an electromechanical pulley dynamometer, finding ICC of 0.93 with IC95 (0.82-0.97) for internal rotation and 0.84 with IC95 (0.60-0.93) for external rotation compared to the measurement with ID. In this study, the authors used a contraction time of 6 seconds of isometry, against 3 seconds in the current study and did not describe the angular speed at which the ID was assessed.
In a recent study with 14 athletes, Romero-Franco et al23. described a measurement protocol for movements of the upper limbs in a seated position using a digital dynamometer with a tension cell. Study findings showed a Pearson correlation> 0.90 for the shoulder movements. Such values were also higher than the results found in the present study, however is important to highlight the study by Romero-Franco et al22 assessed entirety physically active individuals, which suggest greater physical fitness in comparison to a sample of healthy but sedentary volunteers of the present study. Furthermore, other limitations such as the use of dynamometer with a force cell and absence of angular velocity description when performing ID assessment precluded any comparison between the studies and the protocols adopted.
The peak torque assessments obtained with HHD and ID showed considerable differences for all shoulder movements evaluated possibly due to different types of contractions produced as stated by Corvino et al26 in a cross-sectional study with 14 healthy volunteers that noticed the rate of strength development was different according to the type and speed of contraction produced. The authors26 demonstrated that the lower the velocity and amplitude of muscle contraction, the greater the torque generated. This statement is based on the greater recruitment of muscle motor units in isometric contractions and with smaller angular velocities attributed. Such physiological behavior is analogous to that observed in the present study, in which a higher mean torque peak was found during an HHD compared to an isokinetic evaluation. Such difference between the means does not reflect the relevant clinical difference, given the lack of specific reference values in the literature for each type of contraction evaluated.
The present study is a pioneer to establish the accuracy of HHD for shoulder movements in healthy individuals using a protocol suggested by Vasconcelos et al16. In this study, 40 individuals underwent reconstruction of the anterior cruciate ligament, was found that sensitivity was between 50 and 88% for the identification of the imbalance and specificity 63 to 100% for the identification of muscle balance. The intervals are similar to those found for the accuracy of HHD for diagnostic assessment of shoulder muscles for healthy individuals in the present study.
Moreover, this study is the first to investigate concurrent validation and establish the diagnostic accuracy of HHD compared to the gold standard for all shoulder movements in healthy individuals. Additionally, it has been shown that HHD may be considered a useful tool to evaluate muscle imbalance and enhance early identification and treatment of this muscle deficiency. The main limitations of the study were the different positions required in the assessments carried out (ID and HHD) and the mean age of sample included.
In conclusion, study findings suggested HHD is a useful tool for measuring muscle performance in healthy individuals given the considerable concurrent validity of the instrument compared to a gold standard method. Further, HDD also demonstred acceptable accuracy to identifying muscle imbalance for the various shoulder movements and sufficient agreement rate. Results suggest the implementation of HDD as a low-cost, straightforward and valid resource to establish shoulder muscle performance in clinical practice.