Accuracy of Two Methods in Estimating Target Muscle Force During Shoulder Submaximal Isometric Contractions.

Background: Submaximal isometric exercises are used for pain control and neuromuscular facilitation. Typically, an ipsilateral maximal voluntary isometric contraction (MVIC) is used as a reference; however, when this is not clinically feasible, an alternative must be considered. Two options are (1) the no reference (NR) method (submaximal contraction at a self-perceived effort level without reference) and (2) the reciprocal reference (RR) method (MVIC on the contralateral side serves as a reference for a submaximal effort-level on the ipsilateral side). No research evidence exists as to which alternative method is more accurate at the shoulder.

Purpose: To determine the accuracy of the NR and RR methods in estimating target muscle force during shoulder ER and IR submaximal isometric contractions among healthy adults.

Study design: Observational cross-sectional.

Methods: Isometric shoulder force was measured via a hand-held dynamometer on 48 healthy participants (36 females and 12 males) mean age of 27.4 ±1.6 years. Both methods (NR and RR), direction of force (IR and ER), and starting test-side (right or left) were randomized. RR testing involved a contralateral MVIC (reference) prior to a 50% submaximal contraction. NR testing entailed a 50% submaximal contraction with no prior reference MVIC.

Results: Actual submaximal efforts were compared to MVIC-based estimated submaximal efforts. Significant moderate - good correlations existed for both the RR (r = 0.691) and NR (r = 0.620) methods, regardless of test-side or shoulder motion. Significant moderate - good correlations were found between both methods for both ER [RR (r = 0.717) and NR (r = 0.614)] and IR [RR (r = 0.669) and NR (r = 0.628)].

Conclusion: Both methods had moderate - good accuracy levels and were not influenced by the test side or direction of force. Either method (RR or NR) can be equally useful for shoulder isometric exercise prescription when an ipsilateral reference cannot be determined.

Level of evidence: 3.