Isokinetic Dynamometry for External and Internal Rotation Shoulder Strength in Youth Athletes: A Scoping Review

Abstract

Background

Accurately measuring shoulder strength in overhead athletes is critical, as sufficient strength is essential for safe and sustained performance during repetitive athletic movements. Isokinetic dynamometry (ID) offers dynamic strength assessments that surpass the capabilities of static methods, such as manual muscle testing and handheld dynamometry. The dynamic assessment provided by ID may enhance upper extremity evaluation, aiding in the prediction of injury risk and the determination of return-to-sport criteria for overhead athletes.

Purpose

The purpose of this review was to examine the existing literature concerning the application of isokinetic shoulder strength testing in rehabilitation and clinical decision-making processes among youth athletes who perform repetitive overhead activities.

Study Design

Scoping review

Methods

A comprehensive literature search was conducted using PubMed and EBSCO Host databases, covering publications from 2000-2024. Search terms included “isokinetic dynamometry,” “shoulder,” and “youth athlete.” Inclusion criteria focused on youth athletes (<18 years) engaged in overhead sports, excluding those with neurological conditions or those designated as college or professional athletes. The PRISMA-ScR guidelines were followed.

Results

A total of 23 articles met the inclusion criteria. Volleyball and swimming were the most studied sports, with the most common testing position being the seated 90/90 position. Variations in testing speeds and outcome measures, such as peak torque and external rotation (ER) ratios, were identified.

Conclusions

Isokinetic dynamometry is a valuable tool for assessing shoulder strength in youth overhead athletes. It provides critical insights into muscle strength dynamics, aiding in injury prevention and rehabilitation. Further research is needed to optimize strength assessment protocols and enhance clinical decision-making for safe return-to-sport practices.

BACKGROUND

Within a larger framework in the decision process for return to sports, recommendations for overhead athletes are reported considering normalized shoulder strength and scapular mechanics, range of motion (ROM), and successful completion of a plyometric program.¹ However, many clinicians find it challenging to accurately measure shoulder strength due to limitations in equipment and expertise.

Shoulder strength assessment generally consists of isometric testing in neutral positions via manual muscle testing (MMT), handheld dynamometry (HHD), or isokinetic dynamometry (ID). Normative values for external rotation (ER) and internal rotation (IR) strength using HHD has been obtained in healthy adults,^2,3 and physically active collegiate male and females.⁴ Studies indicate that HHD provides an assessment of shoulder IR/ER strength assessment that is highly correlated to assessments performed with ID.⁵ The concern, however, with MMT and HHD testing is that reliability of these methods is highly dependent on clinician size and strength.⁶ Additionally, because the testing position is static, HHD lacks the assessment of dynamic components associated with upper extremity movements required during overhead sports.⁷ In addition to torque output as a measure of strength, ID can capture endurance deficits and strength ratio imbalances which are predictors of shoulder injury.⁸ Ellenbecker demonstrated a significant difference in bilateral IR and ER strength measures between MMT grades and isokinetic dynamometry, widely recognized as the gold standard for measuring muscle strength and muscle endurance.⁹

Understanding the balance between the strength of agonist and antagonist muscles is crucial in evaluating and rehabilitating overhead athletes due to the intricate muscular activation patterns necessary for stability of the glenohumeral joint.⁷ One strategy for strength testing of the shoulder in multiple positions throughout the range of motion is ID. Multiple authors have reported normative isokinetic strength assessment data for athletes involved in judo,¹⁰ tennis,^11,12 badminton,^13,14 and volleyball.¹⁵ In baseball, isokinetic strength testing has been performed in high school athletes,¹⁶ collegiate athletes,^17,18 and professional athletes,¹⁹ yet limited information is available regarding normative values of isokinetic shoulder strength in youth athletes.

Gaps remain in the literature, particularly regarding the establishment of normative values for younger, skeletally immature athletes and how these values differ across various stages of development. The purpose of this review was to examine the existing literature concerning the application of isokinetic shoulder strength testing in rehabilitation and clinical decision-making processes among youth athletes who perform repetitive overhead activities.

METHODS

This literature review was conducted in accordance with the recommendations of the “Preferred Reporting Items for Systematic Reviews and Meta Analysis extension for Scoping Reviews (PRISMA-ScR).²⁰

Search Strategy

The literature review was performed with the databases PubMed, and EBSCO Host, which provides a range of databases, e-journals and e-books. Search terms included in the search strategies included phrases such as: “isokinetic dynamometry AND shoulder,” “isokinetic dynamometry AND shoulder AND youth athlete.” (See Appendix A for full search criteria). The search was limited to full-text available human research published between 2000-2024, including participants ages <18 years associated with an organized, overhead sport, and published in the English language. Exclusion criteria were articles pertaining to any neurological condition, and athletes designated as collegiate or professional.

RESULTS

Electronic database searches identified 475 total studies. The scoping review included 23 studies^21–43 after duplicate removal, title/abstract screen, and full text review.

The PRISMA diagram (Figure 1) outlines the search process in its entirety.

Figure 1. PRISMA flow diagram showing the literature search, screening, and eligibility results.

Isokinetic Dynamometry (ID) in Youth Repetitive Overhead Athletes

Of the included studies, volleyball was the most common overhead sport where ID was used for assessment.^{27,29–31,38,43} The second most common sport was swimming^21–25 followed by handball,^{33,34,39,41,43} baseball,^26,28,32,37 tennis,^40,42,43 weightlifting,³⁶ and basketball.³¹ In the articles that fit the inclusion criteria, two pertained to symptomatic patients,^31,35 whereas the remaining articles looked at isokinetic normative values, or the effects of repetitive sport specific movements. Table 1 describes the use of ID in various sports along with the variety of parameters used for testing ID.

Table 1. Use of Isokinetic Dynamometry (ID) in Various Sports and ID Equipment Parameters.

Author	Sample / (Mean Age)	Sport	Type of Dynamometer	Position
Batalha N21	25 males (13.28 yrs)	Swimming	Biodex System-3	Seated 90/90 position
Batalha NM24	36 males (14.25 yrs)	Swimming	Biodex System-3	Seated 90/90 position
Batalha N23	40 males (14.65 yrs)	Swimming	Biodex System-3	Seated 90/90 position
Batalha NM25	40 males (14.65 yrs)	Swimming	Biodex System-3	Seated 90/90
Batalha N22	49 males (14.48 yrs)	Swimming	Biodex System-3	Seated 90/90
Clements AS26	18 males (13-16 yrs)	Baseball	Biodex Multi-Joint Dynamometer	Seated 90/90
de Lira CAB27	28 males (15.5 yrs)	Volleyball	Biodex System-3	Seated (angle not specified)
Dupuis, C28	10 males (15.87 yrs)	Baseball	Kin-Com	Seated 90/90
Duzgun I29	24 athletes (14.5 yrs)	Volleyball	Isomed 2000 Dynamometer	Seated 90/90 position
Eshghi S30	32 males (17.5 yrs)	Volleyball	Biodex System-4	Seated position 90 degrees abduction / 30 degrees shoulder flexion
Guney H31	65 athletes (16.1 yrs)	Volleyball & Basketball	Isomed2000 D&R	Seated 90/90
Lee DR32	23 males (18.2 yrs)	Baseball	Biodex Isokinetic Machine	Seated 90/90 position
Mascarin NC33	26 females (15.3 yrs)	Handball	Biodex Isokinetic Machine	Not Discussed
Mascarin NC34	39 females (15.3 yrs)	Handball	Biodex Isokinetic Dynamometer	Seated 90/90
Mickevičius M35	14 boys (11-12 yrs)	Baseball	Biodex System-3	Seated 90/90
Mohamed IW36	16 males (14.8 yrs)	Weightlifting	Multi Joint System-3 Pro	Seated 45/90 position Repeat testing done @ 90/90
Mulligan IJ37	39 males (15.36 yrs)	Baseball	Kin-Com Dynamometer	Seated: 90 degrees elbow flexion / 30 degrees abduction in scapular plane
Pawlik D38	12 females (12-13 yrs)	Volleyball	Multi-joint 4 Dynamometer Biodex	Seated 90/90
Pontaga I39	14 males (14.6 yrs)	Handball	REV-9000	Seated 90/90 position
Saccol MF40	40 athletes (14 yrs)	Tennis	Cybex-6000	Supine 90/90
van Cingel R41	40 females (17.6 yrs)	Handball	2-Humac Norm dynamometer	Supine 90/90
Vodička T42	20 males (13.23 yrs)	Tennis	Human Norm CSMI	Supine 90/90 position
Yildiz Y43	40 males (17.2 yrs)	Volleyball, Handball, Tennis	Cybex Norm	Supine 90/90

Isokinetic Testing Position

Testing positions throughout studies varied, but trends are seen when testing the upper extremity (UE) of youth athletes with ID as illustrated in Table 1. The most utilized testing position for shoulder IR/ER is the seated position with the shoulder abducted to 90 degrees and the elbow flexed to 90 degrees, commonly known as the 90/90 position. It is postulated that this position, more than the arm at a neutral position, specifically addresses muscle function often required for an overhead athlete.⁴⁴ The use of the 90/90 position also demonstrates strong intraclass coefficient for test/re-test reliability.⁴⁵ Ellenbecker has discussed using a modified position of 30 degrees shoulder abduction, 30 degrees shoulder forward flexion, and 30 degrees diagonal tilt of the dynamometer to determine tolerance of the UE strength test prior to using the ID at a 90/90 position.⁷ Additional alterations to the 90/90 positions used in research are the seated 90/30 (90 degrees elbow flexion and 30 degrees shoulder abduction),³⁷ seated 90/45 (90 degrees elbow flexion and 45 degrees shoulder abduction),³⁶ seated 90/30 with 90 degrees of shoulder abduction and 30 degrees of shoulder flexion,³⁰ and the 90/90 supine position.^40–43

Speeds Used for Testing

Table 2 provides information regarding the variations in the speeds used for testing youth athletes. Most studies assessed participants at multiple speeds for different outcome measures or for different sports-related contexts. Common speeds for assessment of isokinetic peak torque both concentrically and eccentrically include 60 deg/sec,^{22–25,27,29,30,32–34,38–41} 90 deg/sec,^{28,31,37,39,43} 120 deg/sec,^26,36,41 180 deg/sec,^{21–25,28–30,32,37,38,40,42} 240 deg/sec,^{27,33,34,36,39} 300 deg/sec,^38,42 and 360 deg/sec.³⁶

Table 2. Isokinetic Dynamometry Testing Speed and Other Objective Measures.

Author	Testing Speed	Objective Measures
Batalha N21	60 deg/sec 180 deg/sec	PT IR PT ER ER:IR ratio
Batalha N24	60 deg/sec 180 deg/sec	PT ER/IR ratio
Batalha N23	60 deg/sec 180 deg/sec	Max torque ER/IR ratio - norm is 66-75%
Batalha N25	60 deg/sec 180 deg/sec	PT ER/IR ratios
Batalha N22	60 deg/sec 180 deg/sec	PT ER/IR ratio
Clements AS26	120 deg/sec	PT/BW
de Lira CAB27	60 deg/sec 240 deg/sec	Absolute PT Relative PT (divided by BW) Absolute total work Relative total work (divided by BW) Conventional strength ratio Functional strength ratio
Dupuis, C28	90 deg/sec 180 deg/sec	PT IR con / ER ecc (performance ratio) ER con / IR ecc (cocking ratio)
Duzgun I29	60 deg/sec 180 deg/sec	PT / BW Total work
Eshghi S30	60 deg/sec 180 deg/sec	FDR EReccn:IRcon
Guney H31	90 deg/sec	ER:IR Max eccentric ER torque / Max concentric IR torque
Lee D32	60 deg/sec 180 deg/sec	PT
Mascarin N33	60 deg/sec 240 deg/sec	PT and total work
Mascarin N34	60 deg/sec 240 deg/sec	at 60 deg/sec: IR PT CR (ER con / IR con) at 240 deg/sec IR PT IR avg power FR (ER ecc / IR con) ER PT was measured at both speeds
Mickevičius M35	120 deg/sec	PT ER:IR ratio
Mohamed IW36	120 deg/sec 240 deg/sec 360 deg/sec	Relative PT Time to PT
Mulligan I37	90 deg/sec 180 deg/sec	PT and Total work for both movements at both speeds
Pawlik D38	60 deg/sec 180 deg/sec 300 deg/sec	Concentric IR PT
Pontaga I39	60 deg/sec 90 deg/sec 240 deg/sec	PT Avg power
Saccol M40	60 deg/sec 180 deg/sec	PT / BW Total work / BW ER/IR ratio
Vodička T42	180 deg/sec 300 deg/sec	Peak torque ER/IR ratio
Yildiz Y43	90 deg/sec	ER ecc / IR con ER con / IR ecc Both in the late cocking phase of overhead motion and the deceleration phase

Key: PT= peak torque, ER= external rotation, IR= internal rotation, BW= body weight, CR= cocking ratio, FDR= functional deceleration ratio, FR= functional ratio, DCR= dynamic control ratio, CON = concentric, ECC = eccentric

Objective Measures from Isokinetic Dynamometry

Peak Torque

In many studies pertaining to the use of ID in youth athletes, peak torque, normalized via body weight, was the primary objective measure.^26,27,29,40 Peak torque is then used to compute measures of bilateral symmetry, using left and right values, and unilateral strength ratios using shoulder external and internal rotation strength values. Research indicates that repetitive sports-related movements lead to discernible discrepancies in peak torque between shoulder external rotation (ER) and internal rotation (IR) strength when comparing the dominant and non-dominant arms in athletes. Repetitive motions during youth sports such as volleyball,^27,38,43 swimming,^21–25 and tennis⁴⁰ result in increased IR peak torque in the dominant arm of the athlete. Variations exist among age groups within the same sport as evidenced by youth handball athletes demonstrating higher ER and IR peak torque in the dominant arm compared to non-dominant arm in athletes with a mean age of 17.6 years.⁴¹ Conversely, younger players (mean age of 14.6 years) showed no statistically significant difference between ER and IR peak torque.³⁹ In a study of healthy teenage baseball players, Dupuis found greater concentric ER strength in the dominant arm at speeds of 90°/sec and 180°/sec.²⁸ In contrast, Mickevičius observed decreased ER strength among younger baseball players (average age 11.6 years) who had a history of shoulder pain.³⁵ It has been hypothesized that differences in peak torque output results in higher likelihood of overuse type injuries in youth athletes.^24,27,41

ER:IR Ratio

Eight studies^{21–25,35,40,42} used concentric external rotation to concentric internal rotation strength ratios during for their objective measurements whereas six studies^{28,30,31,34,41,43} investigated various forms of eccentric to concentric ratios between ER and IR.

Comparison of shoulder agonist and antagonist muscle groups helps identify specific muscular imbalances.⁷ Variations in the dominant arm’s external rotation to internal rotation (ER:IR) ratios are frequently observed, particularly in cases where imbalances occur in the shoulder’s agonist-antagonist muscle relationships. These imbalances can signify weaknesses that heighten athletes’ vulnerability to injury.^24,40 Ideal ER:IR has been defined as 66%, which remains constant throughout the velocity spectrum⁴⁶; however, this ratio is described in skeletally mature athletes.⁷

A common theme in youth overhead athletes is decreased ER:IR ratio in the dominant arm^{21–24,34,40–42} due to the theorized increase in IR peak torque because of sport specific repetitive motion. Although these imbalances do not result in changes in athletic performance, there is fear that if not addressed in the younger athlete they can result in overuse type injury.⁴⁰

There are unique variations of the ER:IR used in ID assessment of youth athletes as evidenced by discussion of ER:IR acceleration and ER:IR deceleration described by Yildiz in youth volleyball players.⁴³ The concept eccentric to concentric ER:IR is defined as the ratio of balance for muscle activity where the shoulder medial rotators eccentrically control external rotation, while lateral rotators eccentrically control internal rotation, ensuring optimal shoulder function.⁴⁷ This explanation is similar to Dupuis’ definition of Dynamic Control Ratio (DCR) in youth baseball players.²⁸ Both Yildiz and Dupuis investigate the eccentric and concentric relationship of the ER:IR ratio of the shoulder. In each case, the ER:IR ratio into the acceleration phase (concentric ER to eccentric IR) was observed to be higher in the dominant arm,^28,43 whereas the ER:IR ratio into the deceleration phase (eccentric ER to concentric IR) is lower in the dominant arm of the youth volleyball athlete,⁴³ but no difference existed in the youth baseball athlete.²⁸

DISCUSSION

This scoping review highlights the use of isokinetic dynamometry of the youth overhead athlete to obtain multiple objective measures. In adolescent overhead athletes, the repetitive nature of sport-specific movements imposes unique demands on the dynamic muscular control of the glenohumeral joint. This demand is highlighted by fluctuations in the external rotation to internal rotation peak torque ratio, a phenomenon hypothesized to significantly impact the vulnerability of the shoulder complex to injury.^21–25 Various studies support this hypothesis, including symptomatic athletes³⁵ and those with glenohumeral internal rotation deficits.³¹

In sports like baseball, dynamic muscular control of the glenohumeral joint is essential, particularly given the substantial stress experienced across the upper extremity joints during pitching. The angular velocities of shoulder internal rotation and elbow extension can range from 1,000 deg/sec to 7,200 deg/sec in youth and collegiate pitchers, respectively, from maximum external rotation through the acceleration phase.^48,49 While ID testing velocities of 240 deg/sec do not fully replicate these speeds, they still yield valuable data on shoulder musculature function at higher isokinetic speeds.⁵⁰ Table 3 provides a detailed breakdown of these different sport-specific findings.

Table 3. Sports Specific Isokinetic Dynamometry (ID) Strength Findings.

SPORT	STUDY	ID SPORT SPECIFIC STRENGTH OUTCOMES
Volleyball	De Lira27	Higher IR peak torque in dominant arm
	Duzgun29	ER strength increased with weighted jump rope in dominant arm
	Esghi30	Functional Deceleration Ratio (eccentric to concentric strength of IR and ER) in dominant arm Increased ER:IR ratio after strengthening program
	Pawlik38	Higher IR peak torque in dominant arm
Swimming	Batalha21	Higher IR peak torque in dominant arm
	Batalha22	Decreased ER:IR ratio in dominant arm in swimmers vs control Higher IR peak torque in swimmer dominant arm
	Batalha23	Decreased ER:IR ratio in dominant arm for swim only group
	Batalha24	Decreased ER:IR in dominant arm for swimmers- Higher IR peak torque in dominant arm for swimmers
	Batalha25	Decreased ER:IR in dominant arm for swimmers Training improved ER peak torque in dominant arm
Baseball	Clements26	Peak torque of dominant arm not associated with throwing velocity
	Dupuis28	Higher ER peak torque in dominant arm ER:IR lower in dominant arm ER concentric :IR eccentric in cocking phase was higher in dominant arm ER eccentric:IR concentric during deceleration was no different side to side
	Lee32	IR and ER peak torque increased with CKC strengthening in dominant arm
	Mickevičius35	ER eccentric strength was weaker in baseball players dominant arm Decreased ER:IR in dominant arm for baseball players with pain IR eccentric: IR concentric – lower in dominant arm ER eccentric: IR concentric – lower in dominant arm ER concentric: IR eccentric – lower in dominant arm
	Mulligan37	Decreased ER:IR ratio in the dominant arm No significant difference between eccentric and concentric peak torque between dominant vs non-dominant arm
Handball	Mascarin33	ER:IR lower in dominant arm ER eccentric : IR concentric was lower in dominant arm ER:IR ratio unchanged in dominant arm with strength training
	Mascarin34	All participants demonstrated weak IR of the dominant arm ER:IR – 82.8 in experimental group vs 77.6 in control group Strength testing did not result in improvement in IR strength Strength training did not improve strength ratios Strength training improved average power
	Pontaga39	No statistically significant differences between dominant and non-dominant arm peak torque Average power of IR was higher in dominant arm
	Van Cingel41	ER:IR lower in dominant arm of handball players Higher ER and IR peak torque in dominant arm
Tennis	Saccol40	Decreased ER:IR on dominant arm Higher ER strength in youth male tennis players Higher IR strength on dominant arm for both youth male and female athletes
	Vodička42	Decreased ER:IR ratio on dominant arm Higher ER strength in dominant arm Higher IR strength on dominant side
Weightlifting	Mohamed36	ER:IR ratio of dominant arm improved with isokinetic strength program ER:IR ratio of dominant arm improved with isotonic strength program IR and ER peak torque of dominant arm increased in both training groups

Key: ER= external rotation, IR= internal rotation

GAPS IN THE LITERATURE

ID studies of youth athletes skew towards specific sports such as volleyball and swimming. Limited information is available regarding the use of ID for assessment of athletes participating in youth baseball. In reviewing the ages of participants recruited for ID articles in youth athletes, the youngest subjects were 11 years old. However, pitching and repetitive throwing in baseball often begins as early as 6-7 years of age. Studies included in a systematic review suggest that starting repetitive pitching at 10 years of age or younger increases the risk of upper extremity-related injury.⁵¹ In most sports, children are playing and specializing in sport positions at a much younger age than captured in this review, but changes in shoulder IR strength between dominant and non-dominant arm has been seen in youth throwers under the age of 10 using HHD.⁵² Understanding dynamic strength values and muscular balance relationships with measurements from ID in skeletally immature athletes and the changes that occur with repetitive overhead motions can assist in developing injury prevention, rehabilitation programs, and return to sport decision making for youth throwing athletes.

LIMITATIONS

Only studies in English languages were included in the review and the range of articles searched were between 2000-2024. Also, no quality assessment of the studies was performed, which may limit the impact of the findings.

CONCLUSION

Isokinetic dynamometry as an assessment in youth overhead athletes provides insights into shoulder strength (torque) and strength ratios. There is variety in the assessment parameters utilized in the included studies of youth athletes in various sports. By precisely evaluating isokinetic strength, clinicians can gauge the shoulder’s dynamic control, facilitating the tailored design of training programs. Isokinetic assessments allow for detailed analysis of power and the balance between agonist and antagonist muscle groups during movement, enabling the identification of specific strength impairments and guiding targeted interventions to enhance upper extremity loading tolerance for repetitive overhead activities.

Conflict of Interest

The authors declare that they have no conflicts of interest related to the content of this manuscript.