Letter to the Editor: Clinical impact of normative data in ankle strength evaluation

Abstract

This letter offers a critical perspective on the recent work by da Fonseca et al, a study that finally provides a much-needed set of normative isokinetic data for the ankle in healthy, non-athletic adults. The authors meticulously map out gender-specific torque profiles and agonist-antagonist balances, but their most compelling finding, in my view, is the empirical validation of a > 90% symmetry threshold for both the limb symmetry index and a novel muscular deficiency index. This result provides a solid, evidence-based anchor for ankle rehabilitation, a field that has long operated in the shadow of its more thoroughly researched counterpart, the knee. I argue that this study’s chief contribution is its clear challenge to the long-standing clinical reliance on the contralateral limb as a recovery benchmark - a practice complicated by the presence of bilateral deficits. By avoiding repeated emphasis on this issue, the letter now highlights this point more succinctly. While acknowledging the study’s cross-sectional nature as a limitation, I contend that these normative values represent a fundamental step toward more objective, reliable, and ultimately safer return-to-activity decisions.

Key Words: Ankle isokinetic; Normative data; Limb symmetry index; Return-to-sport criteria; Neuromuscular deficits

Core Tip: The work by da Fonseca et al provides a crucial, long-awaited normative dataset for isokinetic ankle strength. What’s particularly striking is their finding that healthy ankles naturally exhibit over 90% bilateral symmetry, as measured by both limb symmetry index and their new muscular deficiency index. This empirically grounds the > 90% threshold, a standard borrowed from knee rehabilitation, as a valid and achievable goal for the ankle. The clinical takeaway is clear: We now have objective, gender-specific benchmarks that allow us to move beyond the flawed assumption that the uninjured limb is a perfect ‘control’. This research equips clinicians with the tools to make more precise and defensible decisions about rehabilitation progress and safe return to activity.

TO THE EDITOR

In the world of orthopedic and sports rehabilitation, our clinical decisions are only as good as the evidence that underpins them. For years, we’ve leaned heavily on isokinetic dynamometry as our most trusted tool for getting an objective read on muscle function[1,2]. It gives us the hard numbers-peak torque, power, muscular balance-that transform subjective assessments into quantifiable data. When it comes to the knee, this approach has led to a rich body of research, culminating in widely accepted normative values and the near-universal adoption of the limb symmetry index (LSI)[3-5]. The > 90% LSI has become a clinical mantra, a go-to benchmark for clearing an athlete to return to the field after injury[6].

But what about the ankle? Here, the story has been quite different. Despite its central role in nearly every form of locomotion and its vulnerability to injury, the ankle has remained something of a “neglected joint” in the realm of isokinetic research. We know that chronic ankle instability is fundamentally a problem of neuromuscular control, tied to imbalances between the muscles that turn the foot out (evertors) and in (invertors), and often accompanied by a loss of power in pushing off (plantar flexion)[7-9]. Yet, we’ve lacked the very thing we need most: A robust, normative database to tell us what a “healthy” ankle actually looks like in terms of strength. This deficiency has created a significant methodological challenge in clinical practice, often necessitating the reliance on the uninjured, contralateral limb to serve as a proxy for the patient’s pre-injury baseline state. Although pragmatic, this approach is fundamentally compromised, particularly in light of recent evidence demonstrating bilateral, or ‘crossover’, neuromuscular deficits following unilateral injury, which render the seemingly ‘good’ limb an unreliable measurement standard[10-12]. It is precisely this long-standing gap that makes the recent study by da Fonseca et al[13], “Normative values of ankle strength and its importance for rehabilitation and return to activity”, so refreshing and clinically significant. The authors have rolled up their sleeves and done the foundational work of charting the isokinetic landscape of the healthy ankle. This communication presents the author’s critical reflection on the study, providing an analysis of its core findings, the implications for professional practice, and a discussion of future research directions.

COMPARISON WITH EXISTING LITERATURE

The findings from da Fonseca et al[13] do not so much overturn existing knowledge as they crystallize it. The idea that chronic ankle instability is linked to weaker evertors and skewed strength ratios is not new; it’s a theme that has recurred in smaller studies for years[14,15]. What this new study brings to the table is scale and purpose. By assessing 200 healthy ankles with the explicit goal of establishing norms, their reported ratios-an evertor/invertor balance of around 88.8% and a dorsiflexor/plantar flexor balance of 36.1%-move from being interesting observations to becoming actionable clinical targets.

But the real breakthrough, in my opinion, is their empirical validation of the > 90% symmetry threshold for the ankle. For too long, we’ve simply extrapolated this number from the knee literature, assuming it would hold true for the ankle without solid evidence[16,17]. The authors demonstrate that in a healthy, uninjured population, both the traditional LSI and their novel, more comprehensive muscular deficiency index (MDI) consistently hover around this 90% mark. This is a powerful finding. It transforms an educated guess into an evidence-based standard, giving us a defensible and achievable goal for our patients recovering from ankle injuries. To further situate these findings within the broader literature, it is worth noting that recent research has also highlighted bilateral deficits and altered neuromuscular patterns in individuals with ankle instability, underscoring the importance of reliable normative benchmarks. For example, Wong et al[18] demonstrated that chronic ankle instability is associated with persistent bilateral torque deficits during inversion - eversion tasks, even in the absence of acute symptoms. Similarly, Gerber et al[19] reported that normative strength ratios vary substantially across athletic and non-athletic groups, suggesting that population-specific baselines are crucial when interpreting limb symmetry values. Integrating these studies reinforces the importance of the normative dataset provided by da Fonseca et al[13] and positions their findings as a much-needed anchor within an otherwise fragmented body of evidence. This work also serves as a direct and necessary challenge to what call the “good limb fallacy” - the routine clinical practice of using the uninjured side as the sole benchmark for recovery.

The literature on anterior cruciate ligament injuries, for example, has made it painfully clear that the contralateral limb is often not a pristine, pre-injury representation; it frequently exhibits its own neuromuscular deficits[20-22]. Da Fonseca et al[13] provide the missing piece of the puzzle: The true baseline. Their data allows clinicians to reframe a central question. Instead of asking “is the injured ankle as good as the other one?”, we can now ask a clearer and more objective question: “Does this patient’s ankle strength fall within the established normative range for their gender?”. This simplified and consistent phrasing improves clarity and aligns the terminology more closely with standard clinical practice. This is not a subtle distinction; it is a fundamental move toward a more rigorous and patient-safe standard of care. For clarity, the key normative values and symmetry thresholds reported by da Fonseca et al[13] are summarized in Table 1, providing a concise reference for clinicians interpreting ankle strength benchmarks.

Table 1 Normative ankle strength values and symmetry thresholds.

Parameter	Normative value (healthy adults)	Clinical interpretation
Peak torque - evertors/inverters	Approximately 88.8% balance	Indicates near-symmetrical agonist-antagonist strength
Peak torque - dorsiflexors/plantar flexors	Approximately 36.1% balance	Reflects expected ratio of ankle mobility vs power muscles
Limb symmetry index	≥ 90%	Empirically validated threshold for return-to-activity considerations
Muscular deficiency index	≥ 90% composite bilateral symmetry	Provides multi-metric bilateral assessment across torque/power/work
Gender differences	Men > women (all strength metrics)	Necessitates gender-specific normative charts

CRITICAL APPRAISAL OF THE STUDY

Methodologically, the study by da Fonseca et al[13] is straightforward and robust. They chose a cross-sectional design, which is perfectly appropriate for their goal of establishing normative data at a specific point in time. Their use of the Biodex system, a gold-standard tool, and adherence to established testing protocols lends credibility to their measurements. The participant pool-healthy, physically active adults free from lower limb complaints-was well-defined and fit for purpose. However, the generalizability of these findings may be limited, as the sample consisted of healthy, physically active adults. The > 90% symmetry threshold may differ in elderly populations, competitive athletes, or individuals with asymmetric neuromuscular demands. Additionally, participant selection may introduce bias that limits external applicability.

The statistical approach was thorough. The authors went beyond simple descriptives, using appropriate non-parametric tests to compare limbs and correlation analyses to probe for relationships with demographic variables. One of the more intriguing results was the lack of any significant correlation between peak torque and either age or body mass index (BMI). The absence of an age effect is likely a reflection of their “healthy and active” cohort, a group that may be successfully staving off typical age-related strength decline. The null finding for BMI is also telling. As the authors wisely point out, BMI is a blunt instrument, incapable of distinguishing between lean muscle mass and adipose tissue[23]. It’s a clear signal that future work in this area must move toward normalizing strength values against more precise measures of body composition, like lean body mass.

I found the introduction of the MDI to be a particularly thoughtful innovation. Where the LSI offers a simple side-to-side comparison for a single movement, the MDI provides a composite score that captures the overall bilateral balance across torque, power, and work. This feels like a more functionally holistic way to think about symmetry and could be a valuable addition to the clinician’s toolkit. However, the study would benefit from greater clarification on how the MDI could be operationalized in real-world practice. For instance, it remains unclear whether clinicians will require specialized software for automated computation or whether the index can be feasibly calculated using standard dynamometer outputs. Additionally, practical considerations such as the time burden of multi-metric testing, the need for clinician training, and the comparative simplicity of LSI relative to MDI should be acknowledged. Discussing how the MDI performs against existing indices-whether it offers superior sensitivity to bilateral deficits or predicts functional outcomes more accurately-would further strengthen its clinical relevance. Overall, the introduction of this index represents a meaningful conceptual expansion, but outlining its practical adoption pathway would help transition it from an appealing idea to a routinely usable tool.

The authors’ interpretation of their data is grounded and clinically oriented. They correctly highlight the pronounced strength differences between men and women, reinforcing that gender-specific reference charts are not just helpful, but essential. Their observation that the non-dominant limb was often slightly stronger is a fascinating nuance. My own interpretation is that this might reflect the different roles our limbs play; the dominant limb is often our ‘skill’ limb, optimized for precision and coordination (like kicking a ball, their criterion for dominance), while the non-dominant limb serves as the ‘power’ or ‘stabilizing’ limb, responsible for generating force and maintaining balance. Regardless of the reason, the key takeaway, as the authors note, is that these differences were small enough to fall within the clinically accepted 10% symmetry band, suggesting they shouldn’t derail our overall rehabilitation goals. The central conclusion-that we now have gender-specific norms and an empirically supported > 90% symmetry target for the ankle-is the study’s powerful and resonant message.

STRENGTHS AND LIMITATIONS OF THE STUDY

The study’s most significant strength is that it bravely tackles a long-neglected area, providing the foundational normative data for the ankle that clinicians have desperately needed. It does so use a robust, gold-standard methodology and offers a clear, empirically supported validation for using the > 90% symmetry threshold - a standard we had previously only borrowed from knee literature. The introduction of the MDI is also a creative and potentially valuable addition to our assessment toolbox. The data itself is immediately actionable, providing tangible, gender-specific targets for rehabilitation. However, the methodological limitations warrant deeper reflection. Because the study adopts a cross-sectional design, it cannot reveal how ankle strength evolves over the course of rehabilitation or across different life stages, limiting our ability to determine what “normal recovery” looks like over time. Likewise, the absence of normalization to lean body mass may obscure important strength variations that are biologically meaningful but not captured by raw torque values. Future research using longitudinal cohorts and body-composition-adjusted metrics would therefore be crucial to refine these normative standards and improve their clinical precision. Finally, the study provides the reference but doesn’t include an injured cohort to demonstrate how far patients typically deviate from these norms, which would have been a powerful illustration of the data’s clinical utility.

CLINICAL IMPLICATIONS AND FUTURE DIRECTIONS

The implications of this study for clinical practice are immediate and clear. We should start using this data. Therapists and physicians can now set rehabilitation goals for ankle strength that are based not on a potentially compromised contralateral limb, but on evidence-based, gender-specific norms. For example, a clinician managing a patient recovering from a lateral ankle sprain may compare the individual’s peak torque ratios to the published normative chart rather than to the contralateral limb alone, enabling more precise determination of readiness to resume activity. The validation of the > 90% LSI/MDI threshold provides a much-needed, defensible benchmark for making those often-difficult return-to-activity decisions, helping to standardize care and, hopefully, reduce re-injury rates. The path forward for research is equally clear. We urgently need similar normative studies conducted in specific athletic populations-soccer players, basketball players, dancers-whose functional demands and strength profiles are undoubtedly different. Longitudinal studies are also needed to map the natural history of ankle strength across the lifespan, from adolescence to old age. Prospectively, it would be fascinating to conduct baseline isokinetic testing on a large cohort of athletes and follow them to see if pre-existing deficits, as defined by these new norms, actually predict future injury. And, of course, the next generation of this research must incorporate more sophisticated body composition analysis, normalizing strength to lean body mass to create even more precise and universally comparable reference standards. Finally, the real test will be to apply these normative data to patient populations in prospective trials to demonstrate definitively that rehabilitating to these objective targets leads to better long-term outcomes.

CONCLUSION

The study by da Fonseca et al[13] is a foundational piece of research that provides a robust and long-overdue set of normative isokinetic strength values for the ankle in healthy adults. By establishing gender-specific benchmarks for peak torque and agonist/antagonist ratios, and by empirically supporting a > 90% symmetry threshold for the ankle, the authors have provided clinicians with the objective tools needed to elevate the standard of care in ankle rehabilitation. This work effectively challenges the over-reliance on the potentially compromised contralateral limb as a reference and paves the way for a more evidence-based, standardized approach to clinical decision-making. While future research is needed to expand these findings to athletic populations and to incorporate more precise normalization methods, this study stands as a critical step forward in filling a significant gap in the orthopedic and sports medicine literature.