Review Article
Biphasic Dose Response, Energetic Constraints, and Cellular Exhaustion in Rehabilitation Science:
A Conceptual Review
Abstract
Rehabilitation interventions—including mechanical loading, therapeutic exercise, manual therapy, and photobiomodulation (PBM) operate within complex, non-linear biological systems. Accumulating evidence across mechanobiology, mitochondrial physiology, and hormesis research demonstrates that cellular responses to stress follow a biphasic (inverted-U) dose–response relationship. Within an optimal stimulus window, cells activate anabolic signaling, enhance protein synthesis, and improve tissue function. When the stimulus is insufficient, adaptive signaling is not triggered; when it is excessive, inflammatory and catabolic pathways dominate. Over time, repeated stress without adequate recovery may result in cellular exhaustion, characterized by diminished adaptive responsiveness secondary to energetic limitation, impaired mitochondrial function, and constrained protein synthesis capacity. This review integrates biphasic dose-response theory, ATP-dependent protein expression, and resource limitation into a unified adaptive capacity model for rehabilitation science. This framework offers a biologically grounded explanation for clinical phenomena such as load intolerance, plateau, delayed recovery, and variability in response to therapeutic interventions. Implications for dosing, progression, and recovery strategies in musculoskeletal rehabilitation are discussed.
Keywords
LMMS (Low Magnitude Mechanical Stimulation), biphasic response, photobiomodulation, cellular exhaustion
Revised 4 August 2026
1 BBA, CEO, Regenerative Technologies Corporation, dba Juvent – Simonson is a 25-year medical device veteran with extensive clinical and research experience. Simonson was the founding CEO of Cytonics, a proteomics research company which discovered the novel protein complex responsible for back and joint pain.
Corresponding author: Rush Simonson, Juvent, 9420 Ooltewah Industrial Drive, Ooltewah, TN 37363. rush.simonson@juvent.com | ã2026 This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution, or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution, or reproduction is permitted which does not comply with these terms.
1. Introduction
Rehabilitation science is fundamentally concerned with the controlled application of stress to restore function. Whether through resistance training, graded exposure, tendon-loading protocols, joint mobilization, or photobiomodulation (PBM), clinicians apply stimuli with the intent of provoking healing and adaptation. Traditional clinical models emphasize progressive overload, tissue irritability, and load management; however, the underlying cellular constraints governing these principles are often underexplored in clinical education.
Biological systems do not respond linearly to stress. Instead, they exhibit non-linear dynamics in which magnitude, duration, frequency, and recovery collectively determine outcome. The concept of hormesis—whereby low-to-moderate stress enhances function while excessive stress impairs it—provides a foundational framework (Calabrese & Baldwin, 2003). The term is derived from the Greek word for “to excite.” Within musculoskeletal tissues, mechanotransducive pathways translate mechanical forces into biochemical signaling cascades that regulate extracellular matrix synthesis, mitochondrial adaptation, and inflammatory balance. Similarly, PBM influences mitochondrial respiration and reactive oxygen species (ROS) signaling in a dose-dependent manner (Hamblin, 2017).
This review synthesizes evidence from hormesis research, cellular energetics, mechanobiology, and PBM literature to propose a unified model of adaptive capacity. Central to this model is the premise that adaptation is constrained not only by stimulus magnitude but also by ATP availability and resource sufficiency required for protein synthesis.
2. Biphasic Dose Response and Hormesis
The biphasic dose response describes a relationship in which low doses of stress produce minimal or insufficient activation, moderate doses enhance biological function, and high doses impair or inhibit function. Calabrese and Baldwin (2003) characterized hormesis as a fundamental biological principle observed across species and tissue types.
In musculoskeletal systems, moderate mechanical loading enhances anabolic signaling pathways, including focal adhesion kinase activation, MAP kinase cascades, and growth factor expression. Bone remodeling theory, particularly Frost’s mechanostat model, suggests defined loading thresholds that determine maintenance, adaptation, or pathological overload. Similarly, tendon research demonstrates that moderate cyclic loading promotes collagen synthesis, whereas excessive strain promotes matrix degradation and inflammatory mediator expression (Cook & Purdam, 2009).
Photobiomodulation exhibits a comparable inverted-U response. At appropriate fluence and irradiance, PBM enhances cytochrome c oxidase activity, increases mitochondrial membrane potential, and transiently elevates ROS as signaling molecules. Excessive dosing, however, may increase oxidative stress or produce inhibitory effects (Hamblin, 2017; Chung et al., 2012). These findings reinforce the necessity of precise dosing parameters rather than the assumption that greater intensity yields greater benefit.
Clinically, the biphasic principle explains why both underloading and overloading fail to produce optimal outcomes. It also clarifies why symptom flare may lag behind molecular signaling shifts, as inflammatory cascades are activated before pain perception emerges.
3. Mechanotransduction and Protein Expression
Adaptation at the tissue level ultimately depends on changes in protein expression. Mechanical forces are sensed via integrins, cytoskeletal elements, ion channels, and associated signaling complexes. These signals converge on transcriptional regulators that modulate the synthesis of structural proteins, enzymes, and receptors.
In tendons and muscles, adaptation requires the synthesis of collagen, actin, myosin, titin, and associated matrix proteins. In bone, remodeling depends on coordinated osteoblastic and osteoclastic activity, each requiring transcriptional and translational machinery. Even neural adaptation to rehabilitation stimuli involves receptor trafficking and synaptic protein synthesis.
Crucially, protein synthesis is energetically expensive. Translation, folding, post-translational modification, and intracellular transport require significant ATP investment. Without sufficient energy (caloric and nutritional) and substrate availability, transcriptional signaling cannot be translated into structural adaptation. Therefore, mechanical signaling alone does not guarantee functional change.
4. Energetic Constraints and ATP Dependency
ATP functions as the universal energetic currency of the cell. Buttgereit and Brand (1995) demonstrated that protein synthesis represents one of the highest ATP-consuming processes in mammalian cells. Rolfe and Brown (1997) further quantified the substantial proportion of resting metabolic expenditure devoted to biosynthetic processes.
During rehabilitation, mechanical loading not only initiates signaling cascades but also imposes metabolic demand. Inflammatory processes, calcium handling, cytoskeletal remodeling, and ROS buffering all compete for ATP. When repeated loading is imposed without adequate recovery, mitochondrial capacity may be insufficient to meet combined biosynthetic and stress-related demands.
Energetic limitation may manifest as: - Impaired protein synthesis - Slower extracellular matrix remodeling - Reduced mitochondrial biogenesis - Increased susceptibility to oxidative damage
This metabolic bottleneck provides a mechanistic explanation for clinical plateau despite appropriate exercise selection and adherence.
5. Cellular Exhaustion: A Cumulative Phenomenon
Cellular exhaustion refers to diminished adaptive responsiveness secondary to repeated stress exceeding recovery capacity. Unlike acute overload, which represents immediate threshold violation, exhaustion reflects cumulative under-recovery.
Mitochondrial function plays a central role in adaptive capacity. Picard et al. (2016) emphasized the growing recognition of mitochondria as regulators of inflammation, signaling, and cellular resilience. Reduced mitochondrial efficiency limits ATP production and impairs redox balance, thereby constraining anabolic signaling.
Clinically, exhaustion may present as: - Delayed onset symptom flare (24–72 hours post-intervention) - Reduced tolerance to previously manageable loads - Persistent soreness despite appropriate progression - Plateau in strength or functional gains
Importantly, increasing stimulus in an exhausted system may exacerbate dysfunction. Instead, restoration of energetic capacity through modified loading, recovery intervals, and metabolic support may be required.
6. Resource Availability Beyond ATP
Energetic sufficiency alone does not ensure adaptation. Protein synthesis requires adequate amino acids, micronutrients, and oxygen delivery. Nutritional insufficiency, systemic inflammation, hormonal dysregulation, and impaired microcirculation may all limit substrate availability.
In aging populations and individuals with metabolic comorbidities, mitochondrial density and efficiency decline, narrowing the adaptive window. These physiological realities underscore the importance of considering systemic health in rehabilitation planning.
Thus, adaptive capacity is governed by four interdependent variables: 1. Stimulus magnitude and frequency, 2. ATP availability; 3. Substrate and micronutrient sufficiency; 4. Recovery duration. Failure at any level constrains adaptation.
7. Photobiomodulation Within an Energetic Framework
PBM provides a useful illustration of the role of energetic modulation in rehabilitation. Appropriate PBM dosing enhances mitochondrial respiration and ATP production, potentially restoring adaptive bandwidth in energetically constrained tissues (Hamblin, 2017). However, excessive fluence or treatment frequency may introduce additional oxidative stress.
Understanding PBM as a metabolic modulator rather than solely an anti-inflammatory modality integrates it coherently within the biphasic and exhaustion framework. Like exercise, PBM requires dosing precision and adequate recovery. Consideration must also be given to stacked modalities, understanding that while different stimuli are applied, each requires a cellular response.
8. Frequency-Specific Micro-Impact and Ion Channel–Mediated Optimization
Recent mechanobiological models propose that low-magnitude micro-impact stimulation delivered within the 32–37 Hz range stimulates connective tissue and bone, operating explicitly within an Arndt–Schulz (biphasic) framework. Within this window, mechanical deformation of the cell membrane activates mechanosensitive ion channels—particularly PIEZO1—resulting in transient calcium influx and downstream signaling activation.
PIEZO1-mediated calcium entry links mechanical deformation to MAPK/ERK pathway activation, thereby bridging physical stimulus and gene expression of structural proteins. Foundational work by Coste et al. (2010) identified PIEZO1 and PIEZO2 as essential components of mechanically activated ion channels, providing a molecular substrate for frequency-dependent mechanotransduction.
Mechanically activated (MA) ion channels are membrane proteins that transduce physical forces—such as stretch, shear stress, compression, and vibration—into intracellular biochemical signals through ion flux. Among these, PIEZO1 has emerged as a principal mechanosensor in mammalian tissues. PIEZO1 is a large trimeric transmembrane protein that directly senses membrane tension and opens a non-selective cation pore in response to mechanical deformation of the lipid bilayer, permitting Ca²⁺ influx and initiating downstream signaling cascades (Coste et al., 2010; Zhao et al., 2018). Unlike ligand-gated or voltage-gated channels, PIEZO1 activation is governed by physical distortion of the plasma membrane itself, making it a primary transducer of mechanical energy into cellular responses.
In skeletal tissue, PIEZO1 plays a critical role in load-induced osteogenesis. Genetic deletion of Piezo1 in osteoblast lineage cells impairs bone formation and blunts the anabolic response to mechanical loading, establishing PIEZO1 as a required component of skeletal mechanotransduction (Li et al., 2019; Sun et al., 2019). Mechanically induced Ca²⁺ entry through PIEZO1 activates signaling pathways involved in osteoblast differentiation and bone formation, including downstream modulation of Wnt/β-catenin and other osteogenic programs. Beyond bone, PIEZO1 mediates endothelial responses to shear stress, regulates vascular tone, contributes to red blood cell volume homeostasis, and influences mesenchymal stem cell fate decisions (Li et al., 2014; Cahalan et al., 2015).
The existence of PIEZO1 as a tension-sensitive ion channel provides a molecular basis for dose-dependent cellular responses to mechanical stimuli. Because channel activation is governed by membrane tension and deformation, both insufficient and excessive mechanical inputs may produce attenuated or maladaptive signaling, consistent with biphasic (hormetic) dose-response patterns observed in mechanobiology. Thus, PIEZO1 and related MA channels offer a mechanistic framework linking graded mechanical stimuli to nonlinear biological adaptation.
8.1 Biphasic Threshold Behavior
Consistent with hormetic principles, insufficient stimulation fails to adequately gate ion channels, while excessive magnitude or duration may provoke oxidative stress and apoptotic signaling. The 32–37 Hz window has been described as operating within an optimal threshold zone, below levels associated with reactive oxygen species (ROS) accumulation and inhibitory signaling.
8.2 Biochemical Mediators
Micro-impact stimulation within this frequency range has been associated with modulation of several regeneration-relevant biomarkers:
· Nitric Oxide (NO): Increased endothelial nitric oxide synthase (eNOS) activity via fluid shear–related signaling, enhancing perfusion and oxygen delivery.
· Insulin-like Growth Factor-1 (IGF-1): Activation of PI3K/Akt signaling pathways supporting protein synthesis and cellular survival.
· Transforming Growth Factor-β (TGF-β): Upregulation of fibroblast-mediated Type I collagen synthesis.
· Myostatin Suppression: Downregulation of inhibitory growth signaling, potentially facilitating anabolic responsiveness.
Earlier vibration literature supports modulation of neuromuscular recruitment and bone adaptation (Lundeberg et al., 1988; Verschueren et al., 2004), while biomechanical transmission characteristics vary with frequency (Goetz et al., 2014). In vitro evidence also suggests that collagen synthesis is frequency-dependent in fibroblastic cells (Ueda et al., 2011).
8.3 Targeted Frequency Windows
While multiple tissues demonstrate sensitivity to mechanical stimulation within the 30–40 Hz range—a frequency band frequently employed in studies of whole-body and local vibration therapy—the current literature does not define narrow sub-frequency bands that preferentially engage distinct mechanotransducive pathways in a tissue-specific manner. Research to date highlights that mechanical vibration frequency, amplitude, and duration all influence neuromuscular, skeletal, and cellular responses, and that the field would benefit from further rigorous investigation to delineate specific frequency–mechanism relationships rather than assume discrete “resonance” windows for particular tissues (Bartel & Mosabbir, 2021; Dade Matthews et al., 2022; Wang et al., 2022). One important consideration is the challenge of determining total cellular loading that may result from resonant loading. The phenomenon of mechanical resonance is real, and while initial loads may be within physiological safety boundaries, resultant loads may be far higher and even damaging. The logic behind choosing a 32-37Hz range is that there are no large organ systems that would be damaged by a resulting resonant development, particularly at the low energies of LMMS (less than 1g)
8.4 Temporal Dosing and Recovery Windows
Session duration and recovery intervals appear critical within a biphasic framework. Proposed protocols suggest 12–15 minute sessions, with 24–48-hour recovery periods to allow completion of protein synthesis and matrix assembly. Exceeding approximately 30 minutes per session may increase ROS accumulation and shift signaling toward inhibitory pathways. This temporal structure mirrors the energetic and biosynthetic constraints outlined earlier in this review.
9. Clinical Implications for Rehabilitation Practice
A unified adaptive capacity model incorporating frequency-specific mechanotransduction yields several practical implications:
1. Precision Dosing: Intensity, frequency, duration, and rest must remain within hormetic thresholds.
2. Ion Channel Targeting: Mechanically activated ion channels such as PIEZO1 represent plausible molecular mediators of frequency-dependent adaptation.
3. Recovery Windows: 24-48-hour intervals may reflect necessary protein synthesis and extracellular matrix assembly and recovery timelines.
4. Plateau Interpretation: Failure to progress may represent energetic or oxidative threshold violations rather than inadequate stimulus.
5. Modality Integration: Micro-impact and PBM may function as metabolic modulators when appropriately dosed within biphasic limits.
6. Consideration must also be given to stacked modalities, understanding that while different stimuli are applied, each requires a cellular response. Clinicians must review modalities for stimulatory overlap and incorporate this into an effective plan.
Rehabilitation thus represents not merely the application of load but the biologically tuned delivery of load within energetic, temporal, and molecular constraints.
10. Conclusion
Cellular adaptation in rehabilitation follows a biphasic, non-linear trajectory constrained by energetic and material resources. Mechanical, photonic, and frequency-specific micro-impact stimuli provide the signal for change, but ATP availability, substrate sufficiency, and redox balance determine whether adaptation can occur. Repeated stress without adequate recovery leads to cellular exhaustion, narrowing the adaptive window, and predisposing tissues to plateau or flare.
Integrating hormesis, mechanotransduction, mitochondrial energetics, ion channel gating, and protein synthesis into a unified framework enhances the biological precision of rehabilitation dosing. Future research should further quantify frequency-dependent signaling thresholds and explore strategies to expand adaptive capacity in chronically stressed tissues.
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