About

Josh R Baxter, PhD, is an Assistant Professor of Orthopaedic Surgery at the University of Pennsylvania's Perelman School of Medicine. He serves as the Director of the Human Motion Laboratory at the Hospital of the University of Pennsylvania. His research focuses on improving rehabilitation loading protocols to treat musculoskeletal injuries, leveraging mechanical loads as a treatment modality in orthopaedics. Baxter's work explores the post-operative loading environment that promotes tissue healing, aiming to identify therapeutic loads that can be translated into precision rehabilitation strategies to maximize patient outcomes. His research involves using preclinical and clinical models to determine loads that are beneficial or injurious for tissue healing. Baxter is currently leading multiple funded projects, including studies on modifying muscle remodeling following Achilles tendon ruptures, tendon loading profiles in Achilles tendinopathy, and linking structural changes to functional outcomes after Achilles tendon repair. His work aims to develop clinically viable treatment paradigms and optimize rehabilitation protocols through a detailed understanding of tissue biomechanics and neuromechanics.

Research topics

• Artificial Intelligence
• Computer Science
• Medicine
• Statistics
• Anatomy
• Mathematics
• Physics
• Computer vision

Selected publications

• Two-week cumulative tendon load estimated from insole sensor contact forces is associated with plantar flexor function in Achilles tendinopathy

  Open MIND · 2026-02-11 · 2 citations

  datasetSenior author

  Tendon loading dictates rehabilitation outcomes in Achilles tendinopathy but is difficult to track in the real world. In this study, we used instrumented insole sensors to monitor Achilles tendon load for two weeks in fifteen individuals with Achilles tendinopathy, who also completed assessments of their plantar flexor strength, dynamic function, and survey-based outcomes. We used insole data to estimate two types of cumulative Achilles tendon load: overall (≥0.3×body weight) and high-level load (≥3×body weight). We determined Pearson correlations between (1) overall and high-level tendon loads, (2) plantar flexor moment, power, work, (3) heel raise height, repetitions, countermovement jump height, and (4) self-reported symptoms and activity. Overall cumulative tendon load moderately correlated to isometric plantar flexor moment (r = 0.543) and weakly to isokinetic and dynamic functions (0.128–0.413). Cumulative high-level tendon load strongly correlated to heel raise height (0.687) and fast isokinetic moment (0.625), and moderately to other functional measures (0.470–0.592). Symptoms weakly correlated to overall (0.392) and moderately to high-level load (0.436). Self-reported activity weakly correlated to overall (0.297) and strongly to high-level load (0.617). Stronger associations with the high-level Achilles tendon load than the overall load suggest that clinical function assessments provide insight into the real-world performance of high-loading activities. In contrast, the disconnect between overall tendon loading and plantar flexor function may explain the variability in recovery outcomes. Self-reported activity and standard heel raises represent high-level tendon load well, yet they do not always suggest functional deficit. Sensor-monitored tendon load shows promise as a new biomarker for real-world plantar flexor function in Achilles tendinopathy.

  DOI

• Biology Votes Repair?

  Journal of Bone and Joint Surgery · 2026-04-02

  articleSenior author

  Every patient who arrives in the office with an acute Achilles tendon rupture receives similar advice—we have compelling evidence that both operative and nonoperative management provide satisfactory results. Inevitably, many patients are skeptical. If the results are similar, why does every professional athlete undergo Achilles repair? In our practice, I usually discussed the improved mechanical strength achieved with repair and concerns about elongation and rerupture in high-level athletes, while acknowledging that surgery carries risks. Achilles tendon rupture management likely represents a case of clinical equipoise. And yet, as a surgeon, I have always believed (or desired to believe) that surgery must be better. The article by Wei et al. lends support to the notion that repair is biologically superior to nonoperative management. The authors studied mice, comparing surgical repair versus nonoperative management after tenotomy. Their study was large, with nearly 200 mice, and the authors should be congratulated on their use of an external fixator to ensure that post-injury positioning was standardized. The authors had 5 groups of mice: (1) control, (2) injury+no repair+90° immobilization, (3) injury+no repair+160° immobilization, (4) injury+repair+90° immobilization, and (5) injury+repair+160° immobilization. The authors provided proof of concept that immobilization in plantar flexion is of paramount importance. The tendons repaired and then immobilized at 90° had less biomechanical strength than the tendons immobilized at 160°, regardless of whether the tendon was repaired. Supporting the healing tendon in plantar flexion also led to better qualitative histological grading scores. Surgically repairing the tendon combined with plantar-flexed immobilization increased tendon-related gene expression, while not repairing the injury led to a stronger inflammatory response. In their discussion, the authors emphasized the importance of a low-tension environment for collagen remodeling and tissue quality. One clinical challenge is how to advise patients who have been immobilized in neutral dorsiflexion rather than plantar flexion prior to presentation. If the patient has been immobilized in neutral for more than a few days, are the results of nonoperative management still noninferior to those of surgery? While more study is needed to answer that and other questions, this study highlights the importance of plantar flexion for creating a low-tension environment to optimize tendon healing. The study has many unavoidable limitations that hinder generalizability to our patients. The Achilles tendon in mice is quite different from the Achilles tendon in humans. Mice are quadrupeds and, as such, the importance of the Achilles tendon for locomotion is lessened; mice walk well with the loss of a leg, but humans do not. Additionally, many Achilles tendon ruptures are related to underlying tendinopathy, while this model created ruptures by a tenotomy. In a spontaneous rupture, the ends of an Achilles tendon look like a horse’s tail, not a transverse cut. It is possible that spontaneous ruptures lend themselves to healing due to overlapping tissue and increased surface area. On the other hand, it is also possible that spontaneous ruptures increase collagen disorganization and therefore healing is of lower quality in humans. Finally, it is difficult to know if the results of this study will hold up in the long term, given the relatively short follow-up timeline. This study tested 2 immobilization paradigms—constant plantar flexion and constant neutral immobilization—that do not fully reflect the standard clinical protocol. Prolonged immobilization in plantar flexion has fallen out of favor clinically and has been replaced with progressive loading that aims to safely introduce the mechanical loading needed to promote tendon remodeling and more organized type-I collagen. The findings in this mouse experiment suggest that continued underloading in plantar flexion is optimal for tendon healing, but our clinical experience and other preclinical experiments suggest otherwise. A 2019 study by Hillin et al. tested the isolated effects of immobilization paradigms following an unrepaired Achilles tendon transection in a rat model1. They found that prolonged plantar flexion immobilization reduced tendon elongation but at the cost of reduced collagen organization, while prolonged neutral immobilization improved collagen organization but with increased elongation. Progressively reducing plantar flexion immobilization in that model improved tendon properties while minimizing elongation. Clinicians should carefully consider the implications of immobilization strategies and practical considerations when considering preclinical reports. Despite the limitations of animal research, however, this paper presents novel findings that are important to our understanding of optimizing tendon healing.

  PublisherDOI

• Wearable sensor dataset: instrumented insole and boot mounted inertial measurement unit collected from patients recovering from Achilles tendon rupture

  Zenodo (CERN European Organization for Nuclear Research) · 2026-03-17

  datasetOpen accessSenior author
  PublisherDOI

• Wearable sensor dataset: instrumented insole and boot mounted inertial measurement unit collected from patients recovering from Achilles tendon rupture

  Zenodo (CERN European Organization for Nuclear Research) · 2026-03-17

  datasetOpen accessSenior author
  PublisherDOI

• Continuous Estimation of Achilles Tendon Loading in Rupture Patients Using a Single Boot-Mounted Accelerometer

  medRxiv · 2026-03-11

  articleOpen accessSenior authorCorresponding

  Abstract Objective Achilles tendon ruptures lead to long-term structural and functional deficits. Prior research that sought to identify optimal rehabilitation techniques was fundamentally limited by the inability to continuously monitor Achilles tendon loading during rehabilitation. Our objective was to develop a data-driven model that predicts per-step peak Achilles tendon loading from only a single, boot-mounted accelerometer. Methods Nineteen patients recovering from an acute Achilles tendon rupture completed in-lab walking trials while wearing an instrumented immobilizing boot. A boot-mounted inertial measurement unit provided acceleration signals used for prediction, while a force-sensing insole provided ground truth tendon-loading data through a validated ankle moment balance. We developed a stance-detection algorithm, as well as a personalized one-dimensional convolutional neural network (1D-CNN) to estimate per-step peak Achilles tendon load. Our training framework incorporated a small patient-specific personalization sample and was evaluated on held-out steps. Results The stance detection algorithm identified stance phases with 99.8% precision and mean timing errors of 27.3 ms for heel strike and 61.9 ms for toe-off. The CNN estimated per-step peak Achilles tendon load with a mean absolute error of 0.14 bodyweights (R 2 =0.68) across rupture patients. Conclusion Continuous, objective estimation of Achilles tendon loading during early rehabilitation is feasible using a single, boot-mounted accelerometer. Model errors were small (9%) relative to the wide range of tendon loading exhibited during immobilizing boot walking. Our proposed approach enables clinicians to continuously monitor mechanical loading during a previously unobservable rehabilitation period and provides a foundation for personalized rehabilitation guidance after Achilles rupture.

  PublisherOA PDFDOI

• Two-week cumulative tendon load estimated from insole sensor contact forces is associated with plantar flexor function in Achilles tendinopathy

  Scientific Reports · 2026-02-18

  articleOpen accessSenior author

  Tendon loading dictates rehabilitation outcomes in Achilles tendinopathy but is difficult to track in the real world. In this study, we used instrumented insole sensors to monitor Achilles tendon load for two weeks in fifteen individuals with Achilles tendinopathy, who also completed assessments of their plantar flexor strength, dynamic function, and survey-based outcomes. We used insole data to estimate two types of cumulative Achilles tendon load: overall (≥ 0.3×body weight) and high-level load (≥ 3×body weight). We determined Pearson correlations between (1) overall and high-level tendon loads, (2) plantar flexor moment, power, work, (3) heel raise height, repetitions, countermovement jump height, and (4) self-reported symptoms and activity. Overall cumulative tendon load moderately correlated to isometric plantar flexor moment (r = 0.543) and weakly to isokinetic and dynamic functions (0.128–0.413). Cumulative high-level tendon load strongly correlated to heel raise height (0.687) and fast isokinetic moment (0.625), and moderately to other functional measures (0.470–0.592). Symptoms weakly correlated to overall (0.392) and moderately to high-level load (0.436). Self-reported activity weakly correlated to overall (0.297) and strongly to high-level load (0.617). Stronger associations with the high-level Achilles tendon load than the overall load suggest that clinical function assessments provide insight into the real-world performance of high-loading activities. In contrast, the disconnect between overall tendon loading and plantar flexor function may explain the variability in recovery outcomes. Self-reported activity and standard heel raises represent high-level tendon load well, yet they do not always suggest functional deficit. Sensor-monitored tendon load shows promise as a new biomarker for real-world plantar flexor function in Achilles tendinopathy.

  PublisherOA PDFDOI

• A Novel Measurement of Altered Achilles Subtendon Load Sharing 6–12 Months Following Rupture

  Journal of Orthopaedic Research® · 2026-03-26

  articleOpen accessSenior authorCorresponding

  Achilles tendon ruptures cause muscle-tendon structural and functional deficits that persist years after the initial injury. A healthy Achilles tendon contains three semi-independent subtendons that slide relative to each other during muscle contractions in healthy adults. However, such sliding decreases postinjury as load sharing-likely caused by intratendinous adhesions-increases between adjacent subtendons. This study quantifies changes in subtendon load sharing 6-12 months following Achilles tendon rupture when patients are cleared by their surgeon to fully return to physical activities. We combined transverse plane ultrasound imaging with neuromuscular electrical stimulation of individual triceps surae muscles and applied a Kanade-Lucas-Tomasi point tracking algorithm to characterize subtendon behavior. We developed a surrogate measure of subtendon load sharing by quantifying differences in point displacement trajectory angles between select regions within the tendon cross section. In patients recovering from rupture injuries (n = 19), subtendon load sharing significantly increased in the injured tendon compared to the contralateral uninjured side during lateral gastrocnemius (p = 0.0094), medial gastrocnemius (p = 0.021), and soleus stimulations (p = 0.048). These differences were not present between right and left legs in the uninjured cohort (n = 17). Linear regression analysis also revealed that the presence of tendon injury was significantly associated with subtendon load sharing, with injured tendons showing up to a 44% decrease in subtendon independence compared to the contralateral uninjured tendon during gastrocnemius stimulations. Statement of Clinical Significance: These results propose a novel biomarker of tendon health and suggest muscle-dependent changes in subtendon function following Achilles tendon rupture.

  PublisherDOI

• Two-week cumulative tendon load estimated from insole sensor contact forces is associated with plantar flexor function in Achilles tendinopathy

  Zenodo (CERN European Organization for Nuclear Research) · 2026-02-11

  datasetOpen accessSenior author

  Tendon loading dictates rehabilitation outcomes in Achilles tendinopathy but is difficult to track in the real world. In this study, we used instrumented insole sensors to monitor Achilles tendon load for two weeks in fifteen individuals with Achilles tendinopathy, who also completed assessments of their plantar flexor strength, dynamic function, and survey-based outcomes. We used insole data to estimate two types of cumulative Achilles tendon load: overall (≥0.3×body weight) and high-level load (≥3×body weight). We determined Pearson correlations between (1) overall and high-level tendon loads, (2) plantar flexor moment, power, work, (3) heel raise height, repetitions, countermovement jump height, and (4) self-reported symptoms and activity. Overall cumulative tendon load moderately correlated to isometric plantar flexor moment (r = 0.543) and weakly to isokinetic and dynamic functions (0.128–0.413). Cumulative high-level tendon load strongly correlated to heel raise height (0.687) and fast isokinetic moment (0.625), and moderately to other functional measures (0.470–0.592). Symptoms weakly correlated to overall (0.392) and moderately to high-level load (0.436). Self-reported activity weakly correlated to overall (0.297) and strongly to high-level load (0.617). Stronger associations with the high-level Achilles tendon load than the overall load suggest that clinical function assessments provide insight into the real-world performance of high-loading activities. In contrast, the disconnect between overall tendon loading and plantar flexor function may explain the variability in recovery outcomes. Self-reported activity and standard heel raises represent high-level tendon load well, yet they do not always suggest functional deficit. Sensor-monitored tendon load shows promise as a new biomarker for real-world plantar flexor function in Achilles tendinopathy.

  PublisherDOI

• In vivo characterization of Achilles subtendon function and morphology within the tendon cross section and along the free tendon

  bioRxiv (Cold Spring Harbor Laboratory) · 2025-06-01

  preprintOpen accessSenior authorCorresponding

  ABSTRACT The Achilles tendon is composed of three distinct fascicle bundles, or “subtendons,” each originating from the head of one of the three triceps surae muscles. In a healthy tendon, these subtendons slide relative to each other during muscle contractions. This subtendon sliding is reduced in older adults and younger adults who suffer an Achilles tendon injury. However, subtendon sliding is challenging to quantify in low-load scenarios that are critical for monitoring subtendon biomechanics in patients with mechanically compromised tendons, like following an Achilles tendon rupture and repair surgery. The purpose of this study was to develop a reliable method to characterize subtendon behavior in vivo using combined transverse plane ultrasound imaging and neuromuscular electrical stimulation of individual gastrocnemii. We used a Kanade-Lucas-Tomasi point tracking algorithm to quantify tendon displacement during isolated muscle stimulations. Next, we applied k-means clustering to characterize heterogeneous subtendon behavior within the tendon cross section. The tendon cross section displayed differential displacement patterns depending on the stimulated muscle (p<0.0001), and these displacements differed along the free tendon during lateral gastrocnemius stimulations (p=0.004). These results reflect possible differences in load-sharing between adjacent subtendons and differing muscle-tendon dynamics among the triceps surae muscles. Finally, this method confirmed bilaterally symmetric subtendon behavior and demonstrated high inter-session reliability (ICC>0.83). Overall, this study furthers our understanding of differential muscle-tendon dynamics of individual Achilles subtendons both within the tendon cross section and along the free tendon. Future work will apply this method to injured populations to develop biomarkers of altered subtendon function. NEW & NOTEWORTHY Achilles subtendon function and morphology are challenging to characterize in vivo . This study employed transverse plane ultrasound imaging and neuromuscular electrical stimulation to characterize behavior of individual subtendons both within the tendon cross section and along the free tendon. It is the first study to demonstrate functional behavior within the Achilles tendon using these combined tools in identifying subtendon dynamics. Additional findings of bilateral symmetry in healthy individuals present this tool’s potential to quantify altered subtendon function post-injury.

  PublisherOA PDFDOI

• Achilles Tendon Surgical Repair Partially Restores Early Plantar Flexor Structure and Function in a Rat Model

  Journal of Orthopaedic Research® · 2025-01-06 · 1 citations

  articleOpen accessSenior authorCorresponding

  Achilles tendon ruptures significantly impair long-term patient function, with two-thirds of patients experiencing persistent functional deficits. Although nonsurgical treatment has gained popularity due to its perceived lower risk of complications, the specific effects of this approach on tendon healing, muscle function, and overall performance remain poorly understood. Directly comparing surgical and nonsurgical treatment options in a clinical population is challenging given the diverse nature of the patient population. Preclinical models are essential to isolate the mechanisms underlying these treatments, enabling a detailed examination of the structural and functional outcomes that are difficult to assess in human studies. Here, we surgically induced Achilles tendon ruptures in 20 adult male Sprague Dawley rats and repaired the rupture in half of these animals. Then, functional outcomes were assessed by measuring plantar flexor torque across the ankle's range of motion using a custom-developed small animal dynamometer, and structural changes were evaluated through measurements of Achilles tendon elongation and plantar flexor muscle mass. We found that surgical treatment led to 11%-35% increased functional plantar flexor torque outcomes compared to nonsurgical treatment. Additionally, plantar flexor muscle mass decreased by 21% in nonsurgically treated animals compared to only 12% in the surgically treated group. Our results suggest that surgically repairing a tendon rupture restores plantar flexor function more effectively than nonsurgical treatment; however, persistent functional deficits in both groups indicate that enhanced rehabilitation strategies are necessary for full functional restoration.

  PublisherOA PDFDOI

Recent grants

• Modifying muscle remodeling following Achilles tendon ruptures

  NIH · $609k · 2020–2026

• Achilles Tendinopathy Center of Research Translation

  NIH · $12.6M · 2023–2027

• Tendon loading profiles that promote healing in Achilles tendinopathy

  NIH · $2.3M · 2021–2026

• Defining neuromechanical mechanisms of Achilles tendinopathy

  NIH · $1.3M · 2022–2026

Frequent coauthors

• Todd J. Hullfish

  University of Pennsylvania

  63 shared

• Susannah L. Gilbert

  Hospital for Special Surgery

  58 shared

• Dean G. Lorich

  Cornell University

  53 shared

• Matthew R. Garner

  Penn State Milton S. Hershey Medical Center

  49 shared

• Patrick C. Schottel

  University of Vermont Medical Center

  49 shared

• Rodrigo Scattone Silva

  Universidade Federal do Rio Grande do Norte

  45 shared

• Karin Grävare Silbernagel

  University of Delaware

  42 shared

• Ke Song

  University of Pennsylvania

  39 shared

Labs

• Baxter LabPI

Education

• PhD, Kinesiology

  Pennsylvania State University

  2012

• MS, Nutrition and Exercise Science

  Oregon State University

  2009

• BS, Exercise Science

  Central Washington University

  2007