Chiropractors Modulate Spinal Manipulation Force Magnitude While Preserving Thrust Duration Timing

Licensed chiropractors asked to halve their thrust force during thoracic spinal manipulation adapt primarily by lowering peak force and force application rate while preserving thrust duration. This biomechanical pattern emerged from an observational cross-sectional study evaluating licensed manual therapy practitioners in France performing high-velocity low-amplitude procedures under simulated conditions. The investigation followed the Strengthening the Reporting of Observational Studies in Epidemiology framework and received institutional approval from the ethics committee of the Institut Franco-Européen de Chiropraxie under certificate CER-24_01_01. By evaluating practitioner performance on an instrumented human analogue mannequin atop a force-sensing platform, researchers quantified baseline thoracic thrust characteristics and examined how clinical scenarios, practitioner sex, age, and clinical experience interact with force output. The findings demonstrate that manual therapists execute deliberate force down-regulation through amplitude scaling rather than temporal elongation.

Manual therapy skills such as spinal manipulation are defined as treatments directed to spinal structures involving forces delivered with specific angulation, amplitude, and speed to an intervertebral articulation, generating distinct neurophysiological and biomechanical responses. Motor expertise within specialized domains represents perceptual motor capabilities accumulated through deliberate practice over extended timeframes, enabling improved decision-making, refined motor control, and enhanced information processing. Similar motor adaptations appear in other expert cohorts; robotic surgery research shows expert surgeons attain superior completion times, speeds, and gesture amplitudes relative to novices, while trained musicians display higher tapping frequencies and elevated timing accuracy. Previous evaluations comparing chiropractic students with experienced clinicians indicate that foundational mechanical metrics—principally peak thrust duration, peak force, and rate of force application—are acquired sequentially during academic curricula, whereas complex attributes like motor retention, coordination stability, error detection, and mechanical transfer across variable contexts develop during subsequent clinical practice.

The study cohort comprised 46 licensed chiropractors registered with a French regional health agency who volunteered during an annual national convention of the French chiropractic association held on March 15 and 16, 2024. The sample maintained an equal sex distribution of 23 female and 23 male clinicians, presenting a group mean age of 39.5 years and a mean practice experience of 14.8 years. Musculoskeletal injury impeding manual execution constituted the formal exclusion criteria, and all enrolled participants provided written informed consent. Demographically, 89.13 percent of participating practitioners graduated from the Institut Franco-Européen de Chiropraxie, while others earned degrees from institutions in Canada and the United States. Furthermore, 86.96 percent reported high-velocity low-amplitude manipulation as their primary modality, while instrument-assisted and mechanically assisted manipulation were utilized by 21.94 percent and 19.56 percent of clinicians, respectively.

Biomechanical data acquisition utilized a standardized Human Analogue Mannequin developed by the Canadian Memorial Chiropractic College, an apparatus previously validated in manual therapy skill acquisition investigations. The mannequin was positioned directly over a Force Sensing Table Technology system comprising a Leander 900 Z Series Chiropractic Table equipped with an integrated AMTI multi-axis force plate. The system computed transmitted mechanical forces within a three-dimensional coordinate framework using specialized software, recording vertical force-time signals across all delivered trials. The analytical framework captured four primary biomechanical metrics: preload force maintained immediately prior to the thrust; peak force representing the absolute maximal force recorded during the thrust; peak thrust duration defined as elapsed time from take-off to peak force; and the rate of force application calculated between take-off and peak amplitude. An a priori statistical power calculation determined that 37 participants were required to detect meaningful linear mixed-effects model differences assuming a mean peak force of 450 Newtons.

The experimental protocol was structured into three consecutive procedural blocks, requiring each practitioner to execute postero-to-anterior thoracic manipulation trials using a consistent chosen technique. Block 1 served as a familiarization stage involving three trial thrusts on the mannequin, during which practitioners received immediate visual displays of biomechanical profiles alongside oral reports of peak force in Newtons. Following familiarization, Block 2 presented a standardized clinical vignette detailing a 31-year-old male rugby player measuring 1.76 meters and weighing 96 kilograms presenting with non-specific mid-back discomfort and palpable thoracic hypomobility at the eighth thoracic vertebra. Participants delivered five consecutive thrusts replicating the force they typically apply to such a patient in daily practice without receiving biofeedback. Following a three-minute rest interval, Block 3 instructed clinicians to deliver five additional thrusts while reducing peak force by approximately 50 percent relative to Block 2, with instructions emphasizing that participants should intentionally alter only force amplitude while avoiding conscious adjustments to other mechanics.

Descriptive evaluations revealed pronounced differences between baseline simulation thrusts and the force modulation task across all recorded parameters. During the simulated rugby player scenario in Block 2, participating chiropractors delivered a mean peak force of 569.1 Newtons with a standard deviation of 142.1 Newtons, accompanied by an average preload force of 169.8 Newtons. The mean peak thrust duration in Block 2 was 132.3 milliseconds, while the average rate of force application reached 3093.8 Newtons per millisecond. When attempting the 50 percent down-regulation task in Block 3, recorded mean peak force dropped to 379.0 Newtons with a standard deviation of 95.6 Newtons, while preload force declined to 140.3 Newtons. The peak thrust duration decreased modestly to 118.3 milliseconds, and the rate of force application fell to 2072.8 Newtons per millisecond across 44 analyzed datasets following two equipment computation exclusions.

Inferential statistical assessments relied on linear mixed-effects modeling executed in R software to evaluate experimental outcomes while accounting for repeated observations across individual clinicians. Extreme statistical outliers situated beyond three times the interquartile range were identified and removed prior to modeling using standard algorithmic packages. Model complexity was systematically refined by comparing candidate specifications via the Akaike Information Criterion and the Bayesian Information Criterion, balancing mathematical goodness of fit against unnecessary parameter complexity to select the most parsimonious fixed-effects configuration. Each biomechanical parameter was evaluated through an independent linear mixed model using participant identification codes as random intercepts to properly manage repeated within-subject variance. Fixed explanatory variables included practitioner gender, chronological age, completed years of practice, clinical technique classifications, conferring educational institution, sequential trial number, and the experimental condition contrast between Block 2 and Block 3.

Linear mixed-effects modeling confirmed that the experimental condition exerted a profound main effect on peak force production, with clinicians generating substantially reduced forces during Block 3 relative to Block 2 (beta = -197.83, SE = 5.30, t = -37.36, p < 0.001). Practitioner sex also significantly influenced peak force (beta = 103.80, SE = 33.56, t = 3.09, p = 0.004), with males producing higher peak forces. Trial repetition showed a small negative effect on peak force (beta = -5.86, SE = 1.87, t = -3.14, p = 0.002), whereas age, practice tenure, and educational background showed no significant associations. Peak thrust duration similarly exhibited a statistically significant decrease between conditions (beta = -15.07, SE = 0.97, t = -15.55, p < 0.001), while remaining unaffected by clinician sex, age, education, or technique. The rate of force application was significantly reduced during modulation (beta = -1074.0, SE = 44.6, t = -24.08, p < 0.001), with significant effects for gender (beta = 645.0, SE = 216.2, t = 2.98, p = 0.006) and therapy technique (beta = -661.2, SE = 280.6, t = -2.36, p = 0.025). Preload force likewise decreased between experimental conditions (beta = -48.27, SE = 3.38, t = -14.28, p < 0.001).

The biomechanical adaptation observed during force modulation closely aligns with classic motor control principles of pulse-height control initially documented by Gordon and Ghez in 1987. Under this neurophysiological control framework, the central nervous system modulates mechanical force output by adjusting the amplitude of descending neural drive while preserving the underlying temporal structure and duration of the movement. During the 50 percent modulation task, chiropractors achieved a substantial 33.4 percent decrease in absolute peak force alongside a 33.0 percent reduction in force development rate, whereas peak thrust duration shortened by approximately 9 percent from 132.3 to 118.3 milliseconds. Furthermore, participants overshot the requested 50 percent reduction target, generating 379 Newtons rather than the mathematical half-force value of approximately 285 Newtons. This outcome mirrors findings by Triano and colleagues in 2015, where clinicians overshot half-force targets by 14 percent, reflecting the ballistic nature of spinal manipulation wherein rapid contractions are executed through preprogrammed neural pulses that frequently exceed intended boundaries without sensory corrections.

The absolute peak force magnitudes recorded during the baseline clinical simulation exceeded values typically observed when manual therapists treat live human subjects, reflecting the unique mechanical context of mannequin simulations. Prior scoping literature examining thoracic manipulation delivered to living humans documented peak forces ranging from 337 to 536 Newtons, preload forces between 137 and 172 Newtons, and thrust durations spanning 87 to 198 milliseconds. The higher forces align with findings from Thurnherr and colleagues in 2025, who showed that manual physiotherapists deliver higher forces to mannequins than to live human subjects across young male, middle-aged male, and older female clinical scenarios. Mannequins lack pain responses, muscle guarding, and biological tissue compliance, permitting clinicians to apply greater loads without patient discomfort. Furthermore, while practitioner sex significantly influenced force amplitude and rate in this cohort, practice experience and chronological age failed to predict biomechanical parameters, demonstrating that basic manual force production in stable settings depends more heavily on individual physical capacity than cumulative clinical years.

Several methodological limitations contextualize the scope of these conclusions, beginning with the sample size of 46 practitioners, which represents roughly three percent of all licensed chiropractors registered in France. Additionally, testing on an inanimate human analogue mannequin does not reproduce the tactile feedback, viscoelastic deformation, or clinical variability encountered when treating human spinal segments in authentic clinical practice. From an educational perspective, the observed trial-to-trial variance and force overshooting underscore the necessity of deliberate motor learning programs incorporating objective biomechanical feedback to help manual therapy students stabilize force rise time and master pulse-height scaling. By focusing instructional curricula on maintaining invariant temporal duration while deliberately modulating force amplitude, educational institutions can foster greater motor reproducibility, reduce performance variability, and enhance practitioner adaptability across diverse clinical presentations.

Subject of Research: Biomechanical force modulation strategies during thoracic spinal manipulation

Article Title: Thoracic spinal manipulation: exploring force-time profiles and force modulation strategies in French chiropractors

Article References: Pasquier, M., Thurnherr, N., Dupèbe, W., Gorrell, L. M., Descarreaux, M., & Lardon, A. (2026). Thoracic spinal manipulation: exploring force-time profiles and force modulation strategies in French chiropractors. Chiropractic & Manual Therapies, 34(1), Article 23. https://doi.org/10.1186/s12998-026-00649-9

Image Credits: AI Generated

DOI: 10.1186/s12998-026-00649-9

Keywords: spinal manipulation, thoracic spine, chiropractic care, biomechanics, force-time profiles, peak force, preload force, rate of force application, thrust duration, pulse-height control, force modulation, high-velocity low-amplitude, manikin simulation, linear mixed models, motor control, manual therapy, clinical simulation