The Combined Influence of Sleep Disturbance and Depression on 12-month Outcomes after Lumbar Spine Surgery.

STUDY DESIGN: Retrospective analysis of prospectively collected data. OBJECTIVE: To examine the combined influence of preoperative sleep disturbance and depression on 12-month patient-reported outcomes after lumbar spine surgery (LSS). SUMMARY OF BACKGROUND DATA: Psychological and behavioral factors are considered major risk factors of poor outcome after LSS. However, there is a need to explore the combined effects of preoperative factors such as sleep disturbance and depression. Understanding the influence of sleep disturbance and depression can inform evidence-based preoperative assessment and shared-decision making of preoperative and postoperative treatment. METHODS: Data from 700 patients undergoing LSS were analyzed. Preoperative sleep disturbance and depression were assessed with PROMIS subscales. Established thresholds defined patients with moderate/severe symptoms. Outcomes for disability (Oswestry Disability Index) and back and leg pain (Numeric Rating Scales) were assessed preoperatively and at 12 months. Separate multivariable linear regressions examined the influence of each factor on 12-month outcomes with and without accounting for the other, and in combination as a 4-level variable: 1) moderate/severe sleep disturbance alone, 2) moderate/severe depression alone, 3) both moderate/severe sleep disturbance and depression, 4) no moderate/severe sleep disturbance or depression. RESULTS: Preoperative sleep disturbance and depression were associated with 12-month disability and pain (P<0.05). After accounting for depression, preoperative sleep disturbance remained associated with disability, while preoperative depression adjusting for sleep disturbance remained associated with all outcomes (P<0.05). Patients reporting both moderate/severe sleep disturbance and moderate/severe depression had 12.6 points higher disability and 1.5 points higher back and leg pain compared to patients without moderate/severe sleep disturbance or depression. CONCLUSION: The combination of sleep disturbance and depression impacts postoperative outcomes considerably. The high-risk group of patients with moderate/severe sleep disturbance and depression could benefit from targeted treatment strategies.

Multifractal Nonlinearity Moderates Feedforward and Feedback Responses to Suprapostural Perturbations.

An adaptive response to unexpected perturbations requires near-term and long-term adjustments over time. We used multifractal analysis to test how nonlinear interactions across timescales might support an adaptive response following an unpredictable perturbation. We reanalyzed torque data from 44 young and 24 older adults who performed a single-leg squat task challenged by an unexpected mechanical perturbation and a secondary visual-cognitive task. We report three findings: (a) multifractal nonlinearity interacted with pre-perturbation torque production and task error to presage greater pre-voluntary feedforward increases and greater voluntary reductions, respectively, in post-perturbation task error; (b) multifractal nonlinearity presaged relatively smaller task error than standard deviations of both pre-perturbation torques and pre-perturbation task error; and (c) increased task demand (e.g., age-related changes in dexterity and dual-task settings) led to multifractal nonlinearity presaging reduced task error. All these results were consistent with our expectations, except that a pre-perturbation knee torque-dependent increase in post-perturbation task error appeared later for older than for younger participants. This correlational multifractal modeling offered theoretical clarity on the possible roles of nonlinear interactions across timescales, moderating both feedforward and feedback processes, and presaging greater stability when the standard deviation is relatively large and task demands are strong. Thus, multifractal nonlinearity usefully describes movement variability even when paired with classical descriptors like the standard deviation. We discuss potential insights from these findings for understanding suprapostural dexterity and developing rehabilitative interventions.

Association of Cognitive Impairment With Rate of Functional Gain Among Older Adults With Stroke.

OBJECTIVES: This study explored the association between cognitive impairment at admission with self-care and mobility gain rate (amount of change per week) during a post-acute care stay (admission to discharge) for older adults with stroke. DESIGN: Retrospective cohort study. SETTING AND PARTICIPANTS: Four inpatient rehabilitation and 6 skilled nursing facilities. A total of 100 adults with primary diagnosis of stroke; mean age 79 years (SD 7.7); 67% women. METHODS: Retrospective cohort study. We evaluated the extent to which cognitive impairment at admission explained variation in weekly gain rate separately for self-care and mobility. Additional covariates were occupational and physical therapy minutes per day, self-care and mobility function at admission, age, and number of comorbidities. RESULTS: Participants were classified as having severe (n = 16), moderate (n = 39), or mild (n = 45) cognitive impairment at admission. Occupational therapy minutes per day (ß = 0.04; P < .01) and Functional Independence Measure (FIM) self-care function at admission (ß = 0.48; P < .01) were both significantly associated with self-care gain rate (Adjusted R2 = 0.18); cognitive impairment group, age, and number of comorbidities were not significant. Only FIM mobility function at admission (ß = 0.29; P < .001) was significantly associated with mobility gain rate (Adjusted R2 = 0.18); cognitive impairment group, physical therapy minutes, age, and number of comorbidities were not significant. CONCLUSIONS AND IMPLICATIONS: These results provide preliminary evidence that patients with stroke who have severe cognitive impairment may benefit from intensive therapy services as well as less severely impaired patients, particularly occupational therapy for improvement in self-care function.

Intraoperative stretching microbreaks reduce surgery-related musculoskeletal pain in otolaryngologists.

OBJECTIVE: The purpose of this study was to examine the role intraoperative otolaryngology stretching microbreak (OSMB) has on musculoskeletal (MSK) pain and discomfort in otolaryngologists. BACKGROUND: Otolaryngology procedures subject surgeons to significant amounts of pain and strain over their years of training. MSK pain is a serious concern for otolaryngologists' career longevity as well as their general wellbeing. METHODS: Participants from two different hospitals and one private practice were recruited to participate in this study. An initial ergonomic survey was obtained to assess baseline MSK pain, and its subjective impact on operative performance. The participants then completed three control days without OSMB exercises, followed by three intervention days with OSMB exercises which were completed at 20-40 minute intervals. Preoperative and postoperative pain rating surveys were completed before and after each procedure and at the end of the day to determine changes in pain and/or discomfort. RESULTS: Ten otolaryngologists (50 % men, 50 % women; mean age 35.6 years) participated in this study. Half of the participants indicated that they were concerned their pain would limit their ability to operate in the future. 70 % of participants indicated that they have not attempted to treat this pain and 60 % did not try any stretching exercises outside the operating room (OR) to mitigate their symptoms. Participants reported neck, upper back, and lower back to be the primary MSK discomfort. OSMB improved participants' pain scores in neck, shoulders, hands, and lower back pain (p < 0.05). CONCLUSION: MSK pain has shown to be a serious concern for the ability of otolaryngologists to continue performing surgery in the future. OSMB may be an effective strategy that can be implemented by otolaryngologists intraoperatively to improve MSK pain and overall well being.

Reporting of exercise dose and dosage and outcome measures for gaze stabilisation in the literature: a scoping review.

OBJECTIVES: The concept of this review is to examine and quantify the reporting of parameters of dose (duration, speed, head excursion) and dosage (daily and weekly frequency, duration) for gaze stabilisation exercises and to report on outcome measures used to assess change in gaze stabilisation following intervention. This review includes any population completing gaze stabilisation exercises. DESIGN: Scoping review. METHODS: We searched key terms in the following databases: PubMed, CINAHL, Scopus and Cochrane. Two researchers reviewed titles, abstracts and full-text articles for inclusion. Data retrieved included: patient diagnosis, specific interventions provided, dose and dosage of gaze stabilisation interventions and outcome measures. RESULTS: From the initial 1609 results, 138 studies were included. Data extraction revealed that only 13 studies (9.4%) reported all parameters of dose and dosage. Most studies used other interventions in addition to gaze stabilisation exercises. Half of the studies did not use a clinical or instrumented outcome measure of gaze stability, using only patient-reported outcome measures. Clinical tests of gaze stability were used in 21.1% of studies, and instrumented measures of gaze stability were used in 14.7% of studies. CONCLUSIONS: Full reporting of the dose and dosage of gaze stabilisation interventions is infrequent, impairing the ability to translate current evidence into clinical care. Most studies did not use a clinical or instrumented measure of gaze stabilisation as outcome measures, questioning the validity of intervention effects. Improved reporting and use of outcome measures are necessary to establish optimal intervention parameters for those with gaze stability impairments.

NIH Toolbox Cognition Battery in Young and Older Adults: Reliability and Relationship to Adiposity and Physical Activity.

BACKGROUND AND PURPOSE: Executive function in normal aging may be modulated by body habitus and adiposity, both factors modified by physical therapist prescriptions. This study measured between-day reliability of executive function metrics in young and older individuals and examined associations between cognition, adiposity, and physical activity. METHODS: Forty-three young and 24 older participants underwent executive function assessment via the National Institutes of Health Toolbox Cognition Battery (Dimensional Change Card Sort, Flanker Inhibitory Control and Attention [Flanker], and List Sorting Working Memory [List Sorting]) at 7-day intervals. Between-day reliability was assessed via intraclass correlation (ICC). Responsiveness was assessed via between-day effect size and Cohen's d. Forward stepwise linear regression examined associations between cognition and age, body mass index, percent body fat, and a self-report measure of physical activity (International Physical Activity Questionnaire Short Form). RESULTS AND DISCUSSION: Executive function scores were higher for young participants than for older participants (all P < .002), consistent with typical age-related cognitive decline. Reliability of cognitive metrics was higher for older participants (ICC = 0.483-0.917) than for young participants (ICC = 0.386-0.730). Between-day effect sizes were approximately 50% smaller for older participants. Percent body fat significantly correlated with the Flanker Unadjusted Scale (P = .004, R2 = 0.0772). Neither vigorous nor total physical activity correlated with any cognitive metric. CONCLUSIONS: Older participants demonstrated greater between-day reliability for executive function measures, while young participants showed greater capacity to improve performance upon repeat exposure to a cognitive test (especially Flanker). Percent body fat correlated significantly with Flanker scores, while body mass index (an indirect measure of body fat) did not. Self-reported physical activity did not correlate with executive function. Cognitive response to physical therapist-prescribed exercise is a fertile ground for future research.

Association of Therapy Time Per Day With Functional Outcomes and Rate of Recovery in Older Adults After Elective Joint Replacement Surgery.

OBJECTIVES: To explore the association between therapy minutes per length of stay (LOS) day (TMLD), functional outcomes, and rate of functional recovery among older adults after elective hip or knee replacement surgery across postacute (PAC) settings. DESIGN: Secondary analysis of data collected for an observational cohort study from 2005 to 2010. SETTING: Four inpatient rehabilitation facilities (IRF) and 7 skilled nursing facilities (SNF). PARTICIPANTS: Adults aged 65 years or older (N=162) with Medicare fee-for-service insurance and a primary diagnosis of elective hip or knee replacement. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: FIM mobility and self-care measures at discharge. RESULTS: The TMLD was divided into high, medium, and low categories. Participants were grouped into low, medium, and high gain rate groups based on their average change in mobility and self-care FIM measures per LOS day. Gain rate and TMLD groups were crossmapped to create 9 gain-TMLD groups separately for mobility and self-care. There were no significant differences in admission mobility or self-care measures by gain rate and TMLD trajectory or by facility type (IRF or SNF). TMLD was not significantly associated with discharge mobility measures. Participants in high gain trajectories attained independence with mobility and self-care tasks at discharge regardless of TMLD. Those in low gain trajectories needed supervision or assistance on all mobility tasks. Older age and greater pain at discharge were significantly associated with lower odds of being in the medium or high gain rate groups. CONCLUSIONS: For clinicians and facility managers who must care for patients with constrained resources, the shift to value-based reimbursement for rehabilitation services in PAC settings has reinvigorated the question of whether the duration of therapy provided influences patient outcomes. Three hours of daily therapy after joint replacement surgery may exceed what is necessary for recovery. Postsurgical pain management remains a significant challenge in older adults.

Hand-held dynamometer to measure pressure pain thresholds: A double-blinded reliability and validity study.

BACKGROUND: Pain is the most common complaint reported in the musculoskeletal setting. Quantitative sensory testing (QST) assists with pain mechanism identification, although QST is typically performed in research settings. It is possible that clinical utilization of QST may improve if clinically accessible tools can be reliably and validly used. OBJECTIVE: To determine if a hand-held dynamometer (HHD) can be a valid and reliable assessment of pressure pain threshold (PPT). DESIGN: Double-blinded validation study. METHODS: Eighteen healthy subjects (25.6 ± 3.4 years old) participated in this study. Two testers independently assessed PPT using a HHD and a digital algometer. Assessments followed previously described pressure algometry protocols. Testers and subjects were each blinded to data during assessments. RESULTS: Intra- and inter-rater reliability were excellent for the foot and face for both devices (ICC's > 0.9). Bland-Altman plots and intraclass correlation coefficients revealed good-excellent agreement with minimal proportional bias when normalizing device force at pain threshold to the circumference of the device applicator (ICC 95%CI: 0.56-0.95). Only poor-good agreement (ICC 95% CI: 0.30-0.76) and significant proportional bias was observed when normalizing to area (pressure). CONCLUSIONS: Based on the results of this study, when force is normalized by circumference of the applicator, a HHD was found to be a valid and reliable tool for measuring PPT. Clinicians may use HHD to detect relevant pain mechanisms at fault in their evaluation and treatment of pain. Additional research in various pathologic populations is warranted.

Age and Cognitive Stress Influences Motor Skill Acquisition, Consolidation, and Dual-Task Effect in Humans.

This study examined motor skill learning using a weight-bearing and cognitive-motor dual-task that incorporated unexpected perturbations and measurements of cognitive function. Forty young and 24 older adults performed a single-limb weight bearing task with novel speed, resistance, and cognitive dual task conditions to assess motor skill acquisition, retention and transfer. Subjects performed a cognitive dual task: summing letters in one color/orientation (simple) or two colors/orientations (complex). Increased cognitive load diminished the rate of skill acquisition, decreased transfer to new conditions, and increased error rate during an unexpected perturbation; however, young adults had a dual-task benefit from cognitive load. Executive function predicted 80% of the variability in dual-task performance. Although initial learning of a weight-bearing cognitive-motor dual-task was poor, longer term goals of improved dual-task effect and retention emerged.

Hybrid stimulation enhances torque as a function of muscle fusion in human paralyzed and non-paralyzed skeletal muscle.

OBJECTIVE: After spinal cord injury (SCI), hybrid stimulation patterns that interpose paired-pulse doublets over a constant-frequency background may enhance the metabolic "work" (muscle torque) performed by paralyzed muscle. This study examined the effect of background stimulation frequency on the torque contribution of the doublet before and after fatigue. DESIGN: Cross-sectional study. SETTING: Research laboratory in an academic medical center. PARTICIPANTS: Five men with chronic sensory and motor-complete SCI and ten non-SCI controls (6 males, 4 females). SCI subjects were recruited from a long-term study of unilateral plantar-flexor training; both limbs were tested for the present study. INTERVENTIONS: Subjects underwent plantar flexor stimulation at 5, 7, 9, and 12 Hz. The four background frequencies were overlaid with 6 ms doublets delivered at the start, middle, or at both the start and middle of each train. The 5 Hz and 12 Hz frequencies were analyzed after fatigue. OUTCOME MEASURES: Mean torque, peak torque, torque fusion index, doublet torque. RESULTS: Trains with doublets at both the start and middle yielded the most consistent enhancement of torque (all P < 0.028). Torque contribution of the doublet was greatest at low stimulus frequencies (all P < 0.016). The low relative fusion of untrained paralyzed muscle preserved the efficacy of the doublet even during fatigue. CONCLUSION: Hybrid stimulus trains may be an effective way to increase contractile work in paralyzed muscle, even after fatigue. They may be useful for rehabilitation strategies designed to enhance the metabolic work performed by paralyzed skeletal muscle.

Speed, resistance, and unexpected accelerations modulate feed forward and feedback control during a novel weight bearing task.

We developed a method to investigate feed-forward and feedback movement control during a weight bearing visuomotor knee tracking task. We hypothesized that a systematic increase in speed and resistance would show a linear decrease in movement accuracy, while unexpected perturbations would induce a velocity-dependent decrease in movement accuracy. We determined the effects of manipulating the speed, resistance, and unexpected events on error during a functional weight bearing task. Our long term objective is to benchmark neuromuscular control performance across various groups based on age, injury, disease, rehabilitation status, and/or training. Twenty-six healthy adults between the ages of 19-45 participated in this study. The study involved a single session using a custom designed apparatus to perform a single limb weight bearing task under nine testing conditions: three movement speeds (0.2, 0.4, and 0.6Hz) in combination with three levels of brake resistance (5%, 10%, and 15% of individual's body weight). Individuals were to perform the task according to a target with a fixed trajectory across all speeds, corresponding to a∼0 (extension) to 30° (flexion) of knee motion. An increase in error occurred with speed (p<0.0001, effect size (eta2): η2=0.50) and resistance (p<0.0001, η2=0.01). Likewise, during unexpected perturbations, the ratio of perturbed/non-perturbed error increased with each increment in velocity (p<0.0014, η2=0.08), and resistance (p<0.0001, η2=0.11). The hierarchical framework of these measurements offers a standardized functional weight bearing strategy to assess impaired neuro-muscular control and/or test the efficacy of therapeutic rehabilitation interventions designed to influence neuromuscular control of the knee.

Physical demand levels in individuals completing a sports performance-based work conditioning/hardening program after lumbar fusion.

BACKGROUND CONTEXT: Pain and disability after lumbar fusion surgery contributes to the over $20 billion dollars spent in health-care costs and estimated $28 billion in lost wages annually. With the goal of returning to work, an intensive program designed to build functional strength may be used. Previous interventions for this subgroup report the outcome measure of return to work (RTW), but do not account for the physical demand of the job to which they are returning. This may account for varying RTW and re-injury rates. PURPOSE: To examine the effectiveness of a sports performance-based work conditioning/hardening (SPWC/H) program on increasing an individual's strength measured by achievement of physical demand level (PDL) job classification of individuals followed by workers' compensation having had lumbar fusion surgery. STUDY DESIGN/SETTING: An uncontrolled multicenter, retrospective observational study of visits from 1999-2002 in an outpatient physical therapy setting. PATIENT SAMPLE: Fifty-four patients having undergone lumbar fusion surgery, managed by workers' compensation, that successfully completed a SPWC/H program. OUTCOME MEASURE: Physiologic measures: Deadlift and overhead press lifts, defined as maximum weight, a patient is able to lift between 8 and 15 repetitions. Functional measures: Calculated deadlift and overhead press volume (DLv, OHv) and estimated one repetition maximum (DLm, OHm). Physical demand level (PDL) for first (pre) and last week (post) are defined as: light (L<20lb occasionally), light/medium (LM>20lb occasionally), medium (M, 50lb occasionally), medium heavy (MH, 75lb occasionally), heavy (H, 100lb occasionally), and very heavy (VH>100lb occasionally), where 'occasionally' for the purposes of this article, is defined as in the 8-15 repetition range. METHODS: Patients completed a greater than or equal to 4 week, 4-5 days/wk, SPWC/H program. This program combines traditional concepts of strength and endurance training of work conditioning (WC) and hardening (WH) programs, with the sports performance concept of periodization in resistance training volume and intensity. Best set overhead and press lifts were obtained from each patient during the first and last week of the program. RESULTS: Significant increase between pre- and post-DLv, DLm, OHv, and OHm (all p<0.0001) existed when grouping all subjects. When adjacent groups are merged into M/MH and H/VH, significant differences existed between groups and pre- and postlifts (p<0.05). There was a median increase of three classifications when grouping by pre-PDL. There was no difference in outcomes found by grouping by single or multiple levels fused. Overall, numbers in each starting PDL were: 41 (75.9%) light, 6 (11.1%) LM, and 7 (13%) in medium. Numbers ending in each PDL group were: 1 (1.9%) light, 2 (3.7%) LM, 7 (13%) medium, 19 (35.2%) medium/heavy, 5 (9.3%) heavy, and 20 (37%) at very high. CONCLUSIONS: Post-lumbar spinal fusion patients are typically at the light PDL (<20lb occasionally) on completion of traditional physical therapy program. After an SPWC/H program, significant increase strength of deadlift and overhead lift volume and one repetition maximum demonstrated a median three-level increase in classification of PDL. We were also able to determine that there was no difference in strength outcome between those with a single- vs. multiple-level fusion surgery. Although the vast majority of individuals entered the program at the lowest PDL (20lb or less occasionally), more than 80% of patients completed the program at PDL of medium (50lb occasionally) or above, and 37% of patients achieved the maximum PDL (over 100lb occasionally). Future studies are needed to determine if increases in strength determined by PDL classification such as these relates to increased RTW rates and decreased re-injury rates.

Feedback-controlled stimulation enhances human paralyzed muscle performance.

Chronically paralyzed muscle requires extensive training before it can deliver a therapeutic dose of repetitive stress to the musculoskeletal system. Neuromuscular electrical stimulation, under feedback control, may subvert the effects of fatigue, yielding more rapid and extensive adaptations to training. The purposes of this investigation were to 1) compare the effectiveness of torque feedback-controlled (FDBCK) electrical stimulation with classic open-loop constant-frequency (CONST) stimulation, and 2) ascertain which of three stimulation strategies best maintains soleus torque during repetitive stimulation. When torque declined by 10%, the FDBCK protocol modulated the base stimulation frequency in three ways: by a fixed increase, by a paired pulse (doublet) at the beginning of the stimulation train, and by a fixed decrease. The stimulation strategy that most effectively restored torque continued for successive contractions. This process repeated each time torque declined by 10%. In fresh muscle, FDBCK stimulation offered minimal advantage in maintaining peak torque or mean torque over CONST stimulation. As long-duration fatigue developed in subsequent bouts, FDBCK stimulation became most effective ( approximately 40% higher final normalized torque than CONST). The high-frequency strategy was selected approximately 90% of the time, supporting that excitation-contraction coupling compromise and not neuromuscular transmission failure contributed to fatigue of paralyzed muscle. Ideal stimulation strategies may vary according to the site of fatigue; this stimulation approach offered the advantage of online modulation of stimulation strategies in response to fatigue conditions. Based on stress-adaptation principles, FDBCK-controlled stimulation may enhance training effects in chronically paralyzed muscle.

Sustained muscle activity minimally influences dynamic position sense of the ankle.

STUDY DESIGN: Cross-sectional study. OBJECTIVE: The purpose of this study was to determine if a sustained fatiguing contraction of the dorsiflexor muscles alters the dynamic position sense (proprioception) and the associated central nervous system processing time of information from the ankle. BACKGROUND: Ankle injury has been hypothesized to be related to altered proprioception as a consequence of fatiguing exercise. Previous reports assessing proprioception include tests of motor performance (balance and limb repositioning) or tests of a joint under static conditions. This study used a novel experimental approach to test the effects of exercise on the somatosensory system of the ankle. METHODS AND MEASURES: Nineteen healthy subjects were tested on their ability to extend the metacarpophalangeal joint of their left index finger when their left ankle was passively plantar flexed (0 degrees-40 degrees, 10 velocities) through a predetermined target angle (20 degrees). Testing occurred before and after a fatiguing contraction of the dorsiflexor muscles. RESULTS: Subjects accurately indicated the ankle target angle up to ankle velocities of 70 degrees/s (300 ms) both before and after the sustained fatiguing contraction. At velocities above 70 degrees/s all subjects could no longer scale to accurately indicate the target angle with the index finger and consequently overshot the target. The central nervous system processing time was estimated to be approximately 85 milliseconds before and after the sustained contraction. CONCLUSIONS: These results indicate that a sustained activity of the dorsiflexion muscles of the ankle minimally affects dynamic position sense and the ability to process dynamic position sense information. Understanding the impact of exercise on sensory system processing will be integral to establishing the scientific basis for rehabilitation programs that purport to train proprioception.

Proprioceptive coordination of movement sequences in humans.

OBJECTIVE: To estimate the processing time and neuromuscular delay required to extract and process sensory information from the ankle in order to coordinate an upper extremity movement sequence. METHODS: Nineteen able-bodied subjects were tested on their ability to perform a motor task that involved extension of their left index finger when their left ankle was passively plantar flexed at random velocities through a predetermined target angle. RESULTS: We found that the able-bodied subjects were able to adjust their finger responses up to ankle velocities of 70 degrees /s (300 ms). Reaction time, defined as the delay between onset of ankle rotation and how quickly the index finger could be extended, was 215 ms. The processing time and conduction delay was estimated to be approximately 85 ms. CONCLUSIONS: These results indicate that the nervous system processes kinesthetic input related to joint rotation of the ankle with the central mechanisms to execute a planned coordinated task with the upper extremity. SIGNIFICANCE: The time required to process proprioceptive information from the leg to perform a coordinated task with the upper extremity may vary throughout the lifespan. Understanding the effects of age, exercise, or injury on proprioceptive processing time may have important clinical implications.

Laboratory Investigation of a New Trommel Model.

Rotary screens, or trommels, are an important unit operation in material and fuel processing. A computer model has been developed based upon fundamental mechanics. The coefficients and variables employed in the model thus have real physical meaning; adjusting them based upon laboratory data allows the model user to draw conclusions about the behavior of the trommel that can be applied to design and operational changes. The laboratory testing program was specifically designed to test the model described in this paper. The model proved able to track the laboratory data. Important phenomena that were validated or revealed included the significance of particle layering, changes in bed sliding with rotational velocity, and the pre-eminence of residence time.