• View in gallery

    Mean relative change in VFP by repetition number within a trial for lame (white circles) and contralateral (black circles) limbs of 20 horses with naturally occurring lameness that trotted over a force plate. Relative change was calculated as the ratio of VFP for a given repetition within a session to that of the first repetition in that session for the same horse. Lame limbs were less lame (increased VFP) after the third repetition, and VFP for the contralateral limb increased from the first repetition at the fourth repetition. Error bars represent SEM. *Value differs significantly (P ≤ 0.01) from that for the first repetition within the same limb.

  • View in gallery

    Mean CV for VFP by repetition number within a trial for the horses represented in Figure 1. Lame limbs had greater variability (ie, the CV increased) from step to step at earlier versus later repetitions, corresponding to horses becoming less lame on the lame limb with repetition. Similarly, CVs for earlier repetitions of the lame limb were larger than those for the contralateral limb, again supporting the lameness data. *Value differs significantly (P ≤ 0.05) from that for the second repetition within the lame limb. †Value differs significantly between limbs in the same horse at the same point. See Figure 1 for remainder of key.

  • View in gallery

    Mean AI for VFP repetition number (A) and session (B) within a trial for the horses represented in Figure 1. The Al-VFP values, which reflect differences between lame and contralateral limbs, did not change with repetition because both the lame and contralateral limbs changed in a parallel (same) direction, indicating that this parameter was a poor estimate of the severity of naturally occurring lameness. See Figure 1 for remainder of key.

  • View in gallery

    Mean subjectively assessed lameness grade by session within a trial for all horses represented in Figure 1 (A) and for horses in which lameness was attributed to osteoarthritis (n = 11; B) or desmitis and tendonitis (1; C). Grades were assigned as follows: 0 = no lameness, 1 = intermittently lame at the trot, 2 = consistently lame at the trot under circumstances of lunging in a dirt arena, 3 = obviously lame at the trot, 3.5 = lame at the walk on turns but not on the straightaway and consistently lame at the trot, 4 = lame at the walk on the straightaway, and 5 = partially or non-weight bearing at the walk. Findings suggested that horses became less lame with repetitions performed every 3 hours (exercise) and returned to 0-hour values with 6 or 12 hours separating repetitions. Horses with osteoarthritis appeared to be primarily responsible for the decrease in lameness, given that horses with tendonitis or desmitis lacked a similar pattern. *†Value differs significantly (*P ≤ 0.05; †P < 0.01) between data points marked by the ends of the brackets. See Figure 1 for remainder of key.

  • View in gallery

    Mean relative change in VFP from the 0-hour (first) session by subsequent session (ie, ratio of the subsequent trial VFP to the 0-hour VFP) for all horses represented in Figure 1 (A) and for horses in which lameness was attributed to osteoarthritis (n = 11; B) or desmitis and tendonitis (1; C). Lame limbs became less lame (ie, the VFP increased) with the earlier (every 3 hours) trial sessions and then were significantly lamer (ie, the VFP decreased) by the last session (at 24 hours). Contralateral limbs became significantly lamer after the session held at 12 hours. In concert, lameness increased in both lame and contralateral limbs at later trial sessions. Findings suggested a subclinical bilateral condition in horses with naturally occurring lameness. The lameness profile observed for all horses was primarily attributable to the horses with osteoarthritis because, unlike horses with osteoarthritis, horses with desmitis or tendonitis became lamer over time in both the lame and contralateral limbs. See Figures 1 and 4 for remainder of key.

  • View in gallery

    Scatter plots, linear regression lines of best fit, and correlations (r values) for changes in VFP with duration of data collection (A), velocity (B), body weight (C), and lameness score (D) for the horses represented in Figure 1. Of the 4 variables, only lameness score was significantly (P ≤ 0.008) correlated with change in VFP, but the strength of that correlation (r = 0.27) was low, indicating a small but significant influence on overall findings. See Figure 1 for remainder of key.

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Effects of repetition within trials and frequency of trial sessions on quantitative parameters of vertical force peak in horses with naturally occurring lameness

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  • 1 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.
  • | 2 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.
  • | 3 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.
  • | 4 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.
  • | 5 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.
  • | 6 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.

Abstract

OBJECTIVE To analyze the effects of vertical force peak (VFP) of repition within trials and between trial sessions in horses with naturally occurring appendicular lameness.

ANIMALS 20 lame horses acclimated to trotting over a force plate.

PROCEDURES Kinetic gait data were collected by use of a force plate regarding affected and contralateral limbs of lame horses that completed 5 valid repetitions in each of 5 sessions performed at 0, 3, 6, 12, and 24 hours, constituting 1 trial/horse. Data were compared within and among repetitions and sessions, and factors influencing VFP values were identified.

RESULTS VFP values differed for lame limbs after 3 valid repetitions were performed within a session and when the interval between sessions was 3 hours. Direction of change reflected less lameness (greater VFP). Lamer horses (≥ grade 4/5) had this finding to a greater degree than did less lame horses. Results were similar for contralateral limbs regarding valid repetitions within a session; however, VFP decreased when the interval between sessions exceeded 6 hours. The coefficient of variation for VFP was ≤ 8% within sessions and ≤ 6% between sessions. The asymmetry index for VFP did not change throughout the study.

CONCLUSIONS AND CLINICAL RELEVANCE Lameness profiles obtained through kinetic gait analysis of horses with naturally occurring lameness were most accurate when valid repetitions were limited to 3 and the interval between sessions within a trial was > 3 hours. Findings suggested that natural lameness may be as suitable as experimentally induced lameness for lameness research involving horses.

Abstract

OBJECTIVE To analyze the effects of vertical force peak (VFP) of repition within trials and between trial sessions in horses with naturally occurring appendicular lameness.

ANIMALS 20 lame horses acclimated to trotting over a force plate.

PROCEDURES Kinetic gait data were collected by use of a force plate regarding affected and contralateral limbs of lame horses that completed 5 valid repetitions in each of 5 sessions performed at 0, 3, 6, 12, and 24 hours, constituting 1 trial/horse. Data were compared within and among repetitions and sessions, and factors influencing VFP values were identified.

RESULTS VFP values differed for lame limbs after 3 valid repetitions were performed within a session and when the interval between sessions was 3 hours. Direction of change reflected less lameness (greater VFP). Lamer horses (≥ grade 4/5) had this finding to a greater degree than did less lame horses. Results were similar for contralateral limbs regarding valid repetitions within a session; however, VFP decreased when the interval between sessions exceeded 6 hours. The coefficient of variation for VFP was ≤ 8% within sessions and ≤ 6% between sessions. The asymmetry index for VFP did not change throughout the study.

CONCLUSIONS AND CLINICAL RELEVANCE Lameness profiles obtained through kinetic gait analysis of horses with naturally occurring lameness were most accurate when valid repetitions were limited to 3 and the interval between sessions within a trial was > 3 hours. Findings suggested that natural lameness may be as suitable as experimentally induced lameness for lameness research involving horses.

Lameness is the most common cause of poor performance of racing and sport horses.1,2 The mainstay for evaluation of gait abnormalities is subjective lameness assessment by an experienced equine practitioner and grading with an interval scale such as the AAEP lameness scale.3 However, studies4–8 have shown that subjective lameness assessment can be insufficient for evaluation of horses with mild or multilimb lameness or subclinical disease or when the observer is inexperienced. Moreover, even when observers are experienced, interobserver variation in lameness scores assigned by use of video recordings can also be high5; therefore, it is recommended that lameness scores be assigned by the same experienced assessor when subjective assessment is performed.5

Because of these controversies regarding subjective lameness assessment, objective assessment is highly recommended. Objective assessment mainly involves kinematic and kinetic techniques. Kinematic analysis can be achieved with an inertial sensor system and videographic analysis involving an optoelectronic system. Such analysis is composed of measurements made with stationary force plates, force-measuring treadmills, strain gauges, dynamometric horseshoes, and pressure-sensitive pads, shoes, or film.9–15 These methods are being integrated in veterinary research to more accurately assess the effectiveness of new drugs, devices, shoeing, and treatments for lame horses, and they supplement subjective scoring methods.16 Kinetic gait analysis by stationary force plate to measure ground reaction force is the most commonly reported objective method for veterinary lameness evaluation and is considered the reference standard.4,8,17 Ground reaction force is equal in magnitude and opposite in direction to the force exerted by the hoof against the ground.18 Therefore, use of stationary force plates provides 3 separate data points, including craniocaudal, mediolateral, and vertical parameters, to characterize the 3-D force applied when horses trot or walk across a force plate.

Repeatability, accuracy, high sensitivity, and specificity have been established for force plate measurements of changes and consistencies in the specific ground reaction forces.4,7,17,19 However, although force plate analysis is one of the most important objective methods of lameness evaluation, few studies have been reported on the effect of repetition on results obtained in the collection of a set of force plate assessments. One reported study8 involved horses with experimentally induced lameness.8 In that study, 8 valid repetitions (correct limb on the plate within velocity criteria) were included in 1 session, and lameness severity decreased significantly irrespective of lameness grade (mild to severe) after 5 valid repetitions. Investigators concluded that lameness evaluations should be limited to 5 valid repetitions/session to avoid altering the lameness profile. Another studya revealed variability in vertical displacement of trotting horses with experimentally induced lameness. Strides were randomly selected from 60 repeated measurements (repetitions), and investigators concluded that at least 15 measurements (repetitions) were required to reflect the mean of all 60 measurements.

To our knowledge, no reports have been published on the influence of repetitive lameness assessments on lameness severity in horses with naturally occurring lameness or over multiple assessment periods (multiple sessions), which are both relevant to any lameness assessment. The purpose of the study reported here was to analyze the effects of successive valid repetitions within 1 session (repetition order within session) and the effect of consecutive repetitive sessions (every 3 hours for 2 sessions, then 6 hours for 1 session, then 12 hours for 1 session) on lameness severity in horses with naturally occurring lameness undergoing kinetic gait analysis by use of a stationary force plate.

Materials and Methods

Animals

Kinetic data used in the present study were obtained from 20 client-owned horses (age range, 2 to 18 years) with naturally occurring appendicular musculoskeletal disease enrolled between December 2013 and May 2014 in an as-yet unreported prospective, single-center clinical trial involving lameness. All horses in that study had undergone kinetic gait analysis by use of a force plate to which they had been acclimated.

Inclusion criteria at enrollment included clinical evidence of musculoskeletal disease (osteoarthritis, desmitis, tendinitis, or foot pain), observable lameness with a grade of 3 to 5 of 5 on the AAEP lameness scale,20 and localization of lameness to the lame limb by means of palpation, flexion tests, and perineural or intra-articular anesthesia within 30 days prior to enrollment. Disease was also confirmed and categorized by use of radiography or ultrasonography. Horses were otherwise systemically healthy or with stable concomitant disease. Exclusion criteria included fracture of a long bone, lack of limb stability, radiographic evidence of a rotating distal phalanx, gas pockets indicating abscess in the affected limb, or opiate administration within 7 days prior to enrollment. Medications, manipulations, or treatments known or promoted to specifically reduce or alleviate pain were discontinued at least 72 hours prior to enrollment.

All experimental procedures were approved by the Institutional Laboratory Animal Care and Use Committee of The Ohio State University. The study was performed in accordance with the Good Laboratory Practice standards outlined by the FDA.21

Lameness evaluation

Prior to the kinetic gait analysis in the study from which the data were obtained, lameness grades had been assigned to each horse by an experienced equine clinician (ALB) at each assessment point by use of a modification of the AAEP lameness scale.8,19 In a 30-m-long lameness examination area with a hard paved surface, horses had been walked in 1 trip (back and forth), trotted in 2 trips, and trotted in 1 trip after joint flexion (only for horses enrolled with osteoarthritis), with 30 seconds for flexion tests of the distal portion of the limb (osteoarthritis in the fetlock, pastern, and coffin joints) and 60 seconds for flexion tests of the proximal portion of the limb (osteoarthritis in the carpus, tarsus, and stifle joints) or after adding pressure for 60 seconds to a tendon or ligament (tendinitis and desmitis). Lameness grades had been assigned as follows: 0 = no lameness, 1 = intermittently lame at the trot, 2 = consistently lame at the trot under circumstances of lunging in a dirt arena, 3 = obviously lame at the trot, 3.5 = lame at the walk on turns but not on the straightaway and consistently lame at the trot, 4 = lame at the walk on the straightaway, and 5 = partially weight bearing or non–weight bearing at the walk.

Kinetic gait trials

An examination aisle (3 × 20 m) with an in-ground, stationary force plateb and computer analysis systemc had been used for data collection in the study from which data for the present study were obtained. In that study, the central force plate and aisle were covered by a mat to prevent horses from slipping or recognizing the plate. The mat was cut around the force plate to prevent distracting forces. Data sampling rate was 500 Hz. Gait velocity was measured by use of 2 photoelectric switches (spaced 5 m apart) that were connected to the computer analysis system. A valid kinetic gait trial involving a force plate was performed for both the lame limb and contralateral limb while the horse was trotting. Each trial (1/horse) was composed of 5 serial sessions of kinetic gait analysis, each with 5 repetitions for both the lame and contralateral limb (10 valid repetitions/session). Timing of the first session was designated as 0 hours, and subsequent sessions were performed 3, 6, 12, and 24 hours later. A valid measurement was defined as a passage by the horse over the force plate during which the hoof of the limb of interest fully contacted the central surface of the plate. Gait velocity was maintained within the range of 2.5 to 3.5 m/s in accordance with velocities used in other studies,8,14,19,22,23 except for 3 horses that were too lame to maintain those velocities. These 3 horses were assigned the reduced velocity range of 2.0 to 3.0 m/s.

Force plate data had been collected in the other study until 5 valid force curves (repetitions) were obtained per limb, and duration of the data collection period was recorded. Vertical force peak was selected as the force plate parameter to represent lameness.8,19

For each repetition within a session in the other study, VFP was calculated by the computer analysis system on the basis of the vertical force-time curve, for which VFP was defined as the highest instantaneous force. The VFP values were standardized by horse body mass by dividing force by body mass, and VFP magnitude was reported as percentage of body weight.

For the present study, VFP was further analyzed for within-horse variability by calculation of the CV. For calculation of the CV for VFP (SD divided by the mean X 100), mean and SD values of VFP were calculated for each of the 5 repetitions to evaluate the effect of repetition within 1 session (CV-VFPwithin session) or for the 5 sessions to evaluate the effect of 5 consecutive sessions within the 24-hour period (CV-VFPamong sessions).

The VFP raw data collected for forelimbs and hind limbs were further evaluated by calculation of the AI for VFP (AI-VFP), which expressed VFP as a difference between the lame and contralateral limb and was calculated within horses as a percentage as follows19,24:

article image

Statistical analysis

Statistical analysis was performed by an investigator (MK) without knowledge of the results of the horses' subjective lameness assessments. Results are reported as mean ± SEM. Data were analyzed by use of a statistical computer program,d and values of P ≤ 0.05 were considered significant for all analyses. Kinetic parameters (VFP, AI-VFP, and CV-VFP) were analyzed by means of repeated-measures ANOVA to identify differences in repetition order within a session (first to fifth repetition) and in serial evaluation of sessions within the 24-hour period. The least significant difference test was subsequently performed to compare repetitions within sessions and among sessions. Afterward, horses were subcategorized into 3 disease groups (osteoarthritis, tendonitis or desmitis injury, and foot pain) for analysis as to repetitive session effect, and ANOVA was repeated for each disease.

The VFP values were normalized by calculation of the common logarithm (log10) and ratios of VFP for a given repetition within a session to that of the first repetition in that session for the same horse and for a given session within a trial to that of the 0-hour session for the same horse. The intention behind these calculations was to have each horse serve as its own control subject and to permit coevaluation of forelimb and hind limb lameness. Other methods of data normalization failed to produce significant statistical results. Data normalized to the 0-hour value were used for graphic purposes. Normalized VFP (relative VFP) is reported as VFP from here on in.

Linear regressione was used to investigate associations between several factors (duration of lameness data collection, lameness grade, body weight, and mean velocity) and VFP within and among sessions. Scatter plots and lines of best fit were graphed, and correlation coefficients (r) are reported.

Results

Horses

Mean ± SD age of the 20 lame horses included in the study was 12.8 ± 4.5 years (median, 14.5 years; range, 4 to 17 years), and mean body weight was 541.0 ± 78.2 kg (median, 550.5 kg; range, 384 to 697 kg). Fourteen horses were castrated males, and 6 horses were females. Five breed groups were represented (7 Quarter Horses, 6 warmblood sport horses, 4 Thoroughbreds, 2 Arabians, and 1 Appaloosa).

Lameness evaluations revealed that 11 horses had osteoarthritis, with locations including the metacarpophalangeal joint (3 horses), pastern joint (3), middle carpal joint (2), and metatarsophalangeal, tarsocrural, or stifle joint (1 each). Five horses had desmitis, with locations including the origin of the suspensory ligament (2 horses) or in the accessory ligament of the deep digital flexor tendon, medial suspensory branch, or lateral suspensory branch (1 each). Three horses had signs of foot pain, including 1 with apparent heel pain, 1 with laminitis, and 1 with osteitis of the third phalanx. The remaining horse had tendonitis of the superficial digital flexor tendon. Overall, 8 horses had lameness of the right forelimb, 6 had lameness of the left hind limb, 5 had lameness of the left forelimb, and 1 had lameness of the right hind limb.

Data collection

All horses in the clinical trial from which the data for this study were acquired completed that study. Mean time required for each horse to complete the subjective lameness evaluation and kinetic gait analysis had been 17.8 minutes (range, 9.5 to 35 minutes). The collected data were normally distributed and met the assumption of homoscedasticity required for ANOVA. For all horses, the VFP for the lame limb was lower than the VFP for the contralateral limb within and among sessions.

Effect of repetition within sessions

The VFP for the lame limb increased significantly between baseline (first valid repetition) and the fourth (P = 0.001) and fifth (P = 0.009) valid repetitions, reflecting less lameness (Figure 1). The VFP for the contralateral limb also increased significantly (P = 0.007) between baseline and the fourth repetition. In conjunction, the within-session CV-VFP for the lame limb decreased significantly (P = 0.05) between the second and fourth repetitions and was significantly (P = 0.05) greater than that of the contralateral limb at the second repetition (Figure 2). The within-session CV-VFP of both the lame and contralateral limbs was < 8%. The AI-VFP was not significantly affected by repetition order (Figure 3).

Figure 1—
Figure 1—

Mean relative change in VFP by repetition number within a trial for lame (white circles) and contralateral (black circles) limbs of 20 horses with naturally occurring lameness that trotted over a force plate. Relative change was calculated as the ratio of VFP for a given repetition within a session to that of the first repetition in that session for the same horse. Lame limbs were less lame (increased VFP) after the third repetition, and VFP for the contralateral limb increased from the first repetition at the fourth repetition. Error bars represent SEM. *Value differs significantly (P ≤ 0.01) from that for the first repetition within the same limb.

Citation: American Journal of Veterinary Research 77, 7; 10.2460/ajvr.77.7.756

Figure 2—
Figure 2—

Mean CV for VFP by repetition number within a trial for the horses represented in Figure 1. Lame limbs had greater variability (ie, the CV increased) from step to step at earlier versus later repetitions, corresponding to horses becoming less lame on the lame limb with repetition. Similarly, CVs for earlier repetitions of the lame limb were larger than those for the contralateral limb, again supporting the lameness data. *Value differs significantly (P ≤ 0.05) from that for the second repetition within the lame limb. †Value differs significantly between limbs in the same horse at the same point. See Figure 1 for remainder of key.

Citation: American Journal of Veterinary Research 77, 7; 10.2460/ajvr.77.7.756

Figure 3—
Figure 3—

Mean AI for VFP repetition number (A) and session (B) within a trial for the horses represented in Figure 1. The Al-VFP values, which reflect differences between lame and contralateral limbs, did not change with repetition because both the lame and contralateral limbs changed in a parallel (same) direction, indicating that this parameter was a poor estimate of the severity of naturally occurring lameness. See Figure 1 for remainder of key.

Citation: American Journal of Veterinary Research 77, 7; 10.2460/ajvr.77.7.756

Mean ± SD velocity over the 5 repetitions was 2.7 ± 0.03 m/s for the contralateral limb and 2.78 + 0.03 m/s for the lame limb. No significant difference in velocity over 5 repetitions was detected for either the lame or contralateral limb.

Effect of repeated sessions at 0, 3, 6, 12, and 24 hours

Irrespective of disease subclassification, subjective lameness scores were significantly lower than 0-hour values during the sessions held at 3 hours (P = 0.02), 6 hours (P = 0.001), and 12 hours (P = 0.003; Figure 4). Within disease groups, horses with osteoarthritis had similar patterns in lameness scores, whereas for horses with tendonitis, desmitis, or foot pain, lameness scores did not change significantly with assessment point, indicating that the horses with osteoarthritis were primarily responsible for the decreases in lameness scores identified at the shorter intervals between sessions. Horses with foot pain had grade 4 to 4.5 lameness throughout the sessions, and these scores did not decrease.

Figure 4—
Figure 4—

Mean subjectively assessed lameness grade by session within a trial for all horses represented in Figure 1 (A) and for horses in which lameness was attributed to osteoarthritis (n = 11; B) or desmitis and tendonitis (1; C). Grades were assigned as follows: 0 = no lameness, 1 = intermittently lame at the trot, 2 = consistently lame at the trot under circumstances of lunging in a dirt arena, 3 = obviously lame at the trot, 3.5 = lame at the walk on turns but not on the straightaway and consistently lame at the trot, 4 = lame at the walk on the straightaway, and 5 = partially or non-weight bearing at the walk. Findings suggested that horses became less lame with repetitions performed every 3 hours (exercise) and returned to 0-hour values with 6 or 12 hours separating repetitions. Horses with osteoarthritis appeared to be primarily responsible for the decrease in lameness, given that horses with tendonitis or desmitis lacked a similar pattern. *†Value differs significantly (*P ≤ 0.05; †P < 0.01) between data points marked by the ends of the brackets. See Figure 1 for remainder of key.

Citation: American Journal of Veterinary Research 77, 7; 10.2460/ajvr.77.7.756

Force plate analysis revealed the same patterns the lameness scores had with respect to the influence of repeated sessions on results. The VFP for the lame limb increased until the session held at 6 hours and then decreased by the session held at 24 hours (P = 0.05), reflecting similar change as the decrease in lameness grade (Figure 5). The VFP for the contralateral limb decreased after the session held at 3 hours, representing a detectable decrease in loading of the contralateral limb that was not detected by subjective lameness assessment. After the session held at 6 hours, body weight did not shift to the contralateral limb. In lame horses with osteoarthritis, VFP followed the same pattern and increased 5% by the session held at 6 hours and then decreased by the session held at 24 hours. In the same group of horses, the VFP for the contralateral limb also followed a pattern similar to that of all horses. In horses with tendonitis, desmitis, or foot pain, the VFP for the lame limb did not significantly change among sessions, but the VFP of the contralateral decreased significantly during some trial sessions. The within-session CV-VFP for the lame limb was significantly greater than that of the contralateral limb at the 0-hour session, but no other significant differences were detected (results not shown). The among-session CV-VFP for both the lame and contralateral limbs was ≤ 6%. The AI-VFP was not significantly affected by session (Figure 3).

Figure 5—
Figure 5—

Mean relative change in VFP from the 0-hour (first) session by subsequent session (ie, ratio of the subsequent trial VFP to the 0-hour VFP) for all horses represented in Figure 1 (A) and for horses in which lameness was attributed to osteoarthritis (n = 11; B) or desmitis and tendonitis (1; C). Lame limbs became less lame (ie, the VFP increased) with the earlier (every 3 hours) trial sessions and then were significantly lamer (ie, the VFP decreased) by the last session (at 24 hours). Contralateral limbs became significantly lamer after the session held at 12 hours. In concert, lameness increased in both lame and contralateral limbs at later trial sessions. Findings suggested a subclinical bilateral condition in horses with naturally occurring lameness. The lameness profile observed for all horses was primarily attributable to the horses with osteoarthritis because, unlike horses with osteoarthritis, horses with desmitis or tendonitis became lamer over time in both the lame and contralateral limbs. See Figures 1 and 4 for remainder of key.

Citation: American Journal of Veterinary Research 77, 7; 10.2460/ajvr.77.7.756

Mean ± SD velocity over the 5 sessions was 2.76 ± 0.02 m/s for the contralateral limb and 2.7 ± 0.03 m/s for the lame limb. No significant difference in mean velocity or duration of data collection over the 5 sessions was evident for either the lame or contralateral limb.

Correlations of variables with change in VFP

No significant correlations were identified between change in VFP between repetition 1 and 4 or 5 and duration of data collection (r = 0.04; P ≤ 0.4), mean velocity (r = 0.08; P ≤ 0.21), or body weight (r = 0.19; P = 0.06; Figure 6). However, initial lameness score was significantly (P < 0.008), although weakly (r = 0.27), positively correlated with the change in lameness severity with repetition. In summary, the change in lameness identified from repetition 1 to repetition 4 or 5 was greater for lamer versus less lame horses.

Figure 6—
Figure 6—

Scatter plots, linear regression lines of best fit, and correlations (r values) for changes in VFP with duration of data collection (A), velocity (B), body weight (C), and lameness score (D) for the horses represented in Figure 1. Of the 4 variables, only lameness score was significantly (P ≤ 0.008) correlated with change in VFP, but the strength of that correlation (r = 0.27) was low, indicating a small but significant influence on overall findings. See Figure 1 for remainder of key.

Citation: American Journal of Veterinary Research 77, 7; 10.2460/ajvr.77.7.756

Discussion

The purpose of the analyses performed in the study reported here was to evaluate of the number of valid repetitions and frequency of trial sessions that might result in altered gait profiles of horses with naturally occurring lameness. Results suggested that having horses perform > 3 valid repetitions/session resulted in a significant reduction in lameness severity, in conjunction with a reduction in within-session CV-VFP (reflecting lower variation) in the later repetitions. Our findings are supported by those in a previous report18 that lamer horses have more step-to-step variation than less lame horses, furthering the recommendation that 3 repetitions should be used in evaluations of naturally occurring lameness.

No significant difference in velocity was identified through 10 repetitions (5/limb; Figure 6), indicating the increase in VFP after the third repetition did not result from an increase in velocity. Findings were also unaffected by duration of lameness data collection. Although body size may affect stride length, which could alter the number of passages or influence the number of misses on the force plate, body weight had no effect on change in VFP in the present study. In the authors' experience, larger and less lame horses that move with greater ease than small lamer horses can generally overstep or avoid the force plate. However, in the analyses of key variables that could affect the collected data, only initial lameness severity (lameness score) was significantly correlated with change in VFP. Lamer horses were more likely to have the significant reduction in lameness after 3 repetitions in a session. Use of lameness grade cutoffs of ≥ 4 or ≥ 3.5 did not significantly increase the odds of having a reduction in lameness after repetition 3 (data not shown), as occurred in a study16 by our laboratory group involving horses with osteoarthritis treated with an autologous protein solution.16

In the study reported here, as in any study, it was difficult, if not impossible, to control the amount of exercise (jogging) from one horse to the next within an experiment. First, not all horses required the same number of passages over the force plate to get sufficient data to constitute a valid trial. Missteps may not have struck the force plate and therefore would not have produced a recording. A limitation of any gait assessment for comparative quantification is the difficulty or inability to control the exact number of steps or strikes within assessments, and the amount of exercise a horse receives may affect its lameness severity. We chose to use duration of data collection as an overall representation of these variables. As the analyses revealed, duration of data collection did not correlate with change in VFP, but rather, lameness severity did. Smaller horses had a nonsignificant (P = 0.06) reduction in lameness with repetition more than larger horses did. Data for smaller horses were generally faster to collect, and those horses were generally lamer than larger horses (data not shown). Regardless, the significant effect of lameness reduction after the third repetition held true.

Because the total number of passages over the force plate required to provide 10 valid ones was variable, we normalized values to the baseline value for each horse to minimize the effect of between-horse variation and maximize the difference within horses. Other investigators have reported that a valid strike on the force plate is typically achieved for every 2 to 6 passages at a walk, trot, and canter.10 Therefore, if 5 repetitions were needed for both the lame and contralateral limbs (for a total of 10 repetitions), as was the situation in the present study, then on average, approximately 20 to 60 strikes would be needed to provide 10 valid repetitions. In the present study, a mean of 20 to 30 seconds was needed to obtain 1 repetition (including for jogging horses) and to interpret and save the data, meaning that approximately 6.5 to 30 minutes would be required to obtain 10 valid repetitions for each horse. The range in duration of data collection was 9.5 to 30 minutes per horse. Therefore, although duration of data collection was variable, we believe it provided sufficient opportunity to obtain 10 valid repetitions.

Repeating sessions as frequently as every 3 hours also resulted in a significant reduction in lameness in the horses of the present study, suggesting that inter-repetition intervals > 3 hours should be used in future evaluations of horses with naturally occurring lameness in which multiple gait analysis sessions are performed. This finding was also observed in a study8 involving horses with experimentally induced lameness. Consideration of these findings would be relevant to future kinetic studies of lameness, research models, and subjective lameness assessment.

In the present study, 5 sessions were performed over a 24-hour period. In the authors' experience, signs of pain in horses with osteoarthritis are generally greatest in the morning. Timings of the first session for affected horses varied. Four of the 20 (20%) horses began between 7:00 am and 8:00 am, 6 (30%) began between 8:00 am and 9:00 am, 6 (30%) began between 9:00 am and 10:00 am, and 3 (15%) began after 10:00 am, indicating that the effect of time of day was small. Horses had a mean of 18 minutes of trotting exercise at each session. This period included subjective lameness assessment and force plate evaluation and was similar to that in previous studies.19,25,26 Our findings suggested that this steady low degree of exercise for lame horses was beneficial in relieving pain, specifically for horses with osteoarthritis, and lameness returned within 6 hours. If an accurate, repeatable reflection of baseline lameness is desired, then evaluation every 6 hours or greater is recommended.

Values of VFP in the present study for repetition within session and among sessions were normalized to baseline to evaluate changes in VFP while controlling for between-horse variability attributable to actual passage number, secondary lameness, and affected limb (forelimb vs hind limb) as well as factors known to influence VFP such as breed, height, age, and training level.18,27–29 Normalization of VFP by body weight (mass) and within horses provided the most accurate data interpretation.

Loading of the affected limb in horses with tendonitis or desmitis in the present study generally decreased with time (sessions) in both the affected and contralateral limb (Figure 5), suggesting that lameness severity worsened with session in horses with soft tissue injury. This was unlike the situation for horses with osteoarthritis, in which lameness severity improved with session. This may have been due to inclusion of horses with active disease of a fairly short duration (inclusion criteria required a minimum of 30 days). Rehabilitation for horses with tendinitis or desmitis typically involves a long-term program that includes walking and trotting, and the exercise period can also be as brief as 15 to 45 minutes, depending on the rehabilitation stage.30 Therefore, the amount of exercise provided in our study, which included subjective lameness assessment, lameness assessment after 60 seconds of pressure was applied to the affected site, force plate analysis, and walking to and from the stall, may have exceeded the amount reported to be beneficial,23,30 resulting in an increase in lameness severity with session. Consequently, horses with tendonitis or desmitis had an increase in lameness severity in the lame limb, thereby forcing a body weight shift to other limbs.25 Interestingly, VFP in the contralateral limb also continued to decrease (less loading) until the last session (at 24 hours), suggesting these horses had bilateral or other subclinical disease. The data indicated that body weight did not shift to the contralateral limb, as would be typical,31 but rather shifted to the diagonal or ipsilateral limb.

Several studies16,19,31,32 involving horses with experimentally induced or naturally occurring lameness have revealed that as the VFP for the lame limb decreases, indicating reduced loading, the load (VFP) on the contralateral limb increases, indicating increased loading. Continued compensation for affected limb lameness by the contralateral limb may have occurred in the present study, given that many horses with orthopedic disease have bilateral lameness,2,33–35 secondary disease, or disease undetectable via ultrasonography or radiography.36–38 The contralateral limb and other limbs in lame horses were evaluated for overt disease in the present study, but subclinical lameness and weight shifting to diagonal and ipsilateral limbs might have existed, supporting the assertion that force plate can be useful for detection of occult bilateral disease.

Regardless of the changes in contralateral limbs, lame limbs remained the predominant site of lameness for all repetitions within and among sessions in the study reported here. The force plate detected subtle changes in contralateral limbs that were undetected during subjective assessment. These data further supported previous findings,4,8,39 suggesting that force plate evaluation and other objective methods, such as inertial sensor assessment, could be used to elucidate subclinical changes in lameness severity. With subjective lameness assessment, the human eye is naturally drawn to the limb in which lameness predominates. The limited sensitivity of human evaluators is most likely caused by the limited temporal resolution of the human eye (15 to 20 Hz), which defines the precision of measurements involving time (sampling rate), compared with more sensitive methods such as force plate (500 Hz) and inertial sensor (200 Hz) systems.39 However, even though objective lameness techniques can detect subtle changes in gaits of lame horses, the equipment is expensive, and the procedures are time and space consuming and require a person able to interpret the data correctly. Moreover, objective methods have the possibility of false-positive results or negative data.4

In comparison, subjective assessment techniques are more affordable, often faster, and allow for evaluation of additional factors, such as turns and lunging, various footings, and other body movements or expressions. Furthermore, subjective assessment performed by an experienced clinician is accurate and reportedly reliable in most situations, excluding subclinical or extremely mild lameness.5,6 Therefore, subjective lameness assessment still can be of assistance when evaluating moderate to severe lameness, as was characteristic of the horses of the present study. Our results for horses with naturally occurring lameness were similar to those of the study7 in which change in subjective lameness grade (AAEP scale) and force plate analysis (VFP) over time in horses with experimentally induced lameness were well correlated; however, that was not an objective of the present study.

Experimentally induced lameness has been used in studies of lameness for many reasons, including the presumed lack of consistency that exists among horses with naturally occurring disease and the possibility of secondary disease or multiply affected limbs. The present study confirmed that the severity of natural lameness changed with repetition of gait analyses and that subclinical changes in VFP existed for limbs other than the identified lame limb. However, the study also revealed that CV-VFP of lame limbs was 8% over 5 repetitions and 6% over 5 sessions, which are low percentages similar to or less than those characteristic of horses with experimentally induced lameness.8 Consequently, we believe that naturally occurring lameness in horses should be suitable for research purposes and as an alternative to experimentally induced lameness.

The AI-VFP has been used as an index for lameness assessment of horses with experimentally induced lameness and helps correct for inclusion of both forelimbs and hind limbs.19,25,26,40 However, this index was unable to reflect the changes in lameness severity in the horses with naturally occurring lameness in the present study. Our study showed that both lame and contralateral limbs changed in the same direction (sounder or lamer) over repetitions within and among sessions, negating the benefit of AI-VFP in differentiating lame limbs from sound ones.19

Results of the present study indicated that 3 repetitions in 1 trial session were adequate for lameness assessment in horses with naturally occurring lameness, and trial sessions separated by > 3 hours provided the most consistent results and reflected the horses' natural lameness severity. A greater number of repetitions resulted in a significant reduction in apparent lameness severity. Exercise for many of the study horses significantly altered the results of the gait analyses. Normalization of lameness data to baseline values for individual horses enhanced the ability to detect changes in lameness severity over repetitions and sessions and facilitated comparisons among horses. These findings can be used to improve subjective and objective lameness assessments, standardize procedures, and improve research models. Given the low variability within repetitions in 1 session and among repeated trial sessions, the findings also suggested that horses with naturally occurring lameness could be used in lameness research instead of horses with experimentally induced lameness. Furthermore, kinetic gait analysis was an effective method for evaluation of changes in naturally occurring lameness, whether severe or mild, and could detect gait changes in contralateral limbs.

Acknowledgments

Supported in part by Elanco Animal Health.

The authors thank Drs. Navid Salmanzadeh, Rebecca Lovasz, and Jane Owens and Amy Marr, Logan Scheuermann, Ryan Yanez, Lindsey Johnson, and Katherine Peachey for technical assistance.

ABBREVIATIONS

AAEP

American Association of Equine Practitioners

AI

Asymmetry index

CV

Coefficient of variation

VFP

Vertical force peak

Footnotes

a.

Starke SD. The effect of induced and alleviated equine lameness on upper body movement and consequences of sampling different stride numbers (oral presentation). 7th Int Conf Canine Equine Locomotion, Strömsholm, Sweden, June 2012.

b.

Model 9287B, Kistler Instrument Corp, Amherst, NY.

c.

Bioware software type 2812A, Kistler Instrument Corp, Amherst, NY.

d.

SPSS, version 22.0, SPSS Inc, Chicago, Ill.

e.

Excel 2013, Microsoft Corp, Redmond, Wash.

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Contributor Notes

Address correspondence to Dr. Bertone (bertone.1@osu.edu).