A low profile human tendon force transducer: the influence of tendon thickness on calibration

Modelling and in vivo evaluation of tendon forces and strain in dynamic rehabilitation exercises: a scoping review

OBJECTIVES

Although exercise is considered the preferred approach for tendinopathies, the actual load that acts on the tendon in loading programmes is usually unknown. The objective of this study was to review the techniques that have been applied in vivo to estimate the forces and strain that act on the human tendon in dynamic exercises used during rehabilitation.

DESIGN

Scoping review.

DATA SOURCES

Embase, PubMed, Web of Science and Google Scholar were searched from database inception to February 2021.

ELIGIBILITY CRITERIA

Cross-sectional studies available in English or Spanish language were included if they focused on evaluating the forces or strain of human tendons in vivo during dynamic exercises. Studies were excluded if they did not evaluate tendon forces or strain; if they evaluated running, walking, jumping, landing or no dynamic exercise at all; and if they were conference proceedings or book chapters.

DATA EXTRACTION AND SYNTHESIS

Data extracted included year of publication, study setting, study population characteristics, technique used and exercises evaluated. The studies were grouped by the types of techniques and the tendon location.

RESULTS

Twenty-one studies were included. Fourteen studies used an indirect methodology based on inverse dynamics, nine of them in the Achilles and five in the patellar tendon. Six studies implemented force transducers for measuring tendon forces in open carpal tunnel release surgery patients. One study applied an optic fibre technique to detect forces in the patellar tendon. Four studies measured strain using ultrasound-based techniques.

CONCLUSIONS

There is a predominant use of inverse dynamics, but force transducers, optic fibre and estimations from strain data are also used. Although these tools may be used to make general estimates of tendon forces and strains, the invasiveness of some methods and the loss of immediacy of others make it difficult to provide immediate feedback to the individuals.

Upper Limb End-Effector Force Estimation During Multi-Muscle Isometric Contraction Tasks Using HD-sEMG and Deep Belief Network

In this study, research was carried out on the end-effector force estimation of two representative multi-muscle contraction tasks: elbow flexion and palm-pressing. The aim was to ascertain whether an individual muscle or a combination of muscles is more suitable for the end-effector force estimation. High-density surface electromyography (HD-sEMG) signals were collected from four primary muscle areas of the upper arm and forearm: the biceps brachii (BB), brachialis (BR), triceps brachii (TB), brachioradialis (BRD), and extensor digitorum communis (EDC). The wrist pulling and palm-pressing forces were measured in elbow flexion and palm-pressing tasks, respectively. The deep belief network (DBN) was adopted to establish the relation between HD-sEMG and the measured force. The representative signals of the four primary areas, which were considered as the input signal of the force estimation model, were extracted by HD-sEMG using the principle component analysis (PCA) algorithm, and fed separately or together into the DBN. An index termed mean impact value (MIV) was proposed to describe the priority of different muscle groups for estimating the end-effector force. The experimental results demonstrated that, in multi-muscle isometric contraction tasks, the dominant muscles with the highest activation degree could track variations in the end-effector force more effectively, and are more suitable than a combination of muscles. The main contributions of this research are as follows: (1) To fuse the activation information from different muscles effectively, DBN was adopted to establish the relationship between HD-sEMG and the generated force, and achieved highly accurate force estimation. (2) Based on the well-trained DBN force estimation model, an index termed MIV was presented to evaluate the priority of muscles for estimating the generated force.

In vivo flexor tendon forces generated during different rehabilitation exercises

We measured in vivo forces in the flexor digitorum profundus and the flexor digitorum superficialis tendons during commonly used rehabilitation manoeuvres after flexor tendon repair by placing a buckle force transducer on the tendons of the index finger in the carpal canal during open carpal tunnel release of 12 patients. We compared peak forces for each manoeuvre with the reported strength of a flexor tendon repair. Median flexor digitorum profundus force (24 N) during isolated flexor digitorum profundus flexion and median flexor digitorum superficialis force (13 N) during isolated flexor digitorum superficialis flexion were significantly higher than during the other manoeuvres. Significantly higher median forces were observed in the flexor digitorum superficialis with the wrist at 30° flexion (6 N) compared with the neutral wrist position (5 N). Median flexor digitorum profundus forces were significantly higher during active finger flexion (6 N) compared with place and hold (3 N). Place and hold and active finger flexion with the wrist in the neutral position or tenodesis generated the lowest forces; isolated flexion of these tendons generated higher forces along the flexor tendons.

LEVEL OF EVIDENCE

III (controlled trial without randomization).

A device to measurein vivoforces in human flexor tendons during carpal tunnel operations

An apparatus has been designed to measure the in vivo forces transmitted along human hand flexor tendons during carpal tunnel release procedures. The tendon will be run through three hooks, the central one of which is attached to a load cell. The rationale is that once these forces are known this will aid in the design of repair techniques and rehabilitation regimens. As a first stage, an in vitro validation study is presented using cord subjected to varying forces to mimic an in vivo flexor tendon under varying conditions of use. Our results show that we can accurately and reproducibly measure the force in the cord.

Finger flexor motor control patterns during active flexion: an in vivo tendon force study

An in vivo tendon force measurement system was used to evaluate index finger flexor motor control patterns during active finger flexion. During open carpal tunnel release surgery (N=12) the flexor digitorum profundus (FDP) and flexor digitorum superficilias (FDS) tendons were instrumented with buckle force transducers and participants performed finger flexion at two different wrist angles (0 degrees or 30 degrees ). During finger flexion, there was concurrent change of metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joint angles, but the FDP and FDS tendon force changes were not concurrent. For the FDS tendon, no consistent changes in force were observed across participants at either wrist angle. For the FDP tendon, there were two force patterns. With the wrist in a neutral posture, the movement was initiated without force from the finger flexors, and further flexion (after the first 0.5s) was carried out with force from the FDP. With the wrist in a flexed posture, the motion was generally both initiated and continued using FDP force. At some wrist postures, finger flexion was initiated by passive forces which were replaced by FDP force to complete the motion.

In vivo flexor tendon forces increase with finger and wrist flexion during active finger flexion and extension

The effects of different hand motions and positions used during early protected motion rehabilitation on tendon forces are not well understood. The goal of this study was to determine in vivo forces in human flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS) tendons of the index finger during active unresisted finger flexion and extension. During open carpal tunnel surgery (n = 12), flexor tendon forces were acquired with buckle force transducers, and finger positions were recorded on video while subjects actively flexed and extended the fingers at two different wrist angles. Mean in vivo FDP tendon forces varied between 1.3N +/- 0.9 N and 4.0 N +/- 2.9 N while mean FDS tendon forces ranged from 1.3N +/- 0.5 N to 8.5 N +/- 10.7 N. FDP force increased with active finger flexion at both wrist angles of 0 degrees or 30 degrees flexion. FDS force increased with finger flexion when the wrist was in 30 degrees flexion, but was unchanged when the wrist was in 0 degrees of flexion. Tendon forces were similar regardless of whether the fingers were moving in the flexion or extension direction. Active finger flexion and extension with the wrist at 0 degrees and 30 degrees flexion may be used during early rehabilitation protocols with limited risk of repair rupture. This risk can be further decreased for a FDS tendon repair by reducing wrist flexion angle.

In vivo forces generated by finger flexor muscles do not depend on the rate of fingertip loading during an isometric task

Risk factors for activity-related tendon disorders of the hand include applied force, duration, and rate of loading. Understanding the relationship between external loading conditions and internal tendon forces can elucidate their role in injury and rehabilitation. The goal of this investigation is to determine whether the rate of force applied at the fingertip affects in vivo forces in the flexor digitorum profundus (FDP) tendon and the flexor digitorum superficialis (FDS) tendon during an isometric task. Tendon forces, recorded with buckle force transducers, and fingertip forces were simultaneously measured during open carpal tunnel surgery as subjects (N=15) increased their fingertip force from 0 to 15N in 1, 3, and 10s. The rates of 1.5, 5, and 15N/s did not significantly affect FDP or FDS tendon to fingertip force ratios. For the same applied fingertip force, the FDP tendon generated more force than the FDS. The mean FDP to fingertip ratio was 2.4+/-0.7 while the FDS to tip ratio averaged 1.5+/-1.0 (p<0.01). The fine motor control needed to generate isometric force ramps at these specific loading rates probably required similar high activation levels of multiple finger muscles in order to stabilize the finger and control joint torques at the force rates studied. Therefore, for this task, no additional increase in muscle force was observed at higher rates. These findings suggest that for high precision, isometric pinch maneuvers under static finger conditions, tendon forces are independent of loading rate.

Finger flexor tendon forces are a complex function of finger joint motions and fingertip forces

In vivo tendon forces provide a view inside the musculoskeletal system revealing muscle function and potential injury etiologies. The studies presented here measured the in vivo tendon force of the flexor digitorum superficialis of the long finger during open carpal tunnel release surgery in ten adult patients. Forces were measured during passive movement of the finger, isometric pinch, and dynamic tapping of the finger. The tendon forces during passive movement of the finger were the largest with the finger fully extended. During isometric pinch, tendon force was linearly related to fingertip force, and was on average 3.3 times larger than the fingertip force. During dynamic activities, however, the relationship between tip and tendon force was nonlinear and often remained elevated when the finger was moving but with no applied force. Tendon forces were the highest with the isometric finger pinch. In conclusion, tendon force is a completed function of both fingertip load and motion of the joints that the tendons cross. A comparison of these results with others published in the literature indicated that rehabilitation processes need to incorporate a systems approach rather than rely on one specific physiologic relationship to minimize finger flexor tendon forces.

An in vivo equine forelimb model for short-term recording of peak isometric force in the superficial and deep digital flexor muscles

OBJECTIVE

To develop and test an experimental model for in vivo short-term recording of peak isometric forces of the digital flexor muscles in the forelimb of adult horses.

STUDY DESIGN

In vivo experimental study.

SAMPLE POPULATION

Four healthy, anesthetized, adult Thoroughbred horses (3 to 7 years old; 527 +/- 87 kg)

METHODS

In dorsal recumbency, ulnar and median nerves were exposed and instrumented with insulated bipolar cuff stimulation electrodes for later connection to an electrical stimulator. In left lateral recumbency, a biplanar fixator was applied to the right humerus and a custom-made, rigid, aluminum frame connected to it, to allow loading of muscles distal to the fixator. Threaded transfixation pins through the radial and metacarpal condyles were clamped to the rigid frame so that the humerus, radius, ulna, and metacarpus were fixed in position. Each digital flexor muscle insertion tendon was transected just above the metacarpophalangeal joint, extracted from the carpal canal, and secured in a metal clamp positioned at the distal myotendinous (MT) junction. Distally, the clamp was connected in series to a load cell and a pneumatic actuator to record force and to maintain muscle length during nerve stimulation. A linear potentiometer was connected in parallel to the actuator to record MT junction position. Initial trials were conducted to identify median and ulnar nerve stimulation variables to achieve maximal muscle contraction. Isometric contractions were performed at different muscle lengths and peak forces registered during 3 seconds of supramaximal dual (ulnar and median) nerve stimulation.

RESULTS

A stimulation voltage of 2.5 to 5.0 V at 50 Hz usually produced maximal force for both the superficial digital flexor (SDF) and deep digital flexor (DDF) muscles. Single ulnar and median nerve stimulation elicited force development not only in the DDF muscle but also in the SDF muscle. At voltages higher than 1 V, normalized force was greatest with combined median and ulnar nerve stimulation for both the DDF and SDF muscles; however, normalized force was greater for median nerve stimulation than ulnar nerve stimulation in the DDF muscle, and the opposite relationship was observed for the SDF muscle. Final recording of dual supramaximal nerve stimulation of SDF and DDF muscles resulted in peak isometric forces of 716 +/- 192 N and 1,577 +/- 203 N, respectively.

CONCLUSIONS

The instrumentation technique and experimental protocol enabled recording of peak isometric forces in the SDF and DDF muscles of anesthetized adult horses.

CLINICAL RELEVANCE

Studies using this model will improve knowledge of SDF and DDF muscle mechanics with insight to functional implications of the complex architecture of these muscles. Knowledge of the dynamic performance of the SDF and DDF muscles would also be useful for the development of new treatment strategies for flexor deformities and tendon injuries in horses.

Dynamic loading of the plantar aponeurosis in walking

BACKGROUND

The plantar aponeurosis is known to be a major contributor to arch support, but its role in transferring Achilles tendon loads to the forefoot remains poorly understood. The goal of this study was to increase our understanding of the function of the plantar aponeurosis during gait. We specifically examined the plantar aponeurosis force pattern and its relationship to Achilles tendon forces during simulations of the stance phase of gait in a cadaver model.

METHODS

Walking simulations were performed with seven cadaver feet. The movements of the foot and the ground reaction forces during the stance phase were reproduced by prescribing the kinematics of the proximal part of the tibia and applying forces to the tendons of extrinsic foot muscles. A fiberoptic cable was passed through the plantar aponeurosis perpendicular to its loading axis, and raw fiberoptic transducer output, tendon forces applied by the experimental setup, and ground reaction forces were simultaneously recorded during each simulation. A post-experiment calibration related fiberoptic output to plantar aponeurosis force, and linear regression analysis was used to characterize the relationship between Achilles tendon force and plantar aponeurosis tension.

RESULTS

Plantar aponeurosis forces gradually increased during stance and peaked in late stance. Maximum tension averaged 96% +/- 36% of body weight. There was a good correlation between plantar aponeurosis tension and Achilles tendon force (r = 0.76).

CONCLUSIONS

The plantar aponeurosis transmits large forces between the hindfoot and forefoot during the stance phase of gait. The varying pattern of plantar aponeurosis force and its relationship to Achilles tendon force demonstrates the importance of analyzing the function of the plantar aponeurosis throughout the stance phase of the gait cycle rather than in a static standing position.

CLINICAL RELEVANCE

The plantar aponeurosis plays an important role in transmitting Achilles tendon forces to the forefoot in the latter part of the stance phase of walking. Surgical procedures that require the release of this structure may disturb this mechanism and thus compromise efficient propulsion.

In vivo tendon force measurement of 2-week duration in sheep

Tendon tension in vivo may be determined indirectly by measuring intratendinous pressure, by using a buckle transducer or by measuring the tendon strain. All of these methods require appropriate calibration, which is highly dependent on various variables. To measure the tendon load in vivo during a period of 2 weeks in sheep, a measurement technique has been developed using a force sensor interposed serially between the humeral head and the tendon end. Within a supporting frame, a flexion-sensitive force transducer is subjected to three-point bending stress. The load is transmitted by sutures from the tendon end through a hole in the sensor frame, orthogonal to the force transducer. In this configuration, the sensor measures the tensile force acting on the tendon, largely independent of the loading direction. The sensor was screwed to the humeral head and connected to the tendon end which was previously released from its insertion site along with a bone chip, using sutures. Connecting wires passed subcutaneously to a skin outlet about 30 cm away from the transducer. The sensor output was linear to the measured load up to 300 N, with maximum hysteresis of 18% full scale. All sensors worked in vivo without drift over a period of up to 14 days with no change in the calibration data. Forces up to 310 N have been recorded in vivo with daily tension measurements. This study shows that serial tendon tension measurement is feasible and allows for reliable, repeatable recording of the absolute tendon tension at the expense of tendon integrity.

Influence of loading rate and cable migration on fiberoptic measurement of tendon force

Several investigators have recently used fiberoptic cables to measure tendon forces in situ. The technique may be subject to significant error due to cable migration and differences in the loading rates used for calibration and those experienced during measurement. This in vitro study examined the impact of these potential sources of error on transducer accuracy. A fiberoptic cable was passed perpendicular to the fibers of four Achilles tendons in the mediolateral direction and each specimen was cyclically loaded to 1000 N. The influence of loading rate on transducer output was investigated by comparing results from tests conducted at 20, 200 and 1000 N/s. The effect of cable migration was examined by comparing the outputs obtained after displacing the cable one tendon width medially and laterally along its path in the tendon and then repeating the 200 N/s testing protocol. It was possible to obtain nonlinear specimen-specific relationships between the fiberoptic output and tendon force. Differences in loading rate resulted in root-mean-square (RMS) errors not larger than 17% maximum load. Hysteresis effects caused RMS errors smaller than 5% maximum load. Cable migration errors were less than 27%. The total RMS error due to the combined effects of loading rate difference and cable movement was less than 32%. Fiberoptic measurement of tendon force is attractive due to its low cost, easy implementation and comparable accuracy relative to other implantable force transducers. Although additional factors such as cable placement, edge artifacts due where the transducer exits the skin and non-uniform loading may also influence fiberoptic output, careful control of loading rate and transducer movement during calibration is imperative if maximum accuracy is to be achieved.

In vivo finger flexor tendon force while tapping on a keyswitch

Force may be a risk factor for musculoskeletal disorders of the upper extremity associated with typing and keying. However, the internal finger flexor tendon forces and their relationship to fingertip forces during rapid tapping on a keyswitch have not yet been measured in vivo. During the open carpal tunnel release surgery of five human subjects, a tendon-force transducer was inserted on the flexor digitorum superficialis of the long finger. During surgery, subjects tapped with the long finger on a computer keyswitch, instrumented with a keycap load cell. The average tendon maximum forces during a keystroke ranged from 8.3 to 16.6 N (mean = 12.9 N, SD = 3.3 N) for the subjects, four to seven times larger than the maximum forces observed at the fingertip. Tendon forces estimated from an isometric tendon-force model were only one to two times larger than tip force, significantly less than the observed tendon forces (p = 0.001). The force histories of the tendon during a keystroke were not proportional to fingertip force. First, the tendon-force histories did not contain the high-frequency fingertip force components observed as the tip impacts with the end of key travel. Instead, tendon tension during a keystroke continued to increase throughout the impact. Second, following the maximum keycap force, tendon tension during a keystroke decreased more slowly than fingertip force, remaining elevated approximately twice as long as the fingertip force. The prolonged elevation of tendon forces may be the result of residual eccentric muscle contraction or passive muscle forces, or both, which are additive to increasing extensor activity during the release phase of the keystroke.

Tensions of the flexor digitorum superficialis are higher than a current model predicts

Existing isometric force models can be used to predict tension in the finger flexor tendon, however, they assume a specific distribution of forces across the tendons of the fingers. These assumptions have not been validated or explored by experimental methods. To determine if the force distributions repeatably follow one pattern the in vivo tension of the flexor digitorum superficialis (FDS) tendon of the long finger was measured in nine patients undergoing open carpal tunnel release surgery. Following the release, a tendon force transducer (Dennerlein et al. 1997 J. Biomechanics 30(4), 395-397) was mounted onto the FDS of the long finger. Tension in the tendon, contact force at the fingertip, and finger posture were recorded while the patient gradually increased the force applied by the fingertip from 0 to 10 N and then monotonically reduced it to 0 N. The average ratio of the tendon tension to the fingertip contact force ranged from 1.7 to 5.8 (mean = 3.3, s.d. = 1.4) for the nine subjects. These ratios are larger than ratios predicted by current isometric tendon force models (mean = 1.2, s. d. = 0.4). Subjects who used a pulp pinch posture (hyper-extended distal interphalangeal joint (DIP)) showed a significantly (p = 0.02) larger ratio (mean = 4.4, s.d. = 1.5) than the five subjects who flexed the DIP joint in a tip pinch posture (mean = 2.4, s.d. = 0.6). A new DIP constraint model, which selects different force distribution based on DIP joint posture, predicts force ratios that correlate well with the measured ratios (r2 = 0.85).