Tendon displacements during voluntary and involuntary finger movements

Abstract

In the human hand, independent movement control of individual fingers is limited. One potential cause for this is mechanical connections between the tendons and muscle bellies corresponding to the different fingers. The aim of this study was to determine the tendon displacement of the flexor digitorum superficialis (FDS) of both the instructed and the neighboring, non-instructed fingers during single finger flexion movements. In nine healthy subjects (age 22–29 years), instructed and non-instructed FDS finger tendon displacement of the index, middle and ring finger was measured using 2D ultrasound analyzed with speckle tracking software in two conditions: active flexion of all finger joints with all fingers free to move and active flexion while the non-instructed fingers were restricted. Our results of the free movement protocol showed an average tendon displacement of 27 mm for index finger flexion, 21 mm for middle finger flexion and 17 mm for ring finger flexion. Displacements of the non-instructed finger tendons (≈12 mm) were higher than expected based of the amount of non-instructed finger movement. In the restricted protocol, we found that, despite minimal joint movements, substantial non-instructed finger tendon displacement (≈9 mm) was still observed, which was interpreted as a result of tendon strain. When this strain component was subtracted from the tendon displacement of the non-instructed fingers during the free movement condition, the relationship between finger movement and tendon displacement of the instructed and non-instructed finger became comparable. Thus, when studying non-instructed finger tendon displacement it is important to take tendon strain into consideration.

Introduction

The human hand has evolved to be able to perform complex hand actions and is essential in daily life to manipulate objects. Despite this capacity for dexterity, there are limitations to the independent movement of the digits (Hager-Ross and Schieber, 2000, van den Noort et al., 2016). These limitations have been attributed to both peripheral mechanical and central neural constraints (van Duinen and Gandevia, 2011). Because of these constraints, movement of one finger (instructed) may cause unintentional movements of the neighboring non-instructed finger(s), a phenomenon called enslaving (van Duinen and Gandevia, 2011, Zatsiorsky et al., 2000).

In previous work, we assessed the relationship between enslaved finger movements and activation patterns of finger specific flexor and extensor muscle regions (van Beek et al., 2017). We observed that the central nervous system is actively resisting movement of the non-instructed fingers by an increased activity of the antagonistic extensor digitorum (ED) muscle (van Beek et al., 2017). Such agonistic-antagonistic coactivation is expected to result in forces exerted at the tendons and, consequently, stretching of the tendons. Thus, tendon displacements in the non-instructed fingers will probably not only be the result of finger and tendon movement, but also of tendon stretch. However, tendon displacements have not been studied in the context of finger enslaving. Higher tendon displacements during active finger flexion compared to passive finger flexion, despite equal ranges of finger movement, have previously been reported (Korstanje et al., 2010). The presence of tendon stretch may explain this discrepancy between active and passive finger movements.

In various pathological conditions, the fingers and/or their tendons are affected, such as arthritis (Arauz et al., 2017), tenosynovitis and carpal tunnel syndrome (Bianchi et al., 2007, Kociolek and Keir, 2016, Korstanje et al., 2012, Singh et al., 2015, Tat et al., 2016). In patients with flexor tendon disorders, tendon displacements have been studied using ultrasound imaging (Korstanje et al., 2012). More insight into tendon behavior during finger movements is needed to more precisely evaluate finger movement pathologies and assess the effects of surgical interventions.

The aims of this study were (1) to investigate tendon displacement of the flexor digitorum superficialis (FDS) of both the finger which was instructed to move and of the non-instructed fingers during single finger flexion movements; and (2) to assess whether tendon stretching is present in the tendon of non-instructed fingers. We hypothesized that (1) the FDS tendons associated with the non-instructed fingers experience tendon stretching, and (2) if effects of tendon stretch are taken into account, there exists a one-on-one relationship between finger movement and tendon displacement.