Selective recruitment of single motor units in human flexor digitorum superficialis muscle during flexion of individual fingers

Abstract

Flexor digitorum superficialis (FDS) is an extrinsic multi-tendoned muscle which flexes the proximal interphalangeal joints of the four fingers. It comprises four digital components, each with a tendon that inserts onto its corresponding finger. To determine the degree to which these digital components can be selectively recruited by volition, we recorded the activity of a single motor unit in one component via an intramuscular electrode while the subject isometrically flexed each of the remaining fingers, one at a time. The finger on which the unit principally acted was defined as the 'test finger' and that which flexed isometrically was the 'active' finger. Activity in 79 units was recorded. Isometric finger flexion forces of 50% maximum voluntary contraction (MVC) activated less than 50% of single units in components of FDS acting on fingers that were not voluntarily flexed. With two exceptions, the median recruitment threshold for all active-test finger combinations involving the index, middle, ring and little finger test units was between 49 and 60% MVC (60% MVC being the value assigned to those not recruited). The exceptions were flexion of the little finger while recording from ring finger units (median: 40% MVC), and vice versa (median: 2% MVC). For all active-test finger combinations, only 35/181 units were activated when the active finger flexed at less than 20% MVC, and the fingers were adjacent for 28 of these. Functionally, to recruit FDS units during grasping and lifting, relatively heavy objects were required, although systematic variation occurred with the width of the object. In conclusion, FDS components can be selectively activated by volition and this may be especially important for grasping at high forces with one or more fingers.

Figure 1. Experimental arrangement and sites of recordings

A, diagram of testing position for the main experiment and for measurement of the maximum force produced by isometric flexion at the proximal IP joint. B, coordinates for electrode insertion sites in the right forearm for the digital components of the flexor digitorum superficialis (FDS) muscle in all subjects. The x-axis represents standardized forearm length and the y-axis the forearm width. Dotted lines represent the midlines in the vertical and horizontal planes of the forearm. The apparent overlap of territories for each unit type reflects the complex, layered structure of FDS and individual variation.

Figure 2. Recording of single motor unit activity

A, single unit activity of the middle finger component of FDS recorded during weak contractions of the middle finger before and after contractions of the index finger. B, recording of single unit activity of the index finger component of FDS during weak contractions of the index finger and during three ramp contractions of the little finger to 50% maximum voluntary contraction (MVC). Note that forces close to 50% MVC were reached before recruitment occurred. Recruitment of the test unit was checked before and after the ramps and in both panels potentials from the unit have been superimposed.

Figure 3. Single subject data

Separate panels show the recruitment thresholds for motor units in the index, middle and ring digital components of FDS during flexion of the index, middle, ring and little fingers at the proximal interphalangeal (IP) joint. The open circles indicate that digit on which the units principally acted and the numbers below reflect the number of units from which recordings were made. The numbers under the filled circles (i.e. the non-test fingers) reflect the number of successful recordings made in which the non-test finger flexed. The line indicates the median threshold for that finger. Units had low force thresholds when the test finger flexed (< 5% MVC), but usually had much higher thresholds when other fingers flexed. Units were classified as ‘not recruited’ if they were not active at contraction strengths of 50% MVC (shown above dotted horizontal lines).

Figure 4. Group data for the recruitment thresholds for motor units in the digital components of FDS

‘All data’ consists of complete and incomplete sets of data, i.e. data from the test unit combined with data from one, two or three active fingers. ‘Complete sets’ consists only of data from the test unit when combined with data from three active fingers. The line indicates the median threshold for that finger. Data are plotted in the same way as in Fig. 3. The total number of finger flexions that were assigned 60% as they did not produce activity in the test units were 36 of 61 and 70 of 120 for the incomplete and complete data sets, respectively.

Figure 5. Percentage of test units not recruited in ramp contractions to 50% MVC

For contractions of non-test fingers, more than 50% of units were not recruited with contraction ramps to 50% MVC, except for little finger units when the ring finger was contracting.

Figure 6. Comparison of motor unit recruitment thresholds of FDS (present study) and flexor digitorum profundus (FDP) (Kilbreath & Gandevia, 1994)

Means and standard deviation for the thresholds of recruitment of the index, middle, ring and little finger units are shown. t tests revealed that FDS was recruited at higher forces than FDP for most combinations: *P < 0.01, +P < 0.05. The difference between the two finger flexors may be underestimated as a value of 60% MVC was arbitrarily used when FDS was not recruited (see Methods). The exception was recruitment of little finger units of FDS which behaved similarly to those of FDP.