Muscle fibre recruitment can respond to the mechanics of the muscle contraction

Abstract

This study investigates the motor unit recruitment patterns between and within muscles of the triceps surae during cycling on a stationary ergometer at a range of pedal speeds and resistances. Muscle activity was measured from the soleus (SOL), medial gastrocnemius (MG) and lateral gastrocnemius (LG) using surface electromyography (EMG) and quantified using wavelet and principal component analysis. Muscle fascicle strain rates were quantified using ultrasonography, and the muscle-tendon unit lengths were calculated from the segmental kinematics. The EMG intensities showed that the body uses the SOL relatively more for the higher-force, lower-velocity contractions than the MG and LG. The EMG spectra showed a shift to higher frequencies at faster muscle fascicle strain rates for MG: these shifts were independent of the level of muscle activity, the locomotor load and the muscle fascicle strain. These results indicated that a selective recruitment of the faster motor units occurred within the MG muscle in response to the increasing muscle fascicle strain rates. This preferential recruitment of the faster fibres for the faster tasks indicates that in some circumstances motor unit recruitment during locomotion can match the contractile properties of the muscle fibres to the mechanical demands of the contraction.

Figure 1

Principal component representation of EMG frequency spectra. (a) Principal component (PC) weightings for PC I (solid line) and PC II (dashed line) from the EMG-intensity spectra for six leg muscles (Wakeling & Rozitis 2004). (b) EMG-intensity spectra that can be reconstructed from vector products of the PC weightings shown in (a) with PC I+0.45 PC II shown in grey and PC I–0.69 PC II shown in black. (c) Vector representation of the spectra in (b) following the same colours as in (b) and indicating angle θ.

Figure 2

Ultrasound image from the medial gastrocnemius during cycling at a pedal speed of 60 r.p.m. and a crank torque of 17 N m. The circles show the digitized points, and the lines show the interpolated aponeurosis and fascicle trajectories. The diamonds show the ends of a muscle fascicle that has a pennation angle of α with the superficial aponeurosis. The angle between the superficial and deep aponeuroses is β−α.

Figure 3

Maximum muscle strain rates during shortening as a function of pedal cadence and crank torque. Symbols show data for the medial gastrocnemius (diamond symbols), the lateral gastrocnemius (square symbols) and the soleus (triangle symbols). Muscle–tendon unit strain rates are shown by the filled symbols and muscle fascicle strain rates are shown by the open symbols. Points show the mean±s.e.m. for six subjects.

Figure 4

The total EMG-intensity per pedal cycle increased with both crank torque and pedal cadence. (a) Crank torque and (b) pedal cadence for the medial (diamond symbols) and lateral (square symbols) heads of the gastrocnemius and the soleus (triangle symbols). Points denote the mean±s.e.m. (n=450 cycles).

Figure 5

Electromyographic signals from the lateral gastrocnemius during cycling. Signals are shown from one subject for three different combinations of crank torque and pedal cadence. Raw signals with the top dead centre pedal position indicated by the vertical lines.

Figure 6

Muscle lengths and activity pattern for the soleus during cycling at a pedal cadence of 60 r.p.m. against a load of 33 N m. (a) Muscle fascicle lengths measured by ultrasound for one subject are shown by the triangles. The modelled muscle fascicle length is shown by the solid black line, r²=0.907. The modelled muscle–tendon unit length is shown by the grey dashed lines (mean±s.e.m. for all six subjects). (b) Normalized EMG-intensity for the soleus during cycling at a cadence of 60 r.p.m. against a load of 33 N m. Intensity is shown as mean±s.e.m. for all six subjects (n=450 cycles).

Figure 7

Principal component representation of the EMG-intensity spectra during cycling at a range of pedal cadences and crank torques for the medial gastrocnemius. (a) Principal components describing the 100 ms bursts of muscle activity centred around the time of maximum EMG-intensity. (b) Principal components describing the total EMG-intensity across each pedal cycle. The principal components were calculated from EMG spectra from all three muscles tested. The loading score panels show the mean±s.e.m. (n=450 cycles) loading scores for the first two principal components: trials with low crank torque and increasing pedal cadence are shown with solid symbols and solid lines and trials with low pedal cadence and increasing crank torque are shown with open symbols and dashed lines. Vectors from the origin to the three extreme points are shown by the grey arrows. The inset panels show the first two principal component weighting spectra PC I (solid line) and PC II (dashed line).