Mean maximal motor unit firing rates (MUFRs) of the human soleus are lower (5-20 Hz) than other limb muscles (20-50 Hz) during brief sustained contractions. With healthy adult aging, maximal MUFRs are 20-40% lower and twitch contractile speed of lower limb muscles are 10-40% slower compared with young adults. However, it is unknown whether the inherently low maximal MUFRs for the soleus are further reduced with aging in association with age-related slowing in contractile properties. The purpose of the present study was to compare the changes in triceps surae contractile properties and MUFRs of the soleus throughout a variety of contraction intensities in six old ( approximately 75 yr old) and six young ( approximately 24 yr old) men. Neuromuscular measures were collected from the soleus and triceps surae during repeated sessions (2-6 sessions). Populations of single MUFR trains were recorded from the soleus with tungsten microelectrodes during separate sustained 6- to 10-s isometric contractions of varying intensities [25%, 50%, 75%, and 100% maximal voluntary isometric contraction (MVC)]. The old men had weaker triceps surae strength (MVC; 35% lower) and slower contractile properties (contraction duration; 20% longer) than the young men. However, there was no difference in average MUFRs of the soleus at 75% and 100% MVC ( approximately 14.5 Hz and approximately 16.5 Hz, respectively). At 25% and 50% MVC, average rates were 10% and 20% lower in the old men compared with young, respectively. Despite a significant slowing in triceps surae contraction duration, there was no age-related change in MUFRs recorded at high contractile intensities in the soleus. Thus the relationship between the whole muscle contractile properties and MUFRs found in other muscle groups may not exist between the triceps surae and soleus and may be muscle dependent.

Motor unit recruitment and motor unit discharge rate (MUDR) have been widely studied in isometric conditions but minimally during velocity-dependent contractions. For isometric contractions, surface electromyography (EMG) activity of the elbow extensors plateaus at near maximal torques (Le Bozec et al. 1980; Le Bozec and Maton 1982). One study (Maton and Bouisset 1975) recorded single motor unit (MU) activity at maximal velocities; however, only the rate of the first interspike interval (ISI) was reported and likely was not representative of the average MUDR of the MU train. The purpose was to calculate average MUDRs of the anconeus during loaded velocity-dependent contractions from zero velocity (isometric) up to maximal velocity (V(max25)) through a large range of motion. A Biodex dynamometer was used to record elbow extension torque, position, and velocity. Single MU potentials were collected from the anconeus with intramuscular EMG, and surface EMG was sampled from the lateral head of the triceps brachii during maximal voluntary isometric contractions (MVCs) and velocity-dependent contractions loaded at 25% MVC over 120° range of motion at five target velocities (0, 25, 50, 75, 100%V(max25)). Elbow extension velocities ranged from 93 to 494°/s and average MUDR ranged from 11.8 Hz at 25%MVC to 39.0 Hz at 100%V(max25.) Overall average MUDRs increased as a function of velocity, although the root mean square of triceps brachii surface EMG plateaued at 50%V(max25). Piecewise regression analysis revealed two distinct linear ranges each described by a unique equation, suggesting that MUDRs of the anconeus enter a secondary range of firing, characterized by a steeper slope as velocity approaches maximum.

Despite an age-related slowing in the contractile properties of the triceps surae, inherently low maximal motor unit firing rates (MUFRs) in the soleus are unchanged. Fatigue following high-intensity contractions is characterized by contractile slowing in conjunction with a reduction in MUFRs in young adults. Here we exploit the ageing model of the soleus to assess changes in neuromuscular function during fatigue and short-term recovery. We hypothesize that a high-intensity sustained contraction will cause minimal reductions in MUFRs in young and old subjects but that recovery of MUFRs will be delayed in aged subjects. We compared the effects of a high-intensity sustained task on the MUFRs of the soleus and triceps surae contractile properties in six young (approximately 24 yr) and six old (approximately 75 yr) men. Various measures of the contractile function of the triceps surae were tested during two to six sessions via maximal voluntary isometric contractions (MVCs) and tibial nerve stimulation. Populations of MUFR trains were recorded from the soleus during brief (approximately 7 s) MVCs, a high-intensity (75% MVC) sustained fatiguing task, and brief MVCs following task failure at 1, 2, 5, and 10 min. Old men had greater time to task failure than the young (approximately 138 and approximately 100 s, respectively). Voluntary activation was near maximal (>99%) for all subjects but at task failure, decreased to approximately 89% in both groups. Maximal MUFRs, for both groups, were reduced by approximately 44% and twitch contraction duration slowed by approximately 30% following task failure. Contraction duration recovered equally for both groups within 2 min, but maximal MUFRs did not recover until 5 min in the old compared with 1 min for the young. The surprising fatigue-induced reduction in MUFRs was similar for both groups, but despite a similar recovery of contractile properties for both, recovery of MUFRs was impaired in the old subjects.

The aim of this study was to examine the effect of shoulder angle on the electromyographic (EMG) activation pattern of the elbow extensors during a fatiguing contraction. Ten young men (23.5 ± 1.7) were tested on two occasions with the elbow angle at 90° and the shoulder at either 0° or 90° of flexion. EMG was recorded by fine wire electrodes inserted into the lateral, medial, and long heads of the triceps brachii and the anconeus. An EMG-torque relationship was determined prior to a sustained isometric contraction at 20% of maximum voluntary contraction (MVC) until target failure. Endurance time was shorter, and postfatigue MVC torque was lower at 90° (40.4 ± 12.7 Nm) versus 0° (47.9 ± 14.7 Nm) of flexion. EMG activity of the long head during the final 10% of the fatiguing contraction was significantly greater at 90° versus 0° with no effect of shoulder angle on any other muscle portions. The findings suggest that measures from one muscle portion of the elbow extensors are not representative of the whole group, and the relative activation of the two-joint long head was changed depending on shoulder angle during a fatigue task.

The fatigue-related reduction in joint range of motion (ROM) during dynamic contraction tasks may be related to muscle length-dependent alterations in torque and contractile kinetics, but this has not been systematically explored previously. Twelve young men performed a repetitive voluntary muscle shortening contraction task of the dorsiflexors at a contraction load of 30% of maximum voluntary isometric contraction (MVC) torque, until total 40 degrees ROM had decreased by 50% at task failure (POST) to 20 degrees ROM. At both a short (5 degrees dorsiflexion) and long muscle length (35 degrees plantar flexion joint angle relative to a 0 degrees neutral ankle joint position), voluntary activation, MVC torque, and evoked tibialis anterior contractile properties of a 52.8 Hz high-frequency isometric tetanus [peak evoked torque, maximum rate of torque development (MRTD), maximum rate of relaxation (MRR)] were evaluated at baseline (PRE), at POST, and up to 10 min of recovery. At POST, we measured similar fatigue-related reductions in torque (voluntary and evoked) and slowing of contractile kinetics (MRTD and MRR) at both the short and long muscle lengths. Thus, the fatigue-related reduction in ROM could not be explained by length-dependent fatigue. Although torque (voluntary and evoked) at both muscle lengths was depressed and remained blunted throughout the recovery period, this was not related to the rapid recovery of ROM at 0.5 min after task failure. The reduction in ROM, however, was strongly related to the reduction in joint angular velocity (R(2) = 0.80) during the fatiguing task, although additional factors cannot yet be overlooked.

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