The effect of muscle length on motor unit discharge characteristics in human tibialis anterior muscle

Abstract

Muscle length influences the contractile properties of muscle in that when muscle is lengthened the relaxation phase of the muscle twitch is prolonged and when muscle is shortened, the relaxation phase is shorter in duration. As a result, the force exerted by active motor units varies with muscle length during voluntary contractions. To determine if motoneuron spike trains were adjusted to accommodate for changes in the contractile properties imposed by shortened and lengthened muscle, motor unit action potentials were recorded from the tibialis anterior muscle at different muscle lengths. Twenty subjects performed isometric ramp contractions at ankle angles of 20 degrees dorsiflexion, neutral between dorsiflexion and plantar flexion, and 30 degrees plantar flexion, which put the tibialis anterior muscle in a shortened, neutral, or lengthened condition, respectively. During isometric contractions where torque increased at 5% MVC/s, motor unit discharge rate at recruitment was greater in shortened muscle than in lengthened muscle (P less than 0.05). Brief initial interspike intervals (less than 40 ms) occurred more frequently in shortened muscle than in either neutral length or lengthened muscle. During steady contractions, motor unit discharge rate was greater per unit torque (N.m) in shortened muscle than in neutral length or lengthened muscle (P less than 0.05). These findings indicate that muscle length does influence the discharge pattern of motor unit spike trains during isometric ramp contractions.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptations in motor unit discharge activity with force control training in young and older human adults

“…Due to the complexity of the EMG interference pattern at even moderate forces, many studies of MU discharge activity have been performed at low force levels and may be biased towards behaviour at threshold levels of activation (Soderberg et al 1991;Laidlaw et al 2000). The high impedance electrode employed in the present study improves selectivity of MU recordings and enables the study of MU discharge patterns at moderate to high force.…”

“…Nonetheless, a representative sample of the subject's MU population should be obtained at each study. Several approaches have been taken to calculating MU discharge rates (Van Cutsem et al 1997;Connelly et al 1999) and range from instantaneous or averaged instantaneous ®ring rates (Howard et al 1988;Vander Linden et al 1991;Laidlaw et al 2000) to average discharge rate over relatively long windows of sustained isometric force (Erim et al 1999). At issue is making an estimate of MU discharge activity with functional relevance to motor control.…”

Nonuniform Weakness in the Paretic Knee and Compensatory Strength Gains in the Nonparetic Knee Occurs after Stroke

“…Such nonuniform weakness may be due to the primary cerebral damage that occurs with stroke. In healthy individuals, it has been shown that twitch duration is reduced during voluntary isometric contractions at short muscle lengths in the biceps brachialis [17, 18] and tibialis anterior [17, 19]. Thus to achieve fusion of twitches when contracting at short lengths, a healthy central nervous system will increase motor unit discharge rates.…”

Effect of different ankle- and knee-joint positions on gastrocnemius medialis fascicle length and EMG activity during isometric plantar flexion

“…Especial attention has been paid to the muscle groups of the triceps surae and quadriceps femoris (Herzog et al, 1990(Herzog et al, , 1991aMaganaris, 2001Maganaris, , 2003Savelberg and Meijer, 2003), as these are very important muscles of the lower extremities for daily but also high performance activities. One of the main difficulties in estimating the force-length relationship of a muscle in vivo is the variation of the voluntary activation during changes in muscle length (Vander Linden et al, 1991;Hasler et al, 1994;Suter and Herzog, 1997;Tamaki et al, 1997;Miaki et al, 1999;Pinniger et al, 2000;Kennedy and Cresswell, 2001;Onishi et al, 2002;Babault et al, 2003). Whereas at short muscle lengths some studies have reported a decrease in EMG activity (Cresswell et al, 1995;Tamaki et al, 1997;Miaki et al, 1999) motor units (Kennedy and Cresswell, 2001), others found the EMG activity (Vander Linden et al, 1991;Hasler et al, 1994;Babault et al, 2003) and the voluntary activation (Suter and Herzog, 1997) to increase.…”

“…One of the main difficulties in estimating the force-length relationship of a muscle in vivo is the variation of the voluntary activation during changes in muscle length (Vander Linden et al, 1991;Hasler et al, 1994;Suter and Herzog, 1997;Tamaki et al, 1997;Miaki et al, 1999;Pinniger et al, 2000;Kennedy and Cresswell, 2001;Onishi et al, 2002;Babault et al, 2003). Whereas at short muscle lengths some studies have reported a decrease in EMG activity (Cresswell et al, 1995;Tamaki et al, 1997;Miaki et al, 1999) motor units (Kennedy and Cresswell, 2001), others found the EMG activity (Vander Linden et al, 1991;Hasler et al, 1994;Babault et al, 2003) and the voluntary activation (Suter and Herzog, 1997) to increase. Nevertheless, the reports from the literature regarding the gastrocnemius medialis (GM) during maximal isometric plantar flexion are in agreement and it is generally accepted that there is a decrease in the activation of the GM at pronounced knee flexion positions, i.e.…”