Responses of intra-abdominal pressure and abdominal muscle activity during dynamic trunk loading in man

The purpose of this study was to determine and compare interactions between the abdominal musculature and intra-abdominal pressure (IAP) during controlled dynamic and static trunk muscle loading. Myoelectric activity was recorded in six subjects from the rectus abdominis, obliquus externus, obliquus internus, transversus abdominis and erector spinae muscles using surface and intra-muscular fine-wire electrodes. The IAP was recorded intra-gastrically. Trunk flexions and extensions were performed lying on one side on a swivel table. An adjustable brake provided different friction loading conditions, while adding weights to an unbraked swivel table afforded various levels of inertial loading. During trunk extensions at all friction loads, IAP was elevated (1.8-7.2 kPa) with concomitant activity in transversus abdominis and obliquus internus muscles-little or no activity was seen from rectus abdominis and obliquus externus muscles. For inertia loading during trunk extension, IAP levels were somewhat lower (1.8-5.6 kPa) and displayed a second peak when abdominal muscle activity occurred in the course of decelerating the movement. For single trunk flexions with friction loading, IAP was higher than that seen in extension conditions and increased with added resistance. For inertial loading during trunk flexion, IAP showed two peaks, the larger first peak matched peak forward acceleration and general abdominal muscle activation, while the second corresponded to peak deceleration and was accompanied by activity in transversus abdominis and erector spinae muscles. It was apparent that different loading strategies produced markedly different patterns of response in both trunk musculature and intra-abdominal pressure

Electromyographic responses of the human triceps surae and force tremor during sustained sub‐maximal isometric plantar flexion

The objective was to investigate electromyographic activity (EMG) and isometric force tremor (IFT) changes during a sustained sub-maximal isometric contraction in two muscles acting upon the same joint but differing in muscle fibre composition. Surface and intra-muscular EMG activity from the gastrocnemius and soleus muscles and IFT were recorded during an exhausting isometric plantar flexion (30% of maximal voluntary contraction). Surface EMG amplitude (RMS) of both gastrocnemius and soleus muscles increased significantly over time. Gastrocnemius EMG RMS increased in a non-linear fashion while soleus EMG RMS increased linearly. A significant linear decrease of surface EMG mean power frequency (MPF) was observed over time for both muscles. The decrease in gastrocnemius MPF was significantly greater than that for soleus. Intra-muscular EMG results showed similar trends. Correlations of intramuscular EMG RMS and MPF with time were, however, characterized by lower correlation coefficients than those from the surface EMG. Isometric force tremor RMS significantly increased non-linearly with duration of contraction, while IFT MPF showed a significant linear decrease with time. Changes in surface EMG RMS were correlated to changes seen in IFT RMS, in particular, for the predominantly fast twitch gastrocnemius muscle. Correlation coefficients of surface EMG MPF and IFT MPF were lower than RMS correlations. The associated changes in IFT and EMG with fatigue indicate alterations in motor unit firing rate, recruitment and synchronization. The muscle specificity of the EMG and IFT changes suggests a coupling to muscle fibre type composition, although differences in the relative force contribution of each muscle could also affect the results

The influence of sudden perturbations on trunk muscle activity and intra-abdominal pressure while standing

Unexpected ventral and dorsal perturbations and expected, self-induced ventral perturbations were delivered to the trunk by suddenly loading a vest strapped to the torso. Six male subjects were measured for intra-abdominal pressure (IAP) and intra-muscular electromyography of the transversus abdominis (TrA), obliquus internus abdominis (OI), obliquus externus abdominis (OE) and rectus abdominis (RA) muscles. Erector spinae (ES) activity was recorded using surface electromyography. Displacements of the trunk and head were registered using a video-based system. Unexpected ventral loading produced activity in TrA, OI, OE and RA, and an IAP increase well in advance of activity from ES. Expected ventral loading produced pre-activation of all muscles and an increased IAP prior to the perturbation. The TrA was always the first muscle active in both the unexpected and self-loading conditions. Of the two ventral loading conditions. forward displacement of the trunk was significantly reduced during the self-loading. Unexpected dorsal loading produced coincident activation of TrA, OI, OE, RA and ES. These results indicate a response of the trunk muscles to sudden expected and unexpected ventral loadings other than the anticipated immediate extensor torque production through ES activation. It is suggested that the increase in IAP is a mechanism designed to improve the stability of the trunk through a stiffening of the whole segment

The effect of an abdominal muscle training program on intra-abdominal pressure

The effect of 10 weeks' specific abdominal strength training (resisted trunk rotations) on intra-abdominal pressure was investigated in 10 healthy males. Isometric rotational force, trunk flexor and extensor torque and intra-abdominal pressure were measured as well as intra-abdominal pressure responses to Valsalva manoeuvres, maximal pulsed pressures, drop jumps and trunk perturbations. The rotational strength increased 29.7%. after training without significant change in intra-abdominal pressure. The isometric flexor strength did not change, while the extensor strength increased 11.0%. Valsalva and pulsed pressures increased 11.6 and 9.2%, respectively. The rate of intra-abdominal pressure development during pulsed pressures, drop jumps and trunk perturbations increased after training. The level of intra-abdominal pressure during the latter two tasks remained unchanged. It is concluded that an increase in strength of the trunk rotators with training improves the ability to generate higher levels of voluntarily induced intra-abdominal pressure and increases the rate of intra-abdominal pressure development during functional situations

Influence of gastrocnemius muscle length on triceps surae torque development and electromyographic activity in man

The present study was designed to determine the relative contribution of the gastrocnemius muscle to isometric plantar flexor torque production at varying knee angles, while investigating the activation of the gastrocnemius muscle at standardised non-optimal lengths. Voluntary plantar flexor torque, supramaximally stimulated twitch torque and myoelectric activity (EMG) from the triceps surae were measured at different knee angles. Surface and intra-muscular EMG were recorded from the soleus muscle and the medial and lateral heads of the gastrocnemius muscle in 10 male subjects. With the ankle angle held constant, knee angle was changed in steps of 30 degrees ranging from 180 degrees (extended) to 60 degrees (extreme flexion), while voluntary torque from a 5-s contraction was determined at 10 different levels of voluntary effort, ranging from 10% of maximal effort to maximal effort. To assess effort, supramaximal twitches were superimposed on all voluntary contractions, and additionally during rest. Maximal plantar flexor torque and resting twitch torque decreased significantly in a sigmoidal fashion with increasing knee flexion to 60% of the maximum torque at 180 degrees knee angle. For similar levels of voluntary effort, the EMG root mean square (RMS) of gastrocnemius was less with increased knee flexion, whereas soleus RMS remained unchanged. From these data, it is concluded that the contribution of gastrocnemius to plantar flexor torque is at least 40% of the total torque in the straight leg position. The decrease of gastrocnemius EMG RMS with decreasing muscle length may be brought about by a decrease in the number of fibres within the EMG electrode recording volume and/or impaired neuromuscular transmission

Excitatory drive to the α-motoneuron pool during a fatiguing submaximal contraction in man

1. This study was undertaken to examine changes of excitatory drive to the triceps surae alpha-motoneuron pool during fatiguing submaximal isometric contractions in man. Eight healthy subjects maintained isometric plantar flexions at 30% of maximum voluntary contraction (MVC) until the limit of endurance (range, 6-9 min)