Force Steadiness Research Articles

Low-frequency Force Steadiness Practice in Plantar Flexor Muscle Reduces Postural Sway during Quiet Standing

The purpose of this study was to assess the effect of low-frequency force steadiness practice in the plantar flexor muscles on postural sway during quiet standing. Healthy young 21 men (21±1 yrs) were randomly assigned to a practice group (n=14) and a nonexercising control group (n=7). Practice groups were divided by frequency of practice: 7 participants practiced once a week, and the other 7 twice a week, for 4 weeks. Steadiness practice required practice group to 5 sets of 60-s contraction at levels corresponding to 10% and 20% maximal voluntary contraction (MVC) in the plantar flexor muscles. The 4-week-long practice period reduced the force fluctuations (assessed as the standard deviation (SD) of the outputted force during steady isometric plantar flexion) and postural sway (assessed as SD of the center of mass velocity during quiet standing). However, these practice effects were not significantly affected by the practice frequencies (1 vs. 2 sessions per week) examined in this study. Further, a linear regression analysis revealed the association between prepractice postural sway and the relative change in postural sway by the practice (r=-0.904) in the practice group. These results suggest that the steadiness practice in plantar flexor muscles improves postural stability during quiet standing, even though the practice is low-frequency (once a week) and low-intensity (within 20% MVC). These practice effects are dependent on prepractice postural stability. Further, the present results have provided the functional significance of force fluctuation in lower limb muscles.

Reduced force steadiness in women with neck pain and the effect of short term vibration

This study compares neck force steadiness in women with neck pain and controls and the way this is influenced by short term vibration of the neck. In the first experiment, 9 women with chronic neck pain and 9 controls performed 10-s isometric cervical flexion at 15 N. Intramuscular EMG was recorded from the sternocleidomastoid muscle. In the second experiment, 10 women with neck pain and 10 controls performed 10-s isometric cervical flexion at 25% of their maximal force before and after vibration to the neck (bursts of 50 Hz with duration 20, 40, 60 and 120 s). Surface EMG was acquired from the sternocleidomastoid and splenius capitis. In both experiments, force steadiness was characterized by the coefficient of variation (CoV) and the relative power in three frequency subbands (low: 0–3 Hz; middle: 4–6 Hz; high: 8–12 Hz) of the force signal. Women with neck pain exhibited decreased force steadiness (Exp 1: patients 3.9 ± 1.3%, controls 2.7 ± 0.9%, P < 0.05; Exp 2: patients 3.4 ± 1.2%, controls 1.7 ± 0.6%, P < 0.01) which was associated with higher power in the low-frequency band (patients 71.2 ± 9.6%, controls 56.7 ± 9.2%, P < 0.01). Following vibration, CoV (2.6 ± 1.1%, P < 0.05) and the power in the low-frequency band of the force signal decreased (63.1 ± 13.9%, P < 0.05) in the patient group. These effects were not present in controls. Motor unit behavior and surface EMG amplitude were similar between groups. In conclusion, women with neck pain have reduced force steadiness, likely due to alterations in Ia afferent input. Vibration, which modulates Ia afferent input, increases force steadiness in patients with neck pain.

Hamstring force control following ACL injury and reconstruction and its relation to quadriceps coactivation and dynamic knee joint stability

Introduction: Typically, isokinetic muscle performance is assessed by measuring the peak or angle-specific torque; yet, another distinct aspect of muscle performance relates to torque steadiness—that is, the ability to control muscle force output. Following anterior cruciate ligament (ACL) injury and surgery, hamstrings force steadiness may reflect dynamic joint stability. Procurement of the combined semitendinosus and gracilis tendon (STGT) graft as an ACL substitute – because the insult incurred to the medial hamstring musculotendinous unit – may potentially alter the hamstrings ability to control muscle force output.

Sex differences in force steadiness in three positions of the forearm

The purpose of this study was to examine force steadiness in three positions of the forearm in young men and women across a variety of force levels. Eight young men and eight young women performed three maximum voluntary contractions (MVCs) in the neutral, supinated, and pronated forearm positions. Viewing a target line on a computer screen, subjects performed submaximal isometric contractions relative to their own MVC at 2.5, 5, 10, 25, 50, and 75% in each of the three forearm positions. Force steadiness was determined as the coefficient of variation (standard deviation around the mean force). A repeated-measures three-way ANOVA was used to assess the differences in force steadiness between sex, position, and force level. Men were stronger than women in all three forearm positions. Overall, men were steadier than women across all force levels and forearm positions. The neutral and supinated positions were equally strong and steady, and the pronated position was the weakest and least steady position. The forearm was most steady between 25 and 75% MVC, and least steady at the lower force levels. When correlations were run between MVC and coefficient of variation at all force levels and all forearm positions, a strong negative relationship was found (r=-0.49). In conclusion, men were stronger, as well as steadier, than women. The neutral and supinated forearm positions were both stronger and steadier than the pronated position. Results suggest that one of the primary factors influencing sex differences in force steadiness is absolute strength.

Reliability of a Test Battery Designed for Quickly and Safely Assessing Diverse Indices of Neuromuscular Function

Spaceflight affects nearly every physiological system. Spaceflight-induced alterations in physiological function translate to decrements in functional performance. Purpose: To develop a test battery for quickly and safely assessing diverse indices of neuromuscular performance. I. Quickly: Battery of tests can be completed in approx.30-40 min. II. Safely: a) No eccentric muscle actions or impact forces. b) Tests present little challenge to postural stability. III. Diverse indices: a) Strength: Excellent reliability (ICC = 0.99) b) Central activation: Very good reliability (ICC = 0.87) c) Power: Excellent reliability (ICC = 0.99) d) Endurance: Total work has excellent reliability (ICC = 0.99) e) Force steadiness: Poor reliability (ICC = 0.20 - 0.60) National

Asymmetry of Force Fluctuation in Knee Extension

The purpose of this study was to investigate the asymmetry of force fluctuation and mechanomyogram (MMG) signal during low intensity isometric knee extension. Twelve healthy males (21+/-1 years) performed unilateral force matching tasks; sustained isometric knee extension for 15 s at levels corresponding to 10% and 20% maximal voluntary contraction (MVC) with the visual feedback of force. During the force matching tasks, MMG was detected by an accelerometer arrangement placed on the vastus lateralis. Power spectrum density of the MMG was obtained by the fast Fourier transformation method, and mean power frequency (MPF) was calculated. Force fluctuation, amplitude of MMG and MPF of MMG were significantly increased in 20% MVC task compared to the 10% MVC task in both limbs. Significant differences between stronger MVC limb and weaker MVC limb were not observed in force fluctuation, amplitude of MMG and MPF of MMG during 10% and 20% MVC tasks. These results suggest that the asymmetry of force steadiness during low intensity isometric knee extension was not observed because there was no difference regarding mechanical characteristics in active muscle between stronger and weaker limbs.

Role of the nervous system in sarcopenia and muscle atrophy with aging: strength training as a countermeasure

Aging is characterized by loss of spinal motor neurons (MNs) due to apoptosis, reduced insulin-like growth factor I signaling, elevated amounts of circulating cytokines, and increased cell oxidative stress. The age-related loss of spinal MNs is paralleled by a reduction in muscle fiber number and size (sarcopenia), resulting in impaired mechanical muscle performance that in turn leads to a reduced functional capacity during everyday tasks. Concurrently, maximum muscle strength, power, and rate of force development are decreased with aging, even in highly trained master athletes. The impairment in muscle mechanical function is accompanied and partly caused by an age-related loss in neuromuscular function that comprise changes in maximal MN firing frequency, agonist muscle activation, antagonist muscle coactivation, force steadiness, and spinal inhibitory circuitry. Strength training appears to elicit effective countermeasures in elderly individuals even at a very old age (>80 years) by evoking muscle hypertrophy along with substantial changes in neuromuscular function, respectively. Notably, the training-induced changes in muscle mass and nervous system function leads to an improved functional capacity during activities of daily living.

Comparison of the influence of two stressors on steadiness during index finger abduction

Although several stressors have been used to examine the influence of arousal on motor performance, including noxious electrical stimulation, cold pressor test, and mental math calculations, no study has compared the influence of different physical stressors on motor output. The purpose of the study was to compare the influence of two stressors (cold pressor test and electrical stimulation) on the steadiness of the abduction force exerted by the index finger. Sixteen subjects (22.8 ± 3.5 years, 8 women) performed steadiness trials before (anticipatory phase), during (stressor phase), and after (recovery phase) each stressor. The steadiness task involved isometric contractions with the first dorsal interosseus muscle, which is the muscle that produces most of the abduction force exerted by the index finger. Subjects were required to match the abduction force on a monitor to a target force set to 5% of the maximal voluntary contraction (MVC) force for 60 s. In contrast to previous studies that examined the influence of stressors on pinch grip steadiness, the two stressors did not decrease steadiness. Furthermore, the absence of a change in steadiness contrasted with the increases in cognitive (State–Trait Anxiety Index, Visual Analog Scale) and physiological (heart rate) arousal during the stressor phase and the subsequent decline during recovery. The null effect of the stressors on index finger steadiness may be due to the relative simplicity of the task compared with those examined previously.

Relationship between Force Fluctuation in the Plantar Flexor and Sustainable Time for Single-leg Standing

The purpose of this study was to investigate the relationships between force fluctuation during isometric plantar flexion and the sustainable time for single-leg standing. Fourteen healthy males (21+/-1 years) performed unilateral (preferred leg) force matching tasks and single-leg quiet standing. Force matching tasks were performed to maintain isometric plantar flexion for 15 s at levels corresponding to 10% and 20% maximal voluntary contraction (MVC) with the visual feedback of force. Force fluctuation during force matching tasks was quantified as the standard deviation of force. Sustainable time for single-leg quiet standing was performed to maintain a single-leg quiet standing barefoot on a platform using the preferred leg with their eyes closed. Force fluctuation was significantly greater in 20% MVC task compared to 10% MVC task. The sustainable time for single-leg quiet standing was strongly correlated with force fluctuation in 20% MVC task (r=-0.56, p=0.04). However, it was not related to force fluctuation in 10% MVC task (r=0.19, p=0.52) or MVC value (r=0.13, p=0.65). These results suggest that a specificity of contraction intensity is observed between force steadiness and the posture stability during single-leg quiet standing; force steadiness during 20% MVC plantar flexion is one of the important components for posture stability during single-leg quiet standing.

Factors responsible for force steadiness impairment with fatigue

In this study we investigated the contribution of muscle activation to the impairment of fine force control with fatigue. In three experiments, we manipulated muscle activation and measured force variability before and after a fatigue protocol. When muscle activation was left free (subjects had to match the same absolute force pre- and post-fatigue), fatigue increased muscle activation at moderate force levels only, and force variability increased regardless of the level of force. When muscle activation was controlled (subjects had to match the same electromyographic activity), fatigue no longer increased force variability, except at low force levels. When voluntary muscle activation was suppressed (muscles were electrically stimulated), force variability was unaffected by fatigue. We conclude that the impairment of force steadiness with fatigue is mainly due to the increase in muscle activation at moderate forces, but there are other central sources of force fluctuation present at low force levels.

Motor unit firing variability and synchronization during short-term light-load training in older adults

We compared motor unit synchronization and firing rate variability within and across synergistic hand muscles during a pinching task following short-term light-load training to improve force steadiness in older adults. A total of 183 motor unit pairs before training and 158 motor unit pairs after training were recorded with intramuscular fine-wire electrodes within and across the first dorsal interosseous (FDI) and adductor pollicis (AdP) muscles during a pinch task performed by ten older adults before and after a 4-week short-term light-load training program. Nine younger adults performed the same experimental sessions 4 weeks apart with no training intervention. Two-minute sustained contractions of 2, 4, 8, and 12% maximal voluntary contraction (MVC) were performed with the non-dominant hand. The coefficient of variation (CV) of force was greater in older than in younger adults and was lower at the 2 and 4% MVC levels in both the finger (0.12 +/- 0.01 vs. 0.08 +/- 0.01, and 0.08 +/- 0.01 vs. 0.05 +/- 0.01, respectively) and thumb (0.11 +/- 0.01 vs. 0.08 +/- 0.01, and 0.09 +/- 0.01 vs. 0.05 +/- 0.01, respectively) compared to higher force levels following training in the older adults. There were no changes in CIS or k'-1 values following training. Motor unit firing rate variability significantly decreased at low force levels in the FDI muscle and also tended to decrease with training in the AdP muscle (p = 0.06). No changes occurred in the younger control group. These findings are the first to show that motor unit synchronization does not change during light-load training. Thus, it is likely that force steadiness in older adults improves by reducing motor unit firing variability rather than by changing motor unit synchronization.

Rebuttal from Taylor

POINT-COUNTERPOINTRebuttal from TaylorPublished Online:01 Jul 2009https://doi.org/10.1152/japplphysiol.91220.2008bMoreSectionsPDF (45 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat Twitch interpolation measures the proportion of maximal possible muscle force produced during voluntary contraction. Drs. de Haan, Gerrits, and de Ruiter conclude this in their second paragraph, but two of three objections to the validity of twitch interpolation are based on the premise that voluntary activation represents something more central, perhaps motoneuron firing or even input to motoneurons.They first object that sensitivity at near maximal contraction intensities is low. The cited model (4) uses a sigmoidal input-output relationship for motoneurons, such that large changes in excitation to the motoneurons produce only small changes in force at high contraction strengths. Hence, twitch interpolation, like voluntary force, has low sensitivity to altered excitatory drive to the motoneuron pool at high forces. However, the model shows a linear relationship between voluntary force and the interpolated twitch and supports the validity of twitch interpolation. Experimental evidence given for low sensitivity is a relatively large EMG increase for the increase in force above 80% maximum in the knee extensors (5). The implication is that small increases in force require large increases in motor unit firing, but other factors like synchronization can also increase EMG (6). Furthermore, other muscles do not show similar EMG-force relationships (2).The second objection that voluntary activation cannot be generalized from one situation to another seems a positive rather than a negative. Under different loading conditions different firing rates will be required for production of full force and one would not expect the same maximal voluntary activation.On the third objection, I concede that nonlinearities in the relationship between the interpolated twitch and voluntary force are common in practice, and extrapolation to predict maximal force is not recommended. The paper cited (5) shows extreme nonlinearity for individuals' knee extensors. Similar curves were seen for biceps brachii where methodological contributors were postulated (1). Such possibilities, including stimulation of antagonists, muscle lengthening, and variable synergist or antagonist contribution to voluntary torque, also hold for knee extension. Triplet stimulation may also contribute to nonlinearity as multiple pulses produce more torque than single pulses during weak but not strong contractions (1). Similarly, short interpulse intervals increase force greatly in unrecruited muscle, but not during repetitive firing (3).Thus twitch interpolation using nerve stimulation cannot measure motoneuronal input or output but only how well the muscle is driven to produce force. Although never ideal in humans, it usefully discriminates altered drive to muscles.REFERENCES1 Allen GM, McKenzie DK, Gandevia SC. Twitch interpolation of the elbow flexor muscles at high forces. Muscle Nerve 21: 318–328, 1998.Crossref | PubMed | ISI | Google Scholar2 Gelli F, Del Santo F, Popa T, Mazzocchio R, Rossi A. Factors influencing the relation between corticospinal output and muscle force during voluntary contractions. Eur J Neurosci 25: 3469–3475, 2007.Crossref | PubMed | ISI | Google Scholar3 Griffin L, Godfrey S, Thomas CK. Stimulation pattern that maximizes force in paralyzed and control whole thenar muscles. J Neurophysiol 87: 2271–2278, 2002.Link | ISI | Google Scholar4 Herbert RD, Gandevia SC. Twitch interpolation in human muscles: mechanisms and implications for measurement of voluntary activation. J Neurophysiol 82: 2271–2283, 1999.Link | ISI | Google Scholar5 Kooistra RD, de Ruiter CJ, de Haan A. Conventionally assessed voluntary activation does not represent relative voluntary torque production. Eur J Appl Physiol 100: 309–320, 2007.Crossref | ISI | Google Scholar6 Yao W, Fuglevand RJ, Enoka RM. Motor-unit synchronization increases EMG amplitude and decreases force steadiness of simulated contractions. J Neurophysiol 83: 441–452, 2000.Link | ISI | Google Scholar Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation More from this issue > Volume 107Issue 1July 2009Pages 357-358 Copyright & PermissionsCopyright © 2009 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.91220.2008bHistory Published online 1 July 2009 Published in print 1 July 2009 Metrics Downloaded 217 times

Eccentric Exercise Alters The Kinematic Structure Of Pointing Movements

It is known that the performance of eccentric muscle contractions often leads to delayed muscle soreness accompanied by declines in strength and isometric force steadiness. However, the effect of eccentric exercise on the performance of a dynamic motor task is not clear. PURPOSE: To examine the effects of a bout of eccentric exercise on kinematic measures related to accuracy (i.e. submovements) during spatially-constrained pointing movements. METHODS: 17 untrained subjects (10 M, 7 F; 26 ± 3 yrs.) performed 20 rapid discrete elbow-extension pointing movements with their right arm before, immediately after (fatigue and muscle damage), and 24-hours after (muscle damage only) fatiguing eccentric exercise on a isokinetic dynamometer. Endpoint (fingertip) movement kinematics were obtained for each pointing trial and examined for secondary submovements indentified by zero crossings in the tangential velocity and acceleration profiles. RESULTS: MVC force declined by 44% immediately after eccentric exercise and remained depressed (22%) 24-hrs after the exercise indicating the existence of muscle damage. Movement times were slower and peak velocity decreased immediately after, and only partially recovered 24-hrs after the exercise compared with before (222 ± 40, 248 ± 37 and 238 ± 42ms, P<0.05; 3.4 ± 0.6, 3.1 ± 0.6 and 3.3± 0.7m/s, P<0.01). Only 8% of the trials exhibited submovements before the eccentric exercise, however that number grew to 87% of trials immediately afterward and 29% of trials 24 hrs after. In those trials requiring submovements, subjects traveled 85 ± 8, 64 ± 6 and 72 ± 11% (P<0.001) of the total movement time before using a secondary submovement. CONCLUSIONS: These results demonstrate that kinematic measures associated with movement accuracy are altered from eccentric exercise and only partially restored after 24 hrs.

Counterpoint: Spectral properties of the surface emg do not provide information about motor unit recruitment and muscle fiber type

The surface electromyogram (EMG) comprises the sum of the electrical contributions made by the active motor units to the interference signal detected by electrodes placed on the skin overlying the muscle. Because it provides a global measure of motor unit activity, this signal is a valuable tool for

Motor Unit Coherence is Not Related to Un-steadiness in Hand

There is contradictory evidence as to whether motor unit coherence contributes to force un-steadiness. Little is known of the effect of coherence across synergistic muscle pairs during multi-digit hand tasks or of variation of motor unit coherence with force level. PURPOSE: To compare intra and across muscle motor unit coherence among pairs of simultaneously firing motor units in first dorsal interosseous (FDI) and adductor pollicis (AdP) muscles during a voluntary low-force pinch task with the non-dominant hand in older adults. METHODS: Seven (2 male, 5 female) healthy older adults (65 ± 3.6 yrs) performed 3 maximum pinching force (MPF) contractions followed by 2-minute pinch contractions at 2%, 4%, 8% and 12% of their respective MPF. Motor units were recorded from the FDI and the AdP muscles using intramuscular fine wire electrodes for a total of 118 motor unit pairs. The coefficient of variation (CV) of the force held was computed at each target level for the thumb and the finger as a measure of force steadiness. RESULTS: Force CV was not significantly different for thumb or finger (p= 0.96). Overall force CV at 2% MPF (0.08 ± 0.01) was greater than that at 8% MPF (0.05 ± 0.005, p= 0.04) and 12% MPF (0.03 ± 0.004, p 0.05). Overall motor unit coherence indices were higher within the FDI (0.13 ± 0.02) and AdP (0.11 ± 0.02) than across the two muscle groups (0.07 ± 0.009) with p= 0.02 and p= 0.09 respectively. CONCLUSION: In older adults, force steadiness was lower at lower target levels whereas, coherence indices did not change. Thus, decreased hand steadiness during low-force pinching tasks cannot be attributed to changes in motor unit coherence. Supported by NIH Grant RO3 AR52883-01.

Hand Steadiness and Coherence Following Motor Learning Training in Older Adults

Motor learning training can improve hand steadiness. Motor unit coherence has been suggested to be related to force un-steadiness. It is possible that motor learning training may change motor unit coherence within and across synergistic muscles. PURPOSE: To compare force steadiness and motor unit coherence in the first dorsal interosseous (FDI) and adductor pollicis (AdP) muscles of older adults during a pinching task before and after a motor learning training program. METHODS: Pre and post tests consisted of 2-minute sustained contractions at 2, 4, 8, and 12 % maximal pinching force (MPF) with the non-dominant hand. Single motor units were recorded from the AdP and FDI muscles with intramuscular finewire electrodes. The training program consisted of 2 sessions per week for 4 weeks. Participants performed 5 contractions each at 2 and 4 % MPF for 20 seconds, as well as 10 slow ramp contractions up to 2 and 4% MPF at their own pace and at a designated velocity of 0.5% MPF/sec. 104 motor unit pairs before training and 95 motor unit pairs after training were analyzed to determine changes within and across muscles. RESULTS: Following motor learning training, the coefficient of variation in the force improved in the 2, 4, and 8 % MPF levels in the finger (0.08 ± 0.01 vs. 0.05 ± 0.005, p =0.009; 0.08 ± 0.01 vs. 0.06 ± 0.003, p< 0.001; and 0.05 ± 0.005 vs. 0.03 ± 0.003, p= 0.02, for the 2, 4, and 8 % levels, respectively) and the 2 and 4 % MPF in thumb ( 0.08 ± 0.01 vs. 0.06 ± 0.003, p= 0.01; and 0.08 ± 0.01 vs. 0.04 ± 0.003, p= 0.01, for the 2 and 4 % levels, respectively). Coherence indices pre-and post-training did not change at any level (p= 0.1, p= 0.5, p= 0.1, p= 0.4 for the 2, 4, 8, and 12 % MPF, respectively). CONCLUSIONS: Motor learning improved hand steadiness; however, it did not impact the incidence of coherence within or across the AdP and FDI muscles. Thus, motor unit coherence was not related to changes in hand steadiness with training. Supported by NIH Grant RO3 AR52883-01.

Effects of experimental muscle pain on shoulder-abduction force steadiness and muscle activity in healthy subjects

We previously demonstrated that the steadiness of shoulder abduction is reduced in patients with subacromial impingement syndrome (SIS), which might be related to shoulder pain associated with the SIS. The aim of the present study was to examine the acute effects of experimental shoulder muscle pain on shoulder motor function in healthy subjects. The fluctuations in exerted force (force steadiness) and electromyographic (EMG) activity from eight shoulder muscles were determined during sub-maximal isometric and dynamic contractions with the shoulder abductors in nine healthy subjects (27.7 +/- 4.2 years, mean +/- 1 SD) before, during and after experimental pain induction. Experimental pain was induced by bolus injections of 6% hypertonic saline into the supraspinatus muscle. Experimental muscle pain reduced shoulder-abduction force steadiness on average by 21% during isometric contractions (P = 0.012) and tended to do so during concentric contractions (P = 0.083). Middle deltoid, and infraspinatus and lower trapezius muscle activity increased (3-5% EMG(max)) during isometric and concentric contractions, respectively (P < 0.05). Thus, experimental shoulder muscle pain reduced the steadiness of isometric shoulder abduction and caused small changes in the abduction activation strategy. The observed effects of experimental pain on shoulder motor function differed from that observed previously in patients with SIS and chronic pain during the same types of contractions. A possible explanation may be that, even though the adopted experimental pain-paradigm may reflect the SIS in terms of the painful structures, it might not reflect the adaptations in the central nervous system seen with chronic pain.

Rate Coding Is Compressed But Variability Is Unaltered for Motor Units in a Hand Muscle of Old Adults

The discharge of single motor units (n = 34) in the first dorsal interosseus muscle and the fluctuations in force during steady contractions were measured across a range of index finger abduction forces in old adults (77.1 +/- 6.9 yr, n = 20). These results were compared with previously reported data on 38 motor units from young adults (25.7 +/- 5.7 yr). Both minimal and peak discharge rates increased with recruitment threshold, but the strength of these relations was notably weaker for the old adults. Minimal discharge rates were similar for young and old adults (P = 0.77), whereas peak discharge rates were lower for old adults (P < 0.01). Consequently, the range of rate coding for each motor unit was substantially less for the old adults (7.1 pps) compared with the young adults (12.1 pps, P < 0.01). However, the variability in motor-unit discharge was similar for young and old adults; the coefficient of variation of the interspike intervals was similar at recruitment (old: 25.4%, young: 27.1%, P = 0.39) and declined with an increase in discharge rate (old: 13.2%, young: 14.2%, P = 0.21). Furthermore, the fluctuations in force during steady isometric contractions (2-95% of maximal force) were similar for young and old adults, except that the relative variability at the lowest force was greater for the old adults. A computational model of motor-unit recruitment and rate coding incorporated the experimental observations and was able to match the measured and simulated values for force steadiness across the operating range of the muscle.

Steadiness of quadriceps contractions in young and older adults with and without a history of falling

Decreased steadiness of muscle force may be associated with ageing and could be a cause of falls in older people. We studied this in isometric and anisometric quadriceps contractions in healthy young and older people. The older group contained people with and without a history of medically unexplained falls. Forty-four young (aged 18-40 years) and 78 older (aged > 70 years) subjects participated. In the latter group 34 people had a history of falling (fallers) and 44 did not (non-fallers). Isometric steadiness was measured by the coefficient of variation (CoV) of force at 10, 25 and 50% maximal voluntary force (MVC). Anisometric steadiness was measured by the SD of acceleration during concentric and eccentric contractions against two external loads (1 and 5 kg). There was an overall trend for the younger subjects to be most steady and the fallers the least but the differences were not consistently significant. Isometric steadiness was unaffected by force in all groups. The fallers were less steady (P < 0.001) than both the young and non-fallers, who had similar values. During anisometric contractions, steadiness was similar with both external loads and types of contraction in all groups. During dynamic contractions the older subjects were less steady (P < 0.002). Only eccentric contractions distinguished between the two older groups, with the fallers being less steady by 31% (P = 0.013). These data indicate ageing per se is associated with decreased anisometric, but not isometric, steadiness. Greater unsteadiness during eccentric contractions in the fallers could be an important mechanism of medically unexplained falls.

Force steadiness, muscle activity, and maximal muscle strength in subjects with subacromial impingement syndrome

We investigated the effects of the subacromial impingement syndrome (SIS) on shoulder sensory-motor control and maximal shoulder muscle strength. It was hypothesized that both would be impaired due to chronic shoulder pain associated with the syndrome. Nine subjects with unilateral SIS who remained physically active in spite of shoulder pain and nine healthy matched controls were examined to determine isometric and isokinetic submaximal shoulder-abduction force steadiness at target forces corresponding to 20%, 27.5%, and 35% of the maximal shoulder abductor torque, and maximal shoulder muscle strength (MVC). Electromyographic (EMG) activity was assessed using surface and intramuscular recordings from eight shoulder muscles. Force steadiness was impaired in SIS subjects during concentric contractions at the highest target force level only, with muscle activity largely unaffected. No between-group differences in shoulder MVC were observed. The present data suggest that shoulder sensory-motor control is only mildly impaired in subjects with SIS who are able to continue with upper body physical activity in spite of shoulder pain. Thus, physical activity should be continued by patients with SIS, if possible, to avoid the loss in neural and muscle functions associated with inactivity.