Competitive swimmers are predisposed to musculoskeletal injuries of the upper limb, knee, and spine. This review discusses the epidemiology of these injuries, in addition to prevention strategies that may assist the physician in formulating rehabilitation programs for the swimmer following an injury.
A literature search was performed by a review of Google Scholar, OVID, and PubMed articles published from 1972 to 2011.
This study highlights the epidemiology of injuries common to competitive swimmers and provides prevention strategies for the sports health professional.
An understanding of swimming biomechanics and typical injuries in swimming aids in early recognition of injury, initiation of treatment, and design of optimal prevention and rehabilitation strategies.
competitive swimmer; injury; prevention; shoulder; knee; spine
Competitive swimming has become an increasingly popular sport in the United States. In 2007, more than 250 000 competitive swimmers were registered with USA Swimming, the national governing body. The average competitive swimmer swims approximately 60 000 to 80 000 m per week. With a typical count of 8 to 10 strokes per 25-m lap, each shoulder performs 30 000 rotations each week. This places tremendous stress on the shoulder girdle musculature and glenohumeral joint, and it is why shoulder pain is the most frequent musculoskeletal complaint among competitive swimmers.
Articles were obtained through a variety of medical search sources, including Medline, Google Scholar, and review articles from 1980 through January 2010.
The most common cause of shoulder pain in swimmers is supraspinatus tendinopathy. Glenohumeral instability and labral tears have also been reported, but a paucity of information remains regarding prevalence and treatment in swimmers.
Because of the great number of stroke repetitions and force generated through the upper extremity, the shoulder is uniquely vulnerable to injury in the competitive swimmer. Comprehensive evaluation should include the entire kinetic chain, including trunk strength and core stability.
swimmer’s shoulder; swim strokes; biomechanics
Low back pain in young athletes is a common complaint and should be taken seriously. It frequently results from a structural injury that requires a high degree of suspicion to diagnose and treat appropriately.
A Medline search was conducted from 1996 to May 2008 using the search terms “low back pain in children” and “low back pain in athletes.” Known texts on injuries in young athletes were also reviewed. References in retrieved articles were additionally searched for relevant articles. Sources were included if they contained information regarding diagnosis and treatment of causes of low back pain in children.
Low back pain is associated with sports involving repetitive extension, flexion, and rotation, such as gymnastics, dance, and soccer. Both acute and overuse injuries occur, although overuse injuries are more common. Young athletes who present with low back pain have a high incidence of structural injuries such as spondylolysis and other injuries to the posterior elements of the spine. Disc-related pathology is much less common. Simple muscle strains are much less likely in this population and should be a diagnosis of exclusion only.
Young athletes who present with low back pain are more likely to have structural injuries and therefore should be investigated fully. Muscle strain should be a diagnosis of exclusion. Treatment should address flexibility and muscle imbalances. Injuries can be prevented by recognizing and addressing risk factors. Return to sport should be a gradual process once the pain has resolved and the athlete has regained full strength.
low back pain; adolescents; pediatric athletes
Swimmer's shoulder is a musculoskeletal condition that results in symptoms in the area of the anterior lateral aspect of the shoulder, sometimes confined to the subacromial region. The onset of symptoms may be associated with impaired posture, glenohumeral joint mobility, neuromuscular control, or muscle performance. Additionally, training errors such as overuse, misuse, or abuse may also contribute to this condition. In extreme cases, patients with swimmer's shoulder may have soft tissue pathology of the rotator cuff, long head of the biceps, or glenoid labrum. Physical therapists involved in the treatment of competitive swimmers should focus on prevention and early treatment, addressing the impairments associated with this condition, and analyzing training methods and stroke mechanics. The purpose of this clinical commentary is to provide an overview of the biomechanics of swimming, the etiology of the clinical entity referred to as swimmer's shoulder, and strategies for injury prevention and treatment.
Swimmer's shoulder; injury prevention; rotator cuff
A group of 453 elite young athletes (231 boys, 222 girls) in five two year age groups from 8-16 years of age was followed up for two years in order to identify self reported injuries over that period. Four sports were studied, namely football (soccer), gymnastics, tennis, and swimming. The injury rate was low with just over half the children suffering one or more injuries per year, with the majority of those injured sustaining one injury only. Over the two year period of intensive sporting activity this amounted to less than one injury per 1000 hours of training. The highest risk of injuries was in football (67%) and the lowest in swimming (37%). Most injuries (70%) were acute and of a minor nature, although overuse injuries did require longer periods off training and competition than acute injuries (20 v 13 days). Footballers appeared to sustain more significant injuries than other sports as judged by the time required to resume training and/or competition (16 days after acute and 57 after overuse). No significant associations were found between injury rate, injury severity, sex, and pubertal status with the single exception of female gymnasts in whom more injuries occurred in the latter stages of puberty. Only four of the 453 athletes reported injury as a reason for retiring from their chosen sports. Most injuries in elite young athletes are minor, their prevalence is low and, at least in the short to medium term, do not constitute a significant health problem.
Overuse injuries of the musculoskeletal system in immature athletes are commonly seen in medical practice.
An analysis of published clinical, outcome, and biomechanical studies of adolescent epiphyseal and overuse injuries was performed through 2008 to increase recognition and provide treatment recommendations.
Adolescent athletes can sustain physeal and bony stress injuries. Recovery and return to play occur more swiftly if such injuries are diagnosed early and immobilized until the patient is pain-free, typically about 4 weeks for apophyseal and epiphyseal overuse injuries. Certain epiphyseal injuries have prolonged symptoms with delayed treatment, including those involving the bones in the hand, elbow, and foot. If such injuries are missed, prolonged healing and significant restrictions in athletic pursuits may occur.
Some of these injuries are common to all weightbearing sports and are therefore widely recognized. Several are common in gymnastics but are rarely seen in other athletes. Early recognition and treatment of these conditions lead to quicker recovery and so may prevent season-ending, even career-ending, events from occurring.
apophysitis; epiphysitis; epihyseolysis; adolescent athletes; overuse
A series of 147 cases of exertion injuries in less than or equal to 15 years old athletes is presented. All injuries occurred during training or athletic performances without trauma and caused symptoms that prevented athletic exercises. There were 67 girls (46%) and 80 boys (54%) in the material. About 90% of them had been training for more than one year before the onset of the symptoms; 65% were interested in track and field athletics, 13% in ball games, 11% in skiing, 4% in swimming, and 3% in orienteering. The rest were interested in other sports. About 33% of the injuries were growth disturbances or osteochondroses seen also in other children. About 15% were anomalies, deformities or earlier osteochondritic changes, which caused first symptoms during the physical exercise; 50% were typical overuse injuries that may bother adult athletes, too; 43% of the injuries were localized in ankle, foot and heel, 31% in knee, 8% in back and trunk, 7% in pelvic and hip region, and the rest in other parts of the body. The injuries were generally slight, no permanent disability was noticed. Rest and conservation therapy cured most cases; operative treatment was used in only eight cases.
To review current concepts of the pathophysiology, diagnosis, and treatment of rotator cuff and impingement injuries in the athlete.
The information we present was compiled from a review of classic and recently published material regarding rotator cuff and impingement injuries. These materials were identified through a search of a personal literature database compiled by the authors, as well as by selective searching of the MEDLINE. In addition, much of the information presented represents observations and opinions of the authors developed over 8 to 10 years of treating shoulder injuries in athletes.
Biomechanics of the normal shoulder and pathophysiology of rotator cuff injuries in the athletic population are discussed, followed by a summary of the important diagnostic features of rotator cuff and impingement injuries. The principles of rehabilitation are extensively presented, along with indications and important technical aspects of selected surgical procedures. General principles and specific protocols of postoperative rehabilitation are also summarized.
Rotator cuff and impingement injuries in the athletic population are multifactorial in etiology, exhibiting significant overlap with glenohumeral instability. Nonoperative treatment is successful in most athletic patients with rotator cuff and impingement injuries. When nonoperative treatment fails, arthroscopic surgical techniques such as rotator cuff repair and subacromial decompression may be successful in returning the athlete to competition.
impingement; rotator cuff tear; glenohumeral instability; arthroscopy
Rowing is one of the original modern Olympic sports and was one of the most popular spectator sports in the United States. Its popularity has been increasing since the enactment of Title IX. The injury patterns in this sport are unique because of the stress applied during the rowing stroke.
This review summarizes the existing literature describing the biomechanics of the rowing stroke and rowing-related injury patterns. Data were obtained from previously published peer-reviewed literature through a search of the entire PubMed database (up to December, 2011) as well as from textbook chapters and rowing coaching manuals.
Rowing injuries are primarily overuse related. The knee, lumbar spine, and ribs are most commonly affected. The injury incidence is directly related to the volume of training and technique.
Familiarity of the injury patterns and the biomechanical forces affecting the rowing athlete will aid in prompt diagnosis and appropriate management.
rowing; rowing injuries; overuse injuries; rib stress fractures; lumbar degenerative disc disease
The purpose of this study was to analyze the relationships between 100-m front crawl swimming performance and relevant biomechanical, anthropometrical and physiological parameters in male adolescent swimmers. Twenty five male swimmers (mean ± SD: age 15. 2 ± 1.9 years; height 1.76 ± 0.09 m; body mass 63.3 ± 10.9 kg) performed an all-out 100-m front crawl swimming test in a 25-m pool. A respiratory snorkel and valve system with low hydrodynamic resistance was used to collect expired air. Oxygen uptake was measured breath-by-breath by a portable metabolic cart. Swimming velocity, stroke rate (SR), stroke length and stroke index (SI) were assessed during the test by time video analysis. Blood samples for lactate measurement were taken from the fingertip pre exercise and at the third and fifth minute of recovery to estimate net blood lactate accumulation (ΔLa). The energy cost of swimming was estimated from oxygen uptake and blood lactate energy equivalent values. Basic anthropometry included body height, body mass and arm span. Body composition parameters were measured using dual-energy X-ray absorptiometry (DXA). Results indicate that biomechanical factors (90.3%) explained most of 100-m front crawl swimming performance variability in these adolescent male swimmers, followed by anthropometrical (45.8%) and physiological (45.2%) parameters. SI was the best single predictor of performance, while arm span and ∆La were the best anthropometrical and physiological indicators, respectively. SI and SR alone explained 92.6% of the variance in competitive performance. These results confirm the importance of considering specific stroke technical parameters when predicting success in young swimmers.
This study investigated the influence of different anthropometrical, physiological and biomechanical parameters on 100-m swimming performance in adolescent boys.
Biomechanical factors contributed most to sprint swimming performance in these young male swimmers (90.3% of variability in performance), followed by anthropometrical (45.8%) and physiological (45.2%) parameters.
Two selected variables (stroke index and stroke rate) explained 92.6% of the variance in competitive performance in these adolescent swimmers.
oxygen uptake; stroke index; energy cost; front crawl
The study aimed to examine changes in selected angular characteristics and duration of the stroke cycle in the back crawl and the front crawl in children learning to swim. Nine boys and two girls, aged 8–13 years, performed seven consecutive swimming tests. The children’s movement technique was recorded with the use of three video cameras. The studied parameters included the angle of incidence between the trunk long axis and the waterline, elbow angle, shoulders roll, stroke cycle duration and stroke length. The results illustrate the development of swimming technique in youth swimmers. The results of the present study indicate the variability and phasing of learning of swimming technique by children.
distance per stroke
Overuse injuries are a frequent occurrence among competitive athletes. When analysing the incidence of overuse injuries in tennis players, it has been determined that a significant number of these injures occur in the upper limb area. In this study, we describe five cases of a stress‐induced injury to the middle and distal humerus occurring mainly due to repetitive serving.
Athletes studied were competitive tennis players and elite junior players, two of whom played at international level. Four of the five were male. In all cases, diagnosis was confirmed by magnetic resonance imaging examination. The treatment of middle and distal humeral stress reactions consisted of physical therapy, which focused on analgesia and muscle strengthening. In addition, we analysed each tennis player's strokes in order to identify modifications that would decrease the amount of stress that the upper limbs were subjected to during the service motion.
The players in our study missed on average 3 weeks of play and at follow‐up after 1 year were able to play symptom free.
Our study highlights the need for coaches, physicians and players to be aware of distal humeral pain and understand treatment options in order to prevent further injury, including stress fractures.
To discuss the anatomy and biomechanics of the acromioclavicular (AC) joint, along with the clinical evaluation and treatment of an athlete with an AC joint injury.
I searched MEDLINE from 1970 through 1999 under the key words “acromioclavicular joint,” “clavicle,” “acromioclavicular separation,” and “acromioclavicular dislocation.” Knowledge base was an additional source.
AC joint injury is common in athletes and a source of significant morbidity, particularly for athletes in overhead sports. Because this injury can masquerade as other shoulder conditions, the examiner must understand the anatomy and biomechanics of the shoulder in order to perform a systematic clinical evaluation and identify the injury.
Careful attention to the clinical evaluation allows the clinician to categorize the athlete's AC joint injury and institute appropriate treatment in a timely fashion, thus permitting the athlete to return to sport as quickly and safely as possible.
acromioclavicular ligaments; coracoclavicular ligaments; acromioclavicular joint separation; clavicle fracture; sternoclavicular dislocation; distal clavicle osteolysis; acromioclavicular joint degenerative disease
Increasing numbers of children are becoming involved in competitive sport. International trends in pre-adolescent sports participation are mirrored in New Zealand, where promising young athletes are being exposed to high-intensity training from an earlier age. As a consequence, overuse injuries which were traditionally described in more mature athletes are now becoming recognized in pre-adolescents. The immature musculoskeletal system is less able to cope with repetitive biomechanical stress. Sites of overuse injury reflect the sites of rapid musculoskeletal development. It therefore behoves all medical practitioners, but particularly those in primary care, to be aware of the young athlete at risk. Inherent in the presentation of such musculoskeletal insult there often lurks an over-enthusiastic parent. We are all well reminded of the covert pressures adults may bring to bear upon children. Psychological, as well as physical injury often results.
The purpose of the present study was to investigate different biomechanical variables of backstroke technique in swimmers specialized in different distance events, in order to investigate the capacity to modify the timing of the arm stroke when changing the swimming velocity from sub-maximal to maximal. Two 25-m backstroke trials respectively at 70% of maximum velocity (V70) and at 100% of maximum velocity (Vmax) were performed by 9 200-m distance swimmers and 9 50-m distance swimmers. Swimming velocity, stroke length, stroke rate, duration of different phases of the arm stroke and selected kinematic variables were assessed in both cases. In the 50-m distance swimmers, the duration of the propulsive phase at Vmax, expressed as a percentage of the duration of the total underwater arm stroke, increased significantly (p = 0.001) with increasing swimming velocity. Specifically, both the pull and push phases were fundamental in the increase of duration of the propulsive phase. When compared to 200-m specialists, 50-m distance swimmers seem to be more able to modify their arm stroke phases duration when increasing the swimming velocity in backstroke.
The 50-m DS are able to find an optimal timing among the stroke phases increasing the duration of the propulsive phase.
The 50-m DS, when increasing the swimming velocity, show a more efficient relationship between propulsive and non propulsive phases with respect to the 200-m DS.
Both pull and push phases are key factors for increasing the duration of the propulsive phase for the 50-m DS.
Arm motion; stroke phases; stroke rate; stroke length; technical analysis
OBJECTIVES: There has been little research on the time course of recovery from injury in athletes. This is especially the case for recovery in arm power in injured swimmers. The purpose of this study was to compare the power output of the injured and non-injured arms of swimmers during recovery from injury by use of a maximal exercise test on a computer interfaced isokinetic swim bench. METHODS: Thirteen swimmers (five men and eight women; age 18.8 (3.2) years; stature 1.76 (0.05) m; body mass 61.7 (5.9) kg; mean (SD)) gave written informed consent and were recruited to this study throughout a three year period. All subjects had experienced non-aquatic soft tissue injury to their dominant-side shoulder or upper arm in the three months before participation, but had been allowed to return to swimming training. All of the subjects had injured their dominant arm and the mean time for absence from training was 3.7 (1.1) weeks. At return to training and at four, eight, and twelve weeks thereafter, subjects performed two all-out 30 second tests on the swim bench by simulating the swimming arm action. From these tests, peak power output (PPO), mean power output (MPO), and power decay (PD) for each arm during the 30 seconds of exercise could be determined by averaging the two tests. The differences between return to training and the four, eight, and twelve week periods were analysed using repeated measures analysis of variance with Tukey b post hoc test. RESULTS: The repeated testing showed 95% confidence intervals of +/- 11.4 W for PPO, +/- 9.5 W for MPO and +/- 0.5 for PD. When the swimmers returned to training the results showed that PPO was 179 (21.9) v 111 (18.1) W (P = 0.02), MPO was 122 (9.8) v 101 (8.8) W (P = 0.01), and PD was 2.5 (0.6) v 5.2 (1.9) (P = 0.001) for non-injured and injured arms respectively (all values mean (SEM)). There were similar differences at four weeks which disappeared after eight weeks, except for that of PPO which was still evident (187.3 (21.9) v 156.8 (18.1) W; P = 0.01). At 12 weeks there were no differences between the non-injured and injured arm on any of the indices of arm power (P > 0.05). CONCLUSIONS: These results suggest that, using the swim bench power test, differences in bilateral arm power output after injury persist for at least eight weeks after return to swimming training. These findings support the need for prolonged rehabilitation after such injury. This would best include physiotherapy and a training programme within which special consideration is given to the recuperation process.
To review the presentation, evaluation, treatment, and prognosis of various nerve injuries about the shoulder in the athletic population. Included are injuries to the axillary, suprascapular, musculocutaneous, long thoracic, and spinal accessory nerves.
This article represents a review of the literature regarding incidence, presentation, and results of treatment of these various nerve injuries. The clinically pertinent anatomy is also presented to better relate mechanism of injury to the occurrence of nerve injury. I searched MEDLINE from 1966 through 1999 and the Journal of Shoulder and Elbow Surgery from 1992 through 1999 for the key words “nerve” and “shoulder.”
A historical review of treatment results is presented as well as a review of treatment options and the results of studies using modern techniques in the management of nerve injuries.
Nerve injuries about the shoulder present as distinct clinical syndromes, although signs and symptoms can be subtle. The athletic trainer and team physician must be able to recognize the presentation of these injuries so that adequate evaluation and prompt treatment can be instituted to maximize the athlete's chance for early return to sport.
axillary nerve; suprascapular nerve; musculocutaneous nerve; long thoracic nerve; spinal accessory nerve; athletic nerve injury
Swimming, a sport practiced in hypogravity, has sometimes been associated with decreased bone mass.
This systematic review aims to summarize and update present knowledge about the effects of swimming on bone mass, structure and metabolism in order to ascertain the effects of this sport on bone tissue.
A literature search was conducted up to April 2013. A total of 64 studies focusing on swimmers bone mass, structure and metabolism met the inclusion criteria and were included in the review.
It has been generally observed that swimmers present lower bone mineral density than athletes who practise high impact sports and similar values when compared to sedentary controls. However, swimmers have a higher bone turnover than controls resulting in a different structure which in turn results in higher resistance to fracture indexes. Nevertheless, swimming may become highly beneficial regarding bone mass in later stages of life.
Swimming does not seem to negatively affect bone mass, although it may not be one of the best sports to be practised in order to increase this parameter, due to the hypogravity and lack of impact characteristic of this sport. Most of the studies included in this review showed similar bone mineral density values in swimmers and sedentary controls. However, swimmers present a higher bone turnover than sedentary controls that may result in a stronger structure and consequently in a stronger bone.
To review the pathoanatomy, classification, and etiologies of lesions of the superior labrum and biceps anchor (SLAP lesions) and to discuss the clinical presentation, with emphasis on physical examination findings and current treatment recommendations.
We searched MEDLINE for English-language articles published from 1985 to 1999 using the key words “superior labral lesion,” “SLAP lesion,” “labral tear,” and “biceps tendon.” Additional information was obtained from cross- referencing pertinent articles and personal communications with experts in the field of shoulder arthroscopy.
The clinical presentation of superior labral lesions often includes a history of trauma or repetitive overuse in athletes associated with complaints of pain and clicking or popping in the shoulder. The diagnosis can be difficult, as clinical findings may overlap with those of acromioclavicular or rotator cuff problems and exist concomitantly with glenohumeral instability.
Superior labral lesions are a relatively newly defined cause of shoulder pain and disability. Knowledge about these lesions and a high index of suspicion are essential to identifying this important cause of shoulder pain. Superior labral lesions are usually confirmed and successfully managed arthroscopically.
shoulder arthroscopy; SLAP lesion; labral tear; biceps tendon
Although the overall injury rate in volleyball and beach volleyball is relatively low compared with other team sports, injuries do occur in a discipline specific pattern. Epidemiological research has revealed that volleyball athletes are, in general, at greatest risk of acute ankle injuries and overuse conditions of the knee and shoulder. This structured review discusses both the known and suspected risk factors and potential strategies for preventing the most common volleyball related injuries: ankle sprains, patellar tendinopathy, and shoulder overuse.
ankle sprain; injury prevention; patellar tendinopathy; shoulder pain; volleyball
The increasingly popular sport of rock climbing is an activity which predisposes participants to overuse injuries. The unique physical demands associated with climbing, as well as a reported 33%-51% incidence of shoulder injuries in these athletes is suggestive of abnormalities in scapulohumeral biomechanics.
To examine the glenohumeral to scapulothoracic (GH:ST) ratio, as represented by end range static positions (ERSP) of the scapula and humerus, in a group of rock climbers and compare it to a group of non-climbers.
The GH:ST ratio of twenty-one experienced rock climbers was compared with 40 non-climbers using a bubble inclinometer to measure scapular upward rotation at the subjects' maximum glenohumeral elevation.
As represented by ERSP, rock climbers had a significantly greater GH:ST ratio than non-climbers. The mean ratio of climbers was 3.7:1 compared with non-climbers at 2.8:1. Scapulothoracic motion appeared to be the source of this difference.
Discussion and Conclusion
A possible explanation for this difference could be related to the extreme and prolonged positioning associated with rock climbing maneuvers that result in shoulder musculature imbalances in strength and flexibility.
rock climbing; shoulder injuries; scapulohumeral dyskinesis
Enthesitis of the direct tendon of the rectus femoris muscle is a rare pathology which mainly affects professional athletes, and it is caused by overuse and repetitive microtrauma. Athletic jumping and kicking exert a great stress on the direct tendon of the rectus femoris muscle, and volleyball and football players are therefore most frequently affected. Enthesitis may occur suddenly causing pain and functional impairment possibly associated with partial or complete tendon injuries, or it may be a chronic condition causing non-specific clinical symptoms.
We present the case of a professional volleyball player who felt a sudden pain in the left side of the groin area during a training session although she had suffered no accidental injury. The pain was associated with impaired ipsilateral limb function. Tendon rupture was suspected, and magnetic resonance imaging (MRI) was performed. MRI showed a lesion at the myotendinous junction associated with marked inhomogeneity of the direct tendon. Ultrasound (US) examination confirmed the presence of both lesions and allowed a more detailed study of the pathology.
This is a typical case of enthesitis which confirms that MRI should be considered the examination of choice in hip pain, particularly when the patient is a professional athlete, thanks to its panoramic visualization. However, also US is an ideal imaging technique for evaluating tendon injuries thanks to its high spatial resolution, and it can therefore be used effectively as a second line of investigation.
Direct tendon; Quadriceps muscle; Enthesitis; Ultrasound; Volleyball
My objective is to review the factors that influence youth participation in sports, to discuss the role coaches may play in youth sports injuries, and to call on athletic trainers and other health professionals to become involved in youth sports in an effort to limit injury risk.
Millions of American youths participate in team sports. Their primary motivation to participate is to have fun. Unfortunately, large numbers of participants have sustained correspondingly large numbers of injuries. Many injuries can be attributed to improper technique and conditioning methods taught by volunteer coaches. Although not the only contributors to injuries, these may be the most amenable to preventive measures, such as formal instruction for coaches in the areas of proper biomechanics and player-coach communication.
I provide an overview of the reasons why children participate in sports, discuss participation motivation, and review the literature on coaches' communication methods that have been proved effective in maximizing learning and enjoyment for young athletes.
This article provides certified athletic trainers with the background knowledge needed to take an active role in youth sports injury prevention at the community level.
young athletes; sports psychology; participation motivation; injury prevention; coaching
The authors have shown that rats can be retrained to swim after a moderately severe thoracic spinal cord contusion. They also found that improvements in body position and hindlimb activity occurred rapidly over the first 2 weeks of training, reaching a plateau by week 4. Overground walking was not influenced by swim training, suggesting that swimming may be a task-specific model of locomotor retraining.
To provide a quantitative description of hindlimb movements of uninjured adult rats during swimming, and then after injury and retraining.
The authors used a novel and streamlined kinematic assessment of swimming in which each limb is described in 2 dimensions, as 3 segments and 2 angles.
The kinematics of uninjured rats do not change over 4 weeks of daily swimming, suggesting that acclimatization does not involve refinements in hindlimb movement. After spinal cord injury, retraining involved increases in hindlimb excursion and improved limb position, but the velocity of the movements remained slow.
These data suggest that the activity pattern of swimming is hardwired in the rat spinal cord. After spinal cord injury, repetition is sufficient to bring about significant improvements in the pattern of hindlimb movement but does not improve the forces generated, leaving the animals with persistent deficits. These data support the concept that force (load) and pattern generation (recruitment) are independent and may have to be managed together with respect to postinjury rehabilitation.
Spinal cord injury; Swimming; Task-specific learning; Rat; Locomotor retraining; Rehabilitation
Using information from physics, biomechanics and evolutionary biology, we explore the implications of physical constraints on sperm performance, and review empirical evidence for links between sperm length and sperm competition (where two or more males compete to fertilise a female's eggs). A common theme in the literature on sperm competition is that selection for increased sperm performance in polyandrous species will favour the evolution of longer, and therefore faster swimming, sperm. This argument is based on the common assumption that sperm swimming velocity is directly related to sperm length, due to the increased thrust produced by longer flagella.
We critically evaluate the evidence for links between sperm morphology and swimming speed, and draw on cross-disciplinary studies to show that the assumption that velocity is directly related to sperm length will rarely be satisfied in the microscopic world in which sperm operate.
We show that increased sperm length is unlikely to be driven by selection for increased swimming speed, and that the relative lengths of a sperm's constituent parts, rather than their absolute lengths, are likely to be the target of selection. All else being equal, we suggest that a simple measure of the ratio of head to tail length should be used to assess the possible link between morphology and speed. However, this is most likely to be the case for external fertilizers in which females have relatively limited opportunity to influence a sperm's motility.