Of the 30 articles that reported injuries to the knee, 22 were surveys that considered amateur and/or professional golfers, either independently of their knee condition (n = 18) or specifically after knee arthroplasty (n = 4). Additionally, eight articles presented case studies where golf was identified as a contributing factor to knee injury.
Injuries Independent of Knee Condition
The rate of knee injury varied from 3 to 18% of the survey population (Table 1), but little information was reported on the nature of the injury or which knee was affected. Five studies reported career injury rates among professionals ranging from 5.5 to 15% [5, 13,14,15,16], and 11 studies reported an injury rate of 3.2–18.9% for amateur golfers [5, 17,18,19,20,21,22,23,24,25,26], with one study suggesting a generally higher injury rate in professionals compared with amateurs (5.5 vs. 3.2%) . Conversely, two studies indicated that less skilled players (players with a higher handicap) may be more prone to knee injury [17, 27]. Three of the five studies comparing male and female golfers showed a higher rate of knee injuries among men [14, 17, 18], whereas equivalent injury rates were observed in male and female players in professional golf groups .
Many surveys questioned participants regarding the mechanisms and timing of their golfing injury, but only limited data refer specifically to the knee. Players surveyed by Batt  attributed their knee injury to an incorrect swing/mis-hit or standing on uneven ground (50%, respectively). Gosheger et al.  reported that 95.7% of players felt their knee injury was due to overuse. Additionally, McCarroll and Gioe  reported that the impact (30.4%) and follow-through (38.5%) phases of swing were the common time points of injury.
In all three surveys conducted by Fradkin et al. [19, 27, 28], the median age of golfers was consistently highest in those experiencing injuries at the knee, with one survey revealing that players aged >65 years were at greatest risk of lower limb injuries. Sugaya et al.  presented similar findings: senior male professional tour players experienced knee injuries at a higher rate (15%) than regular male (8%) and female (8%) professionals. However, in a cohort of golfers with a mean age of 50 years, Batt  found the average age of players with knee injuries was 35.6 years. One study that specifically addressed the issue of knee injuries in right-handed golfers found that the distribution between left (15) and right (17) knees was comparable (only three bilateral) . Since this was the only formal survey reporting on the laterality of a player’s injury, it remains difficult to establish which knee is injured more often.
Orthopedic surgeons commonly recommend golf as a rehabilitative activity following TKA, independent of whether the TKA involved the lead or the trailing knee [1,2,3,4]. In their survey of active amateur golf players after TKA (96% right-handed players, minimum 3 years post-operative), Mallon and Callaghan  found that 15.7% of subjects experienced a mild ache while playing golf and 34.9% experienced aching pain after playing (Table 2). Additionally, there was a statistically significant higher rate of pain during and after play in patients who received left knee TKAs (almost entirely lead knees). Another significant finding was that 54% of all TKAs and 79% of those with cemented implants had experienced radiographic loosening since their procedure (Table 2) . In a separate study, three professionals and 39 amateur golfers with TKAs were surveyed following return to golf. No professionals reported any pain, injury, or revision at the mean follow-up of 4 years . Only 10% of the amateur players reported pain (mean 5 years post-operatively), which was lower than the preoperative levels. Finally, when comparing TKA golfers and age-matched control players, TKA players have been found to experience higher rates of mild and considerable pain (TKA: 49 and 6% vs. controls 7 and 0%, respectively) .
While most studies have failed to report the type of knee injuries, case studies offer some indication as to the structures that are susceptible to damage when playing golf. Fractures and osteochondral fractures of the patella, tibial stress fractures, failure of polyethylene knee arthroplasty components, and medial meniscal tear, have all been reported while playing golf [29, 34,35,36,37,38]. Interestingly, medial meniscus tears have been reported as the most commonly diagnosed injury (17 of 35 injuries) followed by joint degradation due to osteoarthritis (10 of 35)  (Table 1).
In total, 12 studies have reported on knee kinematics during the golf swing [39,40,41,42,43,44,45,46,47,48,49,50]. Kinematics were measured using retroreflective markers and infrared camera systems in all studies except that of Hamai et al. , which employed high frame-rate continuous X-ray imaging. These 12 reports yielded only a small representative population, with large variances in sample size and little consistency in subject selection. These factors made comparisons between groups and assessment of influences on kinematic differences difficult to establish. As a result, only two major comparisons were made: kinematic differences between skill levels according to handicap (HC), and kinematic differences between different clubs. Players were categorised into three skill groups: amateurs (HC >10), skilled amateurs (HC 1–10), and professionals (HC 0). Club types were also classified into three groups: driver, mid-irons (5–7), and pitching wedge or 9-iron.
The majority of reports in the literature address motion of the knee about the flexion–extension axis, but only two studies included kinematics regarding internal/external rotation. No studies reporting tibio-femoral abduction/adduction or translations in any plane during the golf swing were found as a result of this literature review.
Lead Knee Flexion/Extension
Knee flexion angle in the lead knee at the top-BS varied only little across most studies according to both club and skill level of the player (Figs. 2, 3). However, our understanding of flexion angle at impact remains less clear. Here, Somjarod et al.  observed noticeable differences between professional (n = 2) and amateur (n = 2) players, but no such differences were observed in another study assessing a cohort of 308 players of differing skill levels . Additionally, no significant differences in flexion angles were observed during any swing phase when kinematics were measured during swings using either a driver, 5-iron, or pitching wedge . Similar patterns of rapid extension were observed during the mid-DS phase in professional (234 ± 24 deg/s) and amateur (184 ± 30 deg/s) cohorts . However, players achieving a high ball velocity have been shown to have significantly higher rates of lead knee extension than players with a low ball velocity at both the early- and mid-DS phases (164 ± 62 vs. 53 ± 69 and 238 ± 76 vs. 177 ± 47 deg/s, respectively) .
Despite the aforementioned similarities in kinematics with respect to skill level and club, one study indicated sex may indeed play a role. Egret et al.  found that males experienced greater flexion in the lead knee at the top-BS (35° ± 5°) than did females (17° ± 6°), in what the authors hypothesized may be an effort to compensate for decreased hip and shoulder rotation.
Lead Knee Axial Rotation
In their study of a single subject with a lead knee TKA, Hamai et al.  used video-fluoroscopy to measure the relative axial rotation of the tibial and femoral components, with the following measurements: address −6.3°, early-BS −7.4°, late-BS −8.1°, top-BS −13°, and end-FT −2.7°. Positive values indicate internal rotation of the tibia . Using skin marker-based motion capture, Somjarod et al.  reported significant differences in lead knee axial rotation between two professional and two amateur players at the mid-DS (−15° ± 5° vs. −8° ± 2°), impact (2° ± 2° vs. 10° ± 3°), and mid-FT (4° ± 2° vs. 11° ± 4°) phases.
Trail Knee Flexion/Extension
Seven studies also included results for trail knee kinematics [39,40,41, 44, 45, 47, 49]. The trail knee in the sagittal plane exhibited a smaller range of flexion as well as less rapid movements throughout the course of the swing compared with the lead knee (Fig. 4). No significant differences were found when comparing the angular velocity of the trail leg between groups with high and low ball velocity . The results presented by Somjarod et al.  also showed that maximum trail knee angular velocity occurred during the mid-DS phase for both professionals and amateurs, but the magnitude was far less than that of the lead knee (137.8 ± 31.4 vs. 113.1 ± 10.6 deg/s) .
Trail Knee Axial Rotation
The axial rotation of the trailing leg was measured in three subjects with a TKA using video-fluoroscopy : address 9.8° ± 7.7°, early-BS 12.5° ± 7.6°, late-BS 13.9° ± 6.6°, top-BS 16.0° ± 6.7°, and end-FT −5.5° ± 4.9°. Using skin marker-based motion capture, Somjarod et al.  also reported axial rotations of the trail knee: early-BS −3.0° ± 4.1° vs. −2.2° ± 1.9°, mid-BS 2.3° ± 4.0° vs. 6.8° ± 1.0°, top-BS 4.5° ± 3.8° vs. 9.4° ± 1.5°, mid-DS −13.5° ± 1.9° vs. −8.0° ± 2.7°, impact −13.4° ± 2.2° vs. −9.9° ± 2.9°, and mid-FT 12.3° ± 2.4° vs. −9.0° ± 4.2°.
Finally, a study using skin-mounted markers to assess older men found that both lead knee peak internal (20° ± 7°) and external (14° ± 5°) rotations exceeded those of the trail knee (15° ± 6° and 10° ± 6°, respectively) . The effect of club influence on knee axial rotation has not yet been reported in the literature.
The literature search identified six studies that calculated or measured the forces and/or moments occurring at the knee during the golf swing [7, 10, 11, 44, 48, 51]. Four studies used inverse dynamics driven by motion capture and ground reaction forces to calculate the external moments and reaction forces [7, 44, 48, 51]. The remaining two studies reported results measured from subjects with instrumented TKAs [10, 11].
The peak compressive force calculated using inverse dynamics was 100.0 ± 18.9%BW in the lead knee and 71.5 ± 8.7%BW in the trail knee, occurring at 29.5° ± 9.2° and 21.5° ± 6.0° of flexion, respectively  (Table 3). Contrary to these early results, D’Lima et al.  measured tibio-femoral contact forces in the lead knee of up to 440 and 320%BW in the trailing knee. Additionally, the difference between lead knee contact force when using a sand wedge and driver was only 30%BW  (Table 3). A second study measuring a single left-handed player with a right (lead) knee instrumented implant reported contact forces of 320%BW occurring at 27–30° of flexion  (Table 3).
Although only a few quantitative results have been presented, anterior–posterior shear forces in the lead knee calculated using inverse dynamics  suggested magnitudes in a range comparable to that measured using instrumented implants : 39 ± 11 and 34 ± 1%BW, respectively; knee unspecified (Table 3).
Only two studies reported the magnitude of both abduction and adduction moments calculated using inverse dynamics [7, 51]. Lynn and Noffal  reported abduction moments with the lead foot straight at address and externally rotated by 30° that were similar in magnitude to those published by Pfeiffer et al.  (Table S1 in the Electronic Supplementary Material [ESM]). However, this external rotation of the lead foot did significantly reduce the magnitude of adduction moments when compared with the straight foot stance . Conversely, Gatt et al.  calculated larger adduction than abduction moments in the lead knee (Table S1 in the ESM). The comparative magnitude of flexion and extension moments in the lead knee was inconsistent across studies. Flexion moments ranged from 0.10  to 1.26 ± 0.41 Nm/kg , whereas extension moments ranged from 0.27 ± 0.31  to 1.15 Nm/kg  (Table S1 in the ESM) [7, 11, 44, 48, 51]. The magnitude of knee axial rotation moments has only been reported twice in the literature, once calculated using inverse dynamics and once using instrumented TKAs [7, 11]. Although D’Lima et al.  did not indicate the direction of the measured axial moment or the knee in which it was measured, the magnitude (0.17 ± 0.02 Nm/kg) was comparable to that reported by Gatt et al.  for lead knee internal rotation (0.21 ± 0.07 Nm/kg) but less than the external rotation moment (0.36 ± 0.13 Nm/kg). Additionally, Gatt et al.  calculated significantly smaller magnitudes of both internal and external rotation moments in the trailing knee when compared with the lead knee.
Magnitudes of flexion and extension moments about the trail knee during the golf swing were within similar ranges across studies [7, 44, 48] (Table S2 in the ESM). Choi et al.  observed that less skilled golfers exhibited a more random pattern of peak knee flexion moment in relation to knee flexion angle than did their more consistent skilled counterparts.
Three studies reporting muscle activity about the knee during the golf swing were identified. Carlsöö  measured over 300 5-iron swings from a single professional male golfer, but little detail was provided as to the data-collection methods used, and only qualitative data could be extracted from the results. Bechler et al.  utilized fine wire insulated needles inserted directly into muscle bellies to measure three muscles crossing the knee joint (biceps femoris [long head], semimembranosus, and vastus lateralis) during the driver swings of 13 skilled amateur golfers. More recently, surface electrodes were used to measure the activity of six muscles crossing the knee joint (vastus medialis, vastus lateralis, rectus femoris, biceps femoris, semitendinosus, gastrocnemius medialis, and gastrocnemius lateralis) during the swings of players using a pitching wedge, as well as a 7- and 4-iron . The two latter studies expressed muscle activity as a percentage of a maximum muscle voluntary contraction (%MVC).
Qualitative assessment of results presented by Carlsöö  showed that, following top-BS, the flexors of the lead leg (biceps femoris, semimembranosus, and semitendinosus) experience maximum activation, which is maintained until the late-FT. At early-DS, the major extensors of the knee (rectus femoris, vastus medialis, and vastus lateralis) all experience an increase in activity, reaching a peak around impact. Following impact, the biceps femoris, semimembranosus, and semitendinosus muscles remain activated during the early-FT, followed by a decrease in activation until late-FT. Concurrently, the activity of the rectus femoris, vastus lateralis, and vastus medialis remains moderate immediately following impact and gradually decreases as the FT continues to the finish of the swing .
Bechler et al.  measured high levels of activation in the vastus lateralis (88%MVC) during the forward swing, which was maintained into the early-FT (59%MVC). Activity of the biceps femoris and semimembranosus also peaked during phases of the forward swing, with activation levels of 83 and 51%MVC, respectively (Fig. 2). Similar results reported by Marta et al.  showed high levels of quadriceps (vastus medialis, rectus femoris, and vastus lateralis) activity, i.e., 43–58%MVC during the forward swing. Muscles of the hamstrings (biceps femoris and semitendinosus) also showed peak activity of 33–57%MVC during the latter stages of the forward swing (Fig. 2). Additionally, no significant difference in lead leg muscle activity was reported between the use of a pitching wedge, a 7-iron, and a 4-iron .
During the backswing, all three studies measured minimal activity in the extensor muscles but moderate levels of activity in the flexors of the trail knee. Most notably, all three reports measured peak knee flexor activity during the early stages of the DS, which was immediately followed by a major decrease in activation prior to impact [52,53,54]. Extensors of the trail leg showed consistent activation throughout all phases of the forward swing and FT, but the magnitude of peak activity was less than that of the knee flexors [53, 54] (Fig. 4). Marta et al.  reported significant differences in activation levels between the 4-iron and pitching wedge in all muscles besides the vastus lateralis .